Rear suspension question

MikeJ06 · **Posted:** Fri Nov 02, 2007 10:24 am

This is a silly question, but I really don't know anything about solid axle suspensions. Could anyone please explain the whole rear suspension angle to me? What I mean is, when either side of the rear travels, does it get pulled closer to the front of the wheel well under normal conditions or does it remain centered throughout the entire range of travel?

I asked a friend of mine and his response was that I was thinking of the rear arm as being a single pivot point on the axle and not as an entire structure with multiple points. But if there are multiple pivot points at the rear ( I guess it is a U joint?) at what point in suspension travel does the entire rear get pulled forward? What happens in rigs with long travel suspensions when one side is completely extended and another stuffed? Also, does the opposite side become effected when one side is pulled forward and if so how?

It's a newbie question :wink:

MikeJ06 · **Posted:** Fri Nov 02, 2007 7:11 pm

Should I post this in another section, or is it really just that ridiculous of a question?

ATXKJ · **Posted:** Fri Nov 02, 2007 7:35 pm

You'd get more answers in the general section. Lot's of folks just look at specific sections -general or 4x4
(you can click on view all new posts upper right hand corner and see everything in all sections)

To answer your question basically there's always something to try and keep the rear axle centered on a solid axle system - on older suspensions the leaf springs would hold it laterally - on coil spring cars - its typically called a panhard rod - one ends attached to the frame - the other to the axle. Sometime the control bars that keep the axle from rotating are set at angles so that they also hold laterally.

On KJ's it's a Y shaped upper control bar that has 2 contacts on the body/frame and 1 contact on the top of the axle. Keeps the rear end centered and prevents it from rotating as you accelerate. There are also 2 lower control arms - but they go straight forward -'traction bars' to keep the rear axle housing from rotating, but not much help laterally.

now everything flexes so it will move - but the intent is that the wheels move up and down but not forward or backwards.
If I can find a good picture I'll try to post it.

Also the only time the axle really pulls forward - is when you lift the Jeep - if you lift more than a couple of inches - you need longer lower control arms to recenter the rear wheel in the wheelwell.

snowsport · **Joined:** Tue Oct 18, 2005 11:52 pm **Posts:** 1211

^^ A really fine description! The rear end on the KJ is actually a simple setup. Simple but it works well!

MikeJ06 · **Posted:** Fri Nov 02, 2007 9:10 pm

So the normal range of travel for a stock suspenion doesn't allow the rear to drop low enough to cause the axle to be pulled forward at all?

I really appreciate the help guys! For the most part I just started to get into Jeeps and offroading. I've retained a lot of info on lifts, gears, t-cases and traction additives, but not too much on actual suspension mechanics.

ATXKJ · **Posted:** Sat Nov 03, 2007 8:31 am

It's not intended to move forward - however the control arms are operating in arcs and the mounts are either ball joints or rubber cushioned so it does move a little.
also downward drop is limited by shock absorber extension on the low side and the ability if the opposite wheel to go up into the wheel well - normally folks put a stop inside the springs to limit how much the wheel goes up - prevents wheel well damage and prevents the spring from popping out on the 'low' side
Here's a picture from sini - one of the Australian guys showing full extension.

Eddo · **Joined:** Thu Jul 07, 2005 7:01 pm **Posts:** 1214

MikeJ06 wrote:

So the normal range of travel for a stock suspenion doesn't allow the rear to drop low enough to cause the axle to be pulled forward at all?

I really appreciate the help guys! For the most part I just started to get into Jeeps and offroading. I've retained a lot of info on lifts, gears, t-cases and traction additives, but not too much on actual suspension mechanics.

The axle always moves forward and back with suspension travel. However in normal operation it is not that much, hence you would probably never notice it and it doesnt cause any issues.

The axle pivots on a simple radius arm.

Here is basically what the rear of the KJ looks like

The other end of the control arm (far left of the picture) attaches to the frame.

There is also an A shaped third link, but that is there to limit axle twisting and lateral movement.

Long travel supensions have long control arms. The longer the control arm the less horizontial motion.

MikeJ06 · **Posted:** Sat Nov 03, 2007 3:51 pm

Thanks a lot! You all have definately helped. Last question then I promise, and it is more to confirm that I understand it all than anything else.

Lifts that offer more articulation result in more forward movement of the axle in the range that a given side of the axle travels in right? And the degree to which that side can travel is limited (obviously) by the length of the shock, but also by how well the axle can flex in relation to the opposite side and other components.

ATXKJ · **Posted:** Sat Nov 03, 2007 7:39 pm

Yes - although on Liberty lifts it's fairly limited.

The limiting factor on Liberty lifts is the angle the front wheel CV joint can go to before it's damaged - so about 2 1/2 inches on the original 02's a little longer on later models (because they were lowered at the factory) but 2 1/2 is pretty much the standard (there are a couple of folks looking at higher - but it takes a lot more work)

On a wrangler or XJ - to avoid a lot of axle movement on taller lifts- they go to longer control arms - longer radius gives less forward - rearward movement for a given up/down distance.

Rear suspension question

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