Bump Steer

[BAS]

I have just fitted a new quick rack from Moss and decided to see what the bump steer was like. I fitted all the shims supplied buy Hawk to start with & this gave approximately 50 mins toe out per wheel on full bump. Took all the shims out and got a reading of approximately 30 mins toe out per wheel.

Has anyone had to reduce the height of the mounting blocks to improve (reduce) the bump steer or is 30 mins about as good as you can get?

[pimms]

30 mins about as good as you can get?

Hi Brent, what does this 30 mins mean, and how do you measure it? :confused: :o

Thanks

[chris.richard]

Half a degree. one degree = 60 minutes. ;)

[BAS]

Half a degree. one degree = 60 minutes. ;)

yes as Chris said, 360 Degrees / 60 minuets / 60 seconds. Its the surveyor in me.

Just a point, but why do some quote toe in in mm?!
mm over what length?

[David May]

I'm no surveyor but I too find toe in measured in mms hard to visualise. I think its measured at the wheel rim and a clever machine specifies each side differently (assuming that the steering wheel is locked in the centre position.)

[Timberwolf]

I too am confused, bump steer is the change of toe through suspension travel. Is that what your measuring? If not take out the springs, put the steering at dead ahead, attach a plate vertically at the front edge of the rim (not on the tyre), rest a dial guage on the plate and run the suspension through it's move ment watching the dial guage. In an ideal world the dial guage doesn't move, ie zero bump steer, in hideous cases you can see the wheel toeing in and out as it moves. Shim to achiev the least response from the dial guage.

Alan staniforth explains all in his book "Racecar preparation handbook" (sic).

BTW this is probably why mm gets used to discribe bump steer (a dynamic deflection) and degrees to describe toe (a setting relative to the centre line).

[BAS]

I too am confused, bump steer is the change of toe through suspension travel. Is that what your measuring? If not take out the springs, put the steering at dead ahead, attach a plate vertically at the front edge of the rim (not on the tyre), rest a dial guage on the plate and run the suspension through it's move ment watching the dial guage. In an ideal world the dial guage doesn't move, ie zero bump steer, in hideous cases you can see the wheel toeing in and out as it moves. Shim to achiev the least response from the dial guage.

Alan staniforth explains all in his book "Racecar preparation handbook" (sic).

BTW this is probably why mm gets used to discribe bump steer (a dynamic deflection) and degrees to describe toe (a setting relative to the centre line).


Yes I did that, my method.

1 Take off spring
2 Attach dial guage (don't have one) stick laser to side wall of tyre :rolleyes: :D .
3 move suspension up and down. Mark on garage wall where laser hits for full drop, normal ride height & Full Bump.
4 Measure distance from center of wheel to wall.
5 Workout angles.

It was just a rough idea I was after to see if the shimms made it better, but they didn't. The best I could get was without any shimms.

[colin artus]

I seem to recall machining the bottom mounting block to reduce height ( and then shimming it back up). Inevitably you will have some degree of bumpsteer - make sure that it is adjusted so the steer is on droop where it will have minimal effect.

[CorseChris]

...
Just a point, but why do some quote toe in in mm?!
mm over what length?

As David says, it's specified for a given rim diameter. Giving an angle makes more sense to me too, as it's unambiguous.

As for bump steer, try an 80's vinatge Marcos Mantula....... you'd need to specify that in whole radians.

[Timberwolf]

For really wicked bumpsteer have a go in a lowered aircooled beetle, hops around like a frightened rabbit :eek: never really got over a go in a 200 horse one still with the ineffective, fady drum brakes ........

[Sando]

Hi Brent
Think about where your ride height will finally be set and also where the limit of movement of the shock will be on full bump. If you intend running a reasonably low ride height at the front you'll probably only have 2-3" of bump travel. This is the area to concentrate on for the smallest amount of change. As Colin said earlier change in droop is not as crittical, but still undesirable.

Keep at it!!
Rob

[Ken Tomblin]

Hi Brent

Try to get the area of bump spot on, as the droop area is not so critical because the weight on the wheel is unloading.

Regards

[BAS]

Thanks for the all pointers, but can anyone confirm that 30 mins toe out is about as good as you can get on full bump, as with the shimms fitted it is worse.

[Chris J]

Thanks for the all pointers, but can anyone confirm that 30 mins toe out is about as good as you can get on full bump, as with the shimms fitted it is worse.

Brent, I bet they've all forgotten and they didn't make any notes at the time?

