The science behind tyre width for trackdays - who can help?
Discussion
Ray Luxury-Yacht said:
Dunno if this will help or not, but I used to race bikes, and have been a TD instructor too, with a bit of car experience thrown in...
With regard to tyres, there are several main things to consider. Size is only a small part of that equation. If you don't understand fully what I write below - then have a Google round about each point, to give you a greater understanding about the things I am discussing?
1) Assuming the tyres you are using can firstly deal satisfactorily with the transmission of power, especially out of corners - then one of the main considerations is suspension and wheel geometry. I might suggest that the vehicle's camber, castor and weight distribution have some of the greatest effects on both grip and traction, above all else. Secondary to that is whether you have a limited slip differential or not. Without this, it is likely that in a front-wheel-drive vehicle, the inside front wheel might spin up and waste power when accelerating out of a turn.
After all the many thousands of hours I have spent (wasted?
) at race tacks, I can confirm that one of the primary features of a good lap time, is effective acceleration out of turns, especially slower ones. It's (usually) more important than corner speed, and even late braking. The simple fact of the matter is that early traction and acceleration out of a turn will equal a higher top speed along the next corresponding straight. Amongst other things, this is one of the most important factors to consider when cutting lap times. Trust me on this.
2) Tyre pressures have a monstrous effect upon grip and traction too, taking the climate and track temperature into consideration. I bet you that if someone who didn't know what they were doing turned up at a track with a particular car, and I had the same car running smaller / thinner tyres, if I was given some basic setup information and track surface temperatures, then after a bit of testing I would find a suitable tyre air pressure on my car that would blitz the other car with bigger / wider tyres. Not only that, but often a smaller / thinner tyre running at optimum pressures for the track at that particular point can give the driver a better feel too, hence helping to cut lap times even further. Sometimes, compliance is everything - especially in the cold and the wet.
3) Before you go off on a wild goose chase for bigger / wider tyres, consider this: the primary driver of tyre grip and traction, once you have set optimum pressures and decided on compound and sidewall compliance - is the force with which the tyre's contact patch is pushed down into the running surface.
If you don't get that, then you need to think of what's happening at a microscopic level between rubber and ashphalt. Thinking back to those physics lessons at school, friction between two surfaces is created by the irregularities, force and compliance of the two materials. E.g. the tyre and the track have peaks and troughs in them invisible to the naked eye - but it is this interaction and 'keying in' of the two surfaces which creates friction.
You can't really change the track's characteristics, but you can influence the tyre's to create as much friction as possible. A softer more compliant tyre compound, or reduced pressures which allow more running surface deformity, CAN improve this friction co-efficient.
HOWEVER - and providing you have stuck with me - this is NOT ALWAYS improved by just fitting bigger / wider tyres. You might think that a bigger / wider contact patch would give you a greater amount of friction, grip and traction, right? Well, no - as per the physics, it depends upon the WEIGHT of the body (i.e. the car) which is pushing / forcing the tyre down into the track.
A car of a certain weight, is pushing every square inch of the contact patches of the tyres down into the track at a set pressure. Forgetting compounds and tyre pressures for a moment - this force is a set force which will not change, unless the car has some serious aero addenda like wings and spoilers. So, you are stuck with what you have.
If you fit wider tyres to the same car, with no other changes or modifications, expecting more grip and traction, then you may be disappointed. This is primarily because you are now presenting, for example, 20% more tyre contact patch to the track surface. However, the weight of the car which is pushing these bigger contact patches down into the track is the same - hence, you actually have a reduced pressure per square inch of contact patch than you had with thinner tyres.
Therefore, in many cases - overall, you have reduced the friction co-efficient available for grip and traction, despite the wider tyres - and will then suffer paradoxically slower lap times....which might even be compounded by less steering / braking feel, and even a greater propensity to aquaplaning in the wet.
Does that all make sense? Sorry for the long physics lesson reply - I've not long come home and was a bit bored, so thought I might offer all my thoughts on the subject, haha! I don't pretend to be an expert, and I may well get shot down in flames by others who know more - but all I am saying is that these basic principles helped me and my team in deciding upon what tyres to run, where and when.
HTH
Nice reply - re- wider possibly = less grip, you only have to look at Rally cars in snow/ice - they don't use wide wheels, they use quite narrow ones (albeit studded)With regard to tyres, there are several main things to consider. Size is only a small part of that equation. If you don't understand fully what I write below - then have a Google round about each point, to give you a greater understanding about the things I am discussing?
1) Assuming the tyres you are using can firstly deal satisfactorily with the transmission of power, especially out of corners - then one of the main considerations is suspension and wheel geometry. I might suggest that the vehicle's camber, castor and weight distribution have some of the greatest effects on both grip and traction, above all else. Secondary to that is whether you have a limited slip differential or not. Without this, it is likely that in a front-wheel-drive vehicle, the inside front wheel might spin up and waste power when accelerating out of a turn.
After all the many thousands of hours I have spent (wasted?
) at race tacks, I can confirm that one of the primary features of a good lap time, is effective acceleration out of turns, especially slower ones. It's (usually) more important than corner speed, and even late braking. The simple fact of the matter is that early traction and acceleration out of a turn will equal a higher top speed along the next corresponding straight. Amongst other things, this is one of the most important factors to consider when cutting lap times. Trust me on this.2) Tyre pressures have a monstrous effect upon grip and traction too, taking the climate and track temperature into consideration. I bet you that if someone who didn't know what they were doing turned up at a track with a particular car, and I had the same car running smaller / thinner tyres, if I was given some basic setup information and track surface temperatures, then after a bit of testing I would find a suitable tyre air pressure on my car that would blitz the other car with bigger / wider tyres. Not only that, but often a smaller / thinner tyre running at optimum pressures for the track at that particular point can give the driver a better feel too, hence helping to cut lap times even further. Sometimes, compliance is everything - especially in the cold and the wet.
