Why Have a Suspension

[FarmerMatt]

In my line of work I get to play with many different tractors & equipment built to go through "off road" terrain. The majority of the tractors I play with have a single pivot front end with the rear end being ridged mount. In watching the newer built buggies & rock rigs most seem to lean towards minimal compression & are using center limiting straps to limit "unloading". This in essence gives them a center pivot suspension. Keep in mind as we go through this that this is for a slow speed rock rig only. What is rattling around in my demented brain is a center pivot mounted front & rear ends. Thinking this through, the rig would have great clearance with no control arms to slide on. Both front & rear would be center pivot with sway bars & shocks to center & control the rig over the axles. It should make the rig extremely predictable in steep climbs with no front end unloading & anitisquat BS to worry about. Decent should be cool with no front end diving. No axle wrap to worry about either. I would do this with portal axles or maybe even go full hydro drive & build hyd. motor housings out of tubing that will be rolled into arches to give added clearance. The only down side I can see is it could get ruff riding at any speed, but with the tires at 5 psi how bad could it be. I know this is way out of the box & don't know is I'll ever get the balls to actually try it, but the idea has intrigued me for a while now. I'll let you guys chew on that for a while & see what you spit out.

Matt

[ChiXJeff]

Take a look at the bright yellow Willys in the latest FourWheeler TTC. It uses an sliding center pivot kind of like you are describing.

The big problems I see with hydro is that it's heavy and expensive. And you'll have fun building up a pump and reservoir setup that won't spill, can breath, and won't suck air into the pump. My Dad & brother have (what's left anyway) of an FMC nut shaker. Full hydraulics, with 110+ gallons of reservoir for counterweight, a 20gpm Cofeyville pump driven by a Ford 300 inch I6, Spicer trans and axle. It doesn't get as much torque to the ground as you think it would.

[FarmerMatt]

The hyd. wouldn't be a problem for me to whip up. I see the system would be just trading weight. I can't see it being heavier than the componants it would eliminate, but this is a side issue. I'm more interested in the suspension aspect. What are the downsides that anybody can think of other than the ride being a little ruff.

Matt

[TOZOVR]

Originally posted by FarmerMatt
The hyd. wouldn't be a problem for me to whip up. I see the system would be just trading weight. I can't see it being heavier than the componants it would eliminate, but this is a side issue. I'm more interested in the suspension aspect. What are the downsides that anybody can think of other than the ride being a little ruff.

Matt
Steep inclines combined with off camber.

I'd think that would be where the new buggies still rely on articulation, no?

[MrShoeBoy]

If the front and rear where on a pivot wouldnt there have to be some sort of spring keeping the corners of the rig up? As you stated on a tractor, the rear doesnt move and its just the front axle that articulates under the "body" of the tractor. How would that be solved?

As for the bumps, why not make the cab float on the frame like a semi truck does with airbages under the cab attached to the frame. Semis have been around for ever and they seem to work for the truckers driving down the interstate everyday.

AARON

[Beezil]

Matt.....

your harvest time is nearing to a close.....long winter right?

begin building!!!!!

[Kejtar]

Originally posted by ChiXJeff
FMC nut shaker :confused:

[RCP Phx]

Torque is the main reason it wont work.No matter what side/or rotation you use it will want to lift one of the corners(front or rear).

[FarmerMatt]

If the front and rear where on a pivot wouldnt there have to be some sort of spring keeping the corners of the rig up? As you stated on a tractor, the rear doesnt move and its just the front axle that articulates under the "body" of the tractor. How would that be solved?

Sway bars & shocks. Basically you'll just need enough bar to keep the rig centered over the pivots & however stiffer you want it to counteract body roll in off camber situations.

As for the bumps, why not make the cab float on the frame like a semi truck does with airbages under the cab attached to the frame. Semis have been around for ever and they seem to work for the truckers driving down the interstate everyday.

The center pivot mount could be tied into a sub frame using spring eye bushings or something like that, but I'd rather keep it more simplified than having an air bag system suspending the body that may open a whole different bag of worms. Keep in mind that the fastest this rig would probably is is 25-30mph.

FMC nut shaker

Almond tree shaker. Many of these are full hydro driven meaning there is literally a hyhro motor on each driven wheel rather than having an axle shaft & differential. Some have hydro motors attached to a transmission that drives a standard differential. Our old OMC shakers used dana 70 differentials & would strip the teeth off the R&P. We'd have to rebuild 2 or 3 of them a year. If that's not torque than I don't know what is. Although they are using 7:17 gears.

