RB5_245

Ok, I've never had to set up suspension in my life. However I'm building a rally car and have only got the suspension left to do.

The car in question is a 205 1.6 GTI. It will be run in gravel, tarmac and forrest events.

The rear shocks are forrest spec and will stay that way, there is also tarmac springs and inserts, and forrest springs and inserts for the front with eccentric top mounts to allow a bit of adjustment.

I know / \ that way camber helps turn in, but that's about the limits of it. What effect is castor going to have?

And what about ride height at the rear, is it really worth lowering it on tarmac despite having the same dampers?

Thanks

Dave

Chelspeed

Not an expert by any means but this is my understanding:

Camber is the angle of the wheel when you look from the front of the car, your / \ with the top of the wheel leaning in towards the middle of the car is negative camber. Not sure this helps turn in but all of these changes interact and I could be wrong. My understanding is that negative camber gives more grip in corners because, as the car rolls in a corner the outside wheel (which is loaded higher as the weight is thrown onto it) goes more upright putting more tread on the ground.

Castor will affect how eager a car is to turn into a corner. Castor is the angle of the wheels steering axis to the vertical looking from the side of the car. On a 205 with struts it's the angle of the strut to the vertical, you adjust castor by moving the strut top forwards and backwards (in and out is camber I should have said above).

For effect of castor think of a shopping trolley, the wheels are hanging behind the pins they turn on and the trolley goes in a straight line because the wheels follow behind the trolley, this is positive castor. Push the trolley backwards and the wheels are in front of the axis and they keep wanting to turn one or the other, this is negative castor.

So when you're driving more positive castor (strut tops forwards) will mean the car wants to go in a straight line, lots of steering self centreing. Needs to be forced to turn a corner. More negative castor, strut tops backwards makes the car more nervous, it'll be trying to turn at the slightest provocation and steering self cetreing will be less, not good for grandad on the motorway but might be just what you want on a rally car.

Best bet is to try both extremes so you have a feel for what the car feels like then move it gradually until you feel it's best behaved for what you want.

RB5_245

Thanks, That should give me a good starting point.

Any Info on rear ride height?

Chelspeed

Just that the lower the better as it lowers centre of gravity. So think of it as running like this on tarmac and raising it for more ground clearance on gravel rather than t'other way round.

Problem with the pug is the rear torsion bars, not too easy to adjust the ride height compared with a car with coilover rear shocks and adjustable spring seats.

I think the normal thing to do is to find someone else successfully running a similar spec car and copy what they're doing..... 205 Challenge I guess for forest settings but don't think they do much tarmac. What about LAD in Morecambe are they still around? Could try picking their brains.

DuncanG

A minor correction: more positive castor = strut tops backwards.
Positive castor gives dynamic negative camber in the turns and is generally a good thing.

911

Duncan is correct, my Sti v3 runs 8 deg positive caster to kill understeer. The top mounts are WELL back in their turrets ie closer to the windscreen!
911

Chelspeed

So we're all agreed that strut tops back is a good thing. We're just arguing over whether this is positive or negative castor.

Doesn't bother me what it's called as long as we're all agreed what's good and what's bad.

RB5_245

The thing is, If it is actually worth dropping the car at the back for tarmac we'll set the spare rear beam lower and just swap them for event's.

However this means we'd no longer have a spare beam if we broke say a torsion bar on a gravel stage.

If it'll make a big difference to times we'll have one high, one low. If it's not that much different we'll keep both high.

Anyway the main thing was camber and castor which seems to have been answered.

Thanks for the info.

Dave