- Crank asks, how is the easiest way to measure roll centre on a car and what to look for?
So what Crank is asking about there is, I'll start talking about what roll centre
actually is, so for anyone that doesn't know already.
So essentially there's a roll centre defined for each end of the car both at the front
and the rear axle and the concept of a roll centre, it's actually an imaginary
point in space, it's not something that you can physically measure, it's a construction
that you make from measuring other parts of your suspension.
The idea behind it is it's the point that the car rolls about in space at each end of
the car.
So the car's not rolling about a centre of gravity when you go into a turn, it's actually
rolling about this thing we call the roll centre which, the other important point
to understand about the roll centre is it's not static, it's constantly moving, both up
and down and side to side as your cars rolls and moves through the corner,
but the idea is at each point in the suspension travel, it's the instantaneous point that it's
moving about each end of the car.
The important thing to understand is a big part of what we call your geometric weight
transfer, so there is weight transfer through the elastic elements which are things like
your springs and your anti roll bars, bump stops stuff like that, that's what we call the elastic
elements of the suspension.
There is also geometric weight transfer which is heavily impacted by the position of the roll
centre.
Now the easiest way, Crank's question, the best way to measure the roll centre,
so that's something you have to calculate, as far as calculating it statically, it's
relatively simple, essentially you take into account all of your suspension pickup points
relative to some reference.
So usually to start with it's the ground level.
So that's a case of measuring the vertical position from the ground and the horizontal
position from the centreline of the car.
So if it's A arms, that's both inner pivot points for those A arms and the outer
pivot points for both A arms.
If it's McPherson strut, obviously you have to include the top pivot mount as well
and using that and the track width it's actually pretty easy to sketch out on paper
if you can take some basic measurements of your car and construct where the static
roll centre is.
Now that is one part of the puzzle but the more interesting part is actually
determining how much it migrates.
All I mean by migrate is how much it moves as the suspension travels.
Now you can absolutely calculate this by hand on pencil and paper and it is
actually quite a useful exercise to go through to understand how all of thise works for yourself
but this is where really simulation can come in and be a really powerful tool for understanding
the roll centre migration.
So it doesn't have to be a really involved process.
Simulation can be anything as simple as a basic spreadsheet that you make yourself
or there are plenty of really affordable spreadsheet based tools that you can buy
already that have been available and are well developed for a really long time,
right up to incredibly expensive super computer level simulations that are taking into account
a whole lot more than just the roll centre but there's a massive wide spectrum of the
types of simulation, the beauty of that simulation is at any point in the car's
orientation, at its position on track, the idea is that you can see how much
it migrates.
The thing that stuffs a lot of people up with roll centre is that people think that
when they lower the ride height of their car, they're lowering the centre of gravity and
that's going to improve the performance of their car laterally and longitudinally.
So in principle, that's sound reasoning in that the weight transfer, which is something
you want to reduce because of the vertical load sensitivity of the tyres, when you lower
the centre of gravity you're essentially, in theory, you're reducing the weight transfer
and that's true, the weight transfer laterally is dominated largely just by the height of
the centre of gravity and the track width and longitudinally it's the height of the centre
of gravity and the wheel base.
But the important thing to understand is that when you take, let's say something that's based
on a road car that you're taking to the track, you lower it and that's true that you do
lower the centre of gravity but what happens with road car kinematics, and all kinematics
means is the way the suspension moves though its different arcs, depending on the geometry
you have in your car.
When you lower the suspension, you generally have a lot of roll centre migration
and the reason for that is in OEM cars anyway, is because the suspension arms are typically
really short, the problem with that is you get a huge amount of roll centre migration,
they're short because of packaging reasons that you get for road cars so in road
cars you've got really different goals.
You're trying to fit lots of passengers and gear on the inside, you've got to make room
for the engine compartment, passenger's bags and all of that stuff.
So as a result, the suspension arms being, relative to something that would be purpose
designed for racing, the suspension arms end up being really short.
Short suspension arms tend to give you a lot of roll centre migration.
The problem with roll centre migration is, the thing that determines the amount of,
1 of the things that determines the amount of body roll you're getting is the height
difference between the centre of gravity and the roll centre.
So the problem with roll centre migration, when you lower the car is if that was your
centre of gravity and that's your roll centre height, these are the wheels
out here, when you lower the car the roll centre will go a lot lower than it was
originally and that roll moment is defined by the height difference between the centre
of gravity and the roll centre.
So you've lowered the car, now your roll centre is down here, in theory you've reduced
some of the lateral load transfer but you've also increased the moment arm between
the centre of gravity and the roll centre.
The problem with that is now you're actually going to introduce, for all other things
being equal, you're going to introduce more body roll as a result and you're
quite often going to end up with worse performance, just because you've now
introduced a whole lot more roll into the car regardless of the fact, and you're going
to lose everything that you gained by lowering the centre of gravity.
So as a result you'll quite commonly see for commonly modified cars, roll centre
correction kits where it's essentially about correcting the geometry of your car.
So as far as your questions of what to look for, if you have lowered your car and it's
something that's based on a road car, if you've lowered it, you should definitely be
looking at a roll centre correction kit.
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