Torque is the main reason it wont work.No matter what side/or rotation you use it will want to lift one of the corners(front or rear).

I'd like to here more about this. The only time a tire gets lifted on the front of any of my tractors is when we have a huge implement hanging on the back. Why would torque lift a tire in this situation? I know the TJ's have a problem with the short wheelbase & short arms transfering torque from side to side, but thats more a function of the suspension transfering it. In this imaginary setup there realy is no suspension although the swaybars could effect this I guess??

Matt

[RCP Phx]

Sorry,It was earlier this afternoon when I originally started reading this thread.I was thinking of a single pivot point in the frame(articulated knuckle).Now I understand what you meant,it sounds interesting but what about front-to-back "torque",thats what a suspension really does,keeps all wheels working equally!

[JJ13]

As far as the ride is concerned, it shouldn't be a big problem. I drive large forklifts at work all the time. They have no suspension and 100 PSI in the tires. An air seat should be all that you would really need. The fastest of these lifts probably gets up to 25 or 30 mph. The ride gets a little rough after 8 hours, but it isn't too bad. I say build it. :D

[Neil]

Thousands of Hondas in Fresno run around with no suspention, If they can do it why can't you? I even see them bouncing down the freeway at 85 mph on 35 series tires. I say if your kidneys can handle it, go for it.

[ChiXJeff]

Originally posted by FarmerMatt
The hyd. wouldn't be a problem for me to whip up. I see the system would be just trading weight. I can't see it being heavier than the componants it would eliminate, but this is a side issue. I'm more interested in the suspension aspect. What are the downsides that anybody can think of other than the ride being a little ruff.

Matt

You, have trouble with a hydraulic system? Naw, never happen. In your line of work, you HAVE to know something about hydraulics. And you've probably got just about everything you need except maybe a couple of motors in your spare parts racks.

Sure, if you run individual motors at each wheel, that's going to be lighter than an axle. Don't forget you're going to need a bunch of oil in a reservoir, and probably a radiator as well.

On a side note, I've spent the last 20 minutes looking for a picture of a nut shaker, and I've found 2 thumbnails, neither one of which is very good. Basically, it's a low slung vehicle with flotation style tires with a long boom and hydraulic claw hanging off the front. It grabs on to the base of the tree, and shakes the whole thing (hence the name.)

The nut shakers that Matt has experience with are built a LOT heavier than the cherry shakers I've used because nut trees are huge compared to cherries. Taking the teeth off a D70 ring? Wow. The major issue we had with the FMC was the brakes sucked, and the parking brake was more of a suggestion.

The new rigs are tricycles, each wheel has it's own hydraulic motor, with inclined plane canvases. 2 good operators can harvest about two and a half trees per minute, in excess of 100,000 lbs. per day pretty easily.

EDIT: Ha! http://www.fdocitrus.com/fig6.htm Okay, this isn't an FMC. But it gives you a pretty good idea of what a nut shaker looks like.

[Safari Ary]

Matt, you and I have been pondering the same thing it seems. If you eliminate the need for a differential in the middle of the axle housing it sure does make fabricating the perfect suspension a lot easier. Ponder this....

Take a wishbone, similar to Beezil's latest creation, and run the tip of it through the center of the axle housing(just for picturing purposes imagine the shaft on Beez's wishbone going into the pinion shaft hole in the housing and being secured). With one "link" you have now located the axle side to side, back and forth and rotationally. Now let the top of the axle compress an airbag against a crossmember on the frame of the buggy. There are two major problems with this design.

1) Creating a wishbone strong enough to do ALL the locating of the axle, the biggest one I would think, would be pushing one side of the axle back and the other forward, if you catch my drift.

2) You can't possibly have a differential with the wishbone going into the center of the housing.

In your idea, you wouldn't have the airbag or need any of what I just described, but I think that with the airbag design, you eliminate the problem of trying to make the ride bearable. I think my idea is somewhat of a combination of yours and typical suspension setups. My inspiration was the Wiggins Marina Bull forklifts that I used to work around. Front axle solid mounted, rear/steer axle pivot mounted. Anyway, just my .02

Ary

[ChiXJeff]

Here's another oddball thought.

Anybody remember how the front suspension is set up on a Morgan roadster? Independent front that slides on a vertical rail.

If you're running independent hydro motors, run a long coilover shock with the wheel travel on a vertical rail.

