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Discussion
Hi Fella, back to back power gains are not much. About 4bhp IIRC. What it does enable you to do is to run the car at higher RPM's for longer periods without the ECU cutting back the power as the OEM intercooler becomes an interboiler after about 20 minutes on the track. I've run it as a daily driver and done 5 track days with the OEM intercooler and it has only once noticeably cut back and that was at Bedford. Other than that I have not had much cause to go to the Pro Alloy until recently when I upgraded, replaced the turbo's and that was more a matter of convienience than necessity, as it would require a remap after the intercooler is changed.
noble4862 said:
after 2 years of noble ownership and reading the forum have decided to join curently own 2.5 which is at urban racing having chassis treatment and new exhaust and just a bit of advice really about whether its worth getting upgraded intercooler and what power gains this would add
Hi there. what colour is it coz i was up there colecting mine at christmas and saw an orange 2.5 there. I had the intercooler done.Im sure that you would get more power gains as your having the exhaust upgraded as well. I havnt had it remapped yet but when i do i should be able to run more boost with the intercooler upraded along with the fuel pump and exhaust. Stella Artois said:
Hi Fella, back to back power gains are not much. About 4bhp IIRC. What it does enable you to do is to run the car at higher RPM's for longer periods without the ECU cutting back the power as the OEM intercooler becomes an interboiler after about 20 minutes on the track. I've run it as a daily driver and done 5 track days with the OEM intercooler and it has only once noticeably cut back and that was at Bedford. Other than that I have not had much cause to go to the Pro Alloy until recently when I upgraded, replaced the turbo's and that was more a matter of convienience than necessity, as it would require a remap after the intercooler is changed.
Sorry but I have to ask why it needs a remap when changing the intercooler ?
daddysumo said:
Stella Artois said:
Hi Fella, back to back power gains are not much. About 4bhp IIRC. What it does enable you to do is to run the car at higher RPM's for longer periods without the ECU cutting back the power as the OEM intercooler becomes an interboiler after about 20 minutes on the track. I've run it as a daily driver and done 5 track days with the OEM intercooler and it has only once noticeably cut back and that was at Bedford. Other than that I have not had much cause to go to the Pro Alloy until recently when I upgraded, replaced the turbo's and that was more a matter of convienience than necessity, as it would require a remap after the intercooler is changed.
Sorry but I have to ask why it needs a remap when changing the intercooler ?2.5bluenob said:
daddysumo said:
Stella Artois said:
Hi Fella, back to back power gains are not much. About 4bhp IIRC. What it does enable you to do is to run the car at higher RPM's for longer periods without the ECU cutting back the power as the OEM intercooler becomes an interboiler after about 20 minutes on the track. I've run it as a daily driver and done 5 track days with the OEM intercooler and it has only once noticeably cut back and that was at Bedford. Other than that I have not had much cause to go to the Pro Alloy until recently when I upgraded, replaced the turbo's and that was more a matter of convienience than necessity, as it would require a remap after the intercooler is changed.
Sorry but I have to ask why it needs a remap when changing the intercooler ?
I thought the same. But after having a new exhaust anyway it would be a good idea
Yep deffo after an exhaust upgrade if it's more free flowing !
daddysumo said:
Stella Artois said:
Hi Fella, back to back power gains are not much. About 4bhp IIRC. What it does enable you to do is to run the car at higher RPM's for longer periods without the ECU cutting back the power as the OEM intercooler becomes an interboiler after about 20 minutes on the track. I've run it as a daily driver and done 5 track days with the OEM intercooler and it has only once noticeably cut back and that was at Bedford. Other than that I have not had much cause to go to the Pro Alloy until recently when I upgraded, replaced the turbo's and that was more a matter of convienience than necessity, as it would require a remap after the intercooler is changed.
Sorry but I have to ask why it needs a remap when changing the intercooler ?
andygtt said:
daddysumo said:
2.5bluenob said:
daddysumo said:
Stella Artois said:
Hi Fella, back to back power gains are not much. About 4bhp IIRC. What it does enable you to do is to run the car at higher RPM's for longer periods without the ECU cutting back the power as the OEM intercooler becomes an interboiler after about 20 minutes on the track. I've run it as a daily driver and done 5 track days with the OEM intercooler and it has only once noticeably cut back and that was at Bedford. Other than that I have not had much cause to go to the Pro Alloy until recently when I upgraded, replaced the turbo's and that was more a matter of convienience than necessity, as it would require a remap after the intercooler is changed.
