Rear steer

[totta crolla]

No, not going mad!
It occured to me that the rear axle has an element of rear steer due to link arm angles.
lf the bottom link arms point down to the axle at ride height as the car corners and weight is transfered the out side rear spring compresses (the inside elongates) when the spring compresses the body drops and this action effectively makes the bottom link arm 'longer' as it goes through its arc, the other side rises so that side gets 'shorter' this creates an element of rear steer.
ln this case it creates toe out on the outer wheel and toe in on the inner wheel and this will aid turn in.
The effect is tunable by adjusting rear ride height, too low on the back (arms pointing up to axle) and the car might tend towards understeer on turn in.
Any thoughts on this theory?

[davew7]

If you Google short track rear suspension systems, you see multi hole locations on both the chassis and axle for trailing arm adjustments. This aids wheel alignment for better turn-in, plus anti-squat for improved traction. Changing ride height effect both front and rear suspension, plus weight transfer.

[jondee86]

Rear steer aka Roll steer, and interesting enough to force me to consult Google, from
where I gleaned this information...

Cars with rear live axles, also known as solid axles, tend to not have true bump steer.
Since both wheels are connected to a single, rigid member they are incapable of having
any toe angles under normal conditions. Rear live axle suspensions are therefore designed
to exhibit roll understeer. During a curve, the entire axle will turn slightly to face the
inside of the curve so that the car does not turn more sharply than anticipated by the
driver.

It is possible to design a rear live axle suspension that exhibits roll oversteer but
it is highly undesirable for on road use. Roll oversteer is sometimes incorporated on
extreme off-road vehicles because it can allow the rear wheels to help turn the vehicle
extremely sharply in tight trail conditions.

Thinking about this, I can see that obtaining the desired effect with the unequal length
links on an AE86 would take a great deal of careful setting up of ride height, spring rate
and rollbars. As the links move through different arcs, having enough body roll to get
a worthwhile amount of rear steer would also be likely to cause the the links to bind.

Cheers... jondee86

[totta crolla]

I think that just a small amount could be used to advantage, l'm guessing here but 2° (1° in/ 1° out) would probably be enough to make a difference.
I couldn't really get my head around how the short links affect what is going on, they can't physically change the arc that the links run through.
lt is worth noting that the Toyota engineers probably built in an element of roll steer and it will have been biased towards understeer.

[jondee86]

Unequal length trailing links cause the pinion angle to change when the axle moves up or down.



This does not occur if the links are equal length.



The problem with four links is that when one end of the axle moves up or down when the car
hits a bump, or is cornering and one end goes up and the other goes down due to body roll. Then
the links are effectively trying to twist the axle housing. Once the rubber bushings have absorbed
all the compression they can handle, no further "twisting" can occur and the axle binds. Either one
wheel comes off the ground or the brackets holding the links deform. Worst case is that the
bracket can get torn from the chassis.

This is where the "three link" mod comes into play... the front bush on one of the top links) is
drilled full of holes to make it soft enough to absorb more deflection.

Cheers... jondee86