After the success of my 'spring preload' thread, I thought I stir some more technical debate.

This time about ride height? Sound interesting? Read on...

Okay, lets define the 'problem'. On our car there are not many things we can easily change to adjust the balance of the car:

Front and Rear ride heights
Wheel Rates
Toe
Roll Resistance
Wing angles
hmmm

Anyway, the car is slightly oversteery. A little too much to be properly quick, although it's not dangerous really (a lot of the time it manifests itself as a four wheel drift, i.e. zero steering lock whilst turning), and hugely preferable to understeer, I'd like to limit it a bit more.

Wing angles: The rear is currently on maximum. Short of remaking the wing to allow more adjustment, we can't do much there. I do not want to reduce front wing to cure oversteer if I can help it.

Toe: Am considering decreasing the toe-in at the front for turn in reasons, and this will actually increase oversteer in some situations. I am not willing to try more or less rear toe for safety and tyre life reasons.

Roll Resistance: I fine tune the car on the roll bars. It's not close enough to neutral to respond favourably to fine tuning. Therefore not an option at this stage.

Springs: Tempted to increase the front wheel rate by adding spring preload at the front (and resetting the ride height via the pushrods), but as I said before, I'd prefer to add grip at the rear rather than just remove it at the front.

That pretty much leaves ride height. A friend of ours who won a race in an 883 like ours (but before the competition was as quick as it is now) claims he uses the rear ride height to balance the car.
Less rear ride height reduces oversteer, more rear ride height reduces understeer.

The car is a double wishbone suspension, running at a minimum height of 40mm (MSA regulations), i.e. outside where it was designed to work. The rear ride height is currently 60mm. The car is slightly oversteery.

Does it make sense that lowering the rear (which we think will lower the roll center AND the CoG) relative to the front will reduce the oversteer (either by better suspension geometry, additional diffuser efficiency etc)?

Discuss. Photos of the car here if you want to see what you are working with.

Oversteer under braking or under power?, I am thinking it may be down to weight transfer, thats the only thing I can think would cause ride height to influence it.

I think raising the rear ride height would cause less oversteer under power, but more under braking, and vice versa, lowering RH would cause less understeer under breaking, but more under power..........

Or I am talking complete poppy-cock

Off Topic

Tristan, you don't post any "nice" pictures of the car, merely "technical" pictures of the car or dodgy shots of it racing. Never been in the mood for an arty shot of the Reynard?

Raising/lowering the rear would also change the angle of attack of the wings, no?

Interesting.

Aerodynamically, while lowering the rear would increase the diffuser efficiency, it could decrease the rear wing efficiency by taking it out more of the airstream, right? (I've no idea the proportion of diffuser/wing df provided by your car, unregulated diffuser?)

Is the oversteer just in medium-quick to very quick turns, or a general condition?

Also, how much is the live camber affected? It just might be that you lose some camber angle by of lowering the rear thus affecting mechanical grip for the lower speed corners.

It's in the faster corners (e.g. Paddock Hill or Surtees at Brands Hatch Indy). It's on the power, but not caused by excessive rear wheel torque. I can obviously cause power oversteer in slower corners if I want to.

Yes, lowering the rear of the car will slightly reduce the AoA of the wings, and the amount of clean air the rear wing produces. I don't think it's really an aero problem though, as we've raised the wing by about 80mm between the last races to improve rear grip, and it's still oversteery...

I don't know anything about the efficiency of our wings, or diffusers. They're just what the car came with.

Isn't there some way of guesstimating balance based on lines drawn between roll centres and CoG heights...

Jaqk - Not really. I don't like 'arty' shots. It's a racing car, and THE most arty shot of a racing car is it doing it's job - i.e. racing. Feel free to photoshop something if you want to demonstrate something. If I wanted arty shots though I'd have bought a model

Edit: No idea about live cambers. At some point I intend to accurately draw the rear suspension so I can deduce camber change will bump and roll (as well as providing numbers for the CarFactory thingy for rFactor). But that's a job for over the winter.

