Roll Resistance

Hi

Over the winter I'm changing to radial tyres from crossplies. I've worked out what springs I need to take into account the softer sidewalls so that the actual wheel rates and frequencies remain the same (stiffer springs, obviously).

But I also want to work out the roll resistances of the car, taking into account the spring rates and the roll bar dimensions/ratios etc.

I'm sure I've seen some equations that allow you to do that, but I can't find them anywhere. Does anyone know what they are? It's not just the roll bar stiffness I want, but how the springs and bars work together. I think the same equation (or set of equations) will tell you the expected roll angle for a given mass, CoG height & lateral acceration as well, if that helps track them down.

I'm also aware that fancy-pants maths can mean diddly-squat in the real world, and we'll probably just go from a 0.75" bar to a 1.0" bar like everyone else anyway, but I'd like to confirm (ish) that choice mathematically.

Hoping someone can help,
regards, Tristan

Over the winter I'm changing to radial tyres from crossplies. I've worked out what springs I need to take into account the softer sidewalls

I'm intrigued - how did you do that then?

First of all I worked out the wheel rates and frequencies I have at the moment (roughly, but not too roughly).

Then I added the spring rate of a crossply tyre to the sums, so that I got the overall wheel rate and frequency.

I assumed that our car wasn't handling badly (being rather let down by me, the driver), so decided that I wanted to keep the same rates with radial tyres.

So I changed the tyre spring rate to that approximating a radial, and adjusted the spring rate input until the overall wheel rates were the same. And the values work out the same as springs other people have run in the same make of car, semi-confirming my results.

But I don't know what to do with roll-bars, other than blindly (and perhaps stupidly) copy what other people have done a few years ago.

First of all I worked out the wheel rates and frequencies I have at the moment (roughly, but not too roughly).

Then I added the spring rate of a crossply tyre to the sums, so that I got the overall wheel rate and frequency.

I assumed that our car wasn't handling badly (being rather let down by me, the driver), so decided that I wanted to keep the same rates with radial tyres.

So I changed the tyre spring rate to that approximating a radial, and adjusted the spring rate input until the overall wheel rates were the same. And the values work out the same as springs other people have run in the same make of car, semi-confirming my results.

But I don't know what to do with roll-bars, other than blindly (and perhaps stupidly) copy what other people have done a few years ago.

Further intrigued - where did you get the data for the spring rates for your cross ply and radial tyres, and doesn't that vary with the pressure in the tyre?? What were the rates for crossply and radial that you found?

Obviously they are approximate, and they are for the tyre at a tested 'optimum' pressure and temperature. They will vary. Just like my wheel rate will with downforce and rising rate, braking, turning, accelerating. Just like my corner weights will on bumps, cambers, corners, grass... My [very] simple mathematical example is a static, single case, steady state example, taken at static ride height (but works out the rising rate at ride height, which will vary with preload (which is misunderstood) and load... It's becoming quite complex, and I'm loving it, as it agrees with our current setup.

As for the rates, you'll have to ask Avon themselves. They're very friendly and helpful if you ask nicely. They can be contacted on 01225 703101 (and you'll need to ask to speak to a 'technical' person).

As for the rates, you'll have to ask Avon themselves. They're very friendly and helpful if you ask nicely. They can be contacted on 01225 703101 (and you'll need to ask to speak to a 'technical' person).

Can't you just tell me here please?

Thanks.

I'd imagine it depends on your tyres. What size are they. What compound. what type of car. What cold/hot pressures. What cambers.

The numbers I have apply only to my tyres (crossply slicks, medium compound for an old single seater), and are basically Avon's best guess for me. I wouldn't be comfortable in telling people my numbers as fact. It's got nothing to do with secrets - in a couple of hours you can have numbers that apply to you from the horses mouth, not some numbnuts like me.

You sure about "More Suspension" in Cross/Ply's,I'd have thought it was the other way round,considering that ride smoothness is one reason for radial's in the first place.

Ride smoothness is the least of my worries. Grip and balance are vastly more important.

But some people have suggesting doubling the wheel rates (too much in my opinion), others have suggested staying on crossplies.

The aim is to keep the same wheel rates, perhaps slightly increasing them because of the increased load transfer potential, so that pitch and roll are kept in check (the latter helped, of course, by the ARBs).

I fully understand that you are not looking for Ride Smoothness,I am refering to the fact that Radials will give more Sidewall Flex than a Crossply,that's the thing they are supposed to do.A Crossply,in it's construction is appreciably more resistant to tread deflection/sidewall flex.

Oh I see.

Well, from what I've learnt speaking to people in offices (tyre people, suspension gurus (some Lotus/Ex-Lotus guys come round every now and then)) and people in paddocks, they seem to think that the chassis wheel rates need to increase to take into account the softer tyre rate, so that the overall rate remains about them same, but with the bias on slightly stiffer.

Thus the tyre is used more as a chassis spring (which is what you refer to?), whilst the overall suspension system remains roughly similar.

Does that make any sense to you (who no doubt is more experienced than I)?

http://www.ten-tenths.com/forum/showthread.php?p=1477904#post1477904

It's probably about the worse looking sheet I've seen but it works.

I assume that's what your looking for. You need to have the springs and ARB's in N/mm which might be a pain to measure.

It would be interesting to do that for how you ran the car before, and then see what it takes it to get running quick on crossplies. Does it require more or less roll stiffness, a different front:rear balance, more or less geometrical stiffness etc.

On the ARB stiffness measuring I was thinking about how to measure ARB stiffness without a load cell... Put solid links in for the shocks and put it on the flat patch with 4 x 5mm of shims under each wheel and record the corner weights. Then take the shim from one side and put it under (i.e. left wheel goes up 5mm, right wheel goes down 5, so 10mm roll) and see what the effect on the corner weight is.

I.e. if you take give it 10mm of roll and there is 50 kg change, the total rate is 50N/mm or 25N/mm per wheel?

You could then move another 5 mm and see the correlation from 0-40mm of roll. Would this work? Since there is a solid shock there is no change in the spring load. And since your moving one side up and the other down by the same the CG should remain at the centre of the car (is this right though?) and approximately at the same height.

Any thoughts?

If that works you could probably disconnect the ARB and get the average spring stifness too if you wanted to rate the springs (the advantage being if you didn't know the motion ratio's).