Bump steer

[jonners]

Can any of you learned people help me please?

I have read loads about bump steer over the years and understand what it is but having researched the web and various books and old comics can't find a CLEAR explanation of what causes it.

What I need is a clear explanation that will help me understand what I need to do to eliminate it.

The explanations I have read all assume that the relevant car has a steering rack whereas my car has a steering box.

I know that makes no difference to the theory but explanations that refer to remedies such as altering the rack position are no good to me as that is not within the realm of reasonable options with a steering box.

I know the car is suffering from bump steer because I can feel it happening (and I'm not confusing bump steer with 'jump' steer) and I think the root cause is the fact that the car has been lowered considerably (the track control arms are parallel with the ground at rest whereas they should be parallel with the ground under load/dive).

Am taking steps to remedy the angle of the tca's (and I understand that there are other important reasons why this is necessary) but I would like to properly understand what causes bump steer so I can address the scenario properly without just resorting to the hardest springs possible.

Can anyone help?

[The Shrew]

Your correct its the angle of the steering rods that change bump steer. I think im correct in saying with the car standing still the track rods need to be angled down towards the hub end. Like you said changing the rack position would be one way to change this but you cant do that.

Why do you want to change this ? I dont think it would make huge differences if you did manage to change it.

[jonners]

The angle of the tca's has changed not the track/tie rods

Changing the angle of the tca's will make a significant difference to the geometry

[Al Weyman]

The rack or steering arms must run in parallel with the TCA or lower wishbone in an unqual wishbone set up than it should be fine.

[jonners]

Originally Posted by Al Weyman

The rack or steering arms must run in parallel with the TCA or lower wishbone in an unqual wishbone set up than it should be fine.

Not sure what parallel means here - can you elaborate? Are we looking from above or in front? Probably obvious and just me being thick

[tristancliffe]

Not entirely Al. If the steering arms are half way between the wishbones (assuming double wishbone suspension) then the pickup needs to be halfway between the wishbone joints on the hub. The only time the track rod should be parallel to the bottom wishbone is if the inboard pivots of the bottom wishbone and the track rod are at the same height.

As the hub moves up and down in an arc you need to make sure (by raising or lowering either the steering box/rack or the outboard pivots) that the track rod ends go through the same arc.

[JamesH]

There is an explanation on page 57 of "How to build and modify Sportcars usspension and brakes" by Des Hamill - worth getting. I will try to precis it here...!!!

"The alteration of the attitude oif the wheels (Toe in/out) when they encounter a hump or hollow in the road" Its a geometric thing - the relation of the steering rack position and length of tie rods in relation to the suspension system chassis pick points ball joint centres and length of wishbones. If you draw a front on picture of this lot, if the upper outer wishbone balljoint centre (BJC), the tie road BJC and the lower wishbone BJC all line up, as do the inner ones, then you will have NO bumpsteer.

Its easy to see - remove the shock, and move the suspension up and down - you will see the toe changing as you do it. As Tristan said, moving the rack (or box) up and down will alter the geometry and you can see the results easily. By dropping the suspension, you probably moved in to an area of greater bump steer - out of the original design parameters of the car, so you will to move the inner tie rod ball joints up/down/fore/aft appropriately.

James

[GORDON STREETER]

The old trick to the problem was to make up a spacer between the lower arm and upright to restore the lower arm back in its original position. This works well on macpherson struts.
This depends on how much room you have to play with if you have deep inset wheel rims. Wheel rims that are too wide and the wrong offset can cause all sorts of steering problems that is often mistaken for bump steer.

[greenamex2]

I know the car is suffering from bump steer because I can feel it happening


You really can't be sure you have until you have measured it. And you certainly can't fix it until you have confirmed which way they are steering.

I have a DIY guage thats works just as well as the 'real thing', but cost about a tenner to make.

In short it consist of two pieces of wood hinged together that supply the fixed point. There a couple of bolts sticking out of it and it is secured to the ground by a large battery/wife/etc. There is then another piece of wood bolted to the hub that the bolts rest against.

All you then do is jack the wheel up and down (with the spring and, ideally, the damper removed or at least set to minimum, and the steering locked in place) and measure the toe in and out as it rises above and drops below static. That will then tell you whether that side need raising or lowering.

Repeat for the other side then work out how the hell you are going to relocate your rack.

Change eveything a bit and then keep repeating until you have minimised it, or given up.

On my kit car I had to drop one side about 10mm and raise the other 5mm. The effect on 'feel' was quite noticeable.

On the race car the rack is positioned to the inner ball joint is directly in line with the lower TCA pivot and outer ball joint is just about in line with the lower strut ball joint. This means you can lower the car as much as you like and not get bump steer (I've lowered the car about 4 inches, checked it and no bump steer). Which make you wonder why all the 'specialists' sell contraptions that are supposed to get rid of bump steer of ther car!


If you are anywhere in the Herts/Essex/Heathrow triangle you are welcome to borrow my guage. Let me know.

[greenamex2]

Oh, and an alternative to my guage is to clamp a long length of steel to the hub and then draw or tape a parallel line on the ground.

