Tyre Width And Grip

[redgriff500]

I am aware that by increasing tyre width from say 195 to 225 will change the shape of the contact patch on the road.

But is there any quick and easy formula to work out what it will actually do regarding grip levels - both whilst cornering and on acceleration.

(Assuming the same type of tyre)

IIRC it won't improve cornering speeds but will improve grip upon acceleration thus reducing wheelspin but by how much ?

[Anh]

redgriff500 said:

I am aware that by increasing tyre width from say 195 to 225 will change the shape of the contact patch on the road.

But is there any quick and easy formula to work out what it will actually do regarding grip levels - both whilst cornering and on acceleration.

(Assuming the same type of tyre)

IIRC it won't improve cornering speeds but will improve grip upon acceleration thus reducing wheelspin but by how much ?

More tyre width with the same rim wheel size will only increase contact patch area when there is extra load to induce it.

Most cars with OEM tyres are sized as big as they can be, and in the case of hot hatches, too big - so you have to consider what changes you making to the car itself before going wider. If you have a light RWD car that weighs only 500 kilos at the front, and decide to put on 225's on, it may be too wide due to insufficent load for it to provide any extra grip over 195's

There isnt a formula to work out grip levels vs width as there is alot more to it than that, also that tyre performance varies between manufacturer to manufacturer, and is altered through temperature and tyre pressure.

[redgriff500]

I'm increasing the power of a RWD car from 130bhp to 250bhp, torque is more than doubling to 260ftlbs.

Std is 195's on a 6" rim I was thinking of going to 225 on a 7 or 8" rim.

It has an LSD but no electronic aids will be fitted.

[saaby93]

Lots here
http://www.carbibles.com/tyre_bible_pg2.html
It says its all to do with wider tyres having softer sidewalls which heat up quicker

[Anh]

redgriff500 said:

I'm increasing the power of a RWD car from 130bhp to 250bhp, torque is more than doubling to 260ftlbs.

Std is 195's on a 6" rim I was thinking of going to 225 on a 7 or 8" rim.

It has an LSD but no electronic aids will be fitted.

How much does your car weigh, and what is the weight distribution?

[redgriff500]

Approx 1150kg pretty much 50:50 distribution.

[Anh]

saaby93 said:

Lots here
http://www.carbibles.com/tyre_bible_pg2.html
It says its all to do with wider tyres having softer sidewalls which heat up quicker

I would not agree with that, grip is obtained from load and contact patch area - temperature is a by product and should not be used as a performance indicator in such a imperical way.

[Anh]

redgriff500 said:

Approx 1150kg pretty much 50:50 distribution.

Ok, this is just a rule of thumb as I do not know the relative roll resistance and weight transfer of the front and rear track.

Stock 195's at the front, 225's at the rear should work well. The width at the rear wont stop the wheels from spinning when you gun it in a straight line, what the width with do is provide the same grip as the front end but at a smaller slip angle when applying some of the power during a corner, power induced oversteer will be minimised.

Wheel spin in a straight line is normal on a light weight RWD car with a bit of power, you just have to learn to modulate the throttle.

To get more traction, you would have to play around with dampers and differentials I'm afraid.

Edited by Anh on Sunday 1st November 00:18

Edited by Anh on Sunday 1st November 00:19

[GravelBen]

redgriff500 said:

I'm increasing the power of a RWD car from 130bhp to 250bhp, torque is more than doubling to 260ftlbs.

Std is 195's on a 6" rim I was thinking of going to 225 on a 7 or 8" rim.

It has an LSD but no electronic aids will be fitted.

MX5 + turbo?

[saaby93]

Anh said:

saaby93 said:

Lots here
http://www.carbibles.com/tyre_bible_pg2.html
It says its all to do with wider tyres having softer sidewalls which heat up quicker

I would not agree with that, grip is obtained from load and contact patch area - temperature is a by product and should not be used as a performance indicator in such a imperical way.

Why not - it says much the same as you were saying above
For a given load if you increase the width of a tyre the contact patch stays the same so the patch becomes shorter front to back.
2CVs were noted for having very narrow tyres so they could cross ploughed fields. Wide tyres would have spun on the surface.

You see the problem with modern light weight over tyred sports cars. The slightest slippery road surface or leaves and they're off checking out the ditch.

[Mr Whippy]

Anh said:

saaby93 said:

Lots here
http://www.carbibles.com/tyre_bible_pg2.html
It says its all to do with wider tyres having softer sidewalls which heat up quicker

I would not agree with that, grip is obtained from load and contact patch area - temperature is a by product and should not be used as a performance indicator in such a imperical way.

