Offset front subframe bushes - increased castor

[Rusty]

A1QSHIP, thanks for that thread, everything seems to have been covered at one time or another but it's A) finding it and B) finding the people that did it to find out how they faired Nice top mounts too.

Need to measure the gap that I have between the wheels and bumper to make sure that there is some clearance. I'm not sure that I can manage the offset that you have added at 12.5mm but I won't know until I check.

I was thinking last night, 1 deg is approx. Tan-1 1/60 so for arguably a 600mm strut length each 10mm change will yield a 1deg castor change. Need to identify what offset that I can cope with before the wheel arch line becomes victim.

Has anyone got an outline for subframe bushes so that I can look into having some made?

[A1QSHIP]

And again...

http://www.s2forum.com/forum/showthread.php?t=8112&highlight=alloy+bushings

[stiglet]

I would suggest asking 034 but i think they will have a slightly biased opinion!

Also just thinking, these S/F bushes push the whole thing forwards. What happens with your G/B mounts? I doubt thy have the slack in them to compensate.

GT500 - You're right, there is no king pin on Macpherson but it does still apply. I t might be called something else? But its the angle in the steering axis that gives you the scrub radius.

[Error404]

Also just thinking, these S/F bushes push the whole thing forwards. What happens with your G/B mounts? I doubt thy have the slack in them to compensate.

Just what i was thinking



I have a 5 speed subframe sitting at home with stock bushes in if that will help with any measurements Andy?

[Rusty]

Because I've taken the ABY flywheel spacer out, moving the 'box mounts forwards is a great plan for me There is a suprising amount of give in the standard mounts It's either move the whole engine, box, zorst, blah blah etc. or fit some new subframe mounts.

[Keeper of The Kitten]

ISn't KPI and castor the same thing?

No.

KPI is the angle of the steering axis when viewed from the front. Not to be confused with camber which is the angle of the wheel/tyre when viewed from the front.

As it happens with MacPherson struts, the KPI is the strut axis when viewed from the front, as is the castor angle when viewed from the side.

You can get more self-aligning torque by changing wheel offset without changing KPI or castor.

Draw a line through KP axis to the ground and draw a line through the wheel centreline to the ground, and difference between the two will induce self-alignment. The bigger the difference, the bigger the self-aligning effect. Of course, this changes with steering input.

It is also possible for the hub centre to be aft of (or indeed forward of) the castor axis which can increase castor effect without increasing the castor angle.

[Keeper of The Kitten]

(I should point out that cars with driven front wheels tend to have less offset from the KPI to the wheel centreline to reduce the effects of torque steer)

[johne]

This is intriguing.

Hence porsches were mentioned, i think the 928has 4 wheel steering (that must be a b*tch to set up!).

Can anyone shed any more light on this?

928's only had 4 wheel steering in the sense that the compliance of the rubber bushes in the rear suspension was arranged so that under cornering loads the rear wheel under pressure toed in more to help "steer" the rear end round. Rover 400's used a similar trick - as if they could ever corner

[stiglet]

Rover 400's used a similar trick - as if they could ever corner

Rover? Cornering? HAHAHAHAHAHAHAHAHA!!!!!

[GT500]

Thanks Martin! Well explained

[newsh]

Moving the subframe will also move the engine forward, no?

Best way to introduce more caster would be to make offset top mounts I'd have thought.

[Rusty]

No, engine sits on the chassis legs so moving the subframe does not move the engine. These bushes will help after fitting the SM flywheel, will alter the handling in the corners (for the better I hope!) and aren't too much money so I think it's time to have a play! Fancy top mounts are a nice idea but setting them up and paying for them are both going to be a pain.

[Keeper of The Kitten]

@ Rob - no worries!

I am as intrigued as anyone to see what the "real world" results will be from this experiment, nothing beats proper testing!

[Keeper of The Kitten]

I think pushing the subframe forward is the better option in some respects, particularly as it also marginally extends the wheelbase and will also move the weight distribution slightly rearward (as a consequence of pushing the wheels nearer the front).

Maybe the bushes could also be designed to introduce some wedge to improve anti-dive characteristics? All it needs is for the rear bushes to mount the subframe closer to the tub than the front bushes!

[andy10v]

Maybe the bushes could also be designed to introduce some wedge to improve anti-dive characteristics? All it needs is for the rear bushes to mount the subframe closer to the tub than the front bushes!

Has anybody tried this?