Yield strength test for the ME's (and good info for the rest of us).

[CRASH]

OK, I'm beginning to think about my rear suspension for next winter's project. I won't bore you with details, but there will be shaped links involved. As we have seen from recent experience, 2" x .25" tube is not stout enough to withstand the kind punishment we are dishing out when used in a rear LCA application. It's fine for the front, as rocks tend to slide off, but in the rear, under articulation, you are actually driving into rocks under power, and they bend.

So, I want to construct a lower wishbone out of mild steel that will have 50% greater yield strength than 2" x .25" DOM tube. I want to know what thickness of material will achieve this goal given the following design parameters.

Length of member = 38"
Maximum allowable width = 2"
Maximum allowable height = 3"

Assume the stress to be induced perpendicular to the wishbone for simplicity. What material thickness is needed to achieve 50% greater yield strength than 2" x .25" mild steel DOM round tube. Show your work!

CRASH

[BrettM]

2x2 .250 square

or better yet, since your parameters allow, 2x3 .250 rect. tube.

Show your work!


:flipoff2:



Or screw mild steel. 1.75" .188wall heat treated 4130 is 30% stronger than 2" .250wall DOM (source: PolyPerformance), so I'd imagine 1.75" .250wall heat treated 4130 would be about 50% stronger, and 2" .188 or especially 2" .250 would be way more than 50% stronger.

[CRASH]

Oh, and some basic engineering info for the rest of us to ponder (source: Wikepedia):

There are three typical definitions of tensile strength:

* Yield Strength - The stress a material can withstand without permanent deformation.

* Ultimate Strength - The maximum stress a material can withstand.

* Breaking Strength - The stress coordinate on the Stress-strain curve at the point of rupture.


Yield strength, or the yield point, is defined in engineering and materials science as the stress at which a material begins to plastically deform. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permenent and non-reversible. Knowledge of the yield point is vital when designing a component since it generally represents an upper limit to the load that can be applied. It is also important for the control of many materials production techniques such as forging, rolling, or pressing

In structural engineering, yield is the permanent plastic deformation of a structural member under stress. This is a soft failure mode which does not normally cause catastrophic failure unless it accelerates buckling.
Contents

Definition

It is often difficult to precisely define yield due to the wide variety of stress-strain behaviours exhibited by real materials. In addition there are several possible ways to define the yield point in a given material:

* The point at which dislocations first begin to move. Given that dislocations begin to move at very low stresses, and the difficulty in detecting such movement,this definition is rarely used.
* Elastic Limit - The lowest stress at which permenent deformation can be measured. This requires a complex iterative load-unload procedure and is critically dependent on the accuracy of the equipment and the skill of the operator.
* Proportional Limit - The point at which the stress-strain curve becomes non-linear. In most metallic materials the elastic limit and proportional limit are essentially the same.
* Offset Yield Point (proof stress) - Due to the lack of a clear border between the elastic and plastic regions in many materials, the yield point is often defined as the stress at some arbitrary plastic strain (typically 0.2%). This is determined by the intersection of a line offset from the linear region by the required strain. In some materials there is essentially no linear region and so a certain value of plastic strain is defined instead. Although somewhat arbitrary this method does allow for a consistent comparison of materials and is the most common.

http://upload.wikimedia.org/wikipedia/en/0/00/Stress_v_strain_A36_2.png

Steel has a very linear stress-strain relationship up to a sharply defined yield point, as shown in the figure. For stresses below this yield strength all deformation is recoverable, and the material will relax into its initial shape when the load is removed. For stresses above the yield point, a portion of the deformation is not recoverable, and the material will not relax into its initial shape. This unrecoverable deformation is known as plastic deformation. For many applications plastic deformation is unacceptable, and the yield strength is used as the design limitation.

After the yield point, steel and many other ductile metals will undergo a period of strain hardening, in which the stress increases again with increasing strain up to the ultimate strength. If the material is unloaded at this point, the stress-strain curve will be parallel to that portion of the curve between the origin and the yield point. If it is re-loaded it will follow the unloading curve up again to the ultimate strength, which has become the new yield strength.

After steel has been loaded to its ultimate strength it begins to "neck" as the cross-sectional area of the specimen decreases due to plastic flow. Necking is accompanied by a region of decreasing stress with increasing strain on the stress-strain curve. After a period of necking, the material will rupture and the stored elastic energy is released as noise and heat. The stress on the material at the time of rupture is known as the breaking stress. Note that if the graph is plotted in terms of true stress and true strain necking will not be observed on the curve as true stress is corrected for the decrease in cross-sectional area. Necking is also not observed for materials loaded in compression.

Ductile metals other than steel typically do not have a well defined yield point. For these materials the yield strength is typically defined by the "0.2% offset strain". The yield strength at 0.2% offset is determined by finding the intersection of the stress-strain curve with a line parallel to the initial slope of the curve and which intercepts the abscissa at 0.002. A stress-strain curve typical of aluminum along with the 0.2% offset line is shown in the figure below.

Yielded structures have a lower and less constant modulus of elasticity, so deflections increase and buckling strength decreases, and both become more difficult to predict. When load is removed, the structure will remain permanently bent, and may have residual pre-stress. If buckling is avoided, structures have a tendency to adapt a more efficient shape that will be better able to sustain (or avoid) the loads that bent it. Because of this, highly engineered structures rely on yielding as a graceful failure mode which allows fail-safe operation. In aerospace engineering, for example, no safety factor is needed when comparing limit loads (the highest loads expected during normal operation) to yield criteria. Safety factors are only required when comparing limit loads to ultimate failure criteria, (buckling or rupture.) In other words, a plane which undergoes extraordinary loading beyond its operational envelope may bend a wing slightly, but this is considered to be a fail-safe failure mode which will not prevent it from making an emergency landing.

[CRASH]

More (source: Knowledge Article from www.Key-to-Steel.com):

The engineering tension test is widely used to provide basic design information on images/the strength of materials and as an acceptance test for the specification of materials. In the tension test a specimen is subjected to a continually increasing uniaxial tensile force while simultaneous observations are made of the elongation of the specimen. An engineering stress-strain curve is constructed from the load elongation measurements (Fig. 1).

