Suspension Geometry With Various Caster Correnction (4 Viewers)

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Some time ago there was ‘that’ thread, about different options on caster correction and what each did to the front suspension. I had done up a couple of examples on CAD, and I decided to sit down one day and do up three different options. 1st is stock arms w/ either caster bushings, or caster plates. 2nd is longer radius arms. 3rd is 4” drop brackets. So everyone knows where I started, I’ll list my assumptions.

1st assumption - when fully compressed w/ stock bumpstops, that the wheel is centered in the front wheel well.

2nd assumption - 4” lift

3rd assumption - 2” lower bumpstops

4th assumption - 4” up travel, 4” down Stock. Raising with 4" springs, and only lowering bumpstops 2" would add two inches of compression. So I've shown compressed as 6" and extended as 4".

5th assumption - I’m only looking at one side of the truck.

6th assumption - I'm in the process of changing the assumptions on the longer arms, based on what Slee has said below.

7th assumption – stock arms are 33" long - per post below from Slee, it's approximately 33" from pivot to axle center.

8th assumption – axle center is roughly at the same height as the pivot point of the stock arms, at stock height.


This is by no means an end all analysis. It’s just a curiosity on how the radius arm reacts to different changes, and how that effects the placement of the axle, and therefore the tire in relation to the wheel well opening. Seems many of us are trying to fit as big a tire with as little lift as possible, and thought this might help guide some of that activity. It’s been a while since I actually drew this up, so I may be a little off on some of my memories. If there is enough interest in alternate configurations (arms on top of axle), or someone has better info on starting dimensions, I’d be willing to modify with those dimensions / configurations.

This shouldn’t be a source of debate on what’s going on, that’s pretty simple geometry, an arc with a center point, and the computer rarely lies. The dimensions are based on distance from center of the wheel well. They’re not 100% accurate because of the assumptions, but like I said earlier – they’re directionally accurate as far as which configuration moves more fore and aft. Now, as far as debate on which is the best way of modifying the stock set-up, that’s inevitable! :0

Hopefully someone will find some use in this…

ATTENSION - the below are a little off due to the info. Slee was kind enough to supply. I will adjust accordingly, and replace the dwgs - might take a while for me to get to it though.

Edit: I corrected two of the three, the longer arms will take a little more to adjust.

Edit #3 or 5 ;) : All three are now corrected.
 
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Stock Arms - caster bushings or plate

.......
Stock Ride.jpg
Stock up.jpg
Stock down.jpg
 
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Why you should care about this?

This is what happens with the radius arm as you look down from the top when you hit a pothole or bump. And this is why the more movement front and back for the same movement up and down will give more bump steer.
80 series Suspension steering.jpg
 

sleeoffroad

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Heath, fwiw, the assumption on our arms are incorrect. They are made so that the wheel is dropped down vertical for 6" of lift. So at ride height with 6" of lift, the wheel is exactly in the same spot (front to back as it was before, as well as the caster is correct. This does change the arcs.

Also, stock arms are about 33" from center of frame side busing to center between the two axle bushings.
 
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Heath, fwiw, the assumption on our arms are incorrect. They are made so that the wheel is dropped down vertical for 6" of lift. So at ride height with 6" of lift, the wheel is exactly in the same spot (front to back as it was before, as well as the caster is correct. This does change the arcs.

Kool. I'll change it accordingly. That's part of why I wanted to put the assumptions up front, so if any were wrong, they could be corrected. I don't want to be saying innaccurate things about your product or anyone elses. Effectively, it is further from the pivot point, and will have a longer radius to the arc, and will be closer to the front bumper under compression, right? I'll adjust it though so it looks right, but the effect will be simular.

Also, stock arms are about 33" from center of frame side busing to center between the two axle bushings.

Thanks Slee. I'll adjust the arm length in my drawings, and recalculate. What's stock travel - about 4" up 4" down?
 
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Wow, I've only done the dropped brackets and stock location with stock arms so far, and it sure does make a difference the arms being 33" instead of the 24" I origonally used. Much less front to back movement.

Stock arms, stock location:

Ride height - wheel .1" behind center of opening

Compressed - .18" in front of center of opening

Extended - .75" behind center of opening

Overall difference if one side fully compressed, and other fully extended .93"


Stock arms, dropped brackets

Ride height - wheel .24" in front of center of opening

Compressed - .29" behind center of opening

Extended - .0" it's in the center of opening

Overall difference if one side fully compressed, and other fully extended .24"

Interestingly, you'd get more difference if one is at ride height and other is fully compressed .53" difference.