Sorry but I have to ask why it needs a remap when changing the intercooler ?Simon did the same with his but put it on Mikes dyno to check things and it was shown at the dyno day that the std GT03 runs a little rich as std which is why you can fit a free flow one without a remap.
Of cause it may be differnt on the 2.5 or M400.
cant see any reason at all why youd need a remap after a intercooler change unless the original was so restricive as std, and its not that bad just gets heat soak.
Stella Artois said:
daddysumo said:
Stella Artois said:
Hi Fella, back to back power gains are not much. About 4bhp IIRC. What it does enable you to do is to run the car at higher RPM's for longer periods without the ECU cutting back the power as the OEM intercooler becomes an interboiler after about 20 minutes on the track. I've run it as a daily driver and done 5 track days with the OEM intercooler and it has only once noticeably cut back and that was at Bedford. Other than that I have not had much cause to go to the Pro Alloy until recently when I upgraded, replaced the turbo's and that was more a matter of convienience than necessity, as it would require a remap after the intercooler is changed.
Sorry but I have to ask why it needs a remap when changing the intercooler ?
As you have altered the temperature of the charge going in, the fuel ratio would need to be recalculated, in a nutshell
Ah I see by lowering the intake temps a few degrees it must be taking it outside of the map parameters
I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !)
daddysumo said:
Ah I see by lowering the intake temps a few degrees it must be taking it outside of the map parameters I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !)
You should get your one remapped for bald tyres by now I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !)Adrian W said:
daddysumo said:
Ah I see by lowering the intake temps a few degrees it must be taking it outside of the map parameters
I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !) You should get your one remapped for bald tyres by now
You're not wrong there I've been trying to get some 285/35's for the rear but neither toyo or yoko do them in the semi slick variety so will have to go for the 275/35's I think, just waiting on prices and availability from the usual sources.
daddysumo said:
Adrian W said:
daddysumo said:
Ah I see by lowering the intake temps a few degrees it must be taking it outside of the map parameters I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !)
You should get your one remapped for bald tyres by now I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !)Adrian W said:
daddysumo said:
Adrian W said:
daddysumo said:
Ah I see by lowering the intake temps a few degrees it must be taking it outside of the map parameters I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !)
You should get your one remapped for bald tyres by now I think I had better get mine remapped for all this cold weather we are having as that must be altering the temp of the charge as well !!(in a nutshell of course !)andygtt said:
I'm using 295 Corsa's, they last about 10 minutes
how does it ride on those and have you put larger on the front as well?Fronts are standard size
Edited by Adrian W on Wednesday 17th February 10:14
Daddy here is an article on Tyre sizing that you may find interesting
"So many tyres; so many factors to consider! Do you go wider? Do you go for a lower profile? Do you increase your wheel diameter? What pressures do you run? The whole area is quite complicated, and the result is that there is a hell of a lot of misconceptions out there. What is needed is a primer to give some fairly solid grounding in the basics, so that informed decisions can be made.
So, let's go through two of the misconceptions, and see if we can get an understanding of what actually contributes to grip - and what does not.
Myth 1: Wider tyres have a larger contact patch than narrow tyres
What actually influences the size of the tyre's contact patch? Is it the width of the tyre, or the profile? The simple answer that it is neither of these; the size of the tyre's contact patch is related to:
the weight on the wheel
the tyre pressure.
For example, say that the weight on the tyre was 900lb, and the tyre pressure was 10 psi. That internal pressure means that each square inch of area can support 10lb, so, in this case, the contact patch will be 90 square inches. If the tyre pressure was 30 psi, the contact area would be 30 square inches, and if the pressure was 90 psi, the contact area would be 10 square inches. This has been found to be almost exactly correct for most tyres (the exceptions being so-called run-flat tyres, or tyres with extremely stiff sidewalls). For most other tyres, carcass structure will have an effect, but by far the major factor is tyre pressure.
So, as you can see, the size of the contact patch of a tyre is not related to the width of the tyre - it is, in fact, proportional to the tyre pressure. What will change with the fitting of a wider tyre is the shape of the contact patch - it will get wider, but shorter longways.
Myth 2: A larger contact patch = more grip
Okay, most people will come to the conclusion that if you have "more rubber on the road" you will have increased grip. Sorry to say this folks, but to very close to 100% accuracy, the size of the contact patch is irrelevant.