[FarmerMatt]

This is the machine we use in the trees for those not familure with nut shakers. Full hydro drive with a 110 hp cummins diesel motor complete with AC & stereo. ;) Weighs is at a ground shaking 11,400 lbs. :eek: We have 2 of these. We have four of the late 70's modle OMC monoboom shakers. 2 of these new ones replaced 4 of the old...
http://www.orchard-rite.com/nutshaker/rowshaker.cfm



I'm curious as to how a swaybar would react with this type of axle retainment. As tension is applied to a swaybar with a normal suspension it actually pulls the entire suspension down or up depending on compression or droop in any given situation. With what I'm talking about doing the swaybar wouldn't have that lee way. Any of you suspension guru's out there have any comments? Hello Alyn, Mr. Stevens, or anybody out their that knows anything about suspension dynamics.

Matt

[Beezil]

I think it would be fun to be sitting in the tree when one of those things came up on it. Also, I'd be drunk.

[Rev Den]

http://a428.g.akamaitech.net/7/428/34/712564c8f34f20/dsc.discovery.com/fansites/monstergarage/episode/season1/gallery/11_8_hzoom.jpg

Did someone say "Nut Shaker"??

Interesting thread.

Rev

[ChiXJeff]

Beezil, come on up to the farm next July when Dad & Steve are shaking cherries. Nobody quite believes me when I tell them what happens, or how fast. That nut shaker of Matt's is gonna run even faster, it doesn't have a catch frame to deal with.

Den, I saw a clip for the Monster Garage episode. I decided that I really didn't want to bother watching it. More or less, they've built a Shockwave shaker from about 1972. Seriously, that's going to be one of the easier projects. The hardest part would be getting enough hydraulic power to run the head, and counterweighting the front.

Someplace at home, I do have some digital video on a VCD. May have to try and dig it up again. None of my PCs has a DVD decoder, but my player might take it.

BTW, Matt....... how many trees/minute?

[Neil]

What about Nitrogen gas pressurized shocks. They should be enough to keep the rig level and would eleminate the need for the sway bar. My experience in racing is that when the force of a turn pushes up on one wheel, the sway bar lifts the other side of the suspension thereby lowering the car. All of these are in very small amounts. I don’t think that a sway bar will be able to handle the axle travel that you are suggesting. If the forces are high enough the sway bar would bend or snap.

[FarmerMatt]

Beezil, you should feel it around our house when they start hitting trees near by. It was fun to see my wifes city slicker face when she was woken up at 5:00am to the house shaking off it's foundation. :)

Jeff, I'm not sure the trees per minute, but 2 rigs cover 80 acres per day. Shakers are cool, but the really cool hydrolics are in the grape pickers...

Neil, gas shocks would be worth looking at, but the amount of "spring rate needed to counter act body roll would limit articulation. The swaybar has a direct effect on body roll & I could use shocks or springs to adjust with. My concern, & what I really don't know about, is if a sway bar would live fighting a ridged mounted axle that would put a heck of a lot more twisting force through it than it would see with a standard suspension.

Matt

[ChiXJeff]

Originally posted by FarmerMatt
Jeff, I'm not sure the trees per minute, but 2 rigs cover 80 acres per day. Shakers are cool, but the really cool hydrolics are in the grape pickers...


Lordy.......... that's rockin'. My Dad & brother spent 11 days shaking something like 70-80 acres of cherries this summer, but a lot of that time was spent waiting on the canning factory.

I've seen pictures of grape harvesters, and yeah, I know what you mean about the cool hydraulics. I'd love to see one in operation some time.

[Neil]

The only sway bar I can think of that might be able to hold up to that ammount of articulation is the Curry AntiRock. But even then metal only bends and twists so far before it becomes permanent. Are you planning to trailer this to the trails? Because if you are thinking of driving this on the highway that opens a whole other can of worms. As far as springs, do you remember the 70’s Corvette rear end? The leaf spring was mounted left to right instead of front to back. You could use a very weak mono-leaf, just enough to keep the cab from flopping around like a bobble head.

[MaXJohnson]

That's a pretty radical concept.

It changes so much that it'll take some serious pondering (=lot's of beer) to grasp the impact.

A couple of things that come to mind:

With a center pivot, one wheel moving upward in reaction to a rock will have to raise the chassis by 1/2 the wheel travel. Because the chassis reacts to events at one axle, it will impact the axle at the other end.

The tire will be the spring when both wheels impact a bump or dip. The tire will be a significant part of the spring when a single wheel hits.

