Help on set up or not... Ride height.

[ADMA]

I am running 185/60R13 all around and the sump clearance is fine and lower wishbones are parallel to the ground.

If I change to ZZS 185/55R13, the drop is of around 9mm. Will I need to make significant changes on ride height?

My SV has the front dampers' extensions and has adjustable platforms all around.

As much as I would like to use ZZS, I am a bit concerned about ride height adjustability.

Can someone give some advice?

[DCL]

The handling is set by the geometry, rather than the tyre size. That's not to say there's no impact, but there's no need to compensate unless ride height sump clearance is a problem.

Edited by DCL on Tuesday 21st March 16:20

[allen l]

Ride height on mine is about 6.5 centimeters on the lowest point of the sump. A bit too low for my liking but on the other hand... I haven't damaged the sump yet.

[shrink1061]

I've had a similar question myself.

I guess the issue is that if fitting the new tyres causes a problem for ride height, then you need to raise the car. If you raise the car, then you'll fundamentlaly alter the geometry. most simplistically, you'd start removing some camber if you increase ride height. You also start pushing the steering rod out of parallel as well.

My understanding as it increases is that a parallel wishbone isn't necessarily all that important, but a parallel steering rod is, if you're to prevent bump-steer.

These can all be adjusted for by a competent garage though. I'm on 175/60/13 at the moment, so the change to 185/55 is only 1% or so, so not a huge issue for me, but even at my current settings i find the car a tad on the low side, occasionally i can feel the sump edge rest on the back of speed bumps when i go over them slowly!

[DCL]

shrink1061 said:

I've had a similar question myself.

I guess the issue is that if fitting the new tyres causes a problem for ride height, then you need to raise the car. If you raise the car, then you'll fundamentlaly alter the geometry. most simplistically, you'd start removing some camber if you increase ride height. You also start pushing the steering rod out of parallel as well.

My understanding as it increases is that a parallel wishbone isn't necessarily all that important, but a parallel steering rod is, if you're to prevent bump-steer.

What changes fundamentally as you raise or lower the height are the roll centers. As the suspension type is very different at the front to the rear, the relative roll stiffness between the front and rear changes. So there's a different weight transfer, and the balance will change.

We can use this to our advantage to balance the car (by raising or lowering one end, mostly the front). But the problem is often a good set-up can be lost when you change tyres size because you return it to the same height. If you alter the ride hieght, you may find previous balance will never be reached (and end up thinking the new tyres are crap ).

Edit: I suppose what I'm trying to say is each tyre diameter will have its own optimum ride height and adjusting away from a good geometry set-up is a last resort.

Edited by DCL on Wednesday 22 March 11:15

[ADMA]

Thanks for the replies...

I am starting to think that keeping 185/60R13 is a safer bet than to go for the ZZS tyres. As much as I would like to try them.

For what it's worth, without me in the car, over half a tank of petrol, and measured from floor to chassis rail: front 15cm(just at the rear of the front tyre), and rear 19cm where the wing starts.
Whatever this means...

Edited by ADMA on Wednesday 22 March 13:01

Edited by ADMA on Wednesday 22 March 13:02

[HustleRussell]

It means you have +40mm of rake!

[GinG15]

i´m actually in a simlar situation...hence brigning up agan this old thread:

some say: lower wishbones should be parallel to ground
other say: the line between centre of chassi-pivot point and centre of lower baljont on uprite should be parallel

look here:

http://westfield-world.com/Pix/susp/chassis.jpg

if this pivot-point line theory is correct, lower wishbne will be inclned downwards to wheel and car will coem automatically higher

implication: most cars are set lower as necessary, hence clearance problems with sump etc.??

[DCL]

The Caterham uses a spherical bearing and is, more or less, in the middle of the wishbone. But the precise height should be adjusted to give the balance you want after testing. If it is miles away from 'level' then you need to look at other set-up parameters.

[GinG15]

the old caterhams used a trunnion from triumph....how was it there? as also most of those cars have been set that lower wishbone is parallel.
but with the trunnion the wheel-side pivot point is definately not in line with the chassis side pivot point