IRS Bumpsteer adjustment

[PaulProe]

I'm building a Hurricane kit with an IRS - Supercoupe based. The upper arm is not adjustable, only the lower arm. I can change toe and camber.

I am measuring a total bumpsteer change of about 3/32" toe-in (measured over 2" bump and 2" droop).

How is it recommended to correct or adjust to compensate for this? I've worn out my Google button looking for a way to adjust rear bump steer on a T-Bird style IRS.

Thanks

Paul

[mickmate]

Hey Paul isn't toe on an IRS affected mostly by caster? I have some charts somewhere that tell you what to adjust for the toe curve plotted. I'll see if I can find them.

[john chesnut]

I am not familiar with the T bird IRS so my thoughts are not specific to that unit. There should be a "toe" adjustment link. The pivot point on the inner tie rod should be in plane with the upper and lower control inner pivot points. If you were to draw a line from the upper a frame pivot point to the lower a frame pivot point the pivot point of the inner tie rod should fall on that line. If it's not then the arc of the links will not be the same and will effect the toe.
Essentially the goal is to have all three links, upper and lower aframe and toe adjuster link, move in the same plane at the same rate. Usually the inner tie rod mount is fixed so the adjustment is done at the outer tie rod, where it bolts to the hub. This can be done by shimming the tie rod up or down as necessary. Just make sure that the tie rod is still seated and tightened properly in the spindles tapered hole.
I am not an expert but this is my understanding of the geometry.
Good luck! John

[PaulProe]

Mickmate,
I think you're right, but the T-Bird IRS doesn't have a way to set caster. The lower heim's are captured between conical washers so I can't change spacer thickness. And the upper links are welded.

John,
The T-Bird IRS doesn't have a tie rod. It uses adjustment of the lower pivot points to control toe and camber. The outer knuckle doesn't have any adjustments.

The only thing I can think is to set toe great enough that as it bumps/droops, it always stays in a toe-in position? Would this be correct? I think I am correct in saying you don't wan't it crossing neutral and going to a toe-out setting. To keep that from happening, I will have to run a relatively high toe-in.

? ? ?

Paul

[Aussie Mike]

Hi Paul,

I wint through this whole bump steer excersise with the rear of my Classic Revival.

I built new arms because I wanted to narrow the rear end and get rid of the bad bump steer with the stock rear end. Here's a link to the thread Rear suspension mods.

If you graph the change in bump over the wheel travel is it linear or does it peak at the ends of the travel? I managed to tune nearly all my bumpsteer out but it still has a little. This is down near full droop but most of the travel where the suspension works there's hardly any bump.



3/32" doesn't seem like too much if it's down near full droop. I'd just align it so you are always in toe in backing out to zero at full droop.

It's not a nice feeling when you stand on the brakes at high speed and the rear end is doing toe out.

Cheers

[john chesnut]

I knew that I was going to get in trouble giving advice for a unit that I don't have personal experience with!!!!!

[PaulProe]

John,
Your info and post is appreciated. You never have too much info.

My dilemma is the T-Bird based IRS presents some additional challenges due to its limited adjustability.

Still looking for any help on roll steer compensation on a T-Bird or SuperCoupe based suspension.

Paul

[mickmate]

What capability do you have to roll the rear end and change the caster? Would that mess up driveline angles or not possible?As Mike said 3 thirtytooths ain't much, did you see what I was talking about graphing it as he shows. That tells you what to do to adjust/correct it. You seem to be on the right track with setting it at 0 to 3/32 toe in. That is now going to be only 3/64ths at ride height which is pretty good methinks. Morris did a heck of a thread and explanation of setting up that geometry for an IRS on the Gasholes forum. Worth finding and reading if you have a spare hour or two.

[RICK LAKE]

PaulProe Paul if you have the last 2 Kitcar mags, Dean Lampe goes into this procedure on setting up the front end. The rearend should be the same. I have not seen a IRS from a Supercoupe but all you have to do is change a solid rod or link to a threaded one. Speedway selld Heim joints, couplers and threaded rods for maken your own setup. Replace the rear link with your adjustable one and retest up the rearend. You didn't say if you are street driving or autocrossing or roadracing. The suspension is going to be totally different for each one. Just springs alone can be 150-200 lbs different between street and autocrossing. Shocks and alignment are different too. Have you tryed the Supercoupe clubs? Someone there must be roadracing or autocrossing their car and have suspension modes done. As long as the weights for the 4 corners are setup right and a 48-52% weight of front to rear end with your wieght in the drivers seat, you will be in the ball park for a balanced car. I toe in may Jag rearend 1/32" and have the thrust line of the car at "0". This will get you close to perfect. You really need to read tire temps after running the car and see what the car needs in changes of toe, camber, and caster. Rick L.

[strictlypersonl]

With the lower control arm controlling the toe, I don't see any way for such a major bump-steer change unless the LCA is set with a very large caster angle. Are you sure that there isn't excess wheel bearing or pivot-bushing play?