30 replies to this topic

[mat1]

I am sure it has been covered before, but I cannot find it, so I ask.

AFAIK know, friction generated by a tyre is largely dependent on pressure, but not completely. it seems there is an advatage in al larger contact patch. Is that correct, and if so, why? What other factors are there?

maybe you can help me (as I am having a discussion on this topic).

Thanks in advance,

mat1

[Ogami musashi]

If i'm not mistaken,

Area of contact tends to saturate with loads on it (the so called "tire load sensitivity") so a larger contact patch means less loading per unit area thus more grip.

[NRoshier]

is there a tipping point to the size of the contact pact/loading per unit area whereby the grip will reduce?

[Greg Locock]

If you have a tire with no vertical load on it it will generate a bit of lateral force. As you increase the vertical load on it the maximum lateral force will increase, although it tends to approach a maximum level. So if you are thinking in terms of a coeffcient of friction it starts high when the contact patch is small and drops as the contact patch area increases, for a given tire pressure.

If you are in the normal range of a tire, and you increase the pressure, then the cornering stiffness is increased at first, and at high loads you will develop more ultimate lateral force. At low loads you may see a slight drop in both cornering stiffness and max Fy.

All of this ignores wear, temperatures, wet etc.

[phantom II]

A rally car in the snow wears narrow tires. On dirt, the tires are wider with special tread depending on the type of dirt. On tar they are even wider. If they are too wide, they may have less grip. If the tar road gets wet, the tires get narrower and softer. A car on ice with studs in the tire will roll over easy. A Model T Ford will roll over easy on a dirt road and even easier on a tarred road yet the tires have little grip. A high CG car will have less grip than a low CG car. A heavier car will have more grip than a lighter car. A car with down force will have more grip than a car without wings.
The tire compound depicted by the tread wear rating printed on the sidewall will determine how much grip the tire has. The same tire on a different car may have more or less grip be it lateral or longitudinal. A Porsche Boxster will go as fast as a Lotus Elise on the same tires on a tight course even though it weighs 1000lbs more.
Some very clever dude on this BB posted very interesting information on road compounds.
No matter what tires you have in south Florida, your grip will be low because the roads are made of corral. Concrete ovals have less grip than tarred ones.
Street tire manufacturers take all the above into consideration when they design their tires. Auto manufactures design their suspensions to suit that tire.
If one narrows down the above criteria to more manageable levels, such as F1 on a specific track, then the contact patch can be optimized with more accuracy. The area of it these days is determine by regulations but if the rules were kept the same except for the tires, the contact patch would not be much larger than it is even if they got rid of the grooves. The suspension geometry changes in wet conditions because the tire has less grip and the tire won’t deform to the optimum contact patch, which is always the biggest possible for the specific tire.
The wider the tire, the more sensitive it is to camber. It also has more rolling resistance and wind drag. Check tire sizes on LMP 2 cars even though the rules on tires are pretty open. They are all the same.
If you place wider tires on your Exige without adding more power and down force you will go slower in most conditions. Steam roller size tires would give huge amounts of grip even if the surface pressure is low.
There is more to tire grip than coefficient of friction. Some materials are less effected by pressure like melting ice on melting ice and some man made materials than others. The shear forces in the tire tread surface and not the softness determines grip. If the compound is to hard and the shear forces on the tread to high, the tire will have no grip but will last longer. Never get a street tire with more than 220 TW compound. 50 is typical on a DOT slick.
Track surface, track compound, sidewall construction, tire compound and temperature determine slip angle. The smaller the contact patch the higher the slip angle. Tire pressures can perform wonders.

[NRoshier]

Thanks Greg.
P2 you seem to ascribe to the TW = grip theory. The TW notation seems to be big in the US but I have not noticed it anywhere else. Is there any testing done to support the TW = grip thought?

[J. Edlund]

Originally posted by mat1
I am sure it has been covered before, but I cannot find it, so I ask.

