Bump Steering

[Clarky]

My FD seems to suffer from bump steering, and makes the car feel twitchy and unstable, and does not give me confidence in the handling, standard setup as far as i know, on a nice new laid road its really smooth.
I know the Rex has hard suspension, but is this normal, a fault, or is it the state of our roads.
Any help please.
John

[Fish]

John, it could be down to a number of things. But I would recommend getting the geometry checked and make sure that you have some good rubber bands on the wheels.

Geometry Settings

Even the amount of air in your tyres changes the way an FD handles.

Fish

[iaint]

You\'d really have to be hammering it in the corners to hit bump steer on the roads assuming your suspension isn\'t knackered.  For example it\'s a known problem with the Elise suspension but you need to be on sticky rubber on track to bring it out!  It\'s not something you\'d expect to get on the road under even very progressive driving.

What\'s making you think it\'s bump steer you\'re getting?

You could be getting some nasty handling from dead dampers - the rears were gone on mine and it made the car very nervous at speed or as Mr Fishy said it could just be bad geometry.

Iain

[Clarky]

My description of bump steering: is when the steering wheel moves as you hit the bumps and lumps on the road surface making the car unstable in its direction,  its the only way i can describe it, perhaps the term bump steering is wrong.

[Martiny]

My description of bump steering: is when the steering wheel moves as you hit the bumps and lumps on the road surface making the car unstable in its direction,  its the only way i can describe it, perhaps the term bump steering is wrong.

Yes I think thats wrong, I think bump steer is what you get when you bottom out and hit the rubber stops?

[iaint]

Yes I think thats wrong, I think bump steer is what you get when you bottom out and hit the rubber stops?

Yup - when the suspension basically has no more travel left to soak up unevenness in the road surface and is usually found when cornering hard already - i.e. the suspension is fully compressed.  The situation is then basically one of having no suspension and, as you\'re likley already close to the handling limits of the car, you get a sudden decreace in available grip and lose control.  Not good!

[iaint]

My description of bump steering: is when the steering wheel moves as you hit the bumps and lumps on the road surface making the car unstable in its direction,  its the only way i can describe it, perhaps the term bump steering is wrong.

Sounds likely that the suspension geometry is badly out of whack.

Other things to check (after a decent setup) are the springs and dampers.  Incorrect spring rates or worn out dampers will make the car very skittish.

I\'d reccomend having it checked out to be on the safe side as even a working FD can be a handful when things go wrong but bad suspension might make a situation unrecoverable!

[clive]

It won\'t be true "bump" steer as this only occurs normally when a car is built wrongly in the first place by fitting the steering rack at the wrong height in relation to the steering drop arms.

This normally leaves the trackrods facing downwards meaning that when the suspension goes up the track rod fights the angle of the wishbones resulting in the steering arms being moved when they should not be.

This self steering action is bump steer and makes a car totally undriveable and real scary !!  

I know this because many years ago l got this wrong on a scratch built race car that l converted from wishbone suspention to strut type suspension and had to relocate the rack after the thing tried to pitch me into a barrier on its first test!.

First have the tracking checked and then the castor angle as its more likely to be one or the other

Hope this helps

[Clarky]

Thanks to all for help, i\'ll get the alignment checked first.
John

[mullins]

It\'s called "tram tracking" downunder (and is completely different from bump-steer). It usually feels like your car is following every dip in the road and you are not in complete control. Fitting bigger wheel/tyres usually makes it worse.

Simple solution - dial in more castor.