- How does ride height affect performance of a car?
So what we're talking about there is let's say from a static perspective with the car
there on the setup patch, is we can lower the front axle or we can lower the rear
axle independently and part of that is adjusting the rake of the car.
I'm probably just going to try and dig into the mechanical aspects of this rather
than get too, go down the road of talking about aerodynamics which is a whole
different topic but certainly that's where the rake of the car on that relative front
and rear ride height does have a pretty big effect on the performance as well.
The two things about adjusting ride height, this ties into one of my previous answers
that I gave as well.
Lowering the car is a great thing from the perspective of lowering the centre of gravity
so the first thing to understand about car setup is that race tyres or tyres in general
are load sensitive meaning that the more vertical load we add to them, the lower the
effective coefficient of friction you get of the tyres.
Now that's a massive simplification of how a tyre actually works but we can
simplify it down into terms of thinking about things in coefficient of friction.
So the reason that's important is because when we roll the car into a corner or when
we have lateral load on the car, we're automatically transferring load from
the inside tyres to the outside tyres.
The net effect of doing that is a net reduction in the amount of cornering
power we've got.
We always want to reduce the amount of lateral load transfer.
Now the important thing to understand about car setup is there is no way to
reduce the total amount of lateral load transfer, what a lot of people call weight
transfer, without modifying something fundamental about the car.
So in terms of lateral dynamics anyway, the only aspects you've got are the track
width and the centre of gravity height as well as the overall mass of the car.
After that, for a given lateral acceleration, there is nothing you can do to the setup
that will change the amount of lateral load transfer.
The total lateral load transfer across both axles.
So obviously centre of gravity height is a great thing to be able to lower the car in
order to lower that centre of gravity height.
The downside with that is you've generally got to do some sort of roll centre correction
as well.
So maybe I just need to sketch that out, just to make that a little bit clearer because
I know it can be difficult to understand what I'm just talking about this in terms of words.
So let's say we've got our chassis here, now depending on the suspension kinematics
you've got, will depend on how we construct this but essentially we've got, let's say we've
got our centre of gravity placed here, let's say we've got our wheels mounted
out here.
Our roll centre is some instantaneous point in the construction that again depends
on the type of suspension that you've got but it is the point that the whole sprung mass
of the chassis rolls about.
So this is that roll moment that I was trying to describe before.
That's our roll moment.
What happens is when we lower the car, obviously we're lowering the centre of
gravity, great thing for weight transfer.
The total weight transfer anyway.
What we often do in things like, certainly things like AA arm or multi
link in particular, we usually get a huge amount of roll centre movement for a
small amount of suspension movement and that is typical of road cars.
Because we're putting it into a ride height window that the car's not really intended to
be run in.
So what I mean by that is the roll centre, let's say we've lowered our car now, let's
say we've lowered it, I'm going to draw this in a dotted line, let's say we've lowered
it 50 mm, 2 inches roughly, what will usually happen, the roll centre won't
also move down by that amount, usually in a road car style suspension it might move
a huge amount further down.
So now even though let's say our new centre of gravity is down here, this was our
old roll moment arm, now our new roll moment arm is this one here.
This is exaggerated but the idea here is that we're ending up with more body roll as a result.
So as far as changing, you can absolutely make use of front and rear ride height to
affect the balance of the car, just know it, and it i s probably getting a little bit
outside the scope of what I have time to talk about in terms of today's webinar, you are lowering
the centre of gravity but it's also really important to understand that relationship
with the roll centres so that's one thing I'd definitely be very careful of when I'm lowering or
changing the ride height at each end of the car.
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