Grip is a function of load vs coefficient of friction.

Contact patch pressure is important for temperature because low pressure means less patch distortion for transmitting any unit of force = less heat generated.

Temp is important for coefficient of friction.

Make lots of heat distorting a tyre lots as it puts the force through the patch to the floor, and the coefficient of friction of the tyre/floor interface gets bad quickly.

I think which heat you want is confusing. A road tyre isn't designed to run really hot, because the materials are not that soft. Running a road tyre really hot locally (at the contact patch) will make the grip poor.

Think of it like a rubber on a pencil. Rub out with a tiny tip and it'll get hot quickly, but imagine if it gets too hot it's more slippy.
Now rub out with a bigger rubber and it won't get as hot, but can do the same amount of work.

Neither can do more work than the other with the load/coefficient of friction they have to play with (assume both the same in simple terms), but keeping the coefficient of friction good for longer is easier on the bigger rubber.

That said, too low contact patch pressure is bad too, aquaplaning, snow etc, are all problems with too big tyres, as would be running too cool potentially (not getting warm at contact patch through deforming it)

Problem is finding the right trade-off from overheating too small a tyre vs under-loading a big one.

Dave

Edited by Mr Whippy on Sunday 1st November 00:34

[saaby93]

carbibles said:

If there's one question guaranteed to promote argument and counter argument, it's this : do wide tyres give me better grip?
Fat tyres look good. In fact they look stonkingly good. In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car. Contrary to what you might think (and to what I used to think), bigger contact patch does not necessarily mean increased grip. Better yet, fatter tyres do not mean bigger contact patch. Confused? Check it out:

Pressure=weight/area.

That's about as simple a physics equation as you can get. For the general case of most car tyres travelling on a road, it works pretty well. Let me explain. Let's say you've got some regular tyres, as supplied with your car. They're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre is taking about a quarter of your car's weight - in this case 375Kg. In metric, 30psi is about 2.11Kg/cm².
By that formula, the area of your contact patch is going to be roughly 375 / 2.11 = 177.7cm² (weight divided by pressure)
Let's say your standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That means the tread width is 18.5cm side to side. So your contact patch with all these variables is going to be about 177.7cm² / 18.5, which is 9.8cm. Your contact patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back where it sits 'flat' on the road.
Still with me? Great. You've taken your car to the tyre dealer and with the help of my tyre calculator, figured out that you can get some swanky 225/50R15 tyres. You polish up the 15inch rims, get the tyres fitted and drive off. Let's look at the equation again. The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same : 177.7cm²
However you now have wider tyres - the tread width is now 22.5cm instead of 18.5cm. The same contact patch but with wider tyres means a narrower contact area front-to-back. In this example, it becomes 177.7cm² / 22.5, which is 7.8cm.


Imagine driving on to a glass road and looking up underneath your tyres. This is the example contact patch (in red) for the situation I explained above. The narrower tyre has a longer, thinner contact patch. The fatter tyre has a shorter, wider contact patch, but the area is the same on both.

And there is your 'eureka' moment. Overall, the area of your contact patch has remained more or less the same. But by putting wider tyres on, the shape of the contact patch has changed. Actually, the contact patch is really a squashed oval rather than a rectangle, but for the sake of simplicity on this site, I've illustrated it as a rectangle - it makes the concept a little easier to understand. So has the penny dropped? I'll assume it has. So now you understand that it makes no difference to the contact patch, this leads us on nicely to the sticky topic of grip.

The area of the contact patch does not affect the actual grip of the tyre. The things that do affect grip are the coefficient of friction of the rubber compound and the load on the tyre. As far as friction is concerned, the formula is relatively simple - F=uN, where F is the frictional force, N is the Normal force for the surfaces being pressed together and u is the coefficient of friction. In the case of a tyre, the Normal force basically stays the same - mass of the car multiplied by gravity. The coefficient of friction also remains unchanged because it's dependent on the two surfaces - in this case the road and the tyre's rubber.
The coefficient of friction is in part determined by the rubber compound's ability to 'key' with the road surface at a microscopic level.


This explains why you can slide in a corner if you change road surface - for example going from a rough road to a smooth road, or a road surface covered in rain and diesel (a motorcyclist's pet peeve). The slide happens because the coefficient of friction has changed.