AFAIK know, friction generated by a tyre is largely dependent on pressure, but not completely. it seems there is an advatage in al larger contact patch. Is that correct, and if so, why? What other factors are there?

maybe you can help me (as I am having a discussion on this topic).

Thanks in advance,

mat1

Normally, friction forces are independent on the appearent contact area, only increasing with the normal force and the coefficient of friction, the latter being constant even with increased normal forces. This is true since the real contact area does not increase with the appearent contact area, only with the normal force. The real contact area between two materials, such as two metals, are luckily extremly small, as these small contact points provide a very high strength, similar to having the parts welded together. This real contact area also increase with the reduced hardness of the material, while the same reduction in hardness also reduce the strength of the bond at the contact points. This also explains why bearings intended for low friction use a hard base material with a soft top coating (lead alloys on steel for intance), but now it was tires that was the question.

With tires there is a second "friction" generating mechanism called adhesion. Adhesion is for instance what makes tape sticky, and adhesive forces increase with the appearent contact area. Adhesion is the result of five different mechanisms; chemical adhesion, diffusive adhesion, electrostatic adhesion and mechanical adhesion.

It's because of adhesion and a large contact area that for example dragracing tires found on top fuel dragsters can provide impressive friction coefficients of about five.

[Fat Boy]

Competitive tires in any class of racing I've ever known about was at the maximum size allowable by the rules. There may be exceptions, but I'm just not aware of them. I think the maximum grip probably still continues to rise as the tire size goes up, but the downsides would be camber issues, aero drag, rolling resistance, etc. In many cases, the tires would have to be so big as to be difficult or impossible to package before they would actually slow the car down.

[Ben]

In my experience more contact area gives more grip. Whether you put this down to more asperity contacts, less thermal stress per unit area, less contact pressure, etc is difficult to say. It's very hard to get enough data to be objective. Experience is though that more contact area is better.

Hopefully testing two tyre widths with a correvit at some point - might give me more idea.

Ben

[Paolo]

Contact patch size is load dependant for given tyre dimensions and pressure.

In this scenario, increasing the load will (in most situations, and on non-wet, non-dirt roads) decrease the adherence coefficient.

Caution: simplified explanation ahead.



Preliminary note: what follows is a story of friction coefficients. Friction coefficient is a local property of materials and shall not be confused with adherence coefficient, a global property of the tyre.
A given material on a given surface has ( I warned I was going to simplify...) two possible friction coefficients: a standing friction coefficient, acting when there is no relative motion, and a sliding friction coefficient.
Standing friction coefficient is higher than sliding friction coefficient.






Think of the tyre away from the ground. It is a circle. Now put it on the road, load it.

It will change its shape, flattening an area. This is the contact patch. Tyre deformation is quite complex, but in first approximation you can visualize a loaded tyre as a circle with a section cut off and made straight.

Now, make the tyre roll.

Each point of the tyre periphery, except the contact area, will have a speed given by (tyre radius) * (wheel angular speed).

Look now at the contact area.
Radius is shorter there: the distance from the wheel hub is less. The local radius will change along the lenght of the contact patch: it will start as the unloaded wheel radius at the edges of the patch and reach a minimum at the patch's center, on the vertical of the wheel hub.

Now, look at what happens to the peripheral speed along the contact patch.
It will change: the angular speed is the same everywhere, yet the local radius changes.

The tyre is rolling, and it "sees" the road moving at a certain speed under the contact patch. This road speed is the same everywhere. The contact patch speed is not the same everyewhere. So the rubber has a tendence to local sliding.

This local sliding is counteracted by local friction force.
Local friction force is dependent on local pressure (it changes along the contact patch).

In a part of the contact patch, called adherence area, this friction force will be enough to prevent sliding.
It will keep the rubber at the same speed of the road even if it "wants" to slip.

In this adherence area , the standing friction coefficient will be acting.
In other parts of the contact area, friction will not be enough to prevent sliding. This part will be therefore called "sliding area". In the sliding area, sliding friction coefficient will apply.

