JC in NY

When 914's and 2.7 911's stop passing you is when you should start thinking about the first upgrade. Until then just drive it.

E. J. - 993 Alumni

Well Said JC - I have been trying to get that point across to my students for quite sometime, but I always seem to be MUCH more long winded.

E. J.

SundayDriver

I wonder if we are talking about diffent things here. Bump steer is a product of only suspension geometry. Shocks, springs, etc do not have any effect (but having other problems could distract the driver from bump steer issues). Bump steer comes from having the suspension movement on a different radius than the steering connection at the wheel. When the wheel travels up and down, there is a steering input due to that different radius. This is fixed by changing geometry and minimizing this difference. You can also adjust parts such that you control the conditions when you get bump steer. For example, you could minimize bump steer when the wheel travels upwards at the expense of downward bump steer.

Extra equipment, weight, etc has no impact on bump steer, but it does have a big impact on suspension set up. Are you really talking about how the car handles when going over bumps? For example, a car could be bouncing all over bumps and twitching around resulting in the car turning in various directions. The car would be 'steering' itself over bumps, but this is not what is called bump steer.

Hope this helps.

QuikStuf993

Yup, the EVO/tie rods are the solution.

Greg, I sent you email....my JRZ's are for sale.

Jim in St.Louis

John, The best thing that you can do is to leave the car the way you bought it, until you start to feel that you wish that the car would do this or that. As a DE instructor I have found that the intermediate level is one of the most dangerous. You have several events under your belt, as do others around you, sometimes just enough to be a problem. So progress slow and you will have more fun and more car left at the end of day. I will see you at Gateway in April. Jim in St. Louis

Greg Fishman

Mark,
I pulled up this article from Longacre Racing and their description of "Bumpsteer" seems to be very different than yours. The main difference in the EVO uprights is the different tie rods and where they connect to the uprights. As you change the length of the shock assembly (lowering the car) you are changing the geometry and could end up with a bumpsteer problem. Any way read these two articles and tell me what you think, maybe you are saying the same thing in a different way:


"A. Bump Steer Definition

Bump Steer is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the track interacting with improper length or angle of your suspension and steering linkages.

Most car builders design their cars so that the effects of bump steer are minimal. However, you must still take care to bolt on your suspension carefully so as not to create unwanted bump steer. Make sure that you are always using the correct components for a particular car. Bump steer must be designed into the car and cannot be adjusted out if improper parts are used or if pivot points are moved without considering bump steer design principles.

In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm (See diagram below).

The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center.

Sounds complicated? Really it is very simple. To achieve zero bump the front end must be designed correctly. The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires.

To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc. In this scenario the tie rod would shorten much quicker through travel than the suspension and the car would toe in drastically over bumps. The shorter arc of the tie rod would pull on the spindle and toe it in through travel.

I. Bump Simplified

When designing a car, if the centerline of the outer tie rod lines up with the centerline of the lower ball joint, and the inter tie rod lines up with the lower pivot point then the length and angle of the tie rod and suspension will be the same resulting in zero bump. Most car builders design their cars in this fashion.




Here is the website: http://www.longacreracing.com/articles/art.asp?ARTID=13

Here is another article: http://www.derpca.com/Porscheforme/911Tech6-01.htm

Beating Bump Steer
by Steve Grosekemper

San Diego Region
(from THE WIND BLOWN WITNESS)


If you spend enough time at PCA events, you will eventually hear a multitude of strange new words that want to sneak into your daily vocabulary. These are words used by track and bench racers alike. They are best described as racer lingo, or “Porschese.”

You may hear terms like apex, heel and toe, corner balance and bump steer. It is my goal to explain this sometimes-foreign tongue and translate its obscure lingo into plain , normally spoken English.

Bump steer is one of the few Porschese terms that actually is what it says: If you hit a bump, the car steers to one side. On early Porsches, such as 356s, the tie rod ends were of two very different lengths.














356 example showing two uneven length tie rods.


They were installed in the car at an angle. When the car hit a bump and lowered, the short tie rod would straighten more than the long one. This would increase the amount of toe-in at the wheel on the short tie-rod side, causing the car to steer to one side. Quite entertaining on a winding bumpy road!

Later on, the 911 addressed this problem by employing rack and pinion steering with equal length tie rods. In these cars the tie rods were parallel with the ground and caused very little toe change as the car went through bumps.

This diagram shows the change made with introduction of the 911, which introduced rack and pinion steering with equal-length tie rods.

As time went on, people discovered that if you lowered a 911, it would handle much better. However, this caused yet another problem: the tie rods were now at such an angle that a bump in the road would cause excessive toe change.

On a lowered 911, spacers between the steering rack and front suspension member raise steering rack and keep tie rods equal.

While this problem would not cause the car to pull to one side or another, it did create a very darty feeling.

The cure was to install spacers between the steering rack and the front suspension member to raise the steering rack. This levels out the angled tie rods causing less toe change. This is what is done when someone is said to "set the bump steer."



Steve Grosekemper is shop foreman at Dieter's Porsche/BMW Service in San Diego.

For previous tech articles with photos go to www.dietersmotorsports.com and click on Technical Articles


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SundayDriver

Actually Greg, that is exactly what I was saying. Where I was not clear is that changes to spring rate and shock valving do not affect bump steer (I simply said springs and shocks). Since the geometry is not perfect - it is optimized at a given height, changes to ride height impact bump steer. This is why street cars, at stock ride height, almost never have bump steer issues. When you change the ride height, things are no longer optimized so you 'bump steer' (adjust) the car by shimming the steering rack, etc, or change parts to specialty items designed for a lower ride height (such as the EVO uprights).

Sorry I was not clear.

DJ

Greg,

I think you and Mark are saying the same thing...