Figure 1. The engineering stress-strain curve

It is obtained by dividing the load by the original area of the cross section of the specimen.
(1)

The strain used for the engineering stress-strain curve is the average linear strain, which is obtained by dividing the elongation of the gage length of the specimen, d, by its original length.
(2)

Since both the stress and the strain are obtained by dividing the load and elongation by constant factors, the load-elongation curve will have the same shape as the engineering stress-strain curve. The two curves are frequently used interchangeably.

The shape and magnitude of the stress-strain curve of a metal will depend on its composition, heat treatment, prior history of plastic deformation, and the strain rate, temperature, and state of stress imposed during the testing. The parameters, which are used to describe the stress-strain curve of a metal, are the tensile strength, yield strength or yield point, percent elongation, and reduction of area. The first two are strength parameters; the last two indicate ductility.

The general shape of the engineering stress-strain curve (Fig. 1) requires further explanation. In the elastic region stress is linearly proportional to strain. When the load exceeds a value corresponding to the yield strength, the specimen undergoes gross plastic deformation. It is permanently deformed if the load is released to zero. The stress to produce continued plastic deformation increases with increasing plastic strain, i.e., the metal strain-hardens. The volume of the specimen remains constant during plastic deformation, A·L = A0·L0 and as the specimen elongates, it decreases uniformly along the gage length in cross-sectional area.

Initially the strain hardening more than compensates for this decrease in area and the engineering stress (proportional to load P) continues to rise with increasing strain. Eventually a point is reached where the decrease in specimen cross-sectional area is greater than the increase in deformation load arising from strain hardening. This condition will be reached first at some point in the specimen that is slightly weaker than the rest. All further plastic deformation is concentrated in this region, and the specimen begins to neck or thin down locally. Because the cross-sectional area now is decreasing far more rapidly than strain hardening increases the deformation load, the actual load required to deform the specimen falls off and the engineering stress likewise continues to decrease until fracture occurs.
Tensile Strength

The tensile strength, or ultimate tensile strength (UTS), is the maximum load divided by the original cross-sectional area of the specimen.
(3)

The tensile strength is the value most often quoted from the results of a tension test; yet in reality it is a value of little fundamental significance with regard to the strength of a metal. For ductile metals the tensile strength should be regarded as a measure of the maximum load, which a metal can withstand under the very restrictive conditions of uniaxial loading. It will be shown that this value bears little relation to the useful strength of the metal under the more complex conditions of stress, which are usually encountered.

For many years it was customary to base the strength of members on the tensile strength, suitably reduced by a factor of safety. The current trend is to the more rational approach of basing the static design of ductile metals on the yield strength.

However, because of the long practice of using the tensile strength to determine the strength of materials, it has become a very familiar property, and as such it is a very useful identification of a material in the same sense that the chemical composition serves to identify a metal or alloy.

Further, because the tensile strength is easy to determine and is a quite reproducible property, it is useful for the purposes of specifications and for quality control of a product. Extensive empirical correlations between tensile strength and properties such as hardness and fatigue strength are often quite useful. For brittle materials, the tensile strength is a valid criterion for design.
Measures of Yielding

The stress at which plastic deformation or yielding is observed to begin depends on the sensitivity of the strain measurements. With most materials there is a gradual transition from elastic to plastic behavior, and the point at which plastic deformation begins is hard to define with precision. Various criteria for the initiation of yielding are used depending on the sensitivity of the strain measurements and the intended use of the data.

1. True elastic limit based on micro strain measurements at strains on order of 2 x 10-6 in | in. This elastic limit is a very low value and is related to the motion of a few hundred dislocations.
2. Proportional limit is the highest stress at which stress is directly proportional to strain. It is obtained by observing the deviation from the straight-line portion of the stress-strain curve.
3. Elastic limit is the greatest stress the material can withstand without any measurable permanent strain remaining on the complete release of load. With increasing sensitivity of strain measurement, the value of the elastic limit is decreased until at the limit it equals the true elastic limit determined from micro strain measurements. With the sensitivity of strain usually employed in engineering studies (10-4in | in), the elastic limit is greater than the proportional limit. Determination of the elastic limit requires a tedious incremental loading-unloading test procedure.
4. The yield strength is the stress required to produce a small-specified amount of plastic deformation. The usual definition of this property is the offset yield strength determined by the stress corresponding to the intersection of the stress-strain curve and a line parallel to the elastic part of the curve offset by a specified strain (Fig. 1). In the United States the offset is usually specified as a strain of 0.2 or 0.1 percent (e = 0.002 or 0.001).
(4)

A good way of looking at offset yield strength is that after a specimen has been loaded to its 0.2 percent offset yield strength and then unloaded it will be 0.2 percent longer than before the test. The offset yield strength is often referred to in Great Britain as the proof stress, where offset values are either 0.1 or 0.5 percent. The yield strength obtained by an offset method is commonly used for design and specification purposes because it avoids the practical difficulties of measuring the elastic limit or proportional limit.

Some materials have essentially no linear portion to their stress-strain curve, for example, soft copper or gray cast iron. For these materials the offset method cannot be used and the usual practice is to define the yield strength as the stress to produce some total strain, for example, e = 0.005.
Measures of Ductility

At our present degree of understanding, ductility is a qualitative, subjective property of a material. In general, measurements of ductility are of interest in three ways:

1. To indicate the extent to which a metal can be deformed without fracture in metalworking operations such as rolling and extrusion.
2. To indicate to the designer, in a general way, the ability of the metal to flow plastically before fracture. A high ductility indicates that the material is "forgiving" and likely to deform locally without fracture should the designer err in the stress calculation or the prediction of severe loads.
3. To serve as an indicator of changes in impurity level or processing conditions. Ductility measurements may be specified to assess material quality even though no direct relationship exists between the ductility measurement and performance in service.

The conventional measures of ductility that are obtained from the tension test are the engineering strain at fracture ef (usually called the elongation) and the reduction of area at fracture q. Both of these properties are obtained after fracture by putting the specimen back together and taking measurements of Lf and Af .
(5)
(6)

Because an appreciable fraction of the plastic deformation will be concentrated in the necked region of the tension specimen, the value of ef will depend on the gage length L0 over which the measurement was taken. The smaller the gage length the greater will be the contribution to the overall elongation from the necked region and the higher will be the value of ef. Therefore, when reporting values of percentage elongation, the gage length L0 always should be given.