The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre. The higher the coefficient, the more grip which can be generated. The relation that is used is called Amonton's Law, and the equation is:
F=uN,
where F is the force generated, u is the coefficient of friction, and N is the weight on the surface considered (in our case, the weight on the tyre).
So, if you increase the weight on the tyre, then the frictional force will increase as well, in proportion to the increase in weight on the tyre - but the coefficient of friction will remain the same. The level of grip of the tyre (forgetting about suspension niceties - we are only discussing tyres here) is totally dictated by the coefficient of grip of the tyre and the weight acting on it - not the area of the contact between the tyre and the road.
Why Not Narrow Tyres, Then?
So, I hear you argue, why bother with wide, low profile tyres at all? Why not simply have narrow, high profile tyres? The simple reply to that is heat (remember, we are simply talking grip here, not the niceties of handling finesse). The point is that, to get a contact patch of a certain size on the road, you need a certain portion of the tyre to be flat. Taking the contact patch to be basically rectangular (though it is actually partially oval in shape), then the area of that patch will be its length times its width. Now, for a narrow tyre, the contact patch will be quite long compared with a wide tyre.
This introduces two problems for the tyre.
First, to get that long flat section to give the required contact patch, the sidewall of the tyre needs to deform quite a lot. This deformation actually causes the bending and unbending the rubber of the sidewall as it flattens and then the tread curves again. This bending and unbending process results in a lot of heat being generated. (Think about bending and unbending a piece of wire rapidly, and how hot it gets as you do so. If you bend it less, but at the same frequency, less heat will be generated). Obviously, the more it needs to bend, the greater the amount of heat generated.
The second relates to the length itself. There will be a greater percentage of the tyre tread in contact with the road than if the contact patch length were shorter; this reduces the amount that the tread can cool. Also, there is a greater percentage of sidewall at any given time that is actually under bending stresses, again resulting in less opportunity to cool.
So, how much extra bending do you really get, and how much is potential tread cooling reduced? Let's take a theoretical example, and take a 155-width tyre compared with a 225 tyre of the same circumference. Agreed, this is an extreme example, but it will suit our point very well. Assume that the wheel/tyre-unloaded circumference is 60cm. Assume the tyre pressure is 30 psi, and that the weight on the wheel is 600lb, giving an area of 20 square inches (or 129 square cm). Assuming that the contact patch is rectangular, with the wider (225) tyre, the patch will be 5.73cm long, and with the 155 tyre, the patch will be 8.32cm long. Now, the circumference of the wheel-tyre combination is 188cm, so the 225 is heating for 3% of its cycle, and cooling 97%, whereas the 155 is heating for 4.5% of the cycle and cooling for 95.5%. So, you can see that the narrower tyre is generating heat 50% longer than the 225, and is not spending so much of its cycle cooling.
Now, as far as heating of the tyre is concerned, simple geometry shows us that the 155 tyre bends by 0.29cm, and the 225 bends by 0.14cm. Now, assuming that the heating of the tyre is roughly proportional to the deformation, let's find out the results of all of this. We will multiply the deformation by the percentage of time the tyre sidewall is under stress, and divide this number by the percentage of time that the tyre is being cooled. Multiplying the resulting numbers by 100, we get a figure of 1.37 for the 155 tyre, and 0.43 for the 225. Dividing the 155 tyre's number by that of the 225, we find that the heat generation of the 155 is 3.2 times that of the 225! This is quite an amazing result, given that the 225 is only 45% wider than the 155.
As a result on this increased generation of heat, and the reduced capacity for self cooling, the tyres need to be made of a harder rubber compound that is more able to resist heat. This harder compound will, of necessity, have a reduced coefficient of friction, particularly when cold. The tyres that are wider can have a softer compound with better frictional properties. Due to the reduced bending stresses, and greater cooling opportunities, the tyre will tend to stay within a narrow temperature range quite consistently, giving greater cold grip, while managing to have a reduced propensity for overheating. Obviously, this makes for a better performance tyre.
On the issue of wheel size (the diameter, not the width), it is therefore clear that increasing the wheel/tyre diameter combination is beneficial. The reason for this is that the tyre will not have to deform so much to get the required contact patch length, and the percentage of the tyre tread in contact with the road will be less than for a smaller diameter combination.
