Bump Steer

[martin_t]

Hi
Trying to set the bump steer up today on my Jag based GD. Followed the method advised by Andy with two lengths of angle but I cannot get 0 bump steer. The best I can get is with the rack at the very bottom of the adjustment but even with that I have a little toe out when the suspension arms are right at the top. Its pretty dam near at the other extreme (but then again to get in that position I guess I would be off the ground!!!). Just wondered what experience everyone (GD specifically) else has had. Is it possible to get zero bump with the jag geer? What sort of issues will this cause?

Thanks

Martin

[neil still building it]

over how much suspension travel are you talking of ?
1" up and 1" down or more..
neil

[martin_t]

Originally Posted by neil still building it

over how much suspension travel are you talking of ?
1" up and 1" down or more..
neil

More. Basically I'm checking it from maximum upwards (13" between top and bottom suspension bolts) and maximum downwards (10" between top and bottom supsension bolts) as recomended by Andy on the GD video. If I set the suspension to horizontal at the top wishbone, which I guess is the normal ride position, an inch or two either way is fine, its only on the extremes when its out.

Cheers

Martin

[FatBoy]

Here is a link to an excellent thread on setting up suspension on a Cobra:

KMP 259

It's rather long, but well worth a read.

Paul

[slither]

With a Rack and pinion setup you are never going to be able to get "0" bumpsteer throughout the suspension travel. The small amount of toe change at the limits of suspension travel will not affect your handling at all. Also I have found that Cobra's like about 1.5 degrees toe in per wheel for a total of 3 degrees which will also ensure that you are toeing in all the time. The same 1.5 degrees negative camber will make the car handle very predictably and ensure that the tyres wear evenly.

[dave]

Originally Posted by slither

With a Rack and pinion setup you are never going to be able to get "0" bumpsteer throughout the suspension travel.

Yes you can!
But the length of the centre section of the rack has to be absolutely spot on........Amongst other things.



I'm sure the GD set up will adjust to be more than close enough though for us mere mortals.

[Simon R]

Originally Posted by martin_t

More. Basically I'm checking it from maximum upwards (13" between top and bottom suspension bolts) and maximum downwards (10" between top and bottom supsension bolts) as recomended by Andy on the GD video. If I set the suspension to horizontal at the top wishbone, which I guess is the normal ride position, an inch or two either way is fine, its only on the extremes when its out.

Cheers

Martin

Matin,

If you neglect the extrreme 1/2" of droop and bump (you may well be in trouble if you achieve this on the road and bump steer will be the least of your worries!) this leaves you with a suspension range of 2" (shock length). Over this you can get the bumpsteer on the GD down to about 3 arc seconds - this equates to a total change in toe of 0.5mm - i.e. +/- 0.25mm.

Again this is at the extremes - generally it would be more like +/- 0.1mm toe.

Have a look at this graph - being somewhat anal and not convinced about the straight edge method (and my eyesight) I measured the geometry with a mirror and a laser - the graph is the best I could achieve having spent half a day on it. You can see the exagerated bahaviour at full droop.

Sounds to me that this is what you have achievced yourself.

Cheers,

Simon

[martin_t]

Originally Posted by Simon R

Matin,

If you neglect the extrreme 1/2" of droop and bump (you may well be in trouble if you achieve this on the road and bump steer will be the least of your worries!) this leaves you with a suspension range of 2" (shock length). Over this you can get the bumpsteer on the GD down to about 3 arc seconds - this equates to a total change in toe of 0.5mm - i.e. +/- 0.25mm.

Again this is at the extremes - generally it would be more like +/- 0.1mm toe.

Have a look at this graph - being somewhat anal and not convinced about the straight edge method (and my eyesight) I measured the geometry with a mirror and a laser - the graph is the best I could achieve having spent half a day on it. You can see the exagerated bahaviour at full droop.

Sounds to me that this is what you have achievced yourself.

Cheers,

Simon

Hi Simon

I have got it fairly close now. It toes out a little at maximum droop in the last half inch, and toes in at the other extreme. I guess I could play with this for weeks so think i'll leave it for now.
This is something than can be set up professionally afterwards isn't it? If that is the case, I'll stop worrying.

Cheers

Martin

[Sidecarbod]

Originally Posted by martin_t

Hi Simon

I have got it fairly close now. It toes out a little at maximum droop in the last half inch, and toes in at the other extreme. I guess I could play with this for weeks so think i'll leave it for now.
This is something than can be set up professionally afterwards isn't? If that is the case I'll stop worrying.

Cheers

Martin

Martin,

You need to go for a drive in a Sumo MkIII that has not had the "bumpsteer" mod, then you will be very, very happy with the tiny amount of bumpsteer that you've got.

I believe that early Dax's were also very bad.

Regards,

Pete