So you have divided the contact patch in an high friction adherence area and a low friction sliding area.
The smaller the sliding area is, the better overall tyre grip you have.

So sliding = bad.

But what caused sliding ?
If you remember, it was tyre deformation.
And what caused tyre deformation?
Vertical load.
So increasing the vertical load increased the contact patch length, increased tyre deformation, which caused increased sliding, which increased sliding area, which decreased grip.

This explains also why wider tyres are usually better.
A wider tyre may have the same contact patch area, for less contact patch length. So its deformation will be less ( that is, you need to cut a smaller arc from the tyre).
Sliding will be decreased and overall grip will improve.



As I said, there are countless simplifications in this, but I hope it conveys the general idea.

[mat1]

gentlemen,

I am impressed, you have given me a lot of information.

Thanks.

What I gather is:

1. yes, Coulomb doesn't hold for tire grip.
2. we do not know for sure why.

It could be adhesion, it could be contact pach deformation (the explanation of paolo made sense to me), it could be heat, something else, a combination. or is there a definitive explanation?

Does this mean for example the tire companies do not exactly know either?

mat1

[shaun979]

Does anyone have a real life experience of running a tire in its negative load sensitivity range? Or does that occur very rarely? I've only heard it mentioned before and it's not covered in the usual books.

[Paolo]

Mat1,
Coulomb does not fully apply to tyres, as you said, yet I think we know why.
Part of the reason is that tyre rubber acts in some respects as glue, especially in race compounds.
Look at what Greg said: tyres give a non zero lateral force even with zero vertical load.
That's one of the reasons why race sim programmers tinker around with tyre models, adding low speed components to the Pacejka equations.


There are many other reasons, but it is not black magic, just a tad complicated.
Of course this is science: mathemathical representations change, ideas are superseded, yet the current interpretation is quite satisfying (as Tolemaeus said looking at his Solar System model....).
The "glue" effect is currently the most poorly modeled, because it depends a lot on temperature transients, quite difficult to record. In general ,modeling of tyre transients is an ongoing challenge.

[mat1]

Paolo,

Thanks, clear.

Do you think it is possible to estimate how big the "glue" effect is in relation to the Coulomb-effect, and the "sliding-effect'you explained, for tet us say a normal street tire and a tire used for dry weather racing? Just approximately?

mat1

[Paolo]

Let's play Bayes...

For a street tyre in normal operating conditions,

my guesstimates might be zero glue effect, adherence coefficients ranging from 0.7 to 0.95 with 0.7 representing full sliding (say locked tyre under braking) and 0.95 a condition with a lot of adherence area in the contact patch, say 70% (not the whole contact patch, this is impossible).
I am especially dubious of that 70%, it belongs to the *wild* guesstimates category.

In racing conditions?
I won't dare say anything.
A street tyre can literally melt when racing; its performance can vary dramatically with consumption.
If the tyre is shaven (reduced rubber thickness), probably it will not worsen beyond a 10% with consumption, but that's in the *wild* ballpark again.

I suggest you take into account estimates from others here too, if there will be offers.

[mat1]

Originally posted by Paolo
Contact patch size is load dependant for given tyre dimensions and pressure.

In this scenario, increasing the load will (in most situations, and on non-wet, non-dirt roads) decrease the adherence coefficient.

Caution: simplified explanation ahead.



Preliminary note: what follows is a story of friction coefficients. Friction coefficient is a local property of materials and shall not be confused with adherence coefficient, a global property of the tyre.
A given material on a given surface has ( I warned I was going to simplify...) two possible friction coefficients: a standing friction coefficient, acting when there is no relative motion, and a sliding friction coefficient.
Standing friction coefficient is higher than sliding friction coefficient.






Think of the tyre away from the ground. It is a circle. Now put it on the road, load it.

It will change its shape, flattening an area. This is the contact patch. Tyre deformation is quite complex, but in first approximation you can visualize a loaded tyre as a circle with a section cut off and made straight.

Now, make the tyre roll.

Each point of the tyre periphery, except the contact area, will have a speed given by (tyre radius) * (wheel angular speed).