So, what about tyre pressure? Obviously, tyre pressure plays a very important part, but there are clearly limits on both sides of the tyre pressure equation. At the higher end, there is the maximum tyre pressure that can be sustained before there is damage to the carcass. At the low end, you don't want the sidewall almost collapsing, generating massive heat, and have the tyre slipping on the rim. So, you can play around with tyre pressures to optimise your set-up, but there are limitations.
A simple way to find out approximately what pressure is optimal for your combination is to draw a chalkline across the width of the tyre, drive for a bit, and look at the wear pattern of the chalkmark. Wearing more quickly in the centre indicates pressure that is too high, and wear on the edges indicates too low a pressure.
One issue to consider is that, for wet weather driving, despite what you may have heard, it is better to increase your tyre pressure, not reduce it. The reason is that there is a relationship between tyre pressure and the speed at which there is the onset of aquaplaning. In the Imperial system, the equation is 9 times the square root of the tyre pressure. So, if your tyres are at 25 psi, if you drive into a puddle that is deeper than your tread depth, you will aquaplane at 45 mph (72 km/h), whereas if your tyre pressure was 36psi, you would aquaplane at 54 mph (87 km/h). The advantages are obvious.
As far as tyre profile is concerned, the main benefit is one of handling - the lower sidewalls give reduced sidewall deformation under lateral loading, which results in improved steering response and a more stable contact patch.
Conclusion
Summarizing, what factors are important in terms of tyre grip? Tyre width has no direct relation to the amount of grip generated; it is a secondary factor, and the width basically relates to cooling potential and so the tyre compound that can be used. The size of the contact patch has no bearing on the amount of grip generated at all, apart from the extreme of where the compound is getting so hot that it no longer acts as a solid (and therefore doesn't follow Amonton's Law). The tyre pressure has no direct bearing on the level of grip (apart from aquaplaning), but it does have a bearing on the heating and cooling characteristics of the tyre. Having a lower tyre profile gives improved handling through reduced sidewall stress and improved contact patch shape stability."
not quite what you expected !
"So many tyres; so many factors to consider! Do you go wider? Do you go for a lower profile? Do you increase your wheel diameter? What pressures do you run? The whole area is quite complicated, and the result is that there is a hell of a lot of misconceptions out there. What is needed is a primer to give some fairly solid grounding in the basics, so that informed decisions can be made.
So, let's go through two of the misconceptions, and see if we can get an understanding of what actually contributes to grip - and what does not.
Myth 1: Wider tyres have a larger contact patch than narrow tyres
What actually influences the size of the tyre's contact patch? Is it the width of the tyre, or the profile? The simple answer that it is neither of these; the size of the tyre's contact patch is related to:
the weight on the wheel
the tyre pressure.
For example, say that the weight on the tyre was 900lb, and the tyre pressure was 10 psi. That internal pressure means that each square inch of area can support 10lb, so, in this case, the contact patch will be 90 square inches. If the tyre pressure was 30 psi, the contact area would be 30 square inches, and if the pressure was 90 psi, the contact area would be 10 square inches. This has been found to be almost exactly correct for most tyres (the exceptions being so-called run-flat tyres, or tyres with extremely stiff sidewalls). For most other tyres, carcass structure will have an effect, but by far the major factor is tyre pressure.
So, as you can see, the size of the contact patch of a tyre is not related to the width of the tyre - it is, in fact, proportional to the tyre pressure. What will change with the fitting of a wider tyre is the shape of the contact patch - it will get wider, but shorter longways.
Myth 2: A larger contact patch = more grip
Okay, most people will come to the conclusion that if you have "more rubber on the road" you will have increased grip. Sorry to say this folks, but to very close to 100% accuracy, the size of the contact patch is irrelevant.
The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre. The higher the coefficient, the more grip which can be generated. The relation that is used is called Amonton's Law, and the equation is:
F=uN,
where F is the force generated, u is the coefficient of friction, and N is the weight on the surface considered (in our case, the weight on the tyre).
So, if you increase the weight on the tyre, then the frictional force will increase as well, in proportion to the increase in weight on the tyre - but the coefficient of friction will remain the same. The level of grip of the tyre (forgetting about suspension niceties - we are only discussing tyres here) is totally dictated by the coefficient of grip of the tyre and the weight acting on it - not the area of the contact between the tyre and the road.
Why Not Narrow Tyres, Then?
