contradiction in the movement of a 4 link rear end?

[Sam_Q]

I am confused in the way a 4 link rear end works in one area:

I understand that the diff when it goes up in a car such as an ae86 the diff rotates and pinion angle changhes as it moves up due to two different arc of movement from the unequal length top and bottom arms. This is all good and all but what happens when one side is level and the other side moving up? it doesnt seem like a problem at first but looking closer because of the different axis' of movement between the top and bottom arms it wants to rotate but one side does not because it is at a neutral position. As far as I can see it on one side will in that situation want to stay as it is and the other will try and twist.

I can only see this be taken up in the bushes and hence why its recommended to drill out the upper top left trailing arm bushes when the rest are upgraded. But what happens when rose joints (spherical joints) are used in the complete rear end? without any deflection allowance I can only see that one side of the diff can move up by twisting the diff housing, or am I missing something here?

any ideas would be good as I have a headache and have had one for a while now.

[Lachie]

I think what your missing is the wokring of the rose joint itself. The rose joints do not only allow the bars to only move up and down, they also allow the rod to move side to side. i.e the rod do not move in a straight line up and down, they move in an arc up.

Because of the rose joints, the arms on the side that doesnt move simply become slightly offset. when looking at the diff in the horizontal plane.

Don't know if this will make any sense to anyone else, so'll draw some pics to post to get across what I'm saying, stay tuned!

[Sam_Q]

no I dont think that makes a difference because the problem is purely in the sense that they cant shange their length and therefore it will want to twist the housing. I am however open to the fact I am just mistaken.

Drawing would be good!

[Lachie]

Oh, so you are thinking of the ousing twisting about the arms when looking at the diff from the bottom or top?

I'm having some toruble uplodaing my pics, but hopefully I'll sort is out. I can only show you the diff with the chassis rolling form Susprog, but hopefully it helps to get the pont across

Don't forget that the diff is on splines so that it moves fowards and backwards, so it travels in a a big arc. The acutal movement of the diff is made up of two arcs, one for mthe panhard rod/watts link and the other from the trailing arms

[Lachie]

I re-read it and I get what your saying now. You mean the alxe twsiting in the same way the axle rotates!

I'm stumped for now too, let me think a little while

Edit: Yes, it will twist, but it's bugger all. Getting pics now

[Mos]

Lachie - the rose joints only exasperate the effects, they dont make it any better.

The twisting behaviour is well understood. Because of the unequal length of the top and bottom arms, twisting is induced differently on each side and it's taken up by bushes and components twisting. If you remove any flexing in bushes (ie by using rose joints) then something else has to give, otherwise it will resist travel, like a sway bar.

The unequal arms are a packaging constraint and, as far as I know, not an ideal design. I'm not sure if having equal length top and bottom arms completely removes the problem if the arms are not parallel.

Lachie: the diff movement, due to the trailing arms, is more than one arc, depending on what each wheel is doing. I'd be curious to see your pics because I'm struggling to understand what you're getting at.

Mos.

[Sam_Q]

Mos: I completely agree with what your saying and it confirms my theory. I do also beleive that even with equal length arms there is still a difference unless the arms are both parellel which they never are because of the dialed in antisquat.

[Lachie]

Yeah Mos, already agreed that the diff housing will twist (see edit).

I wa thinking Sam was talking about the diff twisting in a a different way, not about the axles. I just misread the question

As far the diff moving on two arcs, it would technically move in three. It would rotate when viewed from above or below, but it's sweet FA, so you really only need to worry about the arc from the trailing arms and the panhard rod when thinking about it in your head.

The trailing arms will most likely flex before the axle twists anyway, but either way, it will just add a little more roll stiffness to the rear. Just run the rear anti-rll bar ever so slighlty softer if you think its a concern

[Lachie]

Mos: I completely agree with what your saying and it confirms my theory. I do also beleive that even with equal length arms there is still a difference unless the arms are both parellel which they never are because of the dialed in antisquat.

Yeah, that rights. Unless they were parallel it would still twist the housing.

[Sam_Q]

so the big question is when one of those T3 multilink rear ends is used and one wheel goes up then what gives to allow for the movement?

[5000gt]

well this has me worried... something i never put any thought into.... just installed my adjustable 5 link (panhard included)....

[oldcorollas]

if the top links are angled in, the effect is minimised somewhat.. and having some compliance in some of the bushings will also help..

but it should be easy to calculate exactly the amount of compliance needed..
then.. test the bushings to see if they bind under those conditions.

[LeeRoy]

I would hazard a guess that as one wheel raises, the force acting downwards on the opposite (unloaded) wheel is increased resulting in it being forced down by both the spring and the increase in force on the arms resisting its movement.

[YelloRolla]

Adding a big sway bar will help keep the rear end in line. In fact his is how 4 links in drag cars are tied in together; a turning diff housing when you are on the back wheels is not a good idea.

edit - that is if I am understanding the problem (it is early). When we are saying twisting, do we mean that the diff is turning? IE the front and rear axles centrelines are no longer parallel.

[rms]

If the arms are not parallel (top and bottom) AND equal length, then the diff housing becomes a large sway bar on one wheel bump.
Arms can be angled up for some anti squat, but you then get roll steer.
Torque reaction will not induce anti squat with parallel arms, the reactions cancel out.

A rubber bush fitted to one end of one arm will delay the twisting moment of the diff.

These problems are overcome with two top arms and a reversed lower 'A' on a live axle, like Bill Sherwoods layout on his De-Dion.