Suspension is for more than just driver comfort. It keeps the tire in contact with the ground. The ideal suspension keeps all four tires in contact, with equal pressure on each corner.

Match suspension frequency with anticipated surface. Smooth surface = higher frequency, rough surface = low frequency. Lower frequency is achieved with lower spring rates and higher unsprung weight. Your center pivot would effectively increase spring rate which would increase suspension frequency compared to a more traditional suspension.

Suspension helps absorb shock loads when torque is transmitted to the tire contact patch. Your rigid mount would put 100% of the shock load at the tire. This would increase the possibility of wheel spin and require "twinkle toes" on the throttle pedal.

There's no reason why your center pivot couldn't utilize traditional springs, or coil-overs, or swaybar or combination of these.

In essence, your design eliminates suspension on two-wheel events and fixes a permanent roll axis. It also eliminates anti-dive, anti-squat and anti-everythingelse. This isn't necessarily a good thing. I have yet to see a valid discussion of the impact of anti-xxxx in relation to climbing. Still waiting for Mr. Peabody, er I mean Ed to lay this out.

Rock crawling suspension design is in it's infancy. Your idea could be as valid as any.

[Ed A. Stevens]

Originally posted by MaXJohnson
That's a pretty radical concept.

It changes so much that it'll take some serious pondering (=lot's of beer) to grasp the impact.

A couple of things that come to mind:

<SNIP>

Suspension helps absorb shock loads when torque is transmitted to the tire contact patch. Your rigid mount would put 100% of the shock load at the tire. This would increase the possibility of wheel spin and require "twinkle toes" on the throttle pedal.

<SNIP>

In essence, your design eliminates suspension on two-wheel events and fixes a permanent roll axis. It also eliminates anti-dive, anti-squat and anti-everythingelse. This isn't necessarily a good thing. I have yet to see a valid discussion of the impact of anti-xxxx in relation to climbing. Still waiting for Mr. Peabody, er I mean Ed to lay this out.

Rock crawling suspension design is in it's infancy. Your idea could be as valid as any.


Why kill brain cells to explain what you ;) already describe (the need for twinkle toe driver control when on the throttle with high anti-squat on hills)? Damn challenges ...

This light touch need is due to the weight's traction resistance load being directly placed on the links with no suspension movement (100% anti-squat), or the even more unstable situation with suspension jacking (over 100% anti-squat) with a growing anti-squat value as the throttle is applied in increasing vigor (forcing the AS% even higher with less throttle). The suspension design should complement the drivers ability to control the vehicle, not accelerate the vehicle into more difficult driver control territory (where the suspension is isolated to the tire flex).

A suspension allows the chassis to store and release energy (for good, or bad). The suspension is a combination of tire, and spring/linkage, and chassis flex. Traditional competition has evolved to eliminate chassis flex, and tire flex (pavement racing low profile doughnuts), so we are usually left to play with the spring/linkage arrangement.

Matt proposes we exploit the tire, and chassis flex, and simplify spring/linkage solutions. It's an approach worth exploring (with a few :cheers: ... later).

Control fails, and energy fails to be stored, when the tires traction adhesion cannot deliver the power to the driving surface: when the tire spins. Energy is stored when the chassis jacks or squats. This is sometimes desirable in a high weight car with a small tire contact patch. It is also sometimes desirable to design in chassis-lean when cornering with a heavy vehicle and a small tire contact patch. The suspension stores the impact shock load, a shock load that would normally force the tire contact adhesion into a less desirable dynamic friction coefficient, a transition that would be difficult for the driver to control (an impact force that would break the static coefficient of friction for the tire material).

We have all experienced the ease of spinning a tire, once the initial rolling friction adhesion limit was exceeded (WhooHoo :), neutral drops & side stepping the clutch, for a smokey throttled burnout once the tire starts spinning). How about the weekend practice on the go-kart slick track? What resulted in a better cornering speed, radical lock to lock steering (Ted Nugent) style, or the smooth transition (Dan Gurney) steering style between corners. The same change in available traction, from fairly good static rolling friction to poor dynamic sliding friction, happens even with less violent of a power threshold transfer (compared to abusive throttle on a poor traction surface). What is easier for driver control, the tire at or below the static traction limit, or crossing the transition between static (rolling) and dynamic (sliding)coefficients for tire contact patch friction?

What if you could dampen, or better yet store, the excess energy that forced the shock load past the tire static friction limit? What if you could keep the tire contact patch at the superior static (rolling) friction coefficient? Cornering's dampening goal is more control to allow the driver to drive the transitional chassis motion, and the acceleration goal is to allow the driver control to more easily throttle & brake, at the tire's superior static traction limit.