The reduction of area does not suffer from this difficulty. Reduction of area values can be converted into an equivalent zero-gage-length elongation e0. From the constancy of volume relationship for plastic deformation A*L = A0*L0, we obtain
(7)

This represents the elongation based on a very short gage length near the fracture.

Another way to avoid the complication from necking is to base the percentage elongation on the uniform strain out to the point at which necking begins. The uniform elongation eu correlates well with stretch-forming operations. Since the engineering stress-strain curve often is quite flat in the vicinity of necking, it may be difficult to establish the strain at maximum load without ambiguity. In this case the method suggested by Nelson and Winlock is useful.

[CRASH]

Since this will be a constructed member, I am not limited to pre-formed shapes. The main trailing portions of the wishbone will see the deflective loads, so they are the only thing I am worried about. Round tube does not fit my design parameters.

In other words, I can use any combination of material thickness commonly availabe in sheet form, and it can be cut into any combination of height and width within, the design parameters.

Oh, an Brett if you were really an engineer, you would have suggested a much more expensive option, requiring months of time to source, and utilizing new tooling for you to play with. :flipoff:

CRASH


2x2 .250 square

or better yet, since your parameters allow, 2x3 .250 rect. tube.



:flipoff2:



Or screw mild steel. 1.75" .188wall heat treated 4130 is 30% stronger than 2" .250wall DOM (source: PolyPerformance), so I'd imagine 1.75" .250wall heat treated 4130 would be about 50% stronger, and 2" .188 or especially 2" .250 would be way more than 50% stronger.

[GSequoia]

Now I know why I don't got into AdFab that much.

:confused:

[goodburbon]

Wow a whole semester of materials science in one sitting.....wanna buy the book? I'm don't need mine now that I have seen this.

[XJ_ranger]

Wow a whole semester of materials science in one sitting.....wanna buy the book? I'm don't need mine now that I have seen this.

just finished my final - anyone want to buy mine? the book store wont take it back.... fawking $150 book!

Andy - Is heat trating aloud?
what amount of strain do you want?

Increasing the yeild stress by 50% but making the material too brittle wont do you a damn thing...

take your metal file that you use to de-burr stuff - the yeild strength of that material is really high, but throw it at the ground and it shatters - its much too brittle for what you are wanting to do, but meets your yeild requirement...

[RuffedUpXJ]

Yeah I have my final next week for MAt Sci. So anyone else need a useless book? Its got a golf ball on the cover. Did you guys have a Lab with it?

[CRASH]

just finished my final - anyone want to buy mine? the book store wont take it back.... fawking $150 book!

Andy - Is heat trating aloud?
what amount of strain do you want?

Increasing the yeild stress by 50% but making the material too brittle wont do you a damn thing...

take your metal file that you use to de-burr stuff - the yeild strength of that material is really high, but throw it at the ground and it shatters - its much too brittle for what you are wanting to do, but meets your yeild requirement...

I think for the sake of cost savings, we should stick to mild steel, no heat treatment. This will definately lower yield strength potential, but it probably puts you in the ballpark on ductility. The graph in my second post is mild steel, the file in your example looks much different, right?

Also, on the strain question, I guess that depends on the strain it takes to deform the 2" tube. I guess we need to find a yield value for 2" mild steel tube, and work up from there........aren't common shapes like tube, channel, and I-beams already worked out for various types of steel?

[XJ_ranger]

The graph in my second post is mild steel, the file in your example looks much different, right?


the numbers are different, but the graph has the same shape... steeper angle = higher modulus of elasticity....

[BrettM]

wait... this is a lower wishbone you're making? If it's a wishbone, I don't understand the design parameters of 2"x3". Do you have a basic/initial sketch of this suspension?

(Is there a Hi9 on the way? only reason I can see for a lower wishbone is driveshaft protection)

[goodburbon]

also need net loaded vehicle weight, tire size, and actual yielded lift
to insure it will hold up to a good smack.

[CRASH]

also need net loaded vehicle weight, tire size, and actual yielded lift
to insure it will hold up to a good smack.



See, I don't think you need that.

We know 2"x.25" mild is not quite strong enough for LCA use. So use that as your basis and add 50% to the yield number to get a target to build toward.

The stuff you mention is a constant for our purposes.

[CRASH]

wait... this is a lower wishbone you're making? If it's a wishbone, I don't understand the design parameters of 2"x3". Do you have a basic/initial sketch of this suspension?

(Is there a Hi9 on the way? only reason I can see for a lower wishbone is driveshaft protection)


The reason for a lower wishbone is two-fold.

A: It will have a shape to it, a down ward sweep, specifically, to help grond clearance while articulated. If you do this with individual links, they flop over when using flex-type joints. Unless you do a set of mini-links or somthing to keep the joints oriented correctly,

B: With a wishbone, you get away with only needing one flex joint at the crossmember, and you can mount sway bar links or anything else you like (air bags, for instance) to the wishbone without worrying about deflection.

I'll be doing a double triangulated system, with an upper wishbone as well. That member is a lot less troublesome than the lower one, however. :D

[goodburbon]

ok.......if you're determined to use the same material, just make it 50% thicker, and that will more than cover your needs :D

[CRASH]

ok.......if you're determined to use the same material, just make it 50% thicker, and that will more than cover your needs :D


You are not understanding. Better sign for that MatSci class anyway!

I don't want to use round tube, and making somthing 50% thicker doesn't increase it's yield strength by 50%.

Where is Phil when I need him? :D

[goodburbon]

I know. I was being facetious and lazy.

[CRASH]

I know. I was being facetious and lazy.

If I wasn't so lazy, I'd do these calcs myself.

Instead I'm relying on ME students that drink too much.

[Zebaru]

I'm no ME, so maybe someone will show up and prove me all wrong, but this is what I have:

It is the section modulus that you need to consider when designing for bending

Section modulus for a .25" wall material:

2" round = 0.537in^3
2" square = 0.771in^3
2"x3" rectangle = 1.438in^3

The maximum allowable moment is then a function of this value and the yield strength. So, for a given yield strength, the allowable moment for a 2x3" rectangle is about 2.5 times greater that of a 2" round tube...

