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Advanced Fabrication and Engineering For when your build is going off the charts. Wildly modified vehicles and straight up buggies welcome.

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  #1  
Old December 4th, 2003, 20:25
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Front suspension geometry...lets get in depth...

Almost every suspension geometry thread I can find is in reference to the rear. I'd like to talk about the front. With track-bar, without track bar, parabolic uppers, 3 mid-length arms, etc, etc...

The information I am looking for is in reference to the geometry involved in properly building a front suspension. The specific questions I have would be this:

Anti-dive, how is it calculated, and what effect does it have on road, and offroad? Where would the optimal anti-dive be set?

Instant Center. This is commonly talked about when building a rear 4 link suspension. This convergence point between the upper and lower control arms have a lot to do with anti-squat in rear application. However, in many situations in the front suspension, the instant center would be located just inches rear of the front links, usually under the front seats. How does the IC effect a front suspension, or is it only in reference to rears? And, what if the lower arm mounted higher than the upper arm, the arms would cross and the IC would end up somewhere around the front floor boards or even forward of that?

What about front steer (like rear steer, as in fore and aft movement of the axle during articulation). Is this as noticeable in a front application since you can easily correct it with steering input?

The reason I want to get in depth with this is that it seems a lot of people just build a front suspension and don't think about the geometry at hand. There are a lot of long arms fabricated at home, where the only paper used was the paper used to wipe their .

I have a bunch of designs on paper, from Beez's parabola, to Goat's 3 mid arm, to a 33" 'J' arm that mounts outside, but next to the frame rail. I want to build it right the first time, or at least get it close

Thanks,
Steve
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  #2  
Old December 4th, 2003, 23:25
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MaXJohnson MaXJohnson is offline
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Front and rear instance centers are derived by using the same method. Project the upper and lower arms out to a point where they converge. The front is no more likely to have a short virtual link than the rear. The rear IC locates where braking and acceleration torque react with the chassis; pushing up to counter-act the rearward pitching of the body during acceleration and pulling down to counter-act the forward pitching of the body during braking. With the front suspension the reverse happens. The virtual link, acting at the front IC pushes upward to counter-act the forward pithing of the body during braking and, in the case of 4WD, pulling down to counter-act the rearward pitching of the body acceleration. In both cases, the slope of a line drawn from the tire contact patch through the IC is used to determine the percentage of anti-squat, lift or dive.

For anti-squat considerations you will typically see the IC compared to a line extending from the rear tire contact patch forward to a point directly above the front axle centerline at the height of the CG. This is valid for rear wheel drive. When in 4-wheel drive, part of the available torque is sent to each axle. In the case of a part-time transfercase, this is usually a 50/50 split. In my opinion, to correctly calculate anti-squat & anti-lift in 4WD, a line should be extended from the contact patch of each wheel to a vertical line midway between each axle centerline. Compare the intersection of the front and rear lines with the height of the CG at this mid-point to calculate the percent contribution of each axle. Take an average of these two forces for a total anti-pitch calculation.

For braking anti-dive in front and anti-lift in the rear, use the above method, but the vertical line of comparison is based on the amount of braking force available to each axle. If the front brakes provide 65% of the braking force, the vertical line should be drawn 65% rearward of the total wheelbase length. Once again, the combined anti-dive and anti-lift force of both axles is used to determine the total anti-pitching force available.

The problem with anti-dive is that it increases the effective spring rate of the front suspension. For street use, you want enough anti-dive to minimize, but not eliminate front end dive under braking without a noticable increase in front end harshness. Lifting an XJ with short 4-link suspension arms will result in a very steep virtual arm. This creates way too much anti-dive. The end result is a harsh hiway ride. Off-road, the dramatic increase in spring rate makes it difficult to maintain tire contact which can reduce traction on downhill decents under braking.

Longer control arms, drop brackets and radius arm designs tend to reduce the virtual arm angle which lessens the amount of anti-dive. In virtually any link design the kind of lift needed for off-road use will result in more anti-dive than you would normally want. At the same time, the virtual arm angle will dictate the amount of anti-lift available when climbing. Anti-lift pulls the front end (sprung weight) down which is a good thing, but lessens the amount of traction available to the front tires which is a bad thing. I have yet to see any discussion of this topic recommend an optimal percentage for these anti-pitching forces. In most cases, it seems that minimizing anti-pitch percentage in the front is desirable.

