Tyre Width Vs Grip

[Jersey Tom]

I leave you to work out the ideal position for the 2011 Pirelli's.

In the garbage. (Unnecessarily harsh but too funny not to take a swing at that ball).

Dislike it almost as much as saying a car is "understeer" or "oversteer"

Please expand.

Brian

It's just completely insufficient. "Car understeers" ... ok, when? Low speed? High speed? Left handers? Right handers? Under the brakes? On initial throttle?

Not to mention you can have various setups with identical "understeer gradient" or various steady state metrics but then handle completely differently.

[Tommy Cookers]

Understeer & Oversteer ?

AGREED !!

(Maurice Olley meant steady state, presumably ?)

[Tommy Cookers]

Are the provided tire vertical stiffness values in a similar range to other road racing slicks found in other classes?

Not really. From my point of view, the objective of setting the suspension of a race car is to match the vehicle to the tyres (mechanically).

Thanks for the tyre stiffness data, recent tyres seem stiffer than one might have guessed.

Mr Coulthard said recently that more wheel travel comes from tyre deflection than from the suspension, what is the main criterion for matching the suspension to the tyres in 2012 ?

(the same person said words to the effect that the F1 Mercedes was more British than German !)

[DaveW]

In the garbage. (Unnecessarily harsh but too funny not to take a swing at that ball).

I'm not completely surprised at your comment, & I'm sure you could state good reasons for it.

From my perspective the tyres are elements of the suspension, and it is very helpful when establishing control over the vehicle dynamics if the tyre stiffness distribution is compatible with the position of the centre of gravity. An exact match is not absolutely necessary, but then a compromise is required.

Statistics are not on your side. Over several years I have tested most mid-engined aero race vehicles from both sides of the Atlantic. I have found that the average ratio of rear/front tyre stiffness was around 1.2. The Bridgestone samples I showed are the only ones with a stiffness ratio less than 1 (although CHAMP street car tyres were close).

The 2005 Bridgestones coincided with a very poor year for Ferrari. The 2006 Bridgestone had a more conventional distribution, and Ferrari's results improved. The 2007 Bridgestones caused many teams to ballast forward. The current FOTA c.g. restrictions was the result.

On the rare occasions that I have be able to influence tyre selection, I have usually managed to improve performance.

Overall, I can't help thinking that your "garbage" might have some merit.

[Jersey Tom]

From my end, focused much more on in-plane than out-of-plane dynamics.. the distribution of cornering stiffness (or effective compliance) in relation to the CG is the big thing. That's basic principles of handling, IMO... and applies on a dead smooth track as much as one with vertical content. Whereas I'd think vertical rate becomes more and less critical depending on how much content the track has.

I was going to say that with regard to distribution of tire rate I'd have a hard time chasing that down given how directly a function that is of inflation pressure and camber - which can be pretty big variables. One could say that the same is true of cornering stiffness - though sometimes to not that strong a degree. It would be interesting to compare the relationship of cornering to vertical stiffness of a tire for such things and what "typically" works out. There certainly can be some correlation between them depending on what you're changing in the tire.

In any event the proportioning of tire response relative to the CG is absolutely crucial on any platform. Fixed weight distribution makes that in the hands of Pirelli. Even then, the distribution one driver prefers will likely be different than another. Or even the same driver on different setups.

But I digress. Not knowing the F&M data of the Pirellis I can't make much proper comment on them, just speculate. Fundamentally though I think for good racing you should have fairly durable, predictable, consistent tires with enough performance that you can get after people without having to worry about wrecking. That's why I say the tires are junk. And really from a competitive product standpoint they are about as far from desirable as you want. Bear in mind that's for good RACING... not necessarily a "good" / unpredictable / random EVENT.

Some continue to argue, "But but... the FIA told them to make crappy tires! It's not their fault!" More speculation, but even if that were entirely the case.. if the FIA came back and said "we need you to do a 180 and give us tires that wear like iron and have piles of well-balanced 'grip'" I'm not sure Pirelli could achieve it. Not necessarily for lack of trying, but at least partially due to the size of the company, resources available, and the immense challenge of it.

[DaveW]

Thanks for that... I would seem that I misinterpreted your comment....

[bill shoe]

I was wondering today if there is a relationship between the width of a tyre and the mechanical grip it provides?

As an example if you had 2 tyres of the same diameter and compound but of differing widths would the wider tyre necessarily provide more grip? I was wondering if the increase in width would provide a lower amount of pressure on each square centimeter of contact with the road and if this would provide less traction?

Given a tire with slip angle relative to the ground. As a section of tread rotates down into the contact patch the ground will force it to change direction, and therefore the tread twists. As the tread leaves the ground it untwists back into alignment with the rest of the tire. This means the tread around the bottom half of the tire makes a kind of s-twist (in plan view) as it approaches, touches, and departs from the ground.