If you are looking for the strongest shape to fit within your 2X3 parameters, then a 2x3 rectangle would be it. You could get away with significantly thinner walls and still have a 50% larger moment carrying capacity than your 2" round tube, but at some point buckling and denting becomes a very real concern.

Travis

[Zebaru]

Well, I went and corrected them, and still got the numbers screwed up.

The round is correct, the 2X2 should be .911 in ^3, and the 2x3 should be 1.698 in^3.

This doesn't change much else I wrote though.

Good thing I dont do this stuff for a living...

Travis

[Phil]

Instead I'm relying on ME students that drink too much.

Who, me?

I've got an idea, but I need to spend a little quality time with my mechanical design text.

What are your constraints?
-mild steel, for cost, availability, and ease of fabrication
-nonlinear links, arcing up towards the underside of the Jeep
-50% better resistance to bending than current link materials

What else?

[XJ_ranger]

these are in compression under acceleration and tension under braking at high speeds too...
also controling the torque of the axle housing rotating about the axle shafts...

I am thinking on this, but for some reason think that I am leaving factors out that will be an issue...

things like - for the shape of material used, when they are in compression, they are acting as an axialy loaded collumn, and you have to compute the radius of gyration from the neutral axis FOR ALL 3 axies! and choose the WEAKEST one of them to find the critical load....

hmmmm - too much brain work - time to go take a final... ill think on it more while im in Clayton this weekend...

[CRASH]

I'm no ME, so maybe someone will show up and prove me all wrong, but this is what I have:

It is the section modulus that you need to consider when designing for bending

Section modulus for a .25" wall material:

2" round = 0.537in^3
2" square = 0.771in^3
2"x3" rectangle = 1.438in^3

The maximum allowable moment is then a function of this value and the yield strength. So, for a given yield strength, the allowable moment for a 2x3" rectangle is about 2.5 times greater that of a 2" round tube...

If you are looking for the strongest shape to fit within your 2X3 parameters, then a 2x3 rectangle would be it. You could get away with significantly thinner walls and still have a 50% larger moment carrying capacity than your 2" round tube, but at some point buckling and denting becomes a very real concern.

Travis


I'm looking for the minimum material thickness in all 4 walls of a rectangular shape. So, for instance, to combat denting, I could use 1/4" for the bottom, and only 1/8" on top. The sides of the rectangle, are, of course the determining factor in combating a strain encountered when I land on a rock at 60 mph, or am powering into a large boulder. So, my inclination is that I need to be at .188" on the side material, which I think plays out in the section modulus calc.

[CRASH]

Who, me?

I've got an idea, but I need to spend a little quality time with my mechanical design text.

What are your constraints?
-mild steel, for cost, availability, and ease of fabrication
-nonlinear links, arcing up towards the underside of the Jeep
-50% better resistance to bending than current link materials

What else?

The links will be bowed in a large radius (my initial calcs show this at about 48" radius), so no point-specific loads. I can use different material for all 4 sides of the rectangle, as all sides will be cut and shaped independantly. This means I could even incorporate a third vertical member into the rectangle section if it meant saving weight with thinner material, while gaining yield.

[Dirk Pitt]

The sides of the rectangle, are, of course the determining factor in combating a strain encountered when I land on a rock at 60 mph, or am powering into a large boulder. So, my inclination is that I need to be at .188" on the side material, which I think plays out in the section modulus calc.

I think I understand you saying here that the sides are more impotrant in bending than the top/bottom. Is this right?

If that is in fact what you are stating, it's not right.

You want the most material the farthest distance from the centroidal axis that runs perpindicular to the force the rock is placing on the link.

For example, a rectangular link that has a top/bottom thickness of 0.25 in. with 0.125 vertical sides has much more resistance to bending than a section with top/bottom 0.125 in., and vertical sides of 0.250, when the load is placed from the bottom.

This is a good topic...

[Paul S]

Seems awfully complicated.
I took a quick drive down to Deaver last week & ordered a set of custom links with built in springs. Super simple.
Don't know the yeild strenth, but it's almost impossible to exceed it.

:confused1

Paul

[XJ_ranger]

I think I understand you saying here that the sides are more impotrant in bending than the top/bottom. Is this right?

If that is in fact what you are stating, it's not right.

You want the most material the farthest distance from the centroidal axis that runs perpindicular to the force the rock is placing on the link.

For example, a rectangular link that has a top/bottom thickness of 0.25 in. with 0.125 vertical sides has much more resistance to bending than a section with top/bottom 0.125 in., and vertical sides of 0.250, when the load is placed from the bottom.

This is a good topic...

so an I beam is the lightest, strongest best way to do this?

[SCW]

I hated my structural classes with a passion, but yes- you want as much steel away from the center as you can get it to resist bending.

An I-beam does a good job of putting steel away from center, but you end up with one strong axis and one weak axis. If you use the 2x3" HSS you do not have any unsupported flange sections and get more strenght for your cross-sectional surface area of steel. Increasing the amount of steel increases the load-carrying capacity, even if you are using the same yeild strength because the capacity is a combination of area and yield strength.

Round tube is also very strong in tension and compression, but less resistive to bending the way the HSS will be. I wouldn't build an I-beam out of it for 2 reasons-

1- it would be difficult for the gains
2-HSS is more supportive in weak axis and I'm not convinced the member won't experience and non-perfectly axial force.

[Dirk Pitt]

I hated my structural classes with a passion, but yes- you want as much steel away from the center as you can get it to resist bending.

An I-beam does a good job of putting steel away from center, but you end up with one strong axis and one weak axis. If you use the 2x3" HSS you do not have any unsupported flange sections and get more strenght for your cross-sectional surface area of steel. Increasing the amount of steel increases the load-carrying capacity, even if you are using the same yeild strength because the capacity is a combination of area and yield strength.

Round tube is also very strong in tension and compression, but less resistive to bending the way the HSS will be. I wouldn't build an I-beam out of it for 2 reasons-

1- it would be difficult for the gains
2-HSS is more supportive in weak axis and I'm not convinced the member won't experience and non-perfectly axial force.