So, I hear you argue, why bother with wide, low profile tyres at all? Why not simply have narrow, high profile tyres? The simple reply to that is heat (remember, we are simply talking grip here, not the niceties of handling finesse). The point is that, to get a contact patch of a certain size on the road, you need a certain portion of the tyre to be flat. Taking the contact patch to be basically rectangular (though it is actually partially oval in shape), then the area of that patch will be its length times its width. Now, for a narrow tyre, the contact patch will be quite long compared with a wide tyre.
This introduces two problems for the tyre.
First, to get that long flat section to give the required contact patch, the sidewall of the tyre needs to deform quite a lot. This deformation actually causes the bending and unbending the rubber of the sidewall as it flattens and then the tread curves again. This bending and unbending process results in a lot of heat being generated. (Think about bending and unbending a piece of wire rapidly, and how hot it gets as you do so. If you bend it less, but at the same frequency, less heat will be generated). Obviously, the more it needs to bend, the greater the amount of heat generated.
The second relates to the length itself. There will be a greater percentage of the tyre tread in contact with the road than if the contact patch length were shorter; this reduces the amount that the tread can cool. Also, there is a greater percentage of sidewall at any given time that is actually under bending stresses, again resulting in less opportunity to cool.
So, how much extra bending do you really get, and how much is potential tread cooling reduced? Let's take a theoretical example, and take a 155-width tyre compared with a 225 tyre of the same circumference. Agreed, this is an extreme example, but it will suit our point very well. Assume that the wheel/tyre-unloaded circumference is 60cm. Assume the tyre pressure is 30 psi, and that the weight on the wheel is 600lb, giving an area of 20 square inches (or 129 square cm). Assuming that the contact patch is rectangular, with the wider (225) tyre, the patch will be 5.73cm long, and with the 155 tyre, the patch will be 8.32cm long. Now, the circumference of the wheel-tyre combination is 188cm, so the 225 is heating for 3% of its cycle, and cooling 97%, whereas the 155 is heating for 4.5% of the cycle and cooling for 95.5%. So, you can see that the narrower tyre is generating heat 50% longer than the 225, and is not spending so much of its cycle cooling.
Now, as far as heating of the tyre is concerned, simple geometry shows us that the 155 tyre bends by 0.29cm, and the 225 bends by 0.14cm. Now, assuming that the heating of the tyre is roughly proportional to the deformation, let's find out the results of all of this. We will multiply the deformation by the percentage of time the tyre sidewall is under stress, and divide this number by the percentage of time that the tyre is being cooled. Multiplying the resulting numbers by 100, we get a figure of 1.37 for the 155 tyre, and 0.43 for the 225. Dividing the 155 tyre's number by that of the 225, we find that the heat generation of the 155 is 3.2 times that of the 225! This is quite an amazing result, given that the 225 is only 45% wider than the 155.
As a result on this increased generation of heat, and the reduced capacity for self cooling, the tyres need to be made of a harder rubber compound that is more able to resist heat. This harder compound will, of necessity, have a reduced coefficient of friction, particularly when cold. The tyres that are wider can have a softer compound with better frictional properties. Due to the reduced bending stresses, and greater cooling opportunities, the tyre will tend to stay within a narrow temperature range quite consistently, giving greater cold grip, while managing to have a reduced propensity for overheating. Obviously, this makes for a better performance tyre.
On the issue of wheel size (the diameter, not the width), it is therefore clear that increasing the wheel/tyre diameter combination is beneficial. The reason for this is that the tyre will not have to deform so much to get the required contact patch length, and the percentage of the tyre tread in contact with the road will be less than for a smaller diameter combination.
So, what about tyre pressure? Obviously, tyre pressure plays a very important part, but there are clearly limits on both sides of the tyre pressure equation. At the higher end, there is the maximum tyre pressure that can be sustained before there is damage to the carcass. At the low end, you don't want the sidewall almost collapsing, generating massive heat, and have the tyre slipping on the rim. So, you can play around with tyre pressures to optimise your set-up, but there are limitations.
A simple way to find out approximately what pressure is optimal for your combination is to draw a chalkline across the width of the tyre, drive for a bit, and look at the wear pattern of the chalkmark. Wearing more quickly in the centre indicates pressure that is too high, and wear on the edges indicates too low a pressure.