[Sando]

You're right there Chris.
Try even bigger shims Brent see which way it goes. :¬)

[Arthur]

Bump steer

You've already had the reference to Allan Staniforth, which is about as good as it gets.
As to the practicalities; I hadn't thought of using my laser and the wall. Good one. I use about 6 foot of straight-edge light timber (not actually so easy to come by, you'll drive the man mad in B+Q sighting every length) and a couple of bungee cords round the wheel.
Block up the chassis, bottle jack under the wheel to be measured. Wood on wheel, drop two plumbs to the floor (oooh, missus) and mark the line with chalk. Wheel up and down on the jack. If wood is parallel to line, minimal bump steer. Wood will in all probability move in and out bodily - that's scrub. No avoiding it.
How good can you get it? Pretty good, and as the lads say, go for minimum around normal ride height, and worry more about bump than droop. It's really up to you - shimming the rod ends works, as does moving the rack height. You will almost always have some at extremes of wheel travel unless you do a single-seater job and put the rack co-incident with and very close to the top suspension arm (or the bottom - look at some eighties formula cars).

Oh yes, and don't forget the rear of the car as well. Depending on the suspension geometry, it may be possible, nay, necessary, to put caster into the rear hub to achieve your required camber and toe. If it's got ANY CASTER AT ALL it will also have bump steer. As I say, may not be avoidable, but be aware, and it may affect how much of what you decide you can run with. If it's bad, it'll have you off the road - a rear steer job is awful if it toes out in bump. If it toes in, you're heading for increasing understeer.
Speaking of which, I talked to one professional builder who'd done a Hawk for a client (I've never done one, this is all hearsay). He reckoned you get two height adjustments for the rear strut, one of which induces absolutely horrific bump steer at the rear. Anyone checked that?

Happy setting-up
Arthur.

[Chris J]

He reckoned you get two height adjustments for the rear strut, one of which induces absolutely horrific bump steer at the rear.

I wonder which one?

[chris.richard]

I'm on the higher mount ( lower ride height) no sign of problems at Knockhill. Can quite believe that my initial off (see thread "arboriculture..." was the result of odd suspension geometry.

[strat6v]

Sounds like strut lengths and ride height hole settings?
I would think the only way you would have problems is if the bottom wishbone's inner pivot was on a different height to the forward link :confused: You'd have to be pretty dumb to do that though?

[Chris J]

I would think the only way you would have problems is if the bottom wishbone's inner pivot was on a different height to the forward link :confused: You'd have to be pretty dumb to do that though?

John

Did you know that some of the Transformer chassis just prior to the year your's was manufactured, have the choice of two settings for the wishbone and only the one setting for the forward link?

The Honda engined Spyder chassis that I had was like this.

[strat6v]

Didn't know that Chris.

I know that perhaps this can be termed as either anti squat or anti dive(as long as the wishbone had the sme angle created)?

This suspension geometry is a black art. One year, rear wheel steering is bad, then somebody designs in some degree of steer or deflection the next year and trumps it as the latest idea?

anyway, i don't seem to see your tangerine dream parked outside ours, when you coming down? or am i coming up for a change??

[Chris J]

The rear wheel 'steering' on an Elise must be right, because the grip levels in one of those is amazing?

I'm still on with the throttle thing on the Orange one. I'm not driving it again untill that's sorted. 'Had to go to a wedding all weekend this weekend, so I got nothing done. I've sorted the clutch travel (I think?), but the brakes need a bit of sorting too. @*!!X:$" kit cars??!!, I dunno??

[SUSIT]

The rear wheel 'steering' on an Elise must be right, because the grip levels in one of those is amazing?

I'm still on with the throttle thing on the Orange one. I'm not driving it again untill that's sorted. 'Had to go to a wedding all weekend this weekend, so I got nothing done. I've sorted the clutch travel (I think?), but the brakes need a bit of sorting too. @*!!X:$" kit cars??!!, I dunno??

Chris, just get a 944 S2 :D :D

[jadefarms]

I seem to recall machining the bottom mounting block to reduce height ( and then shimming it back up). Inevitably you will have some degree of bumpsteer - make sure that it is adjusted so the steer is on droop where it will have minimal effect.
The Hawk 'manual' says the rack mounting is deliberately 3/8" low to allow for shim adjustment of bump steer. On mine, shim adjustment 0-11mm only made things worse, 1.5-3.2 degrees of bump steer from 5-7 ins ride height.
I ended up reducing the height of the lower mounting block by 6mm to give +/- 5mins over the range (twelfth of a degree).
Struan