3) Before you go off on a wild goose chase for bigger / wider tyres, consider this: the primary driver of tyre grip and traction, once you have set optimum pressures and decided on compound and sidewall compliance - is the force with which the tyre's contact patch is pushed down into the running surface.
If you don't get that, then you need to think of what's happening at a microscopic level between rubber and ashphalt. Thinking back to those physics lessons at school, friction between two surfaces is created by the irregularities, force and compliance of the two materials. E.g. the tyre and the track have peaks and troughs in them invisible to the naked eye - but it is this interaction and 'keying in' of the two surfaces which creates friction.
You can't really change the track's characteristics, but you can influence the tyre's to create as much friction as possible. A softer more compliant tyre compound, or reduced pressures which allow more running surface deformity, CAN improve this friction co-efficient.
HOWEVER - and providing you have stuck with me - this is NOT ALWAYS improved by just fitting bigger / wider tyres. You might think that a bigger / wider contact patch would give you a greater amount of friction, grip and traction, right? Well, no - as per the physics, it depends upon the WEIGHT of the body (i.e. the car) which is pushing / forcing the tyre down into the track.
A car of a certain weight, is pushing every square inch of the contact patches of the tyres down into the track at a set pressure. Forgetting compounds and tyre pressures for a moment - this force is a set force which will not change, unless the car has some serious aero addenda like wings and spoilers. So, you are stuck with what you have.
If you fit wider tyres to the same car, with no other changes or modifications, expecting more grip and traction, then you may be disappointed. This is primarily because you are now presenting, for example, 20% more tyre contact patch to the track surface. However, the weight of the car which is pushing these bigger contact patches down into the track is the same - hence, you actually have a reduced pressure per square inch of contact patch than you had with thinner tyres.
Therefore, in many cases - overall, you have reduced the friction co-efficient available for grip and traction, despite the wider tyres - and will then suffer paradoxically slower lap times....which might even be compounded by less steering / braking feel, and even a greater propensity to aquaplaning in the wet.
Does that all make sense? Sorry for the long physics lesson reply - I've not long come home and was a bit bored, so thought I might offer all my thoughts on the subject, haha! I don't pretend to be an expert, and I may well get shot down in flames by others who know more - but all I am saying is that these basic principles helped me and my team in deciding upon what tyres to run, where and when.
HTH
I wonder if there is some formula or graph that shows an optimal point where adding friction material (wider tyre) does not offset the lower down force per square cm - i.e. the point where you go too wide.
98elise said:
But for the same pressure, a wider tyre will not have a larger area (contact patch), it will have a wider but shorter patch.
I don't think that it works like that. If tyres were like balloons with no strength then P=F/A would apply and since the force is a constant then for any pressure the area would be a constant, just a different shape for wider tyres as you say. But tyres are not like balloons with no strength. If the car stood on blocks of wood then the wood would not deform and the larger block of wood would have a larger contact area. Tyres have inherent stiffness so are somewhere between the two extremes.98elise said:
Captain Muppet said:
Kawasicki said:
Wider tyres grip more, as rubber does not follow conventional friction theory.
Yes, there is less pressure per unit area. So the grip per unit area is reduced. However the grip increases because this is more than compensated for by additional area.
Wider tyres may need more ideal suspension geometry and characteristics (camber stiffness is a good example) than narrower tyres to achieve their potential.
I would go with the narrower, cheaper, tyres, and just enjoy the drive. It's not a race.
I hate it when people simplify tyre grip down to high school physics. The coefficient of friction stuff you do a school just doesn't work for a material that makes chemical bonds with the surface it's sat on. Tyres are sticky and with sticky things more area = more sticky.Yes, there is less pressure per unit area. So the grip per unit area is reduced. However the grip increases because this is more than compensated for by additional area.
Wider tyres may need more ideal suspension geometry and characteristics (camber stiffness is a good example) than narrower tyres to achieve their potential.
I would go with the narrower, cheaper, tyres, and just enjoy the drive. It's not a race.
You're also totally right about less grip being more fun. And cheaper.
I would assume you can run a lower pressure in a wider tyre though, as the shorter patch means the side wall must be deforming less. You could effectively increase the length of the contact patch back to the same as the thinner tyre.
I'm no expert so the last bit could be total b
ksGo let the front tyres down on your Elise - some tyres won't even look flat because the tyre construction is strong enough to hold it's shape with that little weight on it. If contact patch were proportional to air pressure flat tyres would be infinitely large. This is another case of a simplification too far to be useful.
sparkyhx said:
Nice reply - re- wider possibly = less grip, you only have to look at Rally cars in snow/ice - they don't use wide wheels, they use quite narrow ones (albeit studded)
I wonder if there is some formula or graph that shows an optimal point where adding friction material (wider tyre) does not offset the lower down force per square cm - i.e. the point where you go too wide.
I think the comparing grip on tarmac to gravel shows more about how gravel behaves than it does tyre grip on tarmac.I wonder if there is some formula or graph that shows an optimal point where adding friction material (wider tyre) does not offset the lower down force per square cm - i.e. the point where you go too wide.
If narrow gravel tyres have more grip then why are tarmac rally tyres big slicks? Partly because the gravel tyres would melt, but even before then they would have significantly less grip.
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