So do wider tyres give better grip?
If the contact patch remains the same size and the coefficient of friction and frictional force remain the same, then surely there is no difference in performance between narrow and wide tyres? Well there is but it has a lot to do with heat transfer. With a narrow tyre, the contact patch takes up more of the circumference of the tyre so for any given rotation, the sidewall has to compress more to get the contact patch on to the road. Deforming the tyre creates heat. With a longer contact patch and more sidewall deformation, the tyre spends proportionately less time cooling off than a wider tyre which has a shorter contact patch and less sidewall deformation. Why does this matter? Well because the narrower tyre has less capacity for cooling off, it needs to be made of a harder rubber compound in order to better resist heating in the first place. The harder compound has less mechanical keying and a lower coefficient of friction. The wider tyres are typically made of softer compounds with greater mechanical keying and a higher coefficient of friction. And voila - wider tyres = better grip. But not for the reasons we all thought.

What about lateral force in cornering?
In terms of the lateral force applied to a tyre during cornering, you eventually come to a point where slip angle becomes important. The plot below shows an example of normalised lateral force (in Kg) versus slip angle (in degrees). Slip angle is best described as the difference between the angle of the tyres that you've set by steering, and the direction in which the tyres actually want to travel. As you corner the lateral force increases on your tyres, and at some point, the lateral force is going to overcome the mechanical grip of the tyres and that point is defined by the peak slip angle, as shown in the graph. ie. there comes a point at which no matter how much vertical load is applied to the tyre (from the vehicle weight), it's going to be overcome by the lateral force and 'break away' and slip. So why do wider tyres perform better when cornering? Well apart from the softer rubber compound giving better mechanical keying and a higher coefficient of friction, they have lower profile sidewalls. This makes them more resistant to deforming under lateral load, resulting in a more predictable and stable contact patch. In other words, you can get to a higher lateral load before reaching the peak slip angle.


In reality, trying to figure this out using static examples and reading some internet hack's website is all but impossible because what's really important here is dynamic setup. In reality the contact patch is effectively spinning around your tyre at some horrendous speed. When you brake or corner, load-transfer happens and all the tyres start to behave differently to each other. This is why weight transfer makes such a difference the handling dynamics of the car. Braking for instance; weight moves forward, so load on the front tyres increases. The reverse happens to the rear at the same time, creating a car which can oversteer at the drop of a hat. The Mercedes A-class had this problem when it came out. The load-transfer was all wrong, and a rapid left-right-left on the steering wheel would upset the load so much that the vehicle lost grip in the rear, went sideways, re-acquired grip and rolled over. (That's since been changed.) The Audi TT had a problem too because the load on it's rear wheels wasn't enough to prevent oversteer which is why all the new models have that daft little spoiler on the back.

[Anh]

saaby93 said:

Why not - it says much the same as you were saying above
For a given load if you increase the width of a tyre the contact patch stays the same so the patch becomes shorter front to back.
2CVs were noted for having very narrow tyres so they could cross ploughed fields. Wide tyres would have spun on the surface.

You see the problem with modern light weight over tyred sports cars. The slightest slippery road surface or leaves and they're off checking out the ditch.

Wide tyres will not give you any more contact patch area unless there is extra load.

The only way to get a wider tyre to increase contact patch area with the same load is to lower the tyre pressure, but to do this means less cornering power.

What we want for more grip is to load the larger tyre one way or another, only then can we exploit the extra grip. Otherwise the wider tyre will simply just alter the balance of the car with the difference in slip angle compared to the narrower tyre.

Edited by Anh on Sunday 1st November 00:42

Edited by Anh on Sunday 1st November 01:06

[Anh]

Mr Whippy said:

Grip is a function of load vs coefficient of friction.

Contact patch pressure is important for temperature because low pressure means less patch distortion for transmitting any unit of force = less heat generated.

Temp is important for coefficient of friction.

Make lots of heat distorting a tyre lots as it puts the force through the patch to the floor, and the coefficient of friction of the tyre/floor interface gets bad quickly.

I think which heat you want is confusing. A road tyre isn't designed to run really hot, because the materials are not that soft. Running a road tyre really hot locally (at the contact patch) will make the grip poor.

Think of it like a rubber on a pencil. Rub out with a tiny tip and it'll get hot quickly, but imagine if it gets too hot it's more slippy.
Now rub out with a bigger rubber and it won't get as hot, but can do the same amount of work.

Neither can do more work than the other with the load/coefficient of friction they have to play with (assume both the same in simple terms), but keeping the coefficient of friction good for longer is easier on the bigger rubber.