One issue to consider is that, for wet weather driving, despite what you may have heard, it is better to increase your tyre pressure, not reduce it. The reason is that there is a relationship between tyre pressure and the speed at which there is the onset of aquaplaning. In the Imperial system, the equation is 9 times the square root of the tyre pressure. So, if your tyres are at 25 psi, if you drive into a puddle that is deeper than your tread depth, you will aquaplane at 45 mph (72 km/h), whereas if your tyre pressure was 36psi, you would aquaplane at 54 mph (87 km/h). The advantages are obvious.
As far as tyre profile is concerned, the main benefit is one of handling - the lower sidewalls give reduced sidewall deformation under lateral loading, which results in improved steering response and a more stable contact patch.
Conclusion
Summarizing, what factors are important in terms of tyre grip? Tyre width has no direct relation to the amount of grip generated; it is a secondary factor, and the width basically relates to cooling potential and so the tyre compound that can be used. The size of the contact patch has no bearing on the amount of grip generated at all, apart from the extreme of where the compound is getting so hot that it no longer acts as a solid (and therefore doesn't follow Amonton's Law). The tyre pressure has no direct bearing on the level of grip (apart from aquaplaning), but it does have a bearing on the heating and cooling characteristics of the tyre. Having a lower tyre profile gives improved handling through reduced sidewall stress and improved contact patch shape stability."
not quite what you expected !
And todays statement of the obvious is! The main objective if you have the resources know how and motivation is to get the tyre to run at the optimum tempreture. On the track we are never satisfied, and quiet often go backwards for no obvious reason.
Just out of interest the sidewalls on Corsa's are so stiff they run with no air in, one reason to check pressures.
Just out of interest the sidewalls on Corsa's are so stiff they run with no air in, one reason to check pressures.
Gadgeroonie said:
Daddy here is an article on Tyre sizing that you may find interesting
"So many tyres; so many factors to consider! Do you go wider? Do you go for a lower profile? Do you increase your wheel diameter? What pressures do you run? The whole area is quite complicated, and the result is that there is a hell of a lot of misconceptions out there. What is needed is a primer to give some fairly solid grounding in the basics, so that informed decisions can be made.
So, let's go through two of the misconceptions, and see if we can get an understanding of what actually contributes to grip - and what does not.
Myth 1: Wider tyres have a larger contact patch than narrow tyres
What actually influences the size of the tyre's contact patch? Is it the width of the tyre, or the profile? The simple answer that it is neither of these; the size of the tyre's contact patch is related to:
the weight on the wheel
the tyre pressure.
For example, say that the weight on the tyre was 900lb, and the tyre pressure was 10 psi. That internal pressure means that each square inch of area can support 10lb, so, in this case, the contact patch will be 90 square inches. If the tyre pressure was 30 psi, the contact area would be 30 square inches, and if the pressure was 90 psi, the contact area would be 10 square inches. This has been found to be almost exactly correct for most tyres (the exceptions being so-called run-flat tyres, or tyres with extremely stiff sidewalls). For most other tyres, carcass structure will have an effect, but by far the major factor is tyre pressure.
So, as you can see, the size of the contact patch of a tyre is not related to the width of the tyre - it is, in fact, proportional to the tyre pressure. What will change with the fitting of a wider tyre is the shape of the contact patch - it will get wider, but shorter longways.
Myth 2: A larger contact patch = more grip
Okay, most people will come to the conclusion that if you have "more rubber on the road" you will have increased grip. Sorry to say this folks, but to very close to 100% accuracy, the size of the contact patch is irrelevant.
The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre. The higher the coefficient, the more grip which can be generated. The relation that is used is called Amonton's Law, and the equation is:
F=uN,
where F is the force generated, u is the coefficient of friction, and N is the weight on the surface considered (in our case, the weight on the tyre).
So, if you increase the weight on the tyre, then the frictional force will increase as well, in proportion to the increase in weight on the tyre - but the coefficient of friction will remain the same. The level of grip of the tyre (forgetting about suspension niceties - we are only discussing tyres here) is totally dictated by the coefficient of grip of the tyre and the weight acting on it - not the area of the contact between the tyre and the road.
Why Not Narrow Tyres, Then?
So, I hear you argue, why bother with wide, low profile tyres at all? Why not simply have narrow, high profile tyres? The simple reply to that is heat (remember, we are simply talking grip here, not the niceties of handling finesse). The point is that, to get a contact patch of a certain size on the road, you need a certain portion of the tyre to be flat. Taking the contact patch to be basically rectangular (though it is actually partially oval in shape), then the area of that patch will be its length times its width. Now, for a narrow tyre, the contact patch will be quite long compared with a wide tyre.
