With adjustable trailing arms, the length adjustment affects the angle of the rear spindle (i.e. the rear caster). Thing is, I have no idea what adjusting the rear caster would do. I have a lot to go in terms of learning about suspension design, so I'm purely speculating when I ask if it has anything to do with loading up the rear on a hard launch, or affecting rear bump steer.

Also, how would you measure rear caster? My friend who aligns my car says that you can't measure rear caster, but I imagine that by toying with the length of the rear toe links, you can calculate rear caster the same as you would calculate front caster. That being said, what's the point of rear caster adjustability?

Finally, what would be a reccomended setting for rear caster, and what would adjustments in either direction do for the car?

Thanks,
-Phil

 

rear caster can do some goofy stuff ....the main thing to worry about is bumpsteer so you pretty much use the castor adjustment to adj. bump steer

as for measuring the "castor" I really wouldn't worry about it too much just adjust it untill your bumpsteer is zeroed out. If you really want to figure it out true castor you have two option you can put the car on swivel plates and disconnect the toe links and measure the camber change or you can built a jig to be 90 deg. to the king pin angle and place a smart level on it.....but like I said the castor number really isn't needed in your situation just bumpsteer the car

 

There really is no caster on the rear.

You're right in seeing that the inclination angle changes and you relate that to caster, but there is no steering axis (kingpin) in the rear; the rear uprights are not turning to the left or right about a steering axis. Therefore there are no caster effects because no weight is jacked and the inclination angle as well as the direction the tire faces doesn't change as the suspension moves. On the front of course this isn't true.

Caster doesn't do anything until the tire steers away from straight ahead. Since the rear doesn't steer...

 

^ I agree I was just using his terminalogy .

there is still a connecting Joint axis( not a king pin Axis as i wrongly stated) which if taken to an extreme angle can cause some strange affects....mainly due to suspension bind.

 

Sereneseven: This begs the question. How do you measure bumpsteer?

 

AHA! well for that you will need a .....bumpsteer gauge, which is made up of a flat plate bolted to the hub face it will need to be roughly as wide as the diameter of your tire and @ least say "24 tall

then you will need a dial gauge mounted to a hieght adj. mount which will need to be....hmm well the bracketry is a little hard explain look here
http://www.longacreracing.com/catalo...id=156&catid=5

next with the car sitting at ride height mark where the spindle is in relation to the chassis(tape on fender works) then jack up car suport on stands remove wheel bolt on plate diconnect ARB you may also have to loosen spring perch if you can, now place jack under upright and raise it untill its back to ride height, place gauge and bracket against plate mark plate where needle is , zero gauge , jack the upright up 1-5 inches and note change in reading then lower back to R.H. and drop upright 1-5 inches, adjust untill you have as little change as possible. in the back always error towards bump toe in ....and your done.