That said, too low contact patch pressure is bad too, aquaplaning, snow etc, are all problems with too big tyres, as would be running too cool potentially (not getting warm at contact patch through deforming it)

Problem is finding the right trade-off from overheating too small a tyre vs under-loading a big one.

Dave

Edited by Mr Whippy on Sunday 1st November 00:34

Sorry what are you trying to acheive with this?

I hope you understand what co-efficent of friction actually is...

Edited by Anh on Sunday 1st November 01:08

[redgriff500]

Yes 195 front and 225 rear was what I was thinking really (for no particular reason other than it seemed right and TVR's with similar power run 225's yet my car tramlines terribly with more than 195's on the front)

Saaby93 - I read that which is pretty much what I remembered except I thought that with the wider contact patch (ie involving more road surface) meant better grip for acceleration / braking which he doesn't seem to cover.

Also he mentions wider tyres have a better slip angle due to the lower profile but mine will effectively have the same profile (in height albeit a lower percentage, say 225/40/15) as I want to keep the wheel size and rolling radius similar.

[Anh]

redgriff500 said:

Yes 195 front and 225 rear was what I was thinking really (for no particular reason other than it seemed right and TVR's with similar power run 225's yet my car tramlines terribly with more than 195's on the front)

It's really just a guide line, you have to remember that the tyre size between front and rear only make sense when applying considerable power, the wider tyres run at lower slip angle compared to the 195 front, but applying power increases the slip angle of the rear tyre. What you could experience is some mild understeer at no-throttle or low throttle, then neutral or power oversteer when applying more throttle.

You could consider softening the front suspension (smaller ARB or softer damper) if you want more traction at the rear when accelerating as this will load the rear tyres more quickly, but may mean even more oversteer on throttle.

Edited by Anh on Sunday 1st November 01:11

Edited by Anh on Sunday 1st November 01:13

[Mr Whippy]

Anh said:

Mr Whippy said:

Suff

Sorry what are you trying to acheive with this?

I'm agreeing with you on one hand, and then disagreeing on the other.

Heat is important.

F = mu*N

N is given, pretty much, and mu ~ temp, and F is your grip by and large.

So you have one thing you can control there, temp.

Optimising the temp of the tyre by choosing the right size tyre for application and mounting for it (wheel size), means you optimise mu, and so grip.


Dave

[Anh]

Mr Whippy said:

Anh said:

Mr Whippy said:

Suff

Sorry what are you trying to acheive with this?

I'm agreeing with you on one hand, and then disagreeing on the other.

Heat is important.

F = mu*N

N is given, pretty much, and mu ~ temp, and F is your grip by and large.

So you have one thing you can control there, temp.

Optimising the temp of the tyre by choosing the right size tyre for application and mounting for it (wheel size), means you optimise mu, and so grip.


Dave

This doesnt really answer any questions. Temperature is a by-product of the tyre interaction with the surface, (wheel spin, scrub or grip) you are looking at tackling the problem backwards.

All anyone needs to be concerned with is that the tyre will provide the required cornering power within the loads that the car is capable of producing on the tyre, and the relative slip angle generated to that of the tyre on the opposite track. This means the smallest tyre required for the job - unless you like extra unsprung weight and frontal area drag.

Any tyre will overheat if it is overloaded, wide or narrow, what is important is the vertical loads to the tyre that the car can produce. The tyres itself must be capable of carrying as much of the car's mass into a corner (cornering force) within the maximum available load that car can provide.

If the tyre is overheating and losing grip, then it is obvious that the tyre is overloaded, the tyre co-efficient of friction is diminishing, gone over the curve etc etc

Which should not be the case with a 1150 kilo 50:50 TVR on 195 tyres at the front, unless it has 200 kilos of downforce at the front axle!

Edited by Anh on Sunday 1st November 01:54

Edited by Anh on Sunday 1st November 01:55

[Mr Whippy]

Anh said:

Mr Whippy said:

Anh said:

Mr Whippy said:

Suff

Sorry what are you trying to acheive with this?

I'm agreeing with you on one hand, and then disagreeing on the other.

Heat is important.

F = mu*N

N is given, pretty much, and mu ~ temp, and F is your grip by and large.

So you have one thing you can control there, temp.

Optimising the temp of the tyre by choosing the right size tyre for application and mounting for it (wheel size), means you optimise mu, and so grip.


Dave

This doesnt really answer any questions. Temperature is a by-product of the tyre interaction with the surface, (wheel spin, scrub or grip) you are looking at tackling the problem backwards.

