Handling question, Bump steer?

[89turbocabmike]

When I'm in a very tight canyon 180 degree canyon turn or high speed freeway ramp the steering wheel pulls to the outside of the turn in a slow rythmic tug after I've set my line. It feels like the tires kind of scrub and catch. It's difficult to put into words. I've always noticed this however it seems much more pronounced since I've upgraded to 245 fronts. I recently corner-balanced and aligned and that hasn't made a difference. Shocks/torsions are new and steering is tight. Any ideas? Would this be connected or considered bumpsteer with high load to the outside wheel affecting toe? I don't think I notice bump steer in a straight line and I haven't done any bump-steer mods. Thanks

[jluetjen]

That's the result of caster.

[dickster]

i was thinking of understeer - just re-read your thread.

[Mike Lindsey]

How much is the car lowered?
What you are describing is typical of bumpsteer. There are only a three options to try and correct that condition. 1- raise the car back up to spec height 2- install steering rack spacers 3- install tie rod spacer kit (best)
The reason you started noticing it more was because you went to the larger 245 tire on the front. I did the same only I went from the 205x16 to a 235x17 and the bumpsteer was much more noticable.
Gook luck.

[jluetjen]

the steering wheel pulls to the outside of the turn in a slow rythmic tug after I've set my line.

This sounds like self-centering effect from caster to me. The rythmic part could be bump-steer.

I've always noticed this however it seems much more pronounced since I've upgraded to 245 fronts.

If it was understeer, putting bigger front tires on it should have lessened it. But if it was from the caster, bigger tires will make is worse since they will pull even hard on the self centering action.

I recently corner-balanced and aligned and that hasn't made a difference.

Corner-balancing won't make a difference in this case. What are your alignment settings? Keep in mind caster is not a bad thing (as long as it is even on both sides). In addition to making the steering self centering it will increase the camber of the outside wheel and reduce the camber of the inside wheel when turning -- and this becomes more pronounces the further you turn the wheels, especially in hairpins. This is a benefit to cars with McPhearson strut front suspensions (like the 911) since they don't gain much camber when compressed, and in many cases even lose camber -- both of which translate into understeer.

If the effort bothers you, you can take it back to have the caster reduced some, but this will involve pretty much a full front alignment again since the caster is adjusted the same way as the camber.

[Tyson Schmidt]

The rhythmic part does sound like bump-steer. The higher steering effort is from wide tires with lots of caster.

With McPherson strut front suspension, going wider generally increases the scrub radius which exacerbates the steering effort and kickback. (Unless you use wheels with more back-spacing) The best thing to do is get rid of as much bump-steer as you can, since everything else is difficult to change, or will be detrimental in other areas.

[ted]

I'll just add the obvious that the larger tires are giving you more feedback than the smallers ones.
The first time I went through T-8 at Big Willow flat in 5th the steering wheel was giving me alot of feedback. We got back and found while measuring the toe over 1 inch compressed and 1 inch extended travel my bump steer was moving from no toe to over 1 mm toe out. That was corrected.
Next time at Big Willow I had much less feedback in T-8.
Everyone should ask to have their bump steer checked at least once. You should know your bump steer specs.
If your front alighnment is going to extreme toe out when compressed under braking its going to be more darty too. Trail braiking into a turn is not a great time to have a darty car.

[89turbocabmike]

Thanks for all the help guys.

I did some measuring today. Ground to top of front wheel arch is 24 7/16 inches on turbo flares. By factory measuring I'm 154mm, so I'm way below euro spec of 108. Didn't realize I was quite so low! This would support the bump steer theory I would guess. The quick fix I would think is the ERP kit? I've heard someone mention rubber boots to protect the heim-joint for longevity? How long do these last in daily driving?

[Tyson Schmidt]

They hold up quite well with track use, so I don't foresee a problem with street driving.

But I recommend checking the tightness of the long through-bolt periodically, since I've seen these loosen up with heavy use. They are not a taper fit like the stock outer tie-rod end.

But yes, they are the easy solution to bump-steer. It makes sense to have a shop check it with a bump-steer gauge. The best you can do without one is to roughly shoot for the lower control arm and the tie rod to be as close to parallel as possible, but keep the tie-rod at slightly less of a downward angle than the control arm. (This is with the car raised off the ground.) This prevents going too far and ending up with bump-oversteer. The Factory designed in a small amount of bump-understeer to keep the steering predictable, and provide some Ackerman during tight cornering. Obviously lowering the car takes this too far and causes problems, but if you over-correct, it can get even worse and make for a very unpredictable car.

[aseem]

What is Ackerman????

And

I have never heard of bumb-oversteer and bump-understeer, but I guess bumb-steer (as I know it: the steering wheel "jumping" in your hand when going over bumps) is just the result of the toe changing when the compression of the front of the car changes. Bump toe-out results in bumb-understeer, and bump toe-in result in bump-oversteer. Am I correct?

[Tyson Schmidt]

Originally posted by aseem
What is Ackerman????

And

I have never heard of bumb-oversteer and bump-understeer, but I guess bumb-steer (as I know it: the steering wheel "jumping" in your hand when going over bumps) is just the result of the toe changing when the compression of the front of the car changes. Bump toe-out results in bumb-understeer, and bump toe-in result in bump-oversteer. Am I correct?

Yes, you nailed it. Bump-understeer means the outside front tire toes out when entering the corner, effectively turning less than intended. This is easily compensated for, and makes it very predictable, so long as it isn't excessive. This toe-out also provides the Ackerman effect, since the car toes out only while cornering. This is desirable since the inside wheels must turn at a sharper angle than the outside wheel in a corner to follow the same path. This is usually achieved by the steering arms and the angle of the tie-rods as it pertains to the positioning of the steering mechanism forward or backward in relation to the outer tie-rod ends. Porsche eliminated Ackerman in the conventional sense, and achieved it with careful tuning of the bump-steer. This has a lot to do with the fact that they located the steering rack behind the axle centerline, (rearsteer) rather than in front of it. (frontsteer).

[aseem]

Thanks Tyson. So what is the best way to achive the correct angle between the lower control arm and the tie-rods? It seems to me that the Bump steer Kits that comes with a fixed sized washer only "approximates" the adjustment as the distance to raise the steering rack depends on how much the car is lowered, or? I guess raising the spindles is the optimum adjustment. I see Mike Lindsey suggest a tie-rod specer adjustment. Any pics of such an adjustment? I have never seen this before.

Edit: BTW Is it neccessary to adjust any parameters in the front end when going with wider wheels? Like toe or caster?
As you might understand, I'm just beginning to grasp the geometry of the Porsche front end.

[89turbocabmike]

Thanks Tyson, I'll call you guys on Monday to schedule bringing my car in.

[dickster]

as I know it: the steering wheel "jumping" in your hand when going over bumps

er no, not quite. this is a common misconseption though.

the bump steer kits are supposed to be fine tuned by changing the length of the spacers.