Very well said.

[Weasel]

trick question, no matter what thickenss you use the yield strength isn't going to change. It will yield at 50-60ksi. Now if you mean you want to reduce the stress in the part by 50%, we can do that. :flipoff2:

[What Rd]

I'm no engineer, but I know logic. Your goal of a 50% improvement is totally arbitrary with no stated support for the assumption that this level will satisfy your strength needs.
You first need to figure out how strong your wishbone needs to be, before you can work out a design to meet that goal.:dunno:

[SCW]

trick question, no matter what thickenss you use the yield strength isn't going to change. It will yield at 50-60ksi. Now if you mean you want to reduce the stress in the part by 50%, we can do that. :flipoff2:

Yield strength is pretty near irrelevant imo. If you don't think you have enough SHEAR CAPACITY, increase the AREA of STEEL. Strength is the yield strength multiplied by the cross-sectional area of the steel. Think about the units, shear at 36ksi is 36 kips per SQUARE INCH. Increase the area and it taks that much more force to cause bending.

Think of rebar- they are all 60ksi yield strength, but if you need more capacity for your application you increase the area of steel. Now think of an I-beam. If you need more shear capacity you need to increase the area of steel in the web. 90% of structural steel is 55ksi steel, you get what you can and adjust the area of steel to meet required forces.

That said, this application should probably be more concerned with the ultimate moment capacity because as I said before, I doubt the forces will be completely axial. If you want good moment resistance use a steel with a high moment of inertia and plenty of steel (cross-sectional area). You can only get HSS in one yield strength anyway, and that really isn't all that high, I think it's 36ksi. Who cares- increase the area.

[Weasel]

Good grief, since when did engineers forget their sence of humor.

Yield strength is still important, although it's not the only number to use. It's only function is to give you an idea of where your stress's are in relation to the material propeties. As you said the Ixx and Iyy for the section propertites are key.

No the force are not axial but there will be bending forces and perhaps even some torsional forces on these members.

A wishbone member isn't exactly ideal as it subjects the members to loads in multiple directions, requiring extra material and weight. But we don't also get to work with ideal situations.

And as was said before to do this right, you really should have an idea of how much more is needed instead of just 50%.

[CRASH]

And as was said before to do this right, you really should have an idea of how much more is needed instead of just 50%.

If someone can show me a good way to calculate the forces experienced by a rear lower control arm in high speed and crawling application in a 5,000 lb Cherokee, I'd love to see it. I don't believe anyone can, as there are too many variables in that application.

So, we design around what we know, namely, that 2" x .25" wall is not quite strong enough. We know we want to be somewhat stronger. 50% stronger may be overkill, or it may not be. We'll have to see what kind of weight penalty there will be for various increases in strength.

I have only seen one post with calcs thus far! Seems to me we need to know the yield strength of a 2" x .25" round tube before we go much farther, right? So, how do we calculate the force necessary to deflect it past the point of plasticity?

[Zebaru]

I agree that bending isn't all that is important here, but considering that these links are typically going to fail when subject to a bending force, it is a good enough place to start.

As everyone had already pointed out, there are a number of things to take into consideration when determining the deflection in a given loading situation, but for simplicity, we can start with the most basic and very simple loading configuration -simply supported and only a single point load. I think that the simply supported assumption would be pretty good if this was one link of a four link setup, but is less appropriate for a wishbone. Without a real design, we can only make guesses at that anyway, so this is a good starting point.

You say 38" long, so we will go with that.

If we use a yield strength of 36ksi, and a section modulus of .536 in^3 for 2" round 0.25 wall tube - that gives us a maximum moment capacity of 1610 ft-lbs to yield. On a simply supported beam with a point load, the maximum moment for a point load will be at the center when the load is at the center. The moment generated by the load is (load *length) / 4. So the load is 4*moment/length or about 2000 lbs.

Travis

[Gary E]

Umm dude are you have some regrets about not getting an ME minor?

What I am envisioning sounds like alot of work. I guess if somebody would make it work I guess it would be you.

As far as the engineering throw out you calculator and get access to a good kinematic program, I think working model was one that was in play back in the school days. It will analyze forces through diffrent planes at each angle of movement instantlly. Each iteration of movement is about 3-4 pages of difficult math for each calculation.

I was really POed when I took that class slaved away for a semester on four page problems and the last day they took me to the computer lab and They showed us how to do our semesters work of problems in one hour. Heaven forbid they show us how to use the computer program and I would be able to do that stuff today.

[Jump This]

Leave me not suggest I have any ME background (I'm spending $20,000 a year for my son to get it for me :D ) BUTin the olden days when we couldn't find the slide rule we would use oval tubing in any place we didn't want any failure from impacts...(think swing arms on dirt bikes....)
Rick R
P.S. use 4130 you tight wad!

[Mr.OverKill]

Umm dude are you have some regrets about not getting an ME minor?

What I am envisioning sounds like alot of work. and to think i was envisioning going back to school to get my real on paper degree :read: instead of the real life experiance title of structural field engineer ( hell i dont even know if i spelled it right :shiver: ) should have gotten the education when i was younger and didnt have a family :dunce:

[Dirk Pitt]

I agree that bending isn't all that is important here, but considering that these links are typically going to fail when subject to a bending force, it is a good enough place to start.


Please explain this. If bending is the failure mode expected, why on earth would it not be the important factor.

Crash, you mentioned placing an airbag at some location on top of the arm. This is going to change the load conditions...

[Weasel]

Please explain this. If bending is the failure mode expected, why on earth would it not be the important factor.

Crash, you mentioned placing an airbag at some location on top of the arm. This is going to change the load conditions...

Yeah I don't get that either, bending is pretty important. if I get a chance I'll draw something in SolidWorks that i think would work well as long as your willing to spend the time to make it.

[CRASH]

Please explain this. If bending is the failure mode expected, why on earth would it not be the important factor.

Crash, you mentioned placing an airbag at some location on top of the arm. This is going to change the load conditions...


