Bump Steer

[MFB]

Hey All,

Here's something to noodle on today...

Bump steer occurs when the center line of the steering rack is below the mounting point of the tie rod to the strut, correct? This causes the tie-rod to be at a positive angle (CCW being positive) to the rack centerline.

What happens if the rack is above the the tie-rod mounting point on the strut, causing a negative tie-rod angle (CCW being positive) to the rack? Is this a better scenario than having the rack below the tie-rod mounting point on the strut? Is having the tie-rods perfectly level the optimum set-up.

If you've seen my other posts, you know that I'm R&R'ing my front suspension. I'm at the point where installing the rack spacers is easily accomplished. My problem is that I'm not sure how low I'm going to set my ride height. If I put in the spacers and decide on the stock height, I could end up with the rack higher than the tie-rod mounting point on the strut.

Does this make any sense? Am I worrying too much?

Any info is appreciated.

Thanks!

[Wil Ferch]

Hi:
I think I know what you're facing..allow me to suggest this:
Optimum ( IMHO) is for the steering linkage to be parallel to the ground at static ride height. If angled either up or down at static ride height...any subsequent suspension movement will "pull-in" ...or "push-out" the steering linkage, causing bump steer. Steering rack spacers are typically used to "partially" correct this, when ( for example) the car is lowered. However, you only can fit rack spacers that are about 1/2" thick, so if you lower the car more than 1/2"...you won't regain optimum. Some people sell the tapered pin set up ( expensive..like under $400) that mounts at the tie-rod-ends, and allows a fine-tune of getting the steering arms straight. I believe they are made by ERP.
--Wil Ferch

[ChrisBennet]

Just thinking out loud but suppose the tie rods "drooped" a little down toward the wheel end. Any bumps would push the wheel; and that end of the tie rod, up into the "sweet spot" where the least deflection occurs. Just a thought...
-Chris

[Wil Ferch]

Chris:
Hi...yes but....
As it moves into the "sweet spot"...it acts in the "non sweet spot" regime of bump steer. Nothing is perfect, as any arc movement of a linkage will exhibit this...but the least amount of movement is anywhere around horizontal. The "target" or goal is not the end-point of the movement, it's the full travel of the movement itself. Hard to explain, but do you see the difference?
--Wil

[ChrisBennet]

Wil,
I see what you're saying. Perhaps an example would better explain what I'm thinking. (Thinking is probably too strong a word for what goes on in there).

Suppose the tie rod is the hand of a clock.
The sweet spot would be with the tie rod horizontal i.e the end would be at 3 o'clock.

If the tie rod end moves up, to 2 o'clock, the tip goes to the left and "pulls" on the steering arm. Classic bump steer. (This is an extreme example for illustrative purposes.) See "A" below.

Now suppose instead, that the tie rod started out with the tip below 3 o'clock, say at 3:30.

Now if the tie rod moves up, to 2:30. While the tie rod is moving from 3:30 to 3:00, the tip would move to the right and "push" on the steering arm. As it keeps moving up to 2:30 it will "pull on the steering arm. See "B" below.

In the latter example, the net deflection is zero because the tip of the tie rod ends up at the same distance from the steering rack i.e the "push" and "pull" cancelled each other out.

-Chris

[s_wilwerding]

Everyone makes the point that American cars are often lowered to match their Euro counterparts. When people lower American cars, they add the bump steer kit. In an already lowered Euro car, is anything done for the bump steer? Is it included from the factory?

[Zeke]

MFB, you are only going to be able to add the spacers and like what you get. To lower the car too much, as you say, and wish to raise the rack more than the height that the spacers afford is to ask for major mods elsewhere in the steering (read sheet metal). A better solution if you are going that low is to raise the spindles and leave the geometry alone. You get factory suspension travel this way as well.

[dickster]

personally, i'd not bother unless you're gonna use the proper drop links. i don't like the idea of raising the rack with washers somehow.

does anyone rave about the results of using the washers? i'd be intersted to hear.

i have heard rave reviews on the drop links however.

jmho.

[nhromyak]

I don't have RAVE reviews about the spacers. However, I did try it without. My bump steer is definately more noticeable without them.

I still have it though...