Look now at the contact area.
Radius is shorter there: the distance from the wheel hub is less. The local radius will change along the lenght of the contact patch: it will start as the unloaded wheel radius at the edges of the patch and reach a minimum at the patch's center, on the vertical of the wheel hub.

Now, look at what happens to the peripheral speed along the contact patch.
It will change: the angular speed is the same everywhere, yet the local radius changes.

The tyre is rolling, and it "sees" the road moving at a certain speed under the contact patch. This road speed is the same everywhere. The contact patch speed is not the same everyewhere. So the rubber has a tendence to local sliding.

This local sliding is counteracted by local friction force.
Local friction force is dependent on local pressure (it changes along the contact patch).

In a part of the contact patch, called adherence area, this friction force will be enough to prevent sliding.
It will keep the rubber at the same speed of the road even if it "wants" to slip.

In this adherence area , the standing friction coefficient will be acting.
In other parts of the contact area, friction will not be enough to prevent sliding. This part will be therefore called "sliding area". In the sliding area, sliding friction coefficient will apply.

So you have divided the contact patch in an high friction adherence area and a low friction sliding area.
The smaller the sliding area is, the better overall tyre grip you have.

So sliding = bad.

But what caused sliding ?
If you remember, it was tyre deformation.
And what caused tyre deformation?
Vertical load.
So increasing the vertical load increased the contact patch length, increased tyre deformation, which caused increased sliding, which increased sliding area, which decreased grip.

This explains also why wider tyres are usually better.
A wider tyre may have the same contact patch area, for less contact patch length. So its deformation will be less ( that is, you need to cut a smaller arc from the tyre).
Sliding will be decreased and overall grip will improve.

Paolo,

Your explanation is clear to me, but now I am stuck with a question.

Given a tire and given a specific vertical load, it seems to me a higher pressure in he tire is better than a lower pressure, because the contact patch will be smaller in the longitudinal axis, so there will be less deformation, en the "sliding effect" you describe wil be less, so more grip.

But on the other hand, usually the assumption is: for grip, try a lower pressure, so you get a larger contact area, which is supposed to give some additional grip.

Do you agree with my reasoning?

mat1

[phantom II]

As you can tell, the subject is quite complex. A number of car magazines are testing the Vette Z06 along side the Nissan GTR and other supercars such as Turbo Porsche Vipers etc. Only Road and Track mentions the sticky tires of the GTR. This car is not available yet on this market but I doubt they will have the same tires. The GTR has Dunlop SP Sport 600 P255 40RF 20 100w frnt and P285 35RF 20 100w rr. Watch for 6000 mile tire life on the GTR the same as the Honda NSX. The Vette has Run Flat Goodyear P275 35 ZR 18 87Y frnt and P325 30ZR 19 84Y rr. The Turbo Porsche has Michelin Pilot P235 35ZR 19 87Y frnt and P305 30ZR 19 102Y rr.
The cars weigh 3960-3350 and 3710 resptvly. The Vette TW rating is 220 and the Porsche is 160. The GTR are not DOT tires on the test cars. Even with 220, you are lucky to get 11000 miles out of the Good Years.
If they all used the same tire, the results would be completely different. The new ZR1 Vette will have Michelin Pilots 285 30R 19 frnt and 335 25R 20 rr. With 160 TW rating.
I changed my Z06 tires from the run flat Good Year F1run flats s to the Michelin Pilot. The wheel alignment had to be revised which included more toe in at the rear and 1’ negative camber all round. The difference was like night and day. Now I have Good Year slicks and down force and I can’t tell you what it’s like because I’m too frightened to exploit them.
The long life radials that Sears and Pep Boys sell have 500 and 600 TW ratings. They guarantee 50 and 60 thousand mile life. It is impossible to stop on these tires especially in the wet. The Sears radial is made by Michelin. They should be outlawed.
These ratings are mandated by DOT and I’m sure that there is some fudging going on including their actual tire size. F1 Michelin and Bridgestone???? Tread wear rating, Temperature rating and Traction rating are separate from speed and load ratings.
New BMWs come with Michelins with 400 Tread wear ratings but A Traction ratings. The tread wear ratings and traction ratings are increasing but generally this is not the case. I am not sure how these ratings are determined

Originally posted by NRoshier
Thanks Greg.
P2 you seem to ascribe to the TW = grip theory. The TW notation seems to be big in the US but I have not noticed it anywhere else. Is there any testing done to support the TW = grip thought?