As a tire gets wider, the contact patch gets shorter. As the contact patch gets shorter, more s-twist occurs in the air and less s-twist occurs during ground contact. When less twist occurs on the ground, the rubber through the contact patch has a more consistent orientation/slip across the ground. This more consistent slip means there is potential to have less rubber and contact force wasted on non-optimum slip angles within the contact patch. More of the contact patch will be at or near optimum slip (if driven well).

I think a wider tire can generally make more lateral force due to this effect. There are many real-world caveats and points of diminishing return that are well discussed in other posts.

[Tommy Cookers]

I'm agreeing with your view, what we call a slip angle is really a distortion angle.

Slip angle should really be called Apparent Slip angle.
Cornering grip basically comes from the tyre contact patch not as a whole slipping across the road.
(Apparent) slip is just a symptom of cornering force, because cornering force can't be delivered without distorting the tyre.

Tyres have become wider and squatter/lower profile over several decades.
These changes in tyre proportions have been made to reduce distortion (about tenfold), that's why today slip angles are a fraction of those when F1 was young. Reducing distortion helped grip (and life), that's why they did it. The rest is detail.

So the 'wider' tyre (ie wider but same sectional height/profile) will have less distortion ('slip angle') for the same demanded cornering force.

Because distortion feeds into the contact patch it hinders the contact patch behaving as a whole, the practical result being that the narrower tyre will have a more progressive behaviour at the limit.
Willie Green said (of Historics vs F1 on dangerous circuits) that progressive behaviour was the vital difference.

So the wider tyre will have a higher limit of grip, but will break away more suddenly, (everything else being equal).

IMO

[Jersey Tom]

As a tire gets wider, the contact patch gets shorter. As the contact patch gets shorter, more s-twist occurs in the air and less s-twist occurs during ground contact. When less twist occurs on the ground, the rubber through the contact patch has a more consistent orientation/slip across the ground. This more consistent slip means there is potential to have less rubber and contact force wasted on non-optimum slip angles within the contact patch. More of the contact patch will be at or near optimum slip (if driven well).

I think a wider tire can generally make more lateral force due to this effect. There are many real-world caveats and points of diminishing return that are well discussed in other posts.

I'm not entirely onboard with this line of reasoning. For one, what is a "non optimum" slip angle? No such thing IMO. A slip angle just is what it is... like jacking forces, they're just there not inherently good nor evil. The "consistent orientation / slip" thing sounds awfully vague.

In any event if you follow Pacejka's line of reasoning, a / the primary driver of tire responsiveness is shear stiffness of the tread in contact with the ground. Definitely correlates with big closed up tread blocks and shaved tires generally being good for "grip."

In any event if that's the case, if you were to magically hold all other things equal and start trading footprint length for width, I think your "grip" / response would come down to what your shear stiffness is doing. I'm sure there's a balance point, and subjectively I'd say that if you have a wide but very short footprint it's not going to be all that great.

Not to mention as you shorten up the footprint your torque feedback goes out the window.

[raymondu999]

If you had a grooved tyre, versus a slick (like what F1 had in 2008 vs 2009) then would it be a similar case? You'd have a smaller contact patch, but because there's more pressure (by virtue of an equal force over a smaller area) then the grip will come from a "longer" contact patch?

[Tommy Cookers]

I think that the contact patch would be the same length (more or less ) !

For a given tyre size and pressure, the length of the contact emerges thus ...........

When the given tyre etc is not loaded, the air pressure essentially loads the tyre sidewall structure, broadly equivalent to tension radially over the whole circumference of sidewall.
When the tyre is on the car, the weight slightly compresses the air in the tyre, and relieves the radial tension in the sidewall on some of its circumference, so reaching a balance where the contact weight equals the total tension relieved .
The circumferential length of sidewall structure so relieved of tension becomes the contact patch.
That is how the pressure determines the length of contact patch (of a given tyre).

So the grooved tyre will have the same contact patch length/envelope as the slick.
If the slick was installed on the car and then attacked by a 'grooving worm' that ate into the tyre a 2008 groove, nothing would change ?

The tyre is not a simple inflated bladder like a toy balloon, it is structurally rather like a (suspension) bridge ?


Surely the slick design is kinder to the rubber or to the designer, so allowing more grip one way or another (unless truly wet) ?
The grooved tyre inherently loses grip quicker when sliding develops(on cornering) than does the slick ?

[munks]

Well, I think we can safely conclude it won't be a direct linear effect. Many have started out with the assumption that you can simply calculate the contact patch area by dividing the load by the pressure (which would basically work for a balloon until pressure approaches zero). While that formula has proven to be pretty inaccurate for tires which are not in fact balloons, there is still a general correlation here: if you decrease pressure, it will *almost* always increase the contact patch size.

So I think the patch will get slightly longer, but not proportionally to the tread area removed via grooves! EDIT: and consequentially to it not being proportional, you'll end up with less contact patch area with a grooved tire compared to a slick tire.