This introduces two problems for the tyre.
First, to get that long flat section to give the required contact patch, the sidewall of the tyre needs to deform quite a lot. This deformation actually causes the bending and unbending the rubber of the sidewall as it flattens and then the tread curves again. This bending and unbending process results in a lot of heat being generated. (Think about bending and unbending a piece of wire rapidly, and how hot it gets as you do so. If you bend it less, but at the same frequency, less heat will be generated). Obviously, the more it needs to bend, the greater the amount of heat generated.
The second relates to the length itself. There will be a greater percentage of the tyre tread in contact with the road than if the contact patch length were shorter; this reduces the amount that the tread can cool. Also, there is a greater percentage of sidewall at any given time that is actually under bending stresses, again resulting in less opportunity to cool.
So, how much extra bending do you really get, and how much is potential tread cooling reduced? Let's take a theoretical example, and take a 155-width tyre compared with a 225 tyre of the same circumference. Agreed, this is an extreme example, but it will suit our point very well. Assume that the wheel/tyre-unloaded circumference is 60cm. Assume the tyre pressure is 30 psi, and that the weight on the wheel is 600lb, giving an area of 20 square inches (or 129 square cm). Assuming that the contact patch is rectangular, with the wider (225) tyre, the patch will be 5.73cm long, and with the 155 tyre, the patch will be 8.32cm long. Now, the circumference of the wheel-tyre combination is 188cm, so the 225 is heating for 3% of its cycle, and cooling 97%, whereas the 155 is heating for 4.5% of the cycle and cooling for 95.5%. So, you can see that the narrower tyre is generating heat 50% longer than the 225, and is not spending so much of its cycle cooling.
Now, as far as heating of the tyre is concerned, simple geometry shows us that the 155 tyre bends by 0.29cm, and the 225 bends by 0.14cm. Now, assuming that the heating of the tyre is roughly proportional to the deformation, let's find out the results of all of this. We will multiply the deformation by the percentage of time the tyre sidewall is under stress, and divide this number by the percentage of time that the tyre is being cooled. Multiplying the resulting numbers by 100, we get a figure of 1.37 for the 155 tyre, and 0.43 for the 225. Dividing the 155 tyre's number by that of the 225, we find that the heat generation of the 155 is 3.2 times that of the 225! This is quite an amazing result, given that the 225 is only 45% wider than the 155.
As a result on this increased generation of heat, and the reduced capacity for self cooling, the tyres need to be made of a harder rubber compound that is more able to resist heat. This harder compound will, of necessity, have a reduced coefficient of friction, particularly when cold. The tyres that are wider can have a softer compound with better frictional properties. Due to the reduced bending stresses, and greater cooling opportunities, the tyre will tend to stay within a narrow temperature range quite consistently, giving greater cold grip, while managing to have a reduced propensity for overheating. Obviously, this makes for a better performance tyre.
On the issue of wheel size (the diameter, not the width), it is therefore clear that increasing the wheel/tyre diameter combination is beneficial. The reason for this is that the tyre will not have to deform so much to get the required contact patch length, and the percentage of the tyre tread in contact with the road will be less than for a smaller diameter combination.
So, what about tyre pressure? Obviously, tyre pressure plays a very important part, but there are clearly limits on both sides of the tyre pressure equation. At the higher end, there is the maximum tyre pressure that can be sustained before there is damage to the carcass. At the low end, you don't want the sidewall almost collapsing, generating massive heat, and have the tyre slipping on the rim. So, you can play around with tyre pressures to optimise your set-up, but there are limitations.
A simple way to find out approximately what pressure is optimal for your combination is to draw a chalkline across the width of the tyre, drive for a bit, and look at the wear pattern of the chalkmark. Wearing more quickly in the centre indicates pressure that is too high, and wear on the edges indicates too low a pressure.
One issue to consider is that, for wet weather driving, despite what you may have heard, it is better to increase your tyre pressure, not reduce it. The reason is that there is a relationship between tyre pressure and the speed at which there is the onset of aquaplaning. In the Imperial system, the equation is 9 times the square root of the tyre pressure. So, if your tyres are at 25 psi, if you drive into a puddle that is deeper than your tread depth, you will aquaplane at 45 mph (72 km/h), whereas if your tyre pressure was 36psi, you would aquaplane at 54 mph (87 km/h). The advantages are obvious.
