8 replies to this topic

[Froilan_G]

Hello people,

first of all a happy New Year to all of you!

Secondly, I have something to worry ourselves over again.
In contemporary F1 cars, and I think every other (race) car, the line connecting the front and rear roll center is inclined upwards toward the rear.
The jacking forces involved will take more weight of the rear than they do on the front, so obviously this inclined roll axis introduces an oversteer element in the car. Speaking about F1, it would seem to me that the front roll center is always a little bit below ground level and the rear quite a bit above it (between 5 - 10 cm ?)

My question is this. Jacking forces take away grip because they counter the normal force on a tyre. With the tyres in F1 generating so much grip, the jacking forces can be quite significant on the rear axle. Now why would a suspension designer want to do that? Why not lower the rear roll center towards ground level where jacking forces are nil, thereby keeping grip and try to re-introduce the lost oversteer element with some other thing (toe, caster, roll stiffness distr. etc.). You can even move the complete roll axis under the ground and thereby creating a down-jacking force.

I can imagine the roll moment on the chassis would be bigger because of an increased roll arm (the line from the CoG towards the roll axis), but you could solve for this by increasing overall roll stiffness. And by the way, the roll moment would increase 20 % by making the rear roll center on the ground, compared to a rear roll center of 8 cm (this is a simple calculation). But is a 20 % increase in roll moment too much or what?

So in short, why is the rear roll center always higher than the front, and why is it always above ground level?

[Ben]

Some more pearls of wisdom from the course I went on the other week:

When a car transfers weight some is transfered by the springs and dampers and some by the suspension arms. The amount of weigt transfer taken by the suspension links is affected by the roll centre height.

A lower roll centre height means more of the load transfer is taken by the springs and shocks and because these are time dependent (they rely of some roll to provide a force) they take longer (we're taking realtive time in milleseconds here) to transfer the load.

A reason I was given for a higher rear roll centre is trying to equalise the rate of load transfer between the front and rear axles by making the load transfer quicker at the rear to make up for the lateral force lag relative to the front axle due to the generation of the yaw moment required to get any lateral force at the rear.

Gary Anderson alludes to this problem in the latest Race Tech, where he talks about a driver feeling he has oversteer because he gets an instant lateral force response from the front wheels and a delayed response from the rears.

This is a different explanation than the one given in Tune to Win for example which talks about a higher rear roll centre being needed to equalise the roll moment because the mass is higher up at the rear. The explanation I've just given seemed more sensible to me because higher rear than front roll centres are often used in touring cars where the mass distribution would indicate the reverse was needed based on Smith's argument in Tune to Win.

Ben

[imaginesix]

Wow Ben, thank you so very much.
That bit about some of the loadings going straight through the wishbones has just put some order into some puzzling questions and theories I had.
You should go to these seminars more often!

[Froilan_G]

What kind of course or seminar was it then, Ben? The Rouelle-thing? Because I heard some good stuff about that one.

[mat1]

Thanks Ben. Very good.

concerning the original question: a high roll axis is just a way to get roll stiffness, apart from the aspect Ben described. To lower the roll axis to unload the outer wheel, and then to add roll stiffness to counteract roll, does not make much sense.

Unless you want to counteract the effect ben describes of course, or if the jacking forces are a problem because the car theds to topple over.

mat1

[Ben]

You answered your own question. It's definitely jacking forces that prevent high roll centres in practice, think VW Beetle cornering too quickly here.

The workshop I went to was run by the Formula Student organisers and we got lectures from Tony Southgate, a guy called Ken Heap from Jag who did all the suspension design on most of their current models. We also had people from Lotus and Prodrive on hand. In and around this we got training in either ADAMS/CAR or Ricardo WAVE depending on our individual interests.

Formula Student is a fantastic competition and we had students from Belgium and Germany as well as Brits, it was a very interesting few days. I would love to go to one of Claude Roulle's lectures but they're so damn expensive.

Ben

[Froilan_G]

So to get back on the roll axis thing, the position of the rear roll center therefore depends on the lateral force lag...

In other words, it would depend on the moment of inertia of the car, the stiffness of the car itself, the stiffness of the suspension, the track and wheelbase, the tyre characteristics (for the jacking), and the required yaw and yaw rate.

And from the latter 2, is it safe to conclude that different circuits/corners require different roll center positions?

[mat1]

Originally posted by Froilan_G
So to get back on the roll axis thing, the position of the rear roll center therefore depends on the lateral force lag...

In other words, it would depend on the moment of inertia of the car, the stiffness of the car itself, the stiffness of the suspension, the track and wheelbase, the tyre characteristics (for the jacking), and the required yaw and yaw rate.

What do you mean, Froilan? The position of the roll center? It depends just on the geometry of the suspension. If you mean the choice of the position of the roll center (by the designer), you're right. Those parameters will influence the desigenr (I hope).

mat

[Ben]

I reckon it might be worth changing the rear roll centre height depending on the circuit. Dallara F3 cars and Champ cars tend to have the option of a couple of different rear roll centres.

There was an instance in Paul Haney's book Inside Racing where the PacWest team changed Mo Gulgelmin's rear roll centre mid practice. I will get round to re-reading the reasons why they did this next week.

In general though the main thing used to adjust the rate of weight transfer at the track is the the low speed damper adjustment. Many comments by the likes of Jeff Ryan (Penske shocks guru) about 7-post rigs being good for getting only the high speed damping right, and one very interesting one about decent shocks not making the car quicker but the driver quicker are starting to make more sense now.

Ben