Your example of mounting (at the chassis) the lower arms higher than the uppers would cause the pinion to angle downward in droop and upward in compression of the suspension. This is opposite of what you would want. It would also create an extremely short virtual link and result in extreme movement of the IC through the range of suspension travel. None of these traits are good.

Short suspension links and steep link angles increase axle steer; front or rear. Since the front axel also has steerable wheels with the potential for bump steer, axle steer can either work with or against bump steer. The two could actually cancel each other out under the right circumstances. Conversely, these two characteristics could be additive, making unwanted steering movement even worse. A vehicle is more sensitive to rear steering imput than front. A forklift is a good example. With this in mind, rear axle steer should be more noticable to the driver than front.

The advantage of Beezil's parabola is in the clearance it provides, not in the geometry of the suspension. An upper link shaped like an "A", a parabola or a horseshoe would all perform the same from a suspension dynamics point of view. The advantage of all three is the elimination of axle housing rotation with one side in droop and the other in compression. The disadvantage is increased bump steer and a single point of failure.

Richard,s 4-link is slightly longer than stock with modest changes in mounting points at the chassis and axle to help level out the arm angles. He has ended up with a design that lessons the amount of anti-pitch without the loss off clearance associated with longer radius arms. Based on his success, you have to agree that it's a pretty good compromise.

J-arms increase the effective length of a comparable rear (relative to the axle tube) mounted link. Longer links result in lesser amounts of longitudinal movement of the link pivot for a given amount of suspensin travel. This means less axle steer and less torsional stress on the axle tube when articulated.

Your questions open up a very broad discussion. I hope this helps.
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  #3  
Old December 6th, 2003, 11:43
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Re: Front suspension geometry...lets get in depth...

Ok, so you're saying, in 4wd, you'd want the IC front and rear to be on a 50/50 center line on the chassis? The rear IC is usually projected to a point above the front axle. This point is usually between 50-80% of the CG, you want more lift, make it closer to the 80% mark, more squat, closer to the 50% mark.

So, what effect does the position, front to rear, of the IC have to do with suspension characteristics? Normally, one would project the rear tire contact patch forward to where the upper and lower links converge (like you said), and that line from the contact patch goes through the front tire. If that line is no longer going through the front tire, and is much steeper, say terminating at a point close to the transfer case, yet still between the 50-80% CG marks, you'd have to change your links to converge at that axle center point. I'd think this would create a steeper lower link.

You have helped a ton, and I need to hit the graph paper a little harder now. I understand more about the rear than front, and I think once I completely understand the rear, I can start transferring that information to front use. I plan on sticking with leaf springs in the rear for now, that may be a future project.

Thanks,
Steve
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  #4  
Old December 6th, 2003, 17:07
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Re: Front suspension geometry...lets get in depth...

Quote:
Originally Posted by Willis
Ok, so you're saying, in 4wd, you'd want the IC front and rear to be on a 50/50 center line on the chassis?
No, what I'm saying is that the usual diagrams that you see on here, PBB, etc is correct for calculating anti-squat for a 2WD vehicle, but not for 4WD. This is not a difference is how you determine the placement of your instance centers, but rather how you compare IC reaction with the CG.

Look at it this way. You can draw a diagram of your rear suspension links in the side view to determine where your IC is. Compare the IC location with the anti-squat neutral line (rear contact patch to CG height above front axle) to determine AS%. Now do the same for the front suspension links to determine front anti-lift. Here's the problem. The values you get assumes that each axle get 100% of the engine torque available.

This is wrong. In a part-time transfercase (no center differential) each axle only recieves 50% of the torque. This means that your calculated AS% and AL% are double their actual value. Some fulltime transfercases split torque on an uneven basis, front to rear, say 60%-40%. This would have to be factored in as well.

If, for example, you calculated 80% anti-squat for the rear axle and 60% anti-lift for the front; the total anti-pitch force available through a 50-50 transfercase would be 1/2 the sum of these two values. My previous post indicated that the total anti-pitch force available would be the average of the front and rear percentages

(80%+60%)/2 = 70%.

