5 replies to this topic

[Froilan_G]

Hi, I have posted this question on other technical forums on the web as well, so you may have seen this one already:

We all know that in F1, the ride height is very important for aerodynamic efficiency etc., but I was near a Formula Ford the other day (read: non-downforce race car), and there they tuned the car by raising or lowering the rear end of the car. (Lowering the rear of the car gives more mid-corner Understeer). We're talking here about 1 or 2 mm, which should have a big effect on balance.

Now how can you affect the balance of the car with the ride height, what is the physical principle behind this?

Now, I've often read about the fact that a higher ride height gives a higher roll center, and with bringing the RC towards the CoG, less roll is created and whammo, there should be an effect on mechanical grip. But I don't buy this.

For one, there is only one CoG, and two RC's, of which the latter form a Roll Axis, about which the chassis will roll. With raising one end of the car, you change the inclination of this roll axis, but you will also change the position of the CoG.
Maybe, you raise the RC more than you raise the CoG, but the only thing I can imagine that happens with changing roll center is that you change the magnitude of the roll moment which is created by the CoG about the roll axis.

When the CoG is directly on the roll axis, there is no roll but there should still be a weight transfer in the car (and probably also some sort of WT distribution).

I know the answer has to do with some of the things above, but I can't find the missing pieces or I can seem to put the pieces which I have together.

So I repeat, for clarity's sake:

What is the effect of ride height on mechanical grip ?

[Ben]

A formula ford still has a flat bottom, so some rake may have a small aero effect but I suspect it's inconsequential.

The most likely effect that I can think of is weight jacking. If you accept the argument that all the cornering force at one end of the car is reacted through the roll centre of the suspension at that end this means that if the roll centre is above or below ground level you get a vertical component of the lateral force that tries to lift or pull down the sprung mass.

You mention that no roll occurs when the roll centre and centre of mass are coincident but as the CoM must be above ground you get a massive jacking effect that unsettles the car (think Beetle swing axles, etc).

So back to your question, I think that raising or lowering the car does move the roll centre and therefore affects the amount of weight jacking. E.g. if the rear roll centre is above ground there is an upward jacking force that will tend to reduce the load on the rear tyres as cornering force increases, i.e. an oversteer effect. Theoretically then you could reduce this effect by reducing the height at the rear.

Interestingly, if you look at this years Williams amongst others it clearly has a front roll centre below ground level, this means that cornering will add vertical load to the tyres as cornering increases and this could help combat understeer to some extent.

Hope this helps in some way, any other ideas people?

Ben

[Ross Stonefeld]

What I can add is that raising the rear of the Formula Ford (no aero to speak of) makes it turn in a bit more crisp, and generally moves the car more towards an overall oversteer situation

I would assume its weight transfer, think how much the nose of the car dives in a road car when you apply the brakes firmly, I guess having increased rear ride height would be similar

[Froilan_G]

Interesting reply, Ben.

Is there a way of calculating the magnitude of this jacking force? I suppose you'd have to know the lateral G's, the height of the roll center etc.
I'd like to know if this effect is significant enough for a balance change, since 1 or 2 mm on the rear of a FFord will do a nice job in changing car handling.

If so, this is probably also the reason why rear roll centers are always higher than front roll centers on (race) cars.

[Ben]

It is true that braking unloads the rear wheels. Jacking does this as well but they are caused by longitudinal and lateral acceleration respectively, and this difference is important. Weight transfer due to lateral acceleration is slightly different again.

As far as calculating the jacking force, you need the lateral force, roll centre height and track width. It's obviously a continuously changing value but it should be fairly easy to get an approximate value.

Ben

[Froilan_G]

Does anyone have an idea to make this approximation of the jacking forces on a suspension?

I've been trying to figure it out but I am not coming very far - and my literature sources don't seem to be very helpful either. They all say how a roll center is found, that a roll center creates jacking but then they fail to explain how this is done etc.