[phantom II]

http://www.safercar....0002fd17898RCRD

[imaginesix]

Originally posted by NRoshier
Thanks Greg.
P2 you seem to ascribe to the TW = grip theory. The TW notation seems to be big in the US but I have not noticed it anywhere else. Is there any testing done to support the TW = grip thought?

Treadwear as indicated by a street tire's UTQG (Uniform Tire Quality Gradient) rating does generally relate to tire grip. UTQG rates tires in three ways, "Traction" with ratings from AA to C, "Temperature" rated A to C and "Treadwear". Treadwear is rated in terms of wear relative to a baseline tire wear set at 100. So a 200 treadwear rating should indicate that a tire will wear twice as slowly as the baseline tire.

The traction rating is a silly test which involves dragging a locked tire across a wet surface and assessing it's grip somehow. The 'AA' rating was added to A B and C in the late 90's because tire technology had evolved so much that there was a quantifyable difference that deserved to be distinguished from regular 'A' traction ratings with a rating of it's own. However in my experience tires with 'AA' traction are too far biased towards wet weather grip, to the detriment of dry weather grip. that is why an 'A' rating is preferable for outright performance in the dry.

The temperature rating is a measure of a tire's ability to dissipate heat, so obviously you want the best heat dissipation for performance use, and that is an 'A' rating.

So, assuming you have found a tire with traction and temperature ratings of 'A' 'A' then it is worth finding a tire with as high treadwear as possible (low treadwear rating) as that will give you a performance-oriented tire, so you can be relatively assured that if treadwear was compromised by design it was done for the sake of improved grip. A 20 treadwear rating is an out-and-out DOT race tire.

Keep in mind though that different OEs treadwear ratings are not necessarily comparable. Though they should all wear at a comparable rate when treadwear is measured at 100, the scale for wear is not clearly defined beyond that so one tire manufacturer's treadwear rating of 200 may be equal to another's treadwear rating at 300.

[Paolo]

Originally posted by mat1


Paolo,

Your explanation is clear to me, but now I am stuck with a question.

Given a tire and given a specific vertical load, it seems to me a higher pressure in he tire is better than a lower pressure, because the contact patch will be smaller in the longitudinal axis, so there will be less deformation, en the "sliding effect" you describe wil be less, so more grip.

But on the other hand, usually the assumption is: for grip, try a lower pressure, so you get a larger contact area, which is supposed to give some additional grip.

Do you agree with my reasoning?

mat1

Usually there is an optimum pressure for a tyre. Exceeding or not reaching it will result in abnormal (meant as "not ideal") carcass shape, and abnormal pressure distribution in the contact patch. Wear and temperatures will not be optimal too. So, either way, you lose performance.

[Greg Locock]

I think Reimpell and Stoll have some measured data for this. I know I have, but I can't show it to you.

This:

If you are in the normal range of a tire, and you increase the pressure, then the cornering stiffness is increased near zero slip, and the max Fy is unchanged. At higher loads you will develop more cornering stiffness and ultimate lateral force as you increase the pressure. At low loads you may see a slight drop in both cornering stiffness and max Fy as you increase the pressure.

If you decrease the tire pressure from nominal then the ultimate Fy drops in all 3 cases, and, oddly, the cornering stiffness increases in all three cases.


is an accurate summary of the test data I have.

[mat1]

Originally posted by Greg Locock


If you decrease the tire pressure from nominal then the ultimate Fy drops in all 3 cases, and, oddly, the cornering stiffness increases in all three cases.