Is it a geometery change?, I know that if you raise the ride height on a bike it makes a big difference to the way the bike handles, but then again, bikes are very different to cars, and on a bike rake makes a massive difference to handling balance.

I still think its weight transfer, combined with lots of other stuff too.

Well, yes it's a geometry change.

I don't know how important rake on the car is compared to the roll center heights... Some books I read say that the rear should be higher than the front as that causes understeer, but mine is already a lot higher, and I want less oversteer. The convention above (which I don't really trust) suggests that lowering the roll center and CoG (but not at the same rates probably) might give me more oversteer.

If only I could afford to go testing everytime I got confused like this...

The closer the tyres are to the ground, the more grip you get. Ideally you want them to be touching the ground.

Interesting shot. Guessing by the depth and angle of the rear diffuser, I imagine that its a very effective bit of kit (relative to the wing). My guess would be getting that closer to the ground would be worthwhile.

The reduction in AoA of rear wing would be countered (and hopefully surpassed) by the diffusers increase in efficiency.

Quote from thisnameistaken :

The closer the tyres are to the ground, the more grip you get. Ideally you want them to be touching the ground.

huh?

He's trying to be funny............

Quote from srdsprinter :

huh?

Ignore me.

Quote from danowat :

He's trying to be funny............

Me to

Quote from Mazz4200 :

Me to

I had a little roffle to myself.

Quote from srdsprinter :

Interesting shot. Guessing by the depth and angle of the rear diffuser, I imagine that its a very effective bit of kit (relative to the wing). My guess would be getting that closer to the ground would be worthwhile.

The reduction in AoA of rear wing would be countered (and hopefully surpassed) by the diffusers increase in efficiency.

You forget if the rear get lowered relative to the front, it might actually have the the opposite effect, as it's the low-pressure area under the diffuser that sucks the car down... Lower rear -> not as much space for the air trapped under the car to expand in the diffuser -> less downforce...

However, the lower CoG and IIRC, the lower rolling center would theoratically help with the oversteer...

lowering the rear sounds like a reasonable thing to try. it should cause a slight weight shift towards the rear, as well as increased negative camber, both of which will give more rear grip. on the other hand, the lower CoG may result in less lateral weight transfer during the corner, which might translate to more responsiveness, but lower ultimate grip, so you might consider a slight reduction in rear arb at the same time as you lower the ride height.

If the problem is medium and high speed corners can you not adjust castor angle or is that not possible on your chassis?

Otherwise take the veterans advise, it came free and with experience and trying it proves the worth of anything else he imparts in the future

I won't comment on the geometry as roll centre heights and the likes are still quite esoteric concepts to me.

But with a car like that I think the most important factor that you are influencing by changing rake and ride height are the aerodynamics of the floor and the diffuser. What shape is the underside of the car? Is it a true Venturi or just a flat bottom with a wing (diffuser) at the end?

If it's a flat bottom increasing rake should increase downforce and shift the balance slightly towards oversteer. The front should be as low as you can get away with. With a Venturi floor (I think) the floor needs to be kept as close as you can to the ground to make the low pressure area under the car work as well as possible.

BTW at paddock you're on the wrong side of the kerb.

Quote from thisnameistaken :

The closer the tyres are to the ground, the more grip you get. Ideally you want them to be touching the ground.

Lol, that clears things up.

Quote from thisnameistaken :

The closer the tyres are to the ground, the more grip you get. Ideally you want them to be touching the ground.

Kev, I owe that tip to all my CTRA lap records...

I wonder how many more stupid things I have to say before they appear in every sig in this forum...

Well, you say some comically awesome things.

Quote from bbman :

You forget if the rear get lowered relative to the front, it might actually have the the opposite effect, as it's the low-pressure area under the diffuser that sucks the car down... Lower rear -> not as much space for the air trapped under the car to expand in the diffuser -> less downforce...

