The Effects of Rear Roll Centre Height

[Duk]

Can someone please explain what effect the raising and lowering of the rear roll centre of a V8 Super Taxi has on the car?
The way I understand it, when they raise the rear roll centre it puts more load onto the front wheel and obviously, when they lower rear RC it puts more load onto the rear tyre. Am I on the right track?
In cars that have 'on the fly' adjustable anti-roll bars, surely the adjustable height rear RC isn't just about the over/understeer charcteristic, or is it?

[Cameron_Datto]

The front part of the chassis is forced to hinge on the front RC, and the rear part is forced to hinge on the rear RC. If the chassis is rigid, it will be forced to hinge on the axis that connects both RCs , that axis is called the roll axis ,The position of the roll axis relative to the cars CG tells a lot about the cornering power of the car; it predicts how the car will react when taking a turn. If the roll axis is angled down towards the front,( Rear Roll Centre Higher that the Front RC ) the front will roll deeper into its suspension travel than the rear, giving the car a 'nose down' attitude in the corner. Because the rear roll moment is small relative to the front, the rear won't roll very far; hence the chassis will stay close to ride height. Note that with a car with very little negative suspension travel (droop) the chassis will drop more efficiently when the car leans over. With the nose of the car low and the back up high, a bigger percentage of the cars weight will be supported by the front tires, more tire pressure means more grip, so the car will have a lot of grip in the front, making it oversteer. A roll axis that is angled down towards the rear will promote understeer. Remember that the position of the roll centers is a dynamic condition , so the roll axis can actually tilt when the car goes through bumps or takes a corner, so it's possible for a car to understeer when entering the corner, when chassis roll is less pronounced, and oversteer in the middle of the corner because the front RC has dropped down a lot, In terms of car handling, this means that the end with highest RC has the most grip initially, when turning in, or exiting the corner, and that the end with the lowest RC when the chassis is rolled will have the most grip in the middle of the corner. a very high roll center in the front will make the car turn in very aggressively, but understeer in the middle of the corner. It's nice if you like an aggressive car you can 'throw' into the corners, but I doubt it's the fastest way round the track. Conversely, if the rear roll center is set very high, the car will turn in very gently, and possibly oversteer after that.

[Rex_Kelway]

That's an excellent reply. Rex was thinking for many time how to answer this question, but Cameron's response is chicken-noodle.

[Duk]

The position of the roll axis relative to the cars CG tells a lot about the cornering power of the car; it predicts how the car will react when taking a turn. If the roll axis is angled down towards the front,( Rear Roll Centre Higher that the Front RC ) the front will roll deeper into its suspension travel than the rear, giving the car a 'nose down' attitude in the corner. Because the rear roll moment is small relative to the front, the rear won't roll very far; hence the chassis will stay close to ride height. Note that with a car with very little negative suspension travel (droop) the chassis will drop more efficiently when the car leans over. With the nose of the car low and the back up high, a bigger percentage of the cars weight will be supported by the front tires, more tire pressure means more grip, so the car will have a lot of grip in the front, making it oversteer. A roll axis that is angled down towards the rear will promote understeer. Remember that the position of the roll centers is a dynamic condition , so the roll axis can actually tilt when the car goes through bumps or takes a corner, so it's possible for a car to understeer when entering the corner, when chassis roll is less pronounced, and oversteer in the middle of the corner because the front RC has dropped down a lot

Awesome!!!
I was thinking along the lines of, more force being applied to the front tyres .
So if you have a car (front engine, rear drive) that is typically hard on it's front tyres, would lowering the rear roll centre (tho still higher than the front) help to reduce excess load on the front tyres?

[oldcorollas]

or would raising front roll centre have same effect?
ie, better to have higher roll centres and lower the moment arm?

[Duk]

or would raising front roll centre have same effect?
ie, better to have higher roll centres and lower the moment arm?

Agreed, but for my 'not so theoretical' example, I've already raised the front RC and am trying to get a good handle on the effects of lowering the rear RC.