Not an airbag, he bag is going on the axle tube. Maybe a sway bar mount and MAYBE a bump stop. I can see how that would change the load, but if I have it measured right, it will go very near the axle end of the wish bone.

Spent some time measuring under the rig last night, and I can't see using an arched member. It's either going to have to be straight links, or a kink about 2/3 of the way back from the member.

This thread has already made me re-think some assumptions, and I've learned something. Now we just need a final calculated answer!

Guessing sucks.

[XJ_ranger]

Not an airbag, he bag is going on the axle tube. Maybe a sway bar mount and MAYBE a bump stop. I can see how that would change the load, but if I have it measured right, it will go very near the axle end of the wish bone.

Spent some time measuring under the rig last night, and I can't see using an arched member. It's either going to have to be straight links, or a kink about 2/3 of the way back from the member.

This thread has already made me re-think some assumptions, and I've learned something. Now we just need a final calculated answer!

Guessing sucks.

do you know how to use solid works?

see PM

[Zebaru]

Please explain this. If bending is the failure mode expected, why on earth would it not be the important factor.

I said bending isn't all that is important...

There are a bunch of things involved here, and it seems possible that one could build a section that handled bending really well, but wouldn't really work for a link.

It isn't sexy, but square or rectuangluar would probalby be the easiest, cheapest solution...

Travis

[Dirk Pitt]

do you know how to use solid works?

see PM

Solidworks will be great for modelling it, but it's not going to tell you what material to put where...

[XJ_ranger]

Solidworks will be great for modelling it, but it's not going to tell you what material to put where...

well - i thought we had agreed on using mild low carbon steel

and with the COSMOS FEA that we did for the Baja, we could choose a material, and look at the diflection based on our loading....

[David Taylor]

Not an airbag, he bag is going on the axle tube. Maybe a sway bar mount and MAYBE a bump stop.

The end of that axle tube is starting to look a little crowded. It's not sexy but how can you beat leafs if your keeping the body ? Are you willing to go through the floor with links ?

[CRASH]

The end of that axle tube is starting to look a little crowded. It's not sexy but how can you beat leafs if your keeping the body ? Are you willing to go through the floor with links ?


Yes, upper wisbone only. And I'm all about sexing it up.

[Weasel]

Cosmos is sort of mickey mouse, Abaqus is much better, and no Solidworks won't tell you where to put the material but it is good to figure out what shape you want. What kind of mounting end are you thinking? Dimensions? Now 2-3 bent members are out but maybe one bend is possible?

[CRASH]

Cosmos is sort of mickey mouse, Abaqus is much better, and no Solidworks won't tell you where to put the material but it is good to figure out what shape you want. What kind of mounting end are you thinking? Dimensions? Now 2-3 bent members are out but maybe one bend is possible?


I'm going to sketch this out and send it to Opie for input into SolidWorks. Basically, picture a very large V, the point of the V is mounted on a crossmember that is 38 inches forward of the centerline of the axle tube. The ends of the "V" are approximately where the spring perches are for the OEM leafs. So about 36 inches, IIRC.

The single bend would have to be 24 inches back from the crossmember in order to clear the floor.

[goodburbon]

pre bending square tubing would be silly since the deformations would serve to weaken the stronger structure, which is why you would choose it over round tube in the first place. Just make mitered cuts, and fab your desired shape with gutssets for excessive accidental lateral loading.

[goodburbon]

I just dreamt up a really wild idea.......what if you made your lower bracket just as planned, and for an upper bracket, a mirror image of the lower, only with the 2 outboard mounting points on the crossmember, and the inboard right over the chunk.......shouldn't even need a panhard rod ......

[CRASH]

Remember people, I have the ability to fabricate the structure completely, there is no need for cutting an mitering. I will simply cut the correct shapes out of pate and TIG them all together.

Just like the big boys do!

http://www.desertrides.com/features/vehicles/gordonTT/imagepages/DSC01987.php

[CRASH]

I just dreamt up a really wild idea.......what if you made your lower bracket just as planned, and for an upper bracket, a mirror image of the lower, only with the 2 outboard mounting points on the crossmember, and the inboard right over the chunk.......shouldn't even need a panhard rod ......


OMG, you're catching on!

You spent too much time debating Jesus, that's the plan.

[goodburbon]

I like that idea, and completely fabbed structure would kick serious ass



oh, and If I have saved one soul from being saved it was worth it.

[CRASH]

I like that idea, and completely fabbed structure would kick serious ass



oh, and If I have saved one soul from being saved it was worth it.

I'm trying to move beyond my angle grinder roots.

A fabbed structure, in my mind, opens up a whole world of possibilities, as it gets you a great combination of strength and weight savings with the proper thickness of metal at every point.

If I could find a competent heat-treat shop, I'd even consider 4130 sheet, as I can get it. The post weld treatment seems like it could get real pricey, real, quick, though. Maybe not, never priced it.

[BrettM]

CRASH, do you know what joint you plan on using for the single end of the wishbones? what about the double end?

[BrettM]

I'm trying to move beyond my angle grinder roots.

A fabbed structure, in my mind, opens up a whole world of possibilities, as it gets you a great combination of strength and weight savings with the proper thickness of metal at every point.

If I could find a competent heat-treat shop, I'd even consider 4130 sheet, as I can get it. The post weld treatment seems like it could get real pricey, real, quick, though. Maybe not, never priced it.
Have you contacted 66CJdean on Pirate? I know he does cryo, I think heat-treat also.

[CRASH]

CRASH, do you know what joint you plan on using for the single end of the wishbones? what about the double end?


1" Uni-ball at the point (106,000lb radial load rated).

Delrin or Poly at the points.

[BrettM]

1" Uni-ball at the point (106,000lb radial load rated).

Delrin or Poly at the points.
hmm, i don't really know jack about Uni-balls, but if it's good enough for trophy trucks, then cool. My friend built a 2.5ton YJ with the 1.25 Evo rebuildable heims, I like that they're easily rebuildable, and definitely plenty of beef. Of course with a wishbone, all of your articulation must be misaligned at the single joint... looks like the Evo goes 28* and a UniBall goes 32* (http://www.pirate4x4.com/tech/billavista/PR-Joints/index.html)

http://www.4x4shots.com/albums/userpics/10008/normal_3.jpg


I used delrin on my wishbone, primarily because I had no triangulation in the lowers, and only around 24" seperation on the wishbone, so I needed something very stiff. I kinda like the idea of having a little bit of poly in a link suspension to reduce shock loads.

