Bump Steer & LSD w/Swepco any additives?
 

-BUMP STEER-
So I thought, this is easy, just turn down the adj on the front end to match the slightly sagging rear, well now it looks cool but is real jittery/nervous, what is the feedback on the steering rack spaces or the bump steer kit offered at PP?

-Swepco w/LSD-
Will I need to add an additional additive or will straight Swepco work with the LSD?

Thanks!

Cliff:D

When you change the ride height you also alter the toe adjustment. You may need to have it realigned.

If you drop much below Euro height (25.5"?), rack spacers are suggested to reduce the effect of bump steer (toe change during jounce/rebound).

Sherwood

When you lower the front, does the toe-in increase or decrease. Mine is set too high, and the front tires exhibit wear on the outer edges, which I assume is excessive toe-in.

Wouldn't the CAMBER be more effected than the TOE?

I agree, I will need get an alignment, I am just exploring my options for setup, then will bring it in.

Still no feedback on the PP kits?

Cliff

The washer type bump steer kits are good for a mildly lowered car. For a really big drop(> .75") as on a track car, pay the extra $ for the ERP style Bump steer kit.

Swepco 201blue) works fine w/ ZF type lsd w/o the use of any additive packages. Someone at the track was telling me about a different Swepco specificly designed for the clutch type lsd's but the conversation was interrupted before I could learn more.

Thanks Bill!

I don't think I have the turbo tie's yet so this might address both issues ;)

Cliff

SmileWavy

Turbo tie rods won't change a car's bump steer. They just make it more direct.

Hi JOHN!

Ya I know, but the Bump Steer Kit comes with Turbo Ties and dropped joint connectors to address the angle.

How do you identify if I have the Turbo Ties yet, is it the boot configuration?

How are you identifying that you have a bump steer issue?

Cliff;
Getting back to basics, if you change the ride-height of the car (either front or rear), you need to get it aligned again. There's no avoiding it. Everyone knows that 911's have bump steer, so by definition if you raise or lower the front end you WILL change the front toe. If the front is toed out, you'll have that nervous wondery feeling that your describing.

I agree with Noah, if your rear end was sagging, first understand why and fix the problem if there is one. Once you have done that, don't adjust the front ride height to match, but rather set the ride height where you want it and have the car cornerweighted. Finally, align the car. There's really no way to get around doing all of those steps since once you change the ride height you'll also change the corner balance and alignment. There's a better then even chance that any blind changes that make will mess up the settings rather then improve them.

Of course you want to do all of these things after you install the bump-steer kit and turbo tie rods.

that sounds like it - "darty" is the phrase i have come across.

i had the same issue and bought the ERP kit - it cured it, but i am still on the look out for some rubber boots to cover the unprotected joints.

Dickster:
I got some infor from Steve Weiner on a supplier for little rubber booties to use on the exposed ball-type joints like the ERP kit.

Once I find it in my files...I'll post it, unless Steve does so ahead of me...

Wil

wil,

that would be great, i dont think they will last long unprotected in the uk climate!

thanks, appreciated.

WHat's the "current best knowledge" on what angle the tie-rods should be at optimally? THe ERP kit (which I have, but not yet installed) looks to drop the tie-rods at about 15 degrees... Just guessing here...

SHould the be at a slight angle? Level? What?

level or slightly down - but not up! (rise towards the centre of the car)

Common wisdom was parallel to ground.....

Later posts ( here too)...indicate may be better to get parallel to the lower A-arm angle. Often this leads to parallel to ground, too....but for a different reason ( only if lower A-arm is also parallel to ground).

Try it both of these ways....

Wil

Geometrically, it doesn't matter how the tie-rods relate to the ground, only the lower A-arm. In general you want them parallel with the A-arms.

Picture the following 3 examples which all start with the tie-rods parallel to the ground:

1) Outside of the A-arm is angled up slightly (ie. car is over lowered.)
a) As the suspension moves in bump (compressing), the upright will be moving inward along the arc of the lower A-arm. If the tie-rods are level, the steering arm on the strut will move in also according to the arc of the tie-rod. Since they are starting from different points of the arc, their travel will be slightly different and so there will be some bump-steer.
b) As the suspension moves in droop the upright will be moving out as the A-arm approaches parallel with the ground, but at the same time the external end of the tie-rod will be moving in as it traverses the lower portion of the arc defined by it's inside pivot. The result will be toe-out in bump.

2) The lower A-Arm is parallel with the ground. Generally the tie rod and the A-arm will stay parallel since the radius of their arcs should be about the same. In fact it doesn't work out perfectly since the top point on the strut is fixed to the chassis as opposed to a double A-arm set-up where the top of the upright is traversing a full arc defined by the upper A-arm. BTW, in cars with double A-arm suspensions, the easiest way to design out bump-steer is to have the tie rods share the same internal pivot point and length as the upper A-arms.

3) The outside of the A-arm is angled down slightly -- as in stock ride-height.
a) The suspension moves in bump: This is essentially the opposite of case 1. Initially as the A-arm approach parallel with the ground, the upright will be moving out while the outer end of the tie-rod will be travelling inward along the upper half of it's arc. The result will be toe-out in droop.
b) The suspension moves in droop: much like 1a, there will still be some bump-steer because the lower A-arm and the tie-rod are traversing different portions of their arcs.

That's a simple mental model. To figure it out for sure you'd need to do the geometry.

The end result -- you can't design out bump-steer in a car with MacPhereson strut front suspension unless you were to put the tie rods at the same level as the lower A-arms. If you can keep the tie-rods parallel with the A-arms though you can keep the bump steer angles small enough that they aren't a problem in most cases.

Wow, some great info..thanks guys!

Jack, I know it's bump-steer cuz it was fine till I lowered it http://www.pelicanparts.com/support/smileys/spankA.gif

and it sounds exactly like the description on PP for the kits.

Sounds like if I want to keep the ride height then measure the angle/distance between the tie rods and steering rack and make adjustments to bring it to a slight angle or level.

Yes, my rear end squeeks :D ....here come the jokes! After the 3.0 was installed with the alum cased 915 (both heavier) it sagged a bit too, so either need to reindex the thing or since I have it apart for the bushings (which ones Jack?) do t-bars as well.

jluetjen you raised some good points!

The excessive toe out could cause this but I have never experienced it this bad and when I did have some bump steer prior to my change it was exactly like I am experiencing now.

If I do all that then decide to put in the bump steer kit then I have to go and align it AGAIN.

Thanks guys... My a-arms are set (by preference) to just above level. I think that's about where my tie-rods are too, but I'm gonna check...

I haven't installed the bump-steer kit, but have the rack spacer and have the lowered steering arm raised spindles....