Mass of car, cornering and braking - Part 2

[SDB]

why were the slicks quicker anyway?

*x-files music* nobody knows!! Spooky isn't it.

We came up with a number of things, like the snow being JUST thin enough that grip was only produced by the contact with the packed snow / ice underneath, but it didn't add up to that. Or maybe the temperature of the snow (different temperatures of snow behave very differently) meant that the leading edge of tyre created a flat surface that provided MORE grip than the fresh packed snow / ice underneath. But it was all guesswork.

Cem

So was Mycroft misquoting you?

I think he was just confused. Apologies that I didn't think it required a response as I would expect that everyone who read it came to the same conclusion as you and I.

All the best

Simon

[SDB]

I'm pleased to say Damian Harty has agreed to send me something I can post up...

He's spent the last little while trawling through this thread and will be sending it over shortly.

Stand by your beds

[Keith_C]

Would extra weight not make a tyre hotter too? Is heat beneficial or detrimental to grip? At what rate does tyre heat increase relative to load?

[AlexJ]

Cem, Mycroft was quoting himself in a post from early on in the thread that startd this one.

I am interested in this and clearly Mycroft and SDB and others have wisdom to share.

So here's some more statements...

if we add mass we need something to counteract increased forces required to make that mass change direction, or in physics speak accelerate, this can be left/right or forward/backward.

The bone of contention then is that we may be helped out here by the fact that the increased load on the tyres will provide us with an increase in grip that is a)greater than b)the same as c)less than that required to accelerate our greater mass

a)means we can now get more accceleration before we get wheelspin.
b)means we can get the same accceleration.
c)means we get less accceleration before we get wheelspin.

intuitively we say circuit racing cars try to save weight, drag cars try to save weight so wether these people did the physics or not the forces of competition would have led to heavier cars predominating.

however as we see with SDBs forest tyre vs slick tyre on snow experiment intuition can let us down.

so my searches on google have only come up with these http://bovineracing.com/EOA/pt2/Pt2.htm by Brian Beckman

"The force required is called the adhesive limit. The equation to express this is F<=uW where 'F' is the force required, 'u' is the coefficient of adhesion and 'W' is the weight or load on the contact patch." it goes on to say that while u is not totally constant it is reasonably so.

since F=ma
we get ma<=uW which seems to say either b) or c) are correct

[SDB]

Keith.. great questions..

Would extra weight not make a tyre hotter too?

The easiest way to answer that without going in to the ACTUAL causes, is "yes". A heavier car will heat it's tyres up quicker than a lighter car.

Is heat beneficial or detrimental to grip?

A tyre compound / road surface combination will have an "ideal" operating temperature. Therefore, heat can be both beneficial and detrimantal, depend which side of ideal you currently are

At what rate does tyre heat increase relative to load?

That one I don't think I can answer without going in to the techno babble, but there is no direct equasion that would tell you how heat increases will load that I am aware of.

AlexJ

FABULOUS post.

I may be wrong, but I am getting the impression that you are considering load transfer from front to rear to actually increase the mass at the rear of the car. Deepest apologies if not... the only thing in your post that lead me to think this was the options talking about "prior to wheelspin", etc.

When you get load transfer from front to rear (like when you hit the gas!! ) the mass around the car remains in the same place. All that happens is that the amount of this mass that is being supported by the rear tyres increases and the front descreases. Just like you standing on one leg...

So in cornering, the same amount of mass is required to be resisted, but tyres with more load transfered on to them are able to resist this mass more.

The interesting part is this..

If you add load to the car by increasing mass, do you increase the grip available by enough to resist the extra mass. The answer is no.

If I'm way off the mark here, I'll get me coat!!

Cheers

Simon

[AlexJ]

cheers SDB

I was thinking more of actually adding mass to the whole car than the weight transfer thing

so in the equation ma<=uW
m is the mass of the car where W is the weight on the tyre

[Keith_C]

Originally posted by SDB
So in cornering, the same amount of mass is required to be resisted, but tyres with more load transfered on to them are able to resist this mass more.

The interesting part is this..

If you add load to the car by increasing mass, do you increase the grip available by enough to resist the extra mass. The answer is no.

So when you increase the loading on (say) the rear tyres, you get a corresponding increase in grip.

But, you're saying the increase in grip does not increase at the same rate as the increase in load? So by having a lower mass to start with, the difference between the load on a tyre and grip derived from the load is smaller than an otherwise identical but heavier car?

Have I got that right?

The thing which confuses me is that implies that for a certain amount of power a lighter car would be unable to put it down to the road as well as a heavier car, due to having less grip...

[SDB]

Hi Alex

Understood, taxi waiting!

Keith

So when you increase the loading on (say) the rear tyres, you get a corresponding increase in grip.

There is AN increase in grip, but not "corresponding" in the way you mean the word.

But, you're saying the increase in grip does not increase at the same rate as the increase in load? So by having a lower mass to start with, the difference between the load on a tyre and grip derived from the load is smaller than an otherwise identical but heavier car?

Absolutely

The thing which confuses me is that implies that for a certain amount of power a lighter car would be unable to put it down to the road as well as a heavier car, due to having less grip

OK.. I see what you're saying.

Think about what you actually need power for. It's only purpose is to make the car accelerate. If you have less mass to accelerate, you need less power.