Aftermarket arms, stock mounting location:

Ride height - .53" in front of center center of opening

Compressed - .72" in front of center of opening

Extended - .17" behind center of opening

Overall difference if one side fully compressed, and other fully extended .89" Which makes sense. Longer arms are going to have less fore/aft movement than short arms, that's why Jeeps, which have shorter arms than we do, have aftermarket long arms available.
 
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I am curious why you are relating this to bumpsteer. Bumpsteer is affected by having an unequal angle between steering draglink and panhard. That's why you can't just slap on a front panhard drop bracket with no other effects.

Choosing a drop bracket vs. stock mounted arm changes the arc of the arm. With a drop bracket on a 4" lift, the arm always moves back on compression or extension because it is essentially perpendicular to the axle.

On a lifted arm, the arm has to come forward to compress. Yes, you begin to introduce some harshness, but I don't see or have ever experienced bumpsteer from this, and I have had 16" long Jeep arms on a 6" lift.

Given how long the 80's front arms are, they easily tolerate a 4" lift. But you can argue that flex becomes more limited on an angled arm because the axle has to twist fore/aft for articulation. When the arms are perpendicular at ride height, there is less of this twisting.

None of this is really a caster issue. Caster is something you deal with once you have the rig built the way you need it.
 

landtank

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twisting the axle as he has shown in the previous post is bumpsteer as one side of the front axle is closer to the rear axle than the other side.

At higher speeds this is actually favorable for better control. The pie shape of the two axles has an effect similar to 4 wheel steering. The same is true for leaf sprung vehicles.

At low speeds where this would be the most evident, your going slow and can easily compensate and probably wouldn't even notice it.
 
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I am curious why you are relating this to bumpsteer. Bumpsteer is affected by having an unequal angle between steering draglink and panhard. That's why you can't just slap on a front panhard drop bracket with no other effects.

I relate it to bumpsteer cause what happens when the axle moves up, it either moves up and forward or up and back. If one side is moving up and back or up and forward and the other side is stationary (pothole, or bump on one side of the road), the axle is twisting, or turning relative to the frame, and is causing some amount of steering. The more it moves forward or backward, the more steering your going to get.




Given how long the 80's front arms are, they easily tolerate a 4" lift. But you can argue that flex becomes more limited on an angled arm because the axle has to twist fore/aft for articulation. When the arms are perpendicular at ride height, there is less of this twisting.

That's correct, and that's why the forward and backward motion got so much less (like an inch or more less) when I adjusted to the 33" that Slee gave me rather than the 24" I got somewhere (may have been from my rear, I don't remember).

None of this is really a caster issue. Caster is something you deal with once you have the rig built the way you need it.

Didn't really say it was a caster issue. In fact, on one thread awhile back, I mistakenly said that the more level the arm was the less caster change you had, and then when I went back and laid it out in CAD, I found that the caster change is the same through a 8" travel with dropped brackets or stock brackets.

The title of the thread says it all - suspension geometry with various caster correction. How you choose to accomidate the caster - arms, bushings, plates, drop brackets, flipped arms, cut and turned axles, 3 link, 4 link, and what ever other techniques I'm forgetting or unaware of - determines how the suspension geometry changes through it's motion. As long as you stick with radius arms, the effects we would be most worried about are the fore / aft movement, which can steer the vehicle, and create bind in the bushings, and how the chosen method locates the tire within the wheel well for rubbing.
 
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landtank

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you want the end of the axle that is on the outside of the turn to be forward of the opposing side.

If you look at a leaf sprung vehicle the shackles are at the ends of the vehicle. This is because as the leaf spring flattens out the axle moves out away from the center of the vehicle.

So if the vehicle makes a left hand turn the body roll with shorten the distance between the axles on the inside and lengthen the distance on the outside.


Take my truck for instance. I have a 4" lift with caster plates. So in a left hand turn the body roll will compress the right side which will push the axle forward and on the left hand side the spring will extend causing the axle to move back. This "steering" helps support the maneuver.

Now on a truck with drop brackets where the arms are horizontal, both sides move back the same amount and there is neither help or hindrance.

In a lowered truck, sorry LX, where the arms are lower in the rear the axle twists against the turn, the outside moves back and the inside moves forward, fighting the maneuver.
 