[Wil Ferch]

Guys:
Sorry for the late response.
first...ChrisB
Chris..I see what you say by your "clock" example, but if the net effect cancels out, remember also that during the movement, it will both push and pull, albeit each at half the amount if soley all "push" or pull", as would be the case starting from a different height. So, yes...you have a point, and that is what leveling the steering arms at rest allow...minimizing ( not eliminating) the bump steer, by possibly having both in and out movement, over the normal range of suspension movement ( not all "up"...and not all "down").
As someone else said, there are other better ways to handle this. One is the ERP kit i talked about. If the car gets lowered from ( say) USA to true Euro spec, this would likely be a 1" drop. Rack spacers only retrieve 1/2" of this..so the car is improved but not nearly enough. The ERP kit can fine tune the steering arms to perfectly level...even at more severe drops.
"Dickster"...yes, they make a difference. Huge? Maybe not, but definately noticable. If you start envisioning the geometry, you will see that it must.
Others....does it make sense on a car already at Euro height? Good question. The answer is likely to be that it won't improve it much, if at all, if the steering arms are horizontal at Euro spec....but "yes" if they're not at horizontal in Euro trim.
Yet others...Raising the spindle on the shock strut maintains the original geometry at he new (lowered) condition, with all the sins and attributes it first had ( see above). This can only be done on the upside-down Bilstein struts, as the Koni's and Boges have a "bulge" right above the spindle collar, and can't be moved up. True RSR specs had this raised 12-18 mm, but people have gone as far as 35 mm in aftermarket. Ball joint hits tire rim at severe drops, so you need to limit the drop...OR..go with larger diameter rims to allow some space for the ball joint.
---Wil Ferch

[ChrisBennet]

My car is lowered to something like 25"front 24.5"rear and I haven't noticed any bump steer. (I bought a bump steer kit but never installed it.) This is likely do to the fact that I have stiffer springs (22/29 torsion bars) so I get less suspension movement than stock.
IMO Before you spend your money on the $300+ ERP bump steer kit you should spend it on larger torsion bars instead. Then you probably wouldn't need the bump steer kit if my experience is any indicator.
BTW: Raising the spindles includes heating and bending the control arms as well using a jig - perhaps not a great DIY project..
-Chris

[widebody911]

My car is low - about 24 1/4 in the front. I do notice bump steer on the track, in fact sometimes it gets downright interesting.

[Wil Ferch]

Chris:
Normally, yes, you'd have to heat and rebend the steering arms, but a lowered car with raised spindles can ( I believe) work with the ERP kit without such heating / reshaping done.
--Wil Ferch

[jwetering]

I hate to be the dummy here...but won't bumps cause a change in toe-in regardless of where the tie rods sit when the car is static?

Where is the flaw in my reasoning:

If the tie rods in a lowered car angle up from the rack to the wheel, then a bump which causes the nose of the car to drop will increase this angle, cause more toe in, and cause bump steer.. OK?

Now If the same car has had it's rack spaced so that the tie rods are parallel to the ground, then won't the same bump cause the same angle change, and the same toe in, and the same bump steer?

I will be extremely happy if someone could help me come to terms with this...

[ChrisBennet]

jasper,
There is a toe change no matter what angle but the change in toe is least when the tie rods are horizontal. Imagine if the tie rods were pointing almost up. A little rise on the end produces a big change in the left/right direction.
-Chris

[Wil Ferch]

Chris is correct..let me ad to his post.
As I had said before, any lever arm that swings in an arc will exhibit this to some degree. Again, using Chris' clock analogy...going from 11 o'clock to 1 o'clock is a change of two hours...but the same two hour change change from 2 o'clock to 4 o'clock will result in much less left-to-right movement of the tip of the hour hand....actually no net movement from left to right at all ( it passes through 3 o'clock ...so it moves out and back).
--Wil Ferch

[David Ceruti]

Just a few comments from my experience messing around with open seaters - I have only had my porsche for a few months so my specific experience is negligible
One thing to remember is that the suspension arms are also moving in an arc.
So, if the steering arms and the tie-rod are the same length and both ends of the lower suspension arm and the tie rod are on the same axis you won't have any bump steer.
With saloon cars it is difficult to fit the parts in so the designers need to compromise. They will try to optimise the BS at normal the ride height and when you drop the car you ususlly land up on a sub-optimal part of the bump steer curve. adding spacers or washers can rectify this. Carol Shelby does a really good explanation of this in his book on setting up race cars
If you are really keen you can measure all the main suspension components and make a model from cardboard and then map the bump steer under different conditions

[dickster]

My car is lowered to something like 25"front 24.5"rear and I haven't noticed any bump steer. (I bought a bump steer kit but never installed it.)

problem is, would you know it if it were there. i bet most people wouldn't notice until they drove a "great steering" car to compare with.

[jwetering]

OK...those are three very sensible answers to my question.

I'm drawing a picture on a piece of papare and I am ready to describe bump steer to anyone who asks.....now I just need some friends, family or co-workers who care about this sort of thing.

thanks for de-mystifying the effect guys.