Self centering

[Stumaso]

Hi ime a little confused on this one too. The offset of the upper wishbone is 23mm giving a positive caster angle of approx 7 degrees which is all you really need. So why do we need offset mushrooms. Am I missing something?

[alga]

spud69, could you chime in on the topic? You mentioned several times that you recommend the caster angle set so the car just starts to self-centre. How do you adjust the angle? What's the downside of having more caster on the Roadster?

[spud69]

Originally Posted by alga

spud69, could you chime in on the topic? You mentioned several times that you recommend the caster angle set so the car just starts to self-centre. How do you adjust the angle? What's the downside of having more caster on the Roadster?

I seem to remember this has raised it's head a few times previously and yes technically you're right to bring it up again. You can turn the offset mushroom to the rear to move the steering angle forward a bit to influence the self centering. As you know it doesn't actually move the top pivot point but in my experience of the cars i've helped to set up it does help - the reason why offset mushroom started to be used. Getting the bump steer and toe fine tuned will have the best effect over stability.

I dont claim to be an expert just have some experience to use and pass on.

Cheers.......Andy

[Big Vern]

Originally Posted by spud69

I seem to remember this has raised it's head a few times previously and yes technically you're right to bring it up again. You can turn the offset mushroom to the rear to move the steering angle forward a bit to influence the self centering. As you know it doesn't actually move the top pivot point but in my experience of the cars i've helped to set up it does help - the reason why offset mushroom started to be used. Getting the bump steer and toe fine tuned will have the best effect over stability.

I dont claim to be an expert just have some experience to use and pass on.

Cheers.......Andy

The mushroom should be orientated away from the engine.
The mushroom is not meant to be used to overcome the lack of caster. The mushroom takes into account the upright doesn't pivot about its centre line in the donor vehicle. Martin Keenan devised the mushroom to overcome this problem.
The lack of caster is a design flaw in my view and the design should have been corrected years ago or at least a sticky put in place so new builders would know to build in extra caster as they built their chassis. Rotating the mushroom and adding undersirable toe out to get it to self centre is just bodging round the issue.
5 degrees should be considered a minimum but some people arn't getting anywhere near as much.

[spud69]

Originally Posted by Big Vern

The mushroom should be orientated away from the engine.
The mushroom is not meant to be used to overcome the lack of caster. The mushroom takes into account the upright doesn't pivot about its centre line in the donor vehicle. Martin Keenan devised the mushroom to overcome this problem.
The lack of caster is a design flaw in my view and the design should have been corrected years ago or at least a sticky put in place so new builders would know to build in extra caster as they built their chassis. Rotating the mushroom and adding undersirable toe out to get it to self centre is just bodging round the issue.
5 degrees should be considered a minimum but some people arn't getting anywhere near as much.

You're right Vern, 7:30 this morning was a bit early for my brain working to be honest.

The roadster handles fine the way it is and you don't really want a great deal of self center, in my opinion it's mainly for lazy people driving big cars... There is nothing wrong with toe-ing out a bit more on the rack extensions just to get through IVA, although you cant do this with the mushroom (just the camber which you can also do with the threaded insert).

Suns out this afternoon and i've finished my work for the day so i'm of out in mine to see HandyAndy I'm sure it will be handling fine

Andy

[Johno]

Hi Alga,
Did you sort out your self centering problem?
I've been reading up on it and it all seems a bit daunting to say the least

[alga]

No, didn't have much time to play with it TBH.

Take a look on this old thread: http://www.locostbuilders.co.uk/view....php?tid=97634
The consensus is that the proper fix is more caster, and one figure given is that 22-24 mm of displacement between top and bottom ball joints should do it (we have 20 mm standard). I'll try slackening the wishbone bushes and ensuring that their play is at the end of most camber angle.

One more idea to try from there -- rotating the mushroom a little bit inwards might increase the KPI angle (as the upper ball joint will have to be threaded further in to return the camber to where it was), while the mechanical trail will be reduced minimally. I'm not sure there will be space on the thread though -- the locknuts are already at the end of the thread, with just -2° camber and mushrooms straight ahead.

I reduced toe to zero, and now the car displays "a degree of self centering evident": if I rotate the wheel by 15 degrees, when driving, it springs back by some 3 or 5 degrees sometimes. This could be enough for IVA, but is nowhere near where I would consider it to have any safety effect.