[Billzilla]

Adding to the above, yes when you lower a car that has a live-axle rer-end it will tend to understeer more into the corners and oversteer out. It's typically hard to move the rear roll centre on a live-axle car so you're often better off not lowering the front too much.
A car with an independant rear end can be quite different though.

[Duk]

Adding to the above, yes when you lower a car that has a live-axle rer-end it will tend to understeer more into the corners and oversteer out. It's typically hard to move the rear roll centre on a live-axle car so you're often better off not lowering the front too much.
A car with an independant rear end can be quite different though.

Cheers Bill. It's a DeDion rear end with a Watts linkage, so probably the only time you'd call it relatively easy.

[Billzilla]

Cheers Bill. It's a DeDion rear end with a Watts linkage, so probably the only time you'd call it relatively easy.

Ooh, nearly the bestest suspension ever!

In that case, the rear roll centre will come down at the same rate as the car being lowered - I'm assuming the Watts Link is mounted on the chassis here.

Is it a bit like the one on my racing car?

[oldcorollas]

bill, is there any benefit of having such a low RC on a sedan? or just useful on open wheeler due to the very low CG?

[Duk]

Ooh, nearly the bestest suspension ever!

In that case, the rear roll centre will come down at the same rate as the car being lowered - I'm assuming the Watts Link is mounted on the chassis here.

I'm a little lost .
If the pivot point of the Watts linkage is mounted on the horizontal bar of the rear suspension, shouldn't the RC stay the same as it's (nearly) always the same distance from the ground.
I can see how the roll couple changes as the suspension moves up and down or the rear is lowered, or am I ass about?

BTW, that is the lowest rear RC I've ever seen on a live axle. Hope you don't smash anything

[Billzilla]

I'm a little lost .
If the pivot point of the Watts linkage is mounted on the horizontal bar of the rear suspension, shouldn't the RC stay the same as it's (nearly) always the same distance from the ground.
I can see how the roll couple changes as the suspension moves up and down or the rear is lowered, or am I ass about?

BTW, that is the lowest rear RC I've ever seen on a live axle. Hope you don't smash anything

There's two ways to mount the Watts - on the diff or on the chassis. If it's on the chassis then the roll centre moves with the chassis and that's the proper way of doing it. (most car maker don't do it this way). It's the proper way because as the front roll centre moves around, so does the rear so they stay roughly in par wth each other.
On my racer above, it's on the diff as the car doesn't pitch or roll much at all, so the front roll centre don't move much either. The rear roll centre does not move relative to the ground, but moves around a bit vertically ref: the chassis but not enough to be a worry.

Heh, if you think that rear RCC is low, have a look at a Mumford Link - they can have the rear RC below the ground on a live axle.

bill, is there any benefit of having such a low RC on a sedan? or just useful on open wheeler due to the very low CG?

It reduces the roll coupling, and yes it's a good thing for a proper racing car as you don't want them to roll for a few reasons; increasingly so these day so you can control the airflow under the car properly.

One really cool thing I spotted on the latest F1 cars is that they've deliberately made the roll centres sit way above the centre of gravity, so the car will actually roll into the corners rather than out, like normal cars do. They must have amazingly low CoG's to make all that work!
Here's a photo of one, you can see how the front arms angle upwards instead of downwards, to give it a higher RC.

[Billzilla]

Not the best photo, but here's one of the Mumford on the back of my Mallock, before we swapped the rear end for a Hewland and so went to independent suspension.



The RC is where the two angled lateral arms intersect.

[rolleraction]

So RC obviously below the ground on your car if I am looking at it right Bill? (I'm following the 2 shiny bars right?)

[Billzilla]

So RC obviously below the ground on your car if I am looking at it right Bill? (I'm following the 2 shiny bars right?)

Not quite, it's just the way that photo looks. Part of the reason the Mumford was invented was that Michael Mumford wanted to come up with a link that gave a lower rear RC on a live axle rear, but keep the rear undertray clean.
You can vary the angles of the lateral links to get the RC height you want.
The one I've got has the Watts Link still sitting out in the breeze but I'm not overly worried about it.