911 Bump Steer, advanced solution (beyond rack spacers)

I noticed in the latest "Automation" sales flyer that they have a kit for replacing the tie rod ends with spherical rod ends. This is supposed to address the fact that the rack cannot be spaced far enough on a lowered car to eliminate the possibility of bump steer. It looks like a well thought out kit and in theory makes sense to me.

I have spacers on the rack in my 930 (lowered 25.5" frt, 25" rear, ground to fender lip), but I still notice that I do experience kick back in the steering wheel on occasion from bump steer.

The kit for both ends is not cheap (I would not expect it to be from the quality the components have to be), so I would like to hear an endorsement from anyone who has installed this particular kit, or who in general could provide me with comment. Also is anyone aware of this type of kit being available from other suppliers?

Thanks, John

John:

For what it's worth, I have had my 1983 911SC since newly picked up at the factory, and once it arrived on these shores, I had the car lowered to "European" spacifications. Flying into the face of conventional wisdom, I did not put any steering rack spacers on it, and I have NEVER experienced bump steer.

I run the car at PCA track events, at autocrosses, on the highway, in the mountains, etc. and I have never experienced this so called,dreaded, bump steer. When the car hits a rock or large pebble, sure there is a commensurate hit in the wheel (I know I ran over something), but the car never wanders off the straight and narrow. Maybe I lead a clean life. But after over 30 years of membership in the PCA and having owned a variety of Porsches, I don't believe in this thing called "bump steer". I just don't think it exists, and is probably something manufacturers of "bump steer spacers" have convinced us we need, like others have "convinced" us we need SUVs.

Sorry, but I can't replicate bump steer....Andras

John, I have to question what you cited in your first paragraph as 'fact' ... did that come from the advertising 'hype' for that new and expensive kit with the spherical rod ends? Because if there was a design defficiency like that, the kit wouldn't fix it anyway!!! But, in reality, there is no defficiency at all! The factory addressed the lowered ride height and extremely low profile racing tires in the '73-'75 RSR cars by altering the position of the spindle on the strut housing. Suspension evolution and changes were dicussed in Paul Frere's book. Much technical information can be found in Paul Van Valkenburg or Mark Donohue's books on handling and vehicle dynamics.

It is as Andras states indirectly, a matter of individuals' taste how sensitive one may or may not be to bump steer. The perfect front suspension has never been designed, so some degree of bump steer is present in all designs, and a factor called 'scrub radius' is a factor in how significant the effect may be, so tire and wheel selection play a part in the noticeability of the effect. Toe-in settings are also significant ... most sources recommend 1/16" to 1/8" of toe-in, but some people set it to zero, which would increase instability somewhat, and possibly enhance the effect of bump-steer. Instability, twitchiness, and oversteer are fun in an autocross setting ... not as much so on the highway!

In the final analysis, I doubt if the 'kit' would help a car that left the factory with one of the best steering and front suspension designs ever produced on the planet. Advertising hype is not known for factual representations of engineering issues, regardless of whether the target market is Porsche owner/mechanics or computer software users!

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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 01-25-2000).]