Sumo Bumpsteer etc. etc.......

[janandchris]

So, does anyone know how to solve Mk2 bumpsteer problems?

[Sidecarbod]

Originally Posted by janandchris

So, does anyone know how to solve Mk2 bumpsteer problems?

Your sort of assuming that you have bumpsteer problems!

Although the grip of death on the steering wheel would suggest that you do!

Support the car at ride height on bricks on one front side of the car only, have both front wheels on the ground, one will be under the weight of the car the other will not.

Attach a long stick to the wheel so that its horizontal and pointing forward. Jack the wheel up under the wish bone and watch what happens to the end of the stick. It should move upwards, if it moves up and outwards (or inwards) then you have bumpsteer. (In fact it should move outwards a tiny fracttion as the wheel itself is also moving outwards as it goes up....maybe 5mm outwards).

If the stick was say 4 metres long (a laser beam stick) then the end of it would move out 100mm even though the wheel and only moved out 5mm.

The longer the stick the easier it is to see the error.

Have the steering pointing striaght ahead during the test.

Pete

[Noel]

I seem to recall that someone listed the differences between a Mk2 and Mk3 chassis on here a little while ago. As I recall the majority of it was extra cross bracing (in footwells, transmission tunnel, boot area etc). Also I am pretty sure the Mk2 had 120mm posts and the bumpsteer was even worse, leading Pilgrim to modify them down to 110mm, which might also explain a few things here??

[Sidecarbod]

Originally Posted by Noel

I seem to recall that someone listed the differences between a Mk2 and Mk3 chassis on here a little while ago. As I recall the majority of it was extra cross bracing (in footwells, transmission tunnel, boot area etc). Also I am pretty sure the Mk2 had 120mm posts and the bumpsteer was even worse, leading Pilgrim to modify them down to 110mm, which might also explain a few things here??

I believe that the early MKIII was also 120mm. I have also heard that the MKII did not have as bad a problem so who knows what's going on?...Pilgrim?

The fact that it has a different donnor front end would put me off just hacking away at the chassis without measuring the issue first.

It would be nice if the same mod fixes the MKII though.

Pete

[KevinW]

isnt bump steer affected by ride height?
so how should that be set?

[Sidecarbod]

Originally Posted by KevinW

isnt bump steer affected by ride height?
so how should that be set?

If you have no bumpsteer then you can set the ride height to whatever you like. By definition the angle (tow) of the wheels is not effected by the suspension travel.

If you have bumpsteer then messing about with the ride height will effect the tow in/out of the wheels and therefore mess up the tracking.

Setting the tracking on a car the has bumpsteer is going to make the tracking right at that ride height only. As soon as the wheel height is changed by either a bump or a suspension spring adjustment the tracking will be out.

Pete

[stu e]

The MK2 doesn't have posts. The steering rack bolts to the chassis cross member with 4 bolts. I guess you could fit spacers if the rack is too low, but if it's too high the only way to move it would involve moving the cross member.
HTH Stu.

[Sidecarbod]

Originally Posted by stu e

The MK2 doesn't have posts. The steering rack bolts to the chassis cross member with 4 bolts. I guess you could fit spacers if the rack is too low, but if it's too high the only way to move it would involve moving the cross member.
HTH Stu.

Stu, You have solved the mystery of the missing posts!

I remeber when this subject came up last time someone decided that they did not want to cut the posts on a MKIII so instead they removed the rack and cut the support blocks that are part of the rack, this will have the same effect. If it does not work it can always be spaced back up. As to how much has to come off a MKII is a total guess unless someone does some experiments! (Like you say it could already be too low or it could be just right and bumpsteer is not the issue)

I also noticed that Noels rack was slightly different to mine in that it did not have support blocks, it had tube like spacers with a band that went round the rack to either end of the spacer tube. I don't think you could cut this sort of rack down.

What rack does a MKII use anyway?

Pete

[KevinW]

Originally Posted by Sidecarbod

If you have no bumpsteer then you can set the ride height to whatever you like. By definition the angle (tow) of the wheels is not effected by the suspension travel.

If you have bumpsteer then messing about with the ride height will effect the tow in/out of the wheels and therefore mess up the tracking.

Setting the tracking on a car the has bumpsteer is going to make the tracking right at that ride height only. As soon as the wheel height is changed by either a bump or a suspension spring adjustment the tracking will be out.

Pete

Thanks Pete - got it - I did say it was a dumb question.
I've just adjusted my shocks and tyre pressures now. Once i have fixed the current electrical problem (tacho cutting out and idiot lights going on when lights switch on) I will be on to the bump steer mod.

Kevin

[stu e]

What rack does a MKII use anyway?

Pete[/quote]
The MKII uses a Cortina rack MK4/5.
I ended up having to put spacers under my rack as the steering arms didn't clear the chassis.
Stu.