In addition...

"power is nothing without control"

If you have 5 million horse power and stick it on your car. It won't be able to put any more traction down than it did before, it will just mean that you can continue to BREAK traction at a higher speed. That means that power is irrelevant if the tyres are not able to actually use it.

Does that answer the question?

Cheers

Simon

[Keith_C]

Originally posted by SDB
you need less power.

I'm sorry - I don't understand this sentance - what does it mean?

No, it does make sense, of course. Doh. I suppose in that situation you'd use stickier tyres...

[Keith_C]

Originally posted by SDB
There is AN increase in grip, but not "corresponding" in the way you mean the word.

Well, I meant that there must be an increase in grip proportional to the increase in load.

Or that your whole point, in that the increase in grip *isn't* proportional?

Sorry - trying to get my head round this.

[SDB]

Hi Keith

That's exactly it. The grip does NOT increase proportionally to the load. It increases by a reducing amount the more load you add.

Cheers

Simon

[Keith_C]

Ah-ha! I think I'm getting it now

Is it not possible to calculate what that drop-off is though? Or is that down to the tyre, or anything else? Actually, a better way of phrasing what I mean is 'Is there such a thing as a ratio of load against grip'?

Sorry for bugging you, but I like learning

[AlexJ]

f is grip W is load u is the magic number
F<=uW

edit - Keith this is the ratio, u will change with road/tyre conditions

[SDB]

is it possible? To a fairly accurate degree yes.

Graphs can be produced for a given tyre at given temperatures, on given surfaces, etc.

But it's a physical test that produces the graph rather than calculation due to the imaginably vast number of things no human understands about how tyres behave, and the vast number of things we know about but cannot predict accurately.

Cool isn't it!

One of my favourite things about vehicle dynamics (in fact, possibly my very favourite) is that there is SO MUCH that just isn't known, and the rest are things we know to be true, but might not be in some circumstances .

Books like Doug and Bill Milliken's Race Car Vehicle Dynamics is genuinely an absolute MUST BUY if you're interested in this stuff. Some of it is so complex your head will melt, but most of it is very intuitive.

This doesn't tell the whole story on it's own, though, as when you isolate forces and reactions for illustration and explanation, you cannot possibly include every other possible combination of effects, and each one intereacts with the others. This is the reason why it is impossible to get every single part of the physics of a car in to a computer.

The computer simulations are superb nowadays, but we still need to do the final development by a moron like me blatting the thing around a circuit. - and LONG may that continue!!

All the best

Simon

[SDB]

Attention!

Quote from Damian Harty

---------

"Internet posters: do you suffer from stupid vehicle dynamics engineers? Then start your own world-beating company now, *while you still know everything*."

The point is that real tyres on real cars work entirely in the non-linear portion of the graph. If we could use them down in the linear region, don't you think we would? They don't get up to temperature if they are "too big" for the application. We need the hysteresis in the rubber to make the tyres warm, and the hysteresis has a lot to do with the load sensitivity and non-linearity.

I think your forum thread is muddling up an awful lot of things together and not really adding any clarity or useful learning in the various posts. There is nothing in error in anything Simon has written - but some of the participants seem determined to obscure things.

So, just to be completely clear:

The tyre may or may not behave as a proportional Newtonian device at very low load levels - a few tens of kg - but I don't care; by the times the loads have got that low they are not influencing the car any more and my attention has transferred to a more heavily loaded wheel. Because of the (surprisingly complex) wriggling about in the contact patch then the non-linearity sets in early, especially on road tyres.

As for the arcane wrangling about the difference between the word "grip" and "coefficient of friction", well - I only deal in maths because there is so much needless contention around words. When I have to, I use lengthy compounds like "peak steady state lateral acceleration", never shorthand like "grip" - because so many people mean so many different things. Without definitions he's just another prat in the pub, I think.

I don't intend to weigh in every time some pub-talkers get into an argument, so don't expect me to enter into a lengthy dialogue about all this. My appearance on Scoobynet was purely a favour to Simon (whom I hold in high regard) and my life is busy enough already!

Damian Harty
Chief Engineer, Dynamics, Prodrive AT/SWRT
-------------

On behalf of everyone at gtr.co.uk (I hope I speak for everyone on this) thank you SO MUCH for taking the time out to read and comment.

All the best

Simon

PS. Damian sends his apologies that he will not be able to spend time following this thread closely and will not be able to answer questions, purely due to the limited time he has available.

[Keith_C]

Cheers Simon

[Edit] Cheers to Damian too.

/me goes away happy that I've learnt something new today.

[Mark Ayling]

Fantastic

[AlexJ]

Originally posted by SDB

The point is that real tyres on real cars work entirely in the non-linear portion of the graph. If we could use them down in the linear region, don't you think we would? They don't get up to temperature if they are "too big" for the application. We need the hysteresis in the rubber to make the tyres warm, and the hysteresis has a lot to do with the load sensitivity and non-linearity.

oh yeah, thats the good stuff

[AlexJ]

still there is a question if the grip/load graph is non linear and upredictable and has to be experimentally determined is there ever a time when F>uW ie the grip outpaces the extra load. This would give Mycroft back some points

[Blow Dog]

Superb!

Cem