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I wonder how much of this really hits the steering function on a 33" arm, especially with large tires. A shift in the axle won't turn the steering wheel (bump steer) as both the panhard and draglink move equally. Any amount the axle moves fore/aft will create an opposing movement laterally, with a neutral effect on the steering wheel. One of the many reasons a coil sprung front end with panhard is superior to a leaf sprung front end that doesn't have a panhard to control bumpsteer

In any case, if I draw out a 33" radius arc and then measure say 3" of compression along that arc, how much fore/aft motion did I really get? 3" compression as a max is about right, at least on my suspension. I don't have any boots and the dust line shows the max compression.

It is an interesting point on the handling aspect, though, because we often notice that body roll decreases on the lifted rig, and the angled control arm appears to play a role in this. One of the many reasons you can't make one size fits all assumptions about swaybars.
 
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If you look at a leaf sprung vehicle the shackles are at the ends of the vehicle. This is because as the leaf spring flattens out the axle moves out away from the center of the vehicle..

Depends on which leaf sprung vehicle as to weither the front shackles are in the front, or in the rear, does it not?


Take my truck for instance. I have a 4" lift with caster plates. So in a left hand turn the body roll will compress the right side which will push the axle forward and on the left hand side the spring will extend causing the axle to move back. This "steering" helps support the maneuver.

Yes and no. As you corner, the body starts to lean, and then the suspension changes make the truck turn more ('support the maeauver'), which makes it lean more, which makes it turn more, and on and on until it reaches some limit, or you start letting up on the steering.

If you reverse it, shackels at the rear, you start getting the pie shape you spoke about earlier, and that pie shape, as you said, effectively gives a type of rear steer, only this time, the rear would be turning in-phase with the front. Which is the desired effect of rear steer at speed. It's only at slow speed, parking manuvers and such that any active rear steer goes out of phase.


Now on a truck with drop brackets where the arms are horizontal, both sides move back the same amount and there is neither help or hindrance..

Which right, wrong, or indifferent, is the way Toyota intended it to work.

In a lowered truck, sorry LX, where the arms are lower in the rear the axle twists against the turn, the outside moves back and the inside moves forward, fighting the maneuver.

Again, sorta. Given the front wheels are turned, and you use the front axle the baseline, rather than calling the frame the baseline, the rear is turning in-phase with the front, and giving the 'correct' type of passive rear steer.
 
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I wonder how much of this really hits the steering function on a 33" arm, especially with large tires. A shift in the axle won't turn the steering wheel (bump steer) as both the panhard and draglink move equally.

Nay, you're a little off on what bump steer is.

"A. Bump Steer Definition

Bump Steer is when your wheels steer themselves without input from the steering wheel. "

http://www.longacreracing.com/articles/art.asp?ARTID=13


Hit a bump, and the front axle changes it's relation to the rear axle, and it steers. Thus - bump steer. It's not like IFS bump steer where the tire is actually rotating on the spindle because of where the tie rods pivot relative to the spindle pivoting on the a-arms. It's steering like that wooden push car you built when you were little, that had a solid 2x4 for a front axle, and it had a single pivot in the ceter, and you steered by pulling back one side or the other with a rope (may be showing my age here - as now day's it's all motorized something or other). Or, it's like my Green Machine big wheel - good luck looking up that reference!

Landtank has moved on and is talking about Roll Steer. It's simular, in that it's the axles movement fore and aft as it goes up and down that is causing the steering, but in this case the up and down movement is from the leaning (roll) of the vehicle, and not from hitting a bump (or dip, which could also be called an anti-bump?).
 
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Heath, fwiw, the assumption on our arms are incorrect. They are made so that the wheel is dropped down vertical for 6" of lift. So at ride height with 6" of lift, the wheel is exactly in the same spot (front to back as it was before, as well as the caster is correct. This does change the arcs.

Also, stock arms are about 33" from center of frame side busing to center between the two axle bushings.

Slee - does it look a little better now?
 

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Depends on which leaf sprung vehicle as to weither the front shackles are in the front, or in the rear, does it not?

I've never seen a stock vehicle with leaf spring with the shackles anywhere but at the outer ends. From all that I've read it's for better control at speeds for the reasons I've stated.

Having the axle help in cornering won't create more lean, just less steering input.

There is however an obvious offroad disadvantage to using caster plates over drop brackets. I'm surprised no one has brought that up yet.
 

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