This application of calculating the TOTAL 4WD anti-squat/lift values is derrived from an example in one of Forbe's Aird's books for determining total anti-dive/lift under braking. The method of determining the individual values is correct, but I am having trouble with his interpretation of the total force acting on the chassis being an average of the two individual values.


What all this means is that if you have a significant difference in the link design between the front and rear suspensions (most do) then the common method being tosses about will produce a significant error.
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  #5  
Old December 6th, 2003, 18:07
Beezil Beezil is offline
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Re: Front suspension geometry...lets get in depth...

just remember only a portion of this theoretical masterbatory nonsense applies to a big, trail-only vehicle getting put through the motions.....

plotting "lines and stuff" in a static 3-view renderings is a good start to design, but the physical limitations of your vehicle, and the methods that are available to you when you go to fabricate your design will be deciding factors...

applying a working f&r link suspension system to an xj can be maddening!

starting with a blank canvass (tube buggy) would be the most ideal obviously, but you wil learn a shitload using your xj as a beginning platform.
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  #6  
Old December 6th, 2003, 18:14
XJJPR XJJPR is offline
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Re: Front suspension geometry...lets get in depth...

Quote:
Originally Posted by Beezil
applying a working f&r link suspension system to an xj can be maddening!
Just check out how much Beezil has changed since he did his, and it wasn't for the better either.
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  #7  
Old December 6th, 2003, 18:17
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Re: Front suspension geometry...lets get in depth...

Quote:
Originally Posted by Beezil
just remember only a portion of this theoretical masterbatory nonsense applies to a big, trail-only vehicle getting put through the motions.....
Sounds like you think he should enter into the design phase wearing a blindfold. Seems like I recall some drawings you worked from in the beginnings of your golden arches. Which portion of this "theoretical masterbatory nonsnese" doesn't apply?
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  #8  
Old December 6th, 2003, 18:33
Beezil Beezil is offline
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Re: Front suspension geometry...lets get in depth...

yeah, read it.

I said "only a portion"....

all that stuff I worked on didn't exactly translate well to the ACTUAL VEHICLE when I began building.

and you missed this one:

"plotting "lines and stuff" in a static 3-view renderings is a good start to design"

where exactly, do you see me launching willis into building without a PLAN?

I'm just saying, IN MY EXPERIENCE, plotting lines and stuff is fine.....actually trying to adapt your design to an xj platform SUCKS. You end up compromising most important aspects of your design (or the platform, the XJ) in the process....

if we were building tube buggies from scratch, more of this shit could apply, and we could hold to our designs better, since we'd have a chance to build our rigs AROUND the suspension, if that's your priority.

the point I'm trying to make, since I've been there, is to not get hung up on paper planning too much. Try not to assume you have to post ten-thousand word inquiries on the jeep boards (like I did) and waste vaulable time NOT strategizing the actual CONSTRUCTION on the vehicle. i feel I wasted at least 4 weeks blabbing about it, and desgning something on paper I couldn't fit on my jeep.
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Old December 6th, 2003, 18:38
XJJPR XJJPR is offline
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Re: Front suspension geometry...lets get in depth...

He blabbed so long WE finished his designing in Moab.

right Sean?

hinkley
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  #10  
Old December 6th, 2003, 18:42
Beezil Beezil is offline
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Re: Front suspension geometry...lets get in depth...

hey, all it needed was some schedule 40!

get yo pipe, and yo nalgene
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  #11  
Old December 6th, 2003, 19:07
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Re: Front suspension geometry...lets get in depth...

Quote:
Originally Posted by Beezil
yeah, read it.

I said "only a portion"....
I asked "which portion doesn't apply"...



Willis started this post wanting to discuss front link design. I didn't get the impression he was ready to go full buggy.
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  #12  
Old December 6th, 2003, 19:12
Beezil Beezil is offline
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Re: Front suspension geometry...lets get in depth...

Quote:
I didn't get the impression he was ready to go full buggy
now I know yer just ****in with me....

nope, never pushed him into a buggy either...

just trying to give him a little advice that he doesn't waste his valuable time getting too hung up on every little detail.