[Al Weyman]

Or stand above it and bounce the car up and down and watch the wheels go in and out.

[ian_w]

To describe bump steer - as the wheel moves up and down it actually moves in a slight arc ( viewed from the front/rear ) rather than straight up and down. If the steering arm does no follow this same arc then the wheel will steer as it goes up and down. As has been suggested you can't fix the problem until you have measure exactly what it is doing. Once you measure whats going on you can plot the steer angle against wheel position. If this is a graph is a straight line then typically you will need to adjust either the height of the rack or the outer ball joint. Alternatively if the graph is a curve, i.e. the same toe-in/out in both drop and bump then this implies the distance between the end of the rack and the outer ball joint is too short/long - this is not so easy to fix!

In my experience, what you want to aim for is slight toe-out in bump.

I once make the mistake on a racecar of having a small amount of toe-in in bump as I didn't have exactly the right thickness spacers, this made the car virtually undriveable. As you turn the car into the corner the outside front wheel will go into bump and with toe-in bump steer this causes the car to steer into the corner even more. You ended up turning the wheel to initiate the corner and then immediately backing it off again.

By using a small amount of toe-out you get the opposite effect, i.e. the tyre will steer out of the corner slightly giving a stable response.

[GORDON STREETER]

I'm a bit confused as to what jonners actually means by "bump steer" as if you hit say a sleeping policemen in the road, both wheels will rise together and wont steer one way or the other but will tow out. Roll steer on the other hand, one wheel will tow in and the other tow out actually turning the car but keeping the same track.
We never had this trouble with solid axles !

[jonners]

Thank you all very much for the input - particularly the posts that explain what causes bump steer.

I think I'm starting to understand - is it simply that the arcs prescribed by the tca and the track/tie rod are different....and that in bump you need to ensure that in the critical area of movement the 2 arcs are as close to each other as possible - otherwise the steering mechanism has to operate by virtue of the track/tie rod arc??

Am I on the right lines??

Gordon's suggestion is something we've explored and would solve a lot of problems but it's not possible on our car without fitting either larger diameter wheels - not allowed - or incorrect offset wheels which will help solve one problem but introduce the others Gordon mentions.

Dennis is correct - I should check that bump steer is happening before getting carried away. I am very grateful for the offer of the use of your guage - am in Richmond, Surrey so within range maybe - where are you?

[jonners]

Originally Posted by GORDON STREETER

I'm a bit confused as to what jonners actually means by "bump steer" as if you hit say a sleeping policemen in the road, both wheels will rise together and wont steer one way or the other but will tow out. Roll steer on the other hand, one wheel will tow in and the other tow out actually turning the car but keeping the same track.
We never had this trouble with solid axles !

This could confirm my credentials as a techno dunderhead but I think roll steer and bump steer are the same....

....cue howls of dersion....???

[JamesH]

Originally Posted by GORDON STREETER

I'm a bit confused as to what jonners actually means by "bump steer" as if you hit say a sleeping policemen in the road, both wheels will rise together and wont steer one way or the other but will tow out. Roll steer on the other hand, one wheel will tow in and the other tow out actually turning the car but keeping the same track.
We never had this trouble with solid axles !

Not many sleeping policement on a race track. Lots of bumps that only affect one side of the car though....

Like roll steer. But in a straight line....

[GORDON STREETER]

At the end of the day if you run 700 lb front springs (like me) you wont get a lot of movement anyway !!!!

[GORDON STREETER]

Originally Posted by JamesH

Like roll steer. But in a straight line....

Except in roll steer one wheel droops on the inside of a corner, or should if you don't run an enormous anti roll bar !

[JamesH]

Ok, one wheel hits a bump whilst the other hits a hole....

;-)

James

[dtype38]

If you accidentally run over a saw tooth kerb with bad bumb steer, does it go "in-out-in-out", while the rest of the car "shakes you all about" ?

[jonners]

Originally Posted by dtype38

If you accidentally run over a saw tooth kerb with bad bumb steer, does it go "in-out-in-out", while the rest of the car "shakes you all about" ?

Hmmm...bumb steer and in and out...too technical for me...

[GORDON STREETER]

Originally Posted by JamesH

Ok, one wheel hits a bump whilst the other hits a hole....

;-)

James

In a straight line agreed, but in roll steer the suspension pick up points are starting at a different hight on a corner. The inside will always be higher than the outside.

[R59]

Then just to muddy the waters, someone comes along and throws ackermann angles into the mix.......

Gordon, did you say 700lb springs?.... that's a combined 1400lb at the front, and I didn't think your Anglebox weighed that much overall!

[GORDON STREETER]

Originally Posted by R59

Then just to muddy the waters, someone comes along and throws ackermann angles into the mix.......

Gordon, did you say 700lb springs?.... that's a combined 1400lb at the front, and I didn't think your Anglebox weighed that much overall!

Its about the same. as for Ackerman I thought he fronted Focus ( I can play Sylvia )

[JamesH]

Originally Posted by GORDON STREETER

In a straight line agreed, but in roll steer the suspension pick up points are starting at a different hight on a corner. The inside will always be higher than the outside.

Ok, you win

James