DanH

Ok what causes it? Is it the too soft springs combined with over valved dampers mentioned in the other thread?

If so whats actually happening to the suspension when it bump steers? Why does it try to put me into oncoming traffic?

Can it be improved without making it roll more on track?

Just trying to get a handle on whether or not its worth upgrading my dampers. I realise all suspension design is compromise and I've not run a car as stiff as this before so I'm trying to ascertain whether it can be any better on road without damaging track performance.

The car is by no means intollerable on bumpy roads, but it definitely knocks 10mph off my comfort level when I'm hitting the country lanes unless I know I can use more than my lane.

RTP356

Bump steer does not refer to how a car handles when it hits a bump. Bump steer is the change in suapension settings (ie: camber) as the tire goes through its vertical range of motion.

DanH

Not sure I see a difference as surely the bump is what causes it to go through its vertical motion?

So when I hit a bump is the suspension moving too far causing the geo to radically increase its camber and steer the car onto the other side of the road. How does the damper affect this as that sounds like the spring is too soft for the amount of dynamic camber the suspension creates?

pedsurg

This might be a "996" issue. Had sig bump steer issues with my 02 C2 which was lowered and had pss-9s'. Situation could not be fully corrected by shop. Similar issue easly corrected with "bump steer" kit on current lowered track car, 96 C2. Haven't lowered the 997 yet, so don't know if the bump steer issue has been addressed.
Jack

DanH

I've got a GT3 RS which has optimised geo for its ride height and apparently less bumpsteer. I'm just trying to get to the bottom of why it gets bounced about on bumpy roads so much. Is it just the firm setup, a geo issue or what? Would new dampers fix it? I can't soften the car or it will ground on all bumps...

RTP356

Let me try this again - bump steer is a measurement of the change of camber as the wheel go through its range of motion. Ideally, you want a wheel to have the same camber through that range. Typically only purpose built race cars will come near this ideal. There are various kits availble to help achieve this. Steering or handling problems you have having when you hit bumps is more an issue with your shocks, springs or even sway bars.

DanH

Ok, that makes sense. How does bump steer make its presence known then. What are people talking about when they say a car has a lot of it? How does it feel behind the wheel?

Also why can only race cars come near the ideal? Is it due to lack of suspension travel and long wishbones or something?

thanks.

SundayDriver

What you described is called 'camber gain'. In fact, youi do NOT want the same camber at the wheel travels. You want an increase in camber under compression as that compensates for body roll which is reducing camber on the outside tire.

Bump steer is the change in toe as the suspension travels up and down. The geometry of the suspension 'arms' is different than the tie rods, hence the toe changes. The geometry is designed with a particular ride height in mind. When a car is lowered it will cause more bump steer than intended in the design. Special uprights (such as the EVO uprights) correct this for lowered cars. With purpose built race cars, there is usually enough adjustment to correct this (often done by shimming the steering rack).

What is being described here appears to have nothing to do with either but rather the characteristics of the damper/spring tuning. This feeling can be aggrivated by camber and caster issues.

Hope this helps.

NJ-GT

DanH:

Your driving issues are related to springs/shocks.

My bump steer problems are different. The car steers by itself sometimes when hitting a bump on a turn at speed. In my car the problem is more noticeable than on the stock GT3, because I lowered the front end to the minimum possible on my suspension (103mm). If I increase my front ride height by 2", the bump steer will be drastically reduced, but why would I move almost 1,000 lbs 2" higher on a track car?

SPR

A better way to explain it is how the steering components influence the tire going up and down through a corner. The steering rack is supposed to be level, (relegated to the the inner and outter tie rod arms that come out of the rack), ideally, parallel to the ground. This will allow for an acceptable arc of the steering rods going up and down without being too large. Think of a rod with a fixed point with moving up and down on the non-fixed end. At the very ends of the arc there is greater deflectoin bringing the tie rod in towards the center of the car, this is what happens when you are going through a corner and hit a bump it make the suspension go up and then you get to the end of that arc at the extreme side and that pulls on the spindle making the car waver.

When you lower the car, you actually screw up the arc most of the time. The common fix is to use special spacers to move the rack up or down, or to space the tie rods from the spindles up or down to make them level again and restore the oem arc of travel. THIS is bump steer.

Atgani

Thankyou Sunday Driver and SPR
The term bumpsteer is used incorrectly by so many people (and journalists) who should know better

SPR

Is that a compliment or a knock??? If the former thanks. If the latter, hey just trying to help

RTP356

Sunday Driver, you are correct. I thought bump steer was both camber and toe, but you are right it is only toe and I was describing camber gain.

DanH

Cool, that all makes sense. Thanks guys.

Masa^^

I am sorry to dig up this old thread, but I find it interesting because I have probably the same issues as DanH has described here. Have you found any solution to your problems? I am wondering if my problems are bump steer related or something else. When I am driving on a bumpy surface, in example Nordschleife, my car does not keep on its course. I cannot drive it on maximum, because it is very hard to keep it on track, the car tends to change direction when going through some bumps. Do you guys have any idea what could be the problem? It would be great to have it fixed, because it is very annoying. I have played with anti roll bar settings and alignment, but they wont help much. However, I have noticed that when I am running front anti roll bar on soft the problem is milder.