[CRASH]

hmm, i don't really know jack about Uni-balls, but if it's good enough for trophy trucks, then cool. My friend built a 2.5ton YJ with the 1.25 Evo rebuildable heims, I like that they're easily rebuildable, and definitely plenty of beef. Of course with a wishbone, all of your articulation must be misaligned at the single joint... looks like the Evo goes 28* and a UniBall goes 32* (http://www.pirate4x4.com/tech/billavista/PR-Joints/index.html)

http://www.4x4shots.com/albums/userpics/10008/normal_3.jpg


I used delrin on my wishbone, primarily because I had no triangulation in the lowers, and only around 24" seperation on the wishbone, so I needed something very stiff. I kinda like the idea of having a little bit of poly in a link suspension to reduce shock loads.


Me too.

But now I'm thirsty and still have 5 more hours before I can imbibe. Thanks a lot.

[BrettM]

bleck. I just got back from Europe, and I brought back some amazing Belgian beers. If I still have any left in a couple weeks I'll bring ya one.

[CRASH]

bleck. I just got back from Europe, and I brought back some amazing Belgian beers. If I still have any left in a couple weeks I'll bring ya one.

High zoot beer is lovely for enjoyment.

Does not serve well as fabrication lubrication. :D

[BrettM]

High zoot beer is lovely for enjoyment.

Does not serve well as fabrication lubrication. :D
i go for the rum'n'coke for cheap and refreshing

[Weasel]

Well I drew this up really late last night but it might be close ot what yu are looking for, in the bent sections I would increase the height of the box, then thin it back down towards the plate.

http://img.villagephotos.com/p/2004-10/862678/untitled.JPG

[Weasel]

Rod ends on a wishbone are bad. It puts the center of rotation ahead of where the arms want to rotate causing additonal forces(moment) on the end itself. This is usually self destructive or causing the design to be very overbuilt. With the Uniball or Spehrical Beairng in a plate you can locate it on the intanoues center of the arm like it should be.

[BrettM]

Rod ends on a wishbone are bad. It puts the center of rotation ahead of where the arms want to rotate causing additonal forces(moment) on the end itself. This is usually self destructive or causing the design to be very overbuilt. With the Uniball or Spehrical Beairng in a plate you can locate it on the intanoues center of the arm like it should be.
true in theory, but as long as the rod-end doesn't stick way out from the point the wishbone converges I don't see it being a problem in practical application. besides, the part that needs to be overbuilt in that instance is the shank of the rod-end, which I think 1.25" of 4140 is already overbuilt anyway.

[Joe]

Ouch I am getting a head ache just reading all of this information. And I thought programming Java application was hard.

[XJ_ranger]

:stir:
:stir:
:stir:

http://gearinstalls.com/scottcoil.htm

[BrettM]

:stir:
:stir:
:stir:

http://gearinstalls.com/scottcoil.htm
I've seen that before, but don't see how it applies to this topic. That setup does nothing to achieve better geometry like a links suspension does. That setup does nothing to provide ride height adjustment.

http://gearinstalls.com/scottcoil_files/scottcoil13.jpg

[Captain Ron]

...
If I could find a competent heat-treat shop, I'd even consider 4130 sheet, as I can get it.
...

Got pretzel? :D

--ron

[XJ_ranger]

I've seen that before, but don't see how it applies to this topic. That setup does nothing to achieve better geometry like a links suspension does. That setup does nothing to provide ride height adjustment.

http://gearinstalls.com/scottcoil_files/scottcoil13.jpg

werent we talking about air bags and such to gain height and keep a flexable suspention?

i thought that was this thread...

[BrettM]

werent we talking about air bags and such to gain height and keep a flexable suspention?

i thought that was this thread...
center mount airbags were discussed (i think this thread, I'm too lazy to look through all the posts) but for the purpose of ride-height adjustment, which a coil spring does not accomplish.

[lazyxj]

center mount airbags were discussed (i think this thread, I'm too lazy to look through all the posts) but for the purpose of ride-height adjustment, which a coil spring does not accomplish.

To paraphrase 'gearinstalls' article, "The lower spring cup dish unbolts and spacers can be inserted to create additional lift"

Doesn't that count as ride height adjustment?

[CRASH]

That was the quarter elliptical thread.

Try and keep up. :D

[vetteboy]

I was searching for something else, and found this thread by accident.

What did you end up doing with this idea?

[5-90]

Oh, an Brett if you were really an engineer, you would have suggested a much more expensive option, requiring months of time to source, and utilizing new tooling for you to play with. :flipoff:
CRASH

Not so - engineers are, by nature, lazy. Most of the stuff we have to fight with when working on a vehicle is probably the product of a designer (form follows function, not the other way around, people!) accountants (I can understand saving a few cents over a production life of two million vehicles, but have some understanding of what mechanics go through...) and/or lawyers (and Gawd knows why they do what they do...)

An engineer would design something he'd have to work on himself, and would therefore design it to use a minimum of tools (all common,) and make everything maintenance-related fully and independently accessible.

Don't suppose you have a general idea we can go forward with (I hadn't read the whole thing - just the first page and whatever's up now,) but if you've got "unlimited fab ability," then we can take your basic idea, apply various mill shapes (and materials!) to it, and go forward from there. Also, what sort of stresses to you foresee - we may see something different, but it would still be a jumping-off point...

5-90

[vetteboy]

Not so - engineers are, by nature, lazy. Most of the stuff we have to fight with when working on a vehicle is probably the product of a designer (form follows function, not the other way around, people!) accountants (I can understand saving a few cents over a production life of two million vehicles, but have some understanding of what mechanics go through...) and/or lawyers (and Gawd knows why they do what they do...)

An engineer would design something he'd have to work on himself, and would therefore design it to use a minimum of tools (all common,) and make everything maintenance-related fully and independently accessible.

This is quite true. Being a mechanical engineer myself I'm fighting business, financial, and marketing constraints more often than material ones.