*some* of it doesn't matter

*some* of it can't be applied due to the limitations of the xj platform.

when you actually build one of your own someday, you'll see what I'm talking about.

until then, keep posting the info, I'm still reading, and I'm still learning
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Old December 6th, 2003, 20:45
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Re: Front suspension geometry...lets get in depth...

...still wondering which *some* of it you think doesn't matter.

I've built plenty of my own junk, but will probably never build a buggy. It's not on my list.

Whether Willis or you or me or anyone else is building a buggy, an XJ or a bar-b-que grill that plugs into a hitch receiver, it's still a good idea to define constraints and build within them. You chose to change the constraints on your project. You end up with new constraints and built within them.

Thinking things through and schetching ideas out on paper is a good way to eliminate some bad design ideas before commiting to steel. Tossing around ideas on the internet, schetches on a garage floor, string on plywood; it's all good.
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Old December 6th, 2003, 21:31
XJJPR XJJPR is offline
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Re: Front suspension geometry...lets get in depth...

Quote:
Originally Posted by MaXJohnson
...still wondering which *some* of it you think doesn't matter.

I've built plenty of my own junk, but will probably never build a buggy. It's not on my list.

Whether Willis or you or me or anyone else is building a buggy, an XJ or a bar-b-que grill that plugs into a hitch receiver, it's still a good idea to define constraints and build within them. You chose to change the constraints on your project. You end up with new constraints and built within them.

Thinking things through and schetching ideas out on paper is a good way to eliminate some bad design ideas before commiting to steel. Tossing around ideas on the internet, schetches on a garage floor, string on plywood; it's all good.

I think what Beezil trying say, which I'm a beleiver in, is *some* times you just need to actaully get your hands dirty and *do it* to learn it. You might need the grinder a little more at first but in the end you might just learn something you will never see on paper or the floor.

mark
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Old December 6th, 2003, 21:44
Beezil Beezil is offline
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Re: Front suspension geometry...lets get in depth...

yeah, okay, that's great.....don't know why you keep debating this......

never told him to "forget everything he read here and start building". Its all about the plan. yer making it sound like I want him to scratch his plan, and "to hell" with everything that's been discussed so far.....

once again, for the non-absorbant...

all I'm saying is, *some* of the things you will read and learn about in this thread, cannot be applied to the xj platform due to real physical and/or mechanical limitations.

*some* of these limitations can be overcome by compromising the platform (sawing the shit out of it).

*some* limitations will end up not being overcome due to cost/feasibility issues, or the inability to fabricate everything the design calls for.

I only brought up a tube buggy as a comparison to illustrate that a "blank canvass" is about the only way to employ *almost all* of the lessons learned here.

I'm wanting willis to learn everything he can which is why I'm not trying to add to the information you are already posting. I'm only giving him a "heads up" so that he understands, once he figured things out, and the light bulb goes off, he still has to deal with the realities, and be ready to make a ton of compromises. That process is frustrating, disappointing and time consuming. All thos evehicle dynamics books don't talk about that process when it comes to messing with xjs do they?

relax max, no one is calling your credibility into question. I'm just giving willis a heads up.....ya know, since I've actually done this....

Get back to the discussion at hand, I'm not trying to steal the thread, I'm enjoying it, and still learning.

since you asked, one of the things I think that isn't worth worrying about is anti-dive, for many reasons....I'm assuming on a trail rig, since I'm pretty sure thats what willis is building. There are a few things that prevent a theoretically "perfect" design, dialed in for the desired anti-dive....

pesky things like:

oil pan
axle location
frame rail dimensions
front driveshaft
tire clearance at full lock and stuff (locating links)
exhaust
steering linkages

these are realities, and most of them are a permanant part of the platform. All those groovey suspension dynamics (racecar) books assume that these are secondary features. Try to deal with those things on a cherokee, and treat them all as if they were secondary.

"anti dive".....thats just *one* of the things I didn't worry so much about once I began BUILDING.

I know you've done an excellent job explaining what it is, and I'm sure willis is thankful for the tech. I know I STILL am appreciative of the time spent by a few of you that know how to write about this stuff.....But how can you tell Wilis what to *DO* with this info? If I'm somehow getting in your way, or killing the buzz of the thread by sharing the realities, I'll hit the bench, and take a breather for four weeks until wilis comes back from laying on his back staring at the underside of his XJ for hours wondering what his next move is.
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