This bump steer thing..

[Tam Lin]

Just one example of how reading internet fora can be harmful: There I was, happy-ish about my car's handling characteristics, until I read up on the 924.org board aboput bump steer.

Ever since then, I have managed to convince myself that my 924S, with KW v3, aluminium A arms, and obviously lowered stance, does suffer from it. Since moving the lower part of the steering column might be a bit of a mission, I understand that the other way of tackling this is at the bearing pin/track arm outer end. Anyone have any recipies that actually work?

 

[edh]

Some places will measure your bump steer for you - it was a big talking point on Scoobynet & people used to recommend powerstation in Cheltenham. Their mod was to use shims to lower the rack to eliminate bump steer.

As I understand it, you want to get your track rods parallel with your lower arms (Is that too simplistic?)

Can't say I've really noticed anything like that on my 944. Your car is a track car isn't it? - do you run on many bumpy tracks?

 

[John Sims]

This is perhaps so as, with lowered suspension, your wishbone is no longer horizontal (or below horizontal) in its dormant state. .'. When compressed is continuation on the ark as it lifts up the strut pulls the base of the stut inward.

An extended ball joint such as provided by Rennbay will return the wishbone to its original position but will then put the steering rod out of parallel with the wishbone so can aggravate the situation. Elephant Racing do an extended steering ball joint link to address this.

 

[TTM]

Tam Lin, are your shocks set too soft?

 

[Tam Lin]

Thanks all
@ EDH It's a track car, but is also used for extended trips on the continent: last month we did about 1200 Miles pottering around France.
@TTM You could well be right. I had set the KW v3 up for the 20 Kg lighter 8v, and haven't had time to work out a correct track setting with the 16v up front.

Anybody running the Elephant set-up that John mentioned?

 

[Hilux]

Ever since then, I have managed to convince myself that my 924S, with KW v3, aluminium A arms, and obviously lowered stance, does suffer from it. Since moving the lower part of the steering column might be a bit of a mission, I understand that the other way of tackling this is at the bearing pin/track arm outer end. Anyone have any recipies that actually work?

The lowering of the car is the cause.

Some places will measure your bump steer for you - it was a big talking point on Scoobynet & people used to recommend powerstation in Cheltenham. Their mod was to use shims to lower the rack to eliminate bump steer.

As I understand it, you want to get your track rods parallel with your lower arms (Is that too simplistic?)

Yes (although not quite a rule of thumb)

Bump steer is noticeable when the arc of the track rod scribes an offset arc relative to the arc of the wheel as the wheel goes up and down. It is dialled out as much as possible by ensuring the track rod arc is minimised by making the track rod lengths as short as possible and the steering rack height relative to the wheel is centred on the correct arc the suspension moves through.

To explain more....................If the car is lowered then the steering rack position relative to the original settings changes causing the track rod length to be effectively shortened and pulling the wheel inwards/backwards. To compensate, the track rod is then lengthened so as to ensure this doesnt happen when static BUT as the wheel moves up and down from then on if the car suspension compresses further the longer track rod end is now lower, doesnt scribe the correct arc from the steering rack so tends to move the wheel out of alignment more and the same for rebound. This is bump steer which is felt primarily on compression but can be noticeable on rebound.

The ideal set up is to ensure the steering rack is at a level in relation to the wheel so that whatever required movement up and down are equal amounts/angles from the centre. This moves the wheel at suspension `extremis` the least. In other words to realign the two arc centres and pivot points so movement is minimised.

From my experiences if you lower a car then you`ll need to raise the steering rack. You find out how much by removing the suspension and moving the arms up and down and see where the least movement is at the max extent/center of the arc (hence my comment re Ed`s reply)

This is impractical in a most cases so a geometry session helps. There are alternatives involving in some cases bolting the track rod ball joint on the opposite side of the hub mounting point or extending the track rod ball joints in length. Extended track rod arms do not alter their arcs pivot points relative to the suspension but in some cases they may be required if the original runs out of adjustment.

 

[sawood12]

I had noticeable bump steer when I had my KW fitted. Not a major issue really. A bit disconcerting when it happens for the first time but after that you just learn to expect it under certain circumstances.