However, the lower CoG and IIRC, the lower rolling center would theoratically help with the oversteer...

Yes, that could be a factor. With less volume to accelerate the air into, the downforce might decrease. However, the MSA rules state that all racing cars (unless they have special dispensation, like British F3, Formula Renault, BTCC etc) have to have all sprung parts of the car at a minimum of 40mm ride height at all times (don't ask me why, it doesn't really make sense). Needless to say we only worry about it being 40mm at rest - the car was still bottoming slightly at the bottom of paddock.

Quote from evilgeek :

lowering the rear sounds like a reasonable thing to try. it should cause a slight weight shift towards the rear, as well as increased negative camber, both of which will give more rear grip. on the other hand, the lower CoG may result in less lateral weight transfer during the corner, which might translate to more responsiveness, but lower ultimate grip, so you might consider a slight reduction in rear arb at the same time as you lower the ride height.

Rear ARB will get tweaked next time I drive it to balance the car.
Rear cambers will be reset to 0° (I'm on crossplies this year, which have no tolerance of camber due to stiff sidewalls. Next year we're getting radials, which are worth at least a second per lap, and allow up to 4° of camber if I wanted ).
Don't think I want any more rear weight bias, but I am willing to put up with anything that makes me quicker (up to a point).

Quote from Becky Rose :

If the problem is medium and high speed corners can you not adjust castor angle or is that not possible on your chassis?

We can adjust castor. When we got the car it was 3° on one side, and 5.5° on the other. Not really knowing anything better, we set it to 5.5° on both sides. The steering is a bit heavier now, and I'm thinking of dropping it to 4°. I know castor, as well as producing dynamic camber with steering also provides a jacking effect (I think), and is that what you're referring to?

Quote :

Otherwise take the veterans advise, it came free and with experience and trying it proves the worth of anything else he imparts in the future

I can't just take advice and try it without understanding why it might work. He was wrong about preload, he is very confused about gearing. So I don't really trust his opinions on a scientific basis.

Quote from J.B. :

I won't comment on the geometry as roll centre heights and the likes are still quite esoteric concepts to me.

But with a car like that I think the most important factor that you are influencing by changing rake and ride height are the aerodynamics of the floor and the diffuser. What shape is the underside of the car? Is it a true Venturi or just a flat bottom with a wing (diffuser) at the end?

If it's a flat bottom increasing rake should increase downforce and shift the balance slightly towards oversteer. The front should be as low as you can get away with. With a Venturi floor (I think) the floor needs to be kept as close as you can to the ground to make the low pressure area under the car work as well as possible.

The car is flat bottomed, but subject to the 40mm rule (and with rake, that's currently 60mm at the rear, due to be 55mm) I don't think it's working quite as efficiently as it would at 10mm ride height.

I know that rake will effect downforce through both upper wing efficiency and under-floor effects, but at the huge ride heights we're forced to run I think it's probably more important keeping the diffuser working a bit (i.e. going to 55mm ride height) that to keep the 20mm of rake. Not an easy subject to talk about, especially as most of you can't pop over and look at the car!

BTW at paddock you're on the wrong side of the kerb.

I did that a few times (I think I got closer to the gravel than that on one lap) - bloody scary corner. And the floor of the car didn't like going over the kerb from either direction

Changing he ride high in an Opel (CrazyHarryStyle)

Get a proper big Opel (f.e. Carlton or Commodore)

Tools:

-Toolbox
-Angle sander
-blue paint
-a "MATTIG" Sticker
-red KONIs

Rip the springs out of the suspension, cut them as much you like, clean them from the rust, paint them in blue, use the MATTIG Sticker, welding stabil metal plates on the high fixing points of the suspension ("DOM" in german), go to the technical service with a technical sheet for MATTIG Sport suspension and you have a "legal" 150mm low suspension.