As far as tyre profile is concerned, the main benefit is one of handling - the lower sidewalls give reduced sidewall deformation under lateral loading, which results in improved steering response and a more stable contact patch.
Conclusion
Summarizing, what factors are important in terms of tyre grip? Tyre width has no direct relation to the amount of grip generated; it is a secondary factor, and the width basically relates to cooling potential and so the tyre compound that can be used. The size of the contact patch has no bearing on the amount of grip generated at all, apart from the extreme of where the compound is getting so hot that it no longer acts as a solid (and therefore doesn't follow Amonton's Law). The tyre pressure has no direct bearing on the level of grip (apart from aquaplaning), but it does have a bearing on the heating and cooling characteristics of the tyre. Having a lower tyre profile gives improved handling through reduced sidewall stress and improved contact patch shape stability."
not quite what you expected !
Wow"So many tyres; so many factors to consider! Do you go wider? Do you go for a lower profile? Do you increase your wheel diameter? What pressures do you run? The whole area is quite complicated, and the result is that there is a hell of a lot of misconceptions out there. What is needed is a primer to give some fairly solid grounding in the basics, so that informed decisions can be made.
So, let's go through two of the misconceptions, and see if we can get an understanding of what actually contributes to grip - and what does not.
Myth 1: Wider tyres have a larger contact patch than narrow tyres
What actually influences the size of the tyre's contact patch? Is it the width of the tyre, or the profile? The simple answer that it is neither of these; the size of the tyre's contact patch is related to:
the weight on the wheel
the tyre pressure.
For example, say that the weight on the tyre was 900lb, and the tyre pressure was 10 psi. That internal pressure means that each square inch of area can support 10lb, so, in this case, the contact patch will be 90 square inches. If the tyre pressure was 30 psi, the contact area would be 30 square inches, and if the pressure was 90 psi, the contact area would be 10 square inches. This has been found to be almost exactly correct for most tyres (the exceptions being so-called run-flat tyres, or tyres with extremely stiff sidewalls). For most other tyres, carcass structure will have an effect, but by far the major factor is tyre pressure.
So, as you can see, the size of the contact patch of a tyre is not related to the width of the tyre - it is, in fact, proportional to the tyre pressure. What will change with the fitting of a wider tyre is the shape of the contact patch - it will get wider, but shorter longways.
Myth 2: A larger contact patch = more grip
Okay, most people will come to the conclusion that if you have "more rubber on the road" you will have increased grip. Sorry to say this folks, but to very close to 100% accuracy, the size of the contact patch is irrelevant.
The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre. The higher the coefficient, the more grip which can be generated. The relation that is used is called Amonton's Law, and the equation is:
F=uN,
where F is the force generated, u is the coefficient of friction, and N is the weight on the surface considered (in our case, the weight on the tyre).
So, if you increase the weight on the tyre, then the frictional force will increase as well, in proportion to the increase in weight on the tyre - but the coefficient of friction will remain the same. The level of grip of the tyre (forgetting about suspension niceties - we are only discussing tyres here) is totally dictated by the coefficient of grip of the tyre and the weight acting on it - not the area of the contact between the tyre and the road.
Why Not Narrow Tyres, Then?
So, I hear you argue, why bother with wide, low profile tyres at all? Why not simply have narrow, high profile tyres? The simple reply to that is heat (remember, we are simply talking grip here, not the niceties of handling finesse). The point is that, to get a contact patch of a certain size on the road, you need a certain portion of the tyre to be flat. Taking the contact patch to be basically rectangular (though it is actually partially oval in shape), then the area of that patch will be its length times its width. Now, for a narrow tyre, the contact patch will be quite long compared with a wide tyre.
This introduces two problems for the tyre.
First, to get that long flat section to give the required contact patch, the sidewall of the tyre needs to deform quite a lot. This deformation actually causes the bending and unbending the rubber of the sidewall as it flattens and then the tread curves again. This bending and unbending process results in a lot of heat being generated. (Think about bending and unbending a piece of wire rapidly, and how hot it gets as you do so. If you bend it less, but at the same frequency, less heat will be generated). Obviously, the more it needs to bend, the greater the amount of heat generated.
The second relates to the length itself. There will be a greater percentage of the tyre tread in contact with the road than if the contact patch length were shorter; this reduces the amount that the tread can cool. Also, there is a greater percentage of sidewall at any given time that is actually under bending stresses, again resulting in less opportunity to cool.
So, how much extra bending do you really