All anyone needs to be concerned with is that the tyre will provide the required cornering power within the loads that the car is capable of producing on the tyre, and the relative slip angle generated to that of the tyre on the opposite track. This means the smallest tyre required for the job - unless you like extra unsprung weight and frontal area drag.

Any tyre will overheat if it is overloaded, wide or narrow, what is important is the vertical loads to the tyre that the car can produce. The tyres itself must be capable of carrying as much of the car's mass into a corner (cornering force) within the maximum available load that car can provide.

If the tyre is overheating and losing grip, then it is obvious that the tyre is overloaded, the tyre co-efficient of friction is diminishing, gone over the curve etc etc

Which should not be the case with a 1150 kilo 50:50 TVR on 195 tyres at the front, unless it has 200 kilos of downforce at the front axle!

I don't disagree with that

Optimising the temp of the tyre by choosing the right size tyre for application, pressure, and mounting for it (wheel size), means you optimise mu in the last place it can be optimised, and so optimise grip.

Ie, as you kinda mentioned, the narrowest, smallest tyre, at possibly the lowest pressure, mounted ideally, that doesn't overheat for intended purposes, is all you need.

So assuming you are doing everything else right, heat is the concern imho. If any given tyre is mounted and inflated appropriately for the job at hand, and it overheats doing intended job, then it's not right for the job, and you might go wider/bigger (assuming you have optimised as best you can alignment etc)

If we do silly things like run a really wide tyre on a light car at high pressure, we are in for trouble to start with, with mu moving around because of things we have full control over and can change easily.



Tis a complex subject, I don't believe I understand it amazingly well, I want to buy Pacejka's book shortly to try improve my understanding more, but I do think that if you are managing heat within the bounds of your tyre and what you want it to do, you are 99% of the way there
So heat is important. Doesn't really answer the question though I suppose, oops

Dave

[Anh]

Mr Whippy said:

I don't disagree with that

Optimising the temp of the tyre by choosing the right size tyre for application, pressure, and mounting for it (wheel size), means you optimise mu in the last place it can be optimised, and so optimise grip.

Ie, as you kinda mentioned, the narrowest, smallest tyre, at possibly the lowest pressure, mounted ideally, that doesn't overheat for intended purposes, is all you need.

So assuming you are doing everything else right, heat is the concern imho. If any given tyre is mounted and inflated appropriately for the job at hand, and it overheats doing intended job, then it's not right for the job, and you might go wider/bigger (assuming you have optimised as best you can alignment etc)

If we do silly things like run a really wide tyre on a light car at high pressure, we are in for trouble to start with, with mu moving around because of things we have full control over and can change easily.



Tis a complex subject, I don't believe I understand it amazingly well, I want to buy Pacejka's book shortly to try improve my understanding more, but I do think that if you are managing heat within the bounds of your tyre and what you want it to do, you are 99% of the way there
So heat is important. Doesn't really answer the question though I suppose, oops

Dave

Unfortunately tyre makers do not publish pretty simple to read graphs for us average Joe's to work out which one is best and at what conditions are required to get the grip we want and at what point does the tyre's co-effiecient of friction starts to deteriorate.

I am not saying that temperature is of no importance, after all the tyre compound must reach its optimal temperature to deliver the potential cornering force over a given load - what I strongly disagree on is the notion that a wider tyre provides more grip because it overheats less - which is what one of the earlier posts made by saab93 was asserting.

Wider tyres can grip more if it generates a larger contact patch area, assuming equal tyre pressures. And to do this more tyre load is needed from the car itself, be this from more weight transfer, downforce or more weight.


Another very important and often very overlooked aspect of tyre widths is the slip angles they run at to hold the car's mass during a corner. A narrow tyre often enough must run at a higher slip angle to grip the same as moderately wider tyre, so assuming 50:50 weight distribution, placing different width tyres front and rear will change the balance of the car, i.e it will create understeer or oversteer.

Porsche is an extreme example, huge rear weight bias + RWD on the rear tyres. Very wide tyres are required not because they grip more but because they provide the grip needed at a sensible slip angle compared to the sensibly sized front tyres. You'd hear alot of horror stories of 911's oversteering at high speed corners, this is due to the reasons mentioned above but made worse at high speed when 1) strong rear wing provides too much load to the rear wheels and increases slip angle even more, 2) you need to apply more power at high speeds to maintain the speed and thus increase slip angle even more, hence more oversteer.

Edited by Anh on Sunday 1st November 11:54

Edited by Anh on Sunday 1st November 11:55