Why does my FE FWD car have built in bump steer?

[reflexuk]

I recently raised the ride height of my front engined front wheel drive car. It gained a visible amount of front toe in when I did so. I thought this a little strange as I'd only gone from stock (low for the model range) up to a height(high-ish) for the model range. I did this in the course of choosing a ground clearance when fitting coilovers as my sports variant model becomes a rally car. This particular model was in production at a wide variety of ground clearances in stock form during it's 10year model life. As the wheel rises it clearly toes out, this must happen to the outside wheel in a turn and in reverse on the inside wheel. I presume the amount varies and thus the total toe (tracking) alters through the corner too. Clearly the car would be attempting to steer left and right as it goes down a bumpy road.

Would this be designed into the front geometry on purpose?
Would it cause peculiar handling or be a worthwhile improvement it to modify the car so it doesn't have bump steer?

 

[BrianPetersen]

What you're describing sounds like it would cause roll understeer. It's probably in there by design, but your objectives don't necessarily coincide with those of the original manufacturer. I'd try to get rid of the bump/roll steer, if it was my car. If it has MacPherson strut front suspension, the steering rack is probably up a little too high. If the rack bolts to the subframe from the bottom, try shimming the rack mounting points with washers to drop it down a little ... easily reversible if you don't like the results. It probably won't take much movement to sort it out.

 

[dynatune]

The bumpsteer is with 99% security designed "on purpose" by the manufacturer since it is next to lateral load compliance steer one of the most significant characters to lay-out the "linearity" of the vehicle response on lateral loads. Not the kind of thing you would need for racing though since you will probably suffer from too much understeer already.

Typically a FWD car would have between 3 - 6 °/m bumpsteer (gradient) over wheeltravel (in meters) towards toe-out in compression (roll-understeer). On a McPherson strut FWD your steering rack is usually behind the wheelcenter and by lowering the trackrod joint at the knuckle you will have less toe-out over wheel travel (or raise the steering rack joint giving the same effect). If the rack is at the front of the wheel center you need to do up on the knuckle or down on the rack. A reference value that might help you is the number 1: 1mm of joint re-location will change your bump-steer by roughly 1°/m. So in your case you need to measure the toe-change of one wheel and devide it by the rideheight change in m. This will give you some indication for you bumpsteer gradient and then you easily work out how many mm you need to re-position joints.

Hope this helps,

Cheers,
dynatune,

http://www.dynatune-xl.com

 

[BrianPetersen]

I had my orientation wrong when writing my post. dynatune has it the right way around.

 

[GregLocock]

So to put this into perspective, your suspension probably rolls by about 40mm on each side at 1g, so if it has about 5 deg of bump steer per metre (it could have double that), that is 0.2 deg/g of understeer, if you get rid of the lot.

That is not a huge change, and very few drivers could identify it correctly, but it is in the less benign direction. You should see an improvement in rough road stability, or reduction in 'dartiness'.



Cheers

Greg Locock


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