This is also the goal of suspension improvements in super stock drag racing, and similar limited tire classes (cornering and straight line competition): store the impact energy until it can be released later (when the tire can afford the extra load, or when the shock load can be applied over a longer time period).

When competing a vehicle with a superior traction to weight balance we can ignore storing energy with the suspension: go-karts & F1, Pro-Stock and rail dragsters. The tires traction (and driver skill) is so superior in these arenas that the driver does not need a suspension aid to smooth out the rough spots that can break the tire adhesion. This is the design perimeters documented in most "racing" chassis design manuals: transition loads and driver skill that do not tax the tires adhesion limit. These vehicles are designed to exploit the high traction available (due to the generous tire friction) and can basically run with minimial suspension. The fact that driver skill has placed competition at the ragged edge of the superior tire performance is remarkable.

Rockcrawling and dirt surface racing does not have the advantage of a controlled track surface, and the suspension design must exploit storing shock loads, and the slow release of the energy. Take away the chassis's ability to smooth out the shock loads, and you make the driver control more difficult. This is what happens when the anti-squat percent reaches 100%, or exceeds 100%.

We get to the question ... why does the AS% change when climbing?

Take an FBD for a vehicle linkage system and isolate the IC and CG for level ground. Use a 90 inch wheelbase with 36-inch tires and a 36-inch high CG on the wheelbase centerline (50/50 weight distribution). Draw a linkage system with 75% anti-squat using a level LCA.

Rotate the FBD up 45 degrees (the ground), and recalculate the AS% (and exercise the skills).

The Einsteins will twist their head 45-degrees and tell you the AS% is still 75%. The wheelbase is the same, relative to the ground, and the IC and CG are all the same distance apart with the same linkage angles, relative to the (now angled) ground. The IC is still 27 inches from the front tire contact patch on the ground, and the CG is still 36 inches off the ground measured at the front axle centerline.

What they fail to see is the GC does not care about the simplified convention of measuring the IC and AS% in relation to the ground (or at the front axle centerline perpendicular to the vehicle wheelbase). The convention of using this method for calculating the AS% is a simplification constrained to the level wheelbase condition, where the gravity force (and reaction) is always perpendicular to the wheelbase, and the ground: where we measure the AS% from the ground along the vertical line along the front axle centerline.
Draw the level condition (before rotating and recalculating). The ICL is 27 inches over and CGL of 36 inches = 75% Anti-Squat (check the math).

The non-level ground condition places the force of gravity at an angle to the ground, or the axle centerline height of front and rear axles (the wheelbase) at different heights. The IC is still located the same way, the extension of linkage intersections. The true wheelbase is the same, but the virtual wheelbase perpendicular to the gravity force has changed. The relationship of the linkage angles, relative to the gravity force (not the ground surface) have been altered. The external FBD only cares about the gravity direction (down) at the CG, and the resistance (up) at the tire contact patch.

Have the drawing rotated 45 degrees. Draw a vertical line through the front tire & axle centerline parallel to the direction of the gravity force (down, and now 45 degrees to the wheelbase). Maintain the IC location points and CG (they did not change). Draw a new level line (perpendicular to the gravity force) at the rear tire contact patch (it moves, slightly). Calculate the revised AS% from the new line that is level (at the rear tire contact patch) along the (now much taller) front axle centerline? I measure (rough scale) ICR at 100.5 inches over the CGR at 65 inches = 155% Anti-Squat.

Somewhere during the rotation of the vehicle (in relation to the direction of gravity) that was forced by the terrain change (the 45 degree climb), the Anti-Squat percentage changed from storing 25% of the torque load on the springs (75% AS), to fully carrying the torque load on the suspension links (100% AS, @~22.5 degrees) with no storage in the springs, to storing the torque load in extended springs (150% AS).

Next is where you need to ignore the ground and get a feel for the suspension movement under throttle with gravity pulling 45 degreees to the ground.

This has progressed deep into :cheers: time. It is near worthless without drawings (not possible to scan at work, and I need to run some drawings through the OCE machine).

[MaXJohnson]

If anyone wants something to fiddle with, here's a drawing that's not yet ready for prime time. The mark on the drivers door is the estimated CG. At a glance, it appears that a line drawn from the contact patch through the IC (front spring eye) won't even converge with the front wheel centerline.

http://home.everestkc.net/alynloya/Jeep_Profile_climbing45.JPG