Hmmm, fascinating, not what I expected. Thanks.

mat1

[Paolo]

Originally posted by Greg Locock
I think Reimpell and Stoll have some measured data for this. I know I have, but I can't show it to you.

This:

If you are in the normal range of a tire, and you increase the pressure, then the cornering stiffness is increased near zero slip, and the max Fy is unchanged. At higher loads you will develop more cornering stiffness and ultimate lateral force as you increase the pressure. At low loads you may see a slight drop in both cornering stiffness and max Fy as you increase the pressure.

If you decrease the tire pressure from nominal then the ultimate Fy drops in all 3 cases, and, oddly, the cornering stiffness increases in all three cases.


is an accurate summary of the test data I have.

Interesting, Greg.
That would give a base to the old piece of wisdom stating that to err on the safe side it is better to increase pressure than to decrease it.
It would also be consistent with the "contact patch" explanation.
Of course, wear will worsen, but it will in case of lower pressure too.

[mat1]

Originally posted by Paolo


Interesting, Greg.
That would give a base to the old piece of wisdom stating that to err on the safe side it is better to increase pressure than to decrease it.
It would also be consistent with the "contact patch" explanation.
.

What do you mean here withe the "contact patch" explanation?

mat1

[Paolo]

Originally posted by mat1


What do you mean here withe the "contact patch" explanation?

mat1

My post about adherence area, friction etc.

[mat1]

OK, I see.

mat1

[Aubwi]

Some thoughts I've had on the subject. Correct me if I'm wrong, because I haven't seen any hard data to back them up...

Load sensitivity is dependent on the ability of the contact patch area to grow as more load is applied.

A lower-pressure tire will have greater load sensitivity than a higher-pressure tire of the same construction because at lower-pressure the contact patch more rapidly loses its ability to grow. This growth rate approaches zero when the tire is fully compressed down to the wheel rims. But a lower-pressure tire will still have better grip at sufficiently low loads.

For a pneumatic tire, coefficient of friction increases as load is reduced. I suspect it actually approaches infinity at zero load, because of the glue-like effects of the rubber compound, but I understand this is where I'm really stretching credibility. Just remember "approaches" means it never quite gets there.

[Aubwi]

Originally posted by Fat Boy
Competitive tires in any class of racing I've ever known about was at the maximum size allowable by the rules. There may be exceptions, but I'm just not aware of them. I think the maximum grip probably still continues to rise as the tire size goes up, but the downsides would be camber issues, aero drag, rolling resistance, etc. In many cases, the tires would have to be so big as to be difficult or impossible to package before they would actually slow the car down.

The only series I can think of where the rules weren't the limiting factor was Can Am. According to one driver, the circumference of the front tires was limited by frontal area aerodynamic considerations for the front fenders, and their width was simply limited by the ability of the driver to turn the steering wheel with such an enormous contact patch. The front tires were basically shaped like steamrollers. Wider than they were tall.

[Ogami musashi]

around 2004 some F1 teams studied the possibility to have low width front tyres to get an aero advantage but the decrease in tyre's grip was greater than the drag reduction.


Also if you increase the coeficient of friction you generaly make the car less responsive (especially if you increase contact area) so there's a limit especially for F1 cars that need to change direction quickly.

[OfficeLinebacker]

Originally posted by Aubwi


The only series I can think of where the rules weren't the limiting factor was Can Am. According to one driver, the circumference of the front tires was limited by frontal area aerodynamic considerations for the front fenders, and their width was simply limited by the ability of the driver to turn the steering wheel with such an enormous contact patch. The front tires were basically shaped like steamrollers. Wider than they were tall.

Crazy. Can you find any pics from that era?
j

[Aubwi]

Originally posted by OfficeLinebacker


Crazy. Can you find any pics from that era?
j

I read about it in some interview with a driver I think. Can't find it now. Not sure if he was talking about the Porsche 917/30 , but it appears to fit the bill. Hard to find a good pic with the bodywork off, but you can see how wide the fenders are.