Meanwhile, my XJ has been undergoing major surgery for the last 7 months, and I've designed the entire thing from scratch. No way in hell anyone would buy it if I were to market the system (what customer would really want to cut holes in their floor to run upper links up into the cab somewhere?) but from a functional standpoint it's quite a nice solution. I just bolted EVERYTHING up for the final time over the weekend and it was a joy having all the holes line up, everything fit, and all because I've been really careful and thoughtful the whole time.

I'd still like to see a formed rear wishbone section though.

[XJ_ranger]

I'd still like to see a formed rear wishbone section though.

can you say....



Carbon Fiber Compression Mold?

that would be really really cool... :D

[Weasel]

can you say....



Carbon Fiber Compression Mold?

that would be really really cool... :D

shhh!

The only thing I don't like about the wishbone's is usually the point of rotation is not where the link converge causing the connecting joint to be humongo.

[vetteboy]

shhh!

The only thing I don't like about the wishbone's is usually the point of rotation is not where the link converge causing the connecting joint to be humongo.

True. But I guess you could also build it so that the two halves converge on the housing of a cartridge joint....however seeing as that single connection is providing the bulk of the propulsive force to the chassis, I'd want it to be kind of over built anyway...

[CRASH]

The joint at the conversion will be a 1" Uniball, check the load rating on one of those.

The project is in concept phase, and won't be started until I build a new rear axle. I don't want to cut up my current axle mounts, as it makes the unit less sellable, and I need a larger rear axle for planned tire upgrade in 2008.

[vetteboy]

The project is in concept phase, and won't be started until I build a new rear axle. I don't want to cut up my current axle mounts, as it makes the unit less sellable, and I need a larger rear axle for planned tire upgrade in 2008.

This is why I never regeared my D44. An XJ D44, with locker, disc brakes, 3.55 gears, stock shock mounts and perches, was a very sellable item. Between selling that and my D30 locker it paid for my whole new rear axle assembly.

I didn't quite understand what you're wanting to do here (reason for this thread). Are you doing a wishbone upper and a wishbone lower? It'd be sort of a neat thing to see.

[BrettM]

The joint at the conversion will be a 1" Uniball, check the load rating on one of those.

The project is in concept phase, and won't be started until I build a new rear axle. I don't want to cut up my current axle mounts, as it makes the unit less sellable, and I need a larger rear axle for planned tire upgrade in 2008.
is that gonna come with a front axle upgrade too?

[Weasel]

The joint at the conversion will be a 1" Uniball, check the load rating on one of those.

The project is in concept phase, and won't be started until I build a new rear axle. I don't want to cut up my current axle mounts, as it makes the unit less sellable, and I need a larger rear axle for planned tire upgrade in 2008.

I'm sure it's plenty strong enough but I just wonder if it would cause any additional wear issues, possible fatique issues, and the links might end up being heavier then needed. But I guess the extra beef is probably nothing to be concerned with. I'm so used to designing for super lightweight and optimizing parts I tend to forget that is doesn't always matter on trail rigs.

[Jump This]

I'm sure it's plenty strong enough but I just wonder if it would cause any additional wear issues, possible fatique issues, and the links might end up being heavier then needed. But I guess the extra beef is probably nothing to be concerned with. I'm so used to designing for super lightweight and optimizing parts I tend to forget that is doesn't always matter on trail rigs.

OH MY GAWD!!
Someone moderate this before Taylor reads it....he will pass out!
lightweight doesn't always matter on a trail rig......:lecture:
Rick

[David Taylor]

OH MY GAWD!!
lightweight always matters on a trail rig......:lecture:
Rick


Fixed it for ya Rick

[GroversXJ]

Crash, here's some calcs for you as I haven't seen any yet.
Bending stiffness is determined by SyI/c where Sy= yeild strength, I=moment of inertia of the cross section, c=distance from the neutral axis to the furthest distance on the cross section

For your baseline of 2" dia .25" wall tube you have a moment of inertia of .53594 in^4 and if Sy of the DOM you were using was roughly 42.8ksi (c=1", of half of 2"), the bending stiffness is 22938 lb*in

For a 2" square section with .25" walls you have a moment of inertia of .91109 in^4 and using the same yeild strength as above (with c=1") the bending stiffness is 38994 lb*in, which is over 50% stiffer than the baseline material.

For a rectangular section 2" x 3" with a .120" wall the moment of inertia is 1.41073 in^4 and using the same yeild strength as before (c=1.5") the bending stiffness is 40252 lb*in which is almost twice as stiff as your baseline.

This is the same calculation SAE required us to use when I was in still in school and designing our schools mini baja roll cage. It is a very good way to determine if a specific material you want to use for something is at least as good as things that are acceptable now or have worked in the past with out maintaining the exact same cross section.

I hope it helps.

[GroversXJ]

I noticed you mentioned having a .250" thick bottom section for bashability and possibly a .125" top section so I assumed the same .125" for the walls and the moment of inertia is 1.81219 in^4 (slightly higher than the .120" wall rectangle I showed in the last post) which would give you a bending stiff ness of 51707 lb*in

I also looked at maintaining the your required strength at a loading not going thru the log axis of the cross section. At 30 degrees the .120 wall 2" x 3" rect. tubing will have a moment of inertia of 1.39989 in^4 which isn't much lower than the initial calc of 1.41073 in^4 for the loading along the long axis. I'm pulling my calcs from engineersedge.com calculators as my reference manuals are all at work so I 'm not sure if I believe that small of a reduction in moment of inertia due to rotating it 30 degrees.

Anyways, sorry my calcs are so late as the original post was started months ago, but I thought at least someone may be able to benefit from it even if your already started on your winter project.

[CRASH]

Grover, thanks for some very good calcs, exactly what I needed.

This is looking like it's going to be NEXT winter's project, as I'm about done with upgrades, and I'm ready to hit the desert for some wheeling.

[CRASH]

is that gonna come with a front axle upgrade too?

30 spline stubs and drive flanges will have to do for now.

[GroversXJ]

No problem, sorry I didn't see this thread sooner. I'm just starting on my first few custom parts right now with a traction bar/crossmember/rock rails.