Hi.
We're in the process of preparing our 1986 coupe for a season of ASWMC hillclimbing (roadgoing, so only limited mods to standard allowed) and would greatly appreciate any thoughts on our suspension setup. We've dialled out most of her bad habits, but she still tends to "wander" slightly under deceleration.
Current setup is as follows: 185/60/13 Toyo R888, super minifins, fairly low ride height, hi-los, spax on lowest set, modified bump stops, rose jointed adjustable lower front arms and front and rear tie bars. Standard rear lower arm.
Front: 0.5deg neg camber, 3.5deg pos caster, 2-3mm toe-in, 18psi pressure.
Rear: zero camber, 3-4mm toe-in, 25psi pressure.

What caster, camber and track would you guys recommend and would you use softer front shocks?

When you say "Standard rear lower arm" do you mean the GTM reverse A-frame type?
If so they have far too much compliance in the metalastic bushes and can also bend under acceleration.
My first GTM had bent A-arms when I bought it so I got brand new ones from GTM - They bent first time I left the start line.

IMHO - the only way to go at the rear is to fully rose joint.
That certainly stopped all the rear steering in my GTM.

I went half way with mine (for road use). I made my own wishbones from 25 x 25mm tubing rather than the 25 x 13 of the originals.

I fitted a rose joint at the inner end



and made polyurethane bushes for the outer pivots.

I have some questions
Are the Tie Bars Rod end or rubber bushes as worn rubber bushes cause weaving please specify front and rear.
What lower rear arms? A arm or mini standard parts.
Toe in is that per wheel or total
Sapx settings the same front/rear?

Tyre pressures sound about right, but i have not had any experiance with R888 as I know the A048R can be a bit tempermental with pressure and stability as I have found on the drag strip. then i can share some more info/ideas for you

Neil

Thanks for the great response. Our rear arms are "A-frame" and they are rose-jointed and adjustable. All tie rods are rose-jointed and adjustable. Front bottom arms also rose-jointed and adjustable. Spax shocks are adjusted to the softest setting front and rear (but we suspect this is still a bit firm for the front). The toe-in quoted is overall, not for each wheel. Would love to know what camber, caster and track you all use.

To be quite honest I think the tyre pressures seem a little low especially at the front, if you are finding that the car wanders under deceleration then this is likley to be a front geometry issue, I would say you could find that with the tyres that low you are getting a deflection in the tyre causing the wobble. When I raced we use to use about 26 psi front/rear, however that was with the engine in the front so this could be a little high. Also 0.5 deg neg camber is very little, possibly try a little extra, with the toe in on the front I use to run 1/16" on each wheel and on the rears it was set to zero.

Thanks Andy, interesting thoughts on the front pressures, we'll play with these and see if it makes a difference. Any more tips on front and rear geometry out there?

This is very helpful; I am taking my car to Llandow for a track day in 2 weeks to try it out, then I have an entry in a sprint meeting at the end of March.

All novice stuff - but data like this thread is most useful.

Regards

David

Just remember with any setup its all about the driver and how he/she wants the car to feel, try small changes of each item at a time other wise you will just confuse the issue. If it were me and I was hillclimbing/sprinting then I would opt/start with about 2deg negitive camber on the front, about 1-1.5deg on the rear, slight toe in on the front and netural on the rear, a little caster on the front for a heaver steering feel, zero on the rear, 22 psi front about 25psi rear, shock settings are very personal but possibly 60:40 bias to the rear. Give the car a good run and see how it feels,

Thanks, the car has been used before by the guy who built it; so I have left all of the suspension settings as they were and will try the car then think about it.
The tyre pressure info is very useful although this car has a probably slightly heavier Honda engine.

Cheers


David

Just lost a huge post i wrote in response try again if a little bit mre simplier this time

Miniman, My suggestions for yourself

1.5 neg camber front.
Front toe sounds good, i have just moved to this setting myself.
Front caster you might find gains increasing this to 5 degrees. from memory I run almost 6.
Shocks full soft

Rear 1.5 neg camber
Toe start with 4mm total, once you have some confidance move to 2mm then if its all going well try zero. never go toe out Check that the rear locking of the uprights is secure this could cause weaving.
Shocks 3 clicks up might be worth an experimant.

Tyre pressures leave as is for the memoment this is tha same pressures i run and it works very well with Yoko A539. Harder front will cause nervous at speed I use to run 24psi and now Im 18 I wont go back.

Weaving under braking was historically worn bushes on the tie bars front/rear and rear lower arm bush. as your on rod ends check all for wear any wear play will give unprdicable handling a Gtm is very sensitive to this.


David on your Honda powered Coupe I would apply the same except I would run a rear toe of 4mm total to get some expeiance of the car, I spoke to the origianl owner/builder and he described a handling issue which I have come across on mine and might be worth a little talk off the board for some ideas to make the car more forgiving.

As said before it is driver preferance.

Neil

Thanks, I will pm you.
The previous owner told me he cured the handling issues with 7" rims on the back.

David

Neil,

Thanks for all the info, that'll save us stacks of time getting her ready for the first outing at Wiscombe in April. Very interested to hear you run more caster, we were tempted in that direction and will now be brave enough to try it. We're also wondering whether to ditch the Spax for standard shocks on the front?

Thanks again, Martyn

On the subject of caster, I'm thinking that increasing it gives the steering more "weight" and improves "self-centre". Am I on the right track, or am I missing something obvious and important? (wouldn't be the first time!).

Yes. More caster will increase the self-centring effect of the steering but too much can make the steering very heavy and will introduce a high degree of camber jacking where turning the wheel from side to side will raise and lower that side of the car.