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37 Posts
Discussion Starter #1
Hi all,

Need some help with bump steer and I figured the folks watching this forum might be the best group to ask. I don't have a challenge car - it's an 818, but the front suspension setup is basically the same between cars.



I have the Longacre digital bump steer gauge. I have everything setup right:
- Car is on cement blocks with tile shims to get all four corners exactly level using a laser level beam
- Ride height is adjusted on all four corners using the laser level beam for reference
- Static Caster/Camber/Toe is set where I want

I'm using a Baer bump steer kit (like you Roadster guys do) made for Subarus.

I'm playing with the spacer shims to see what gives the best result. I have read that you want ZERO toe-IN under compression, and if that is not possible, go for a little bit of toe OUT in compression. I've also read that you absolutely want to avoid having the toe change directions in one direction (meaning, through the range of bump, you never want it to first toe-out and then start to toe in, or first toe-in and then start to toe-out). Can anyone validate these claims for me?

The issue I'm having (assuming the above is valid), is that when I add enough spacers to get the toe direction to be consistent under compression (toe-out), I'm getting a LOT more toe change than is acceptable.

It looks like this with 30mm of spacers (compression):
Suspension Travel Toe
0 0
1 .0158 toe-IN
2 .0108 toe-OUT
And with this setup, I'm getting a quarter inch of toe-out @ 2" of rebound


To get to consistent toe-out (I can't get to zero toe), it looks like this with 50mm spacers:
Suspension Travel Toe
0 0
1 .048 toe-OUT
2 .138 toe-OUT
And with this setup, I'm getting .039" of toe-out @ 2" of rebound

Any input would be appreciated! Thank you.
 

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FFCobra Fanatic
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2,031 Posts
Here are my measurements from the challenge car

Bump measurements:
Code:
Ride Height	rear of wheel	Front of wheel	tow
5″		68 10/16	68 11/16	1/16 out
4.5″		68 11/16	68 12/16	1/16 out
4″		68 12/16	68 12/16	0
3.5″		68 12/16	68 13/16	1/16 out
3″		68 12/16	68 13/16	1/16 out
2.5″		68 11/16	68 14/16	3/16 out
2″		68 10/16	68 14/16	4/16 out
 

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37 Posts
Discussion Starter #4
Thanks for the replies guys.

I finally figured it out. Here is my way of explaining bump steer (since everything I found online wasn't really making sense to me):

You shim the tie rod out from the spindle until you stop getting toe out when you go one direction (compression or rebound), and toe in when you go the other direction (compression or rebound). At that point, you should get toe-out in BOTH compression and rebound, or toe-in in BOTH compression and rebound, or, if everything is perfect, zero to change in either direction. If you have toe change at this point, your tie rods are too long or too short, or another way to put it is that the inner tie rod pivot point is too far inboard or too far outboard. Toe-out is from tie rods being too long and toe-in is from them being too short. Can't do much if they are too short but if they are too long, you can add steering rack extenders to move the inner tie rod pivot point out. I think they did this on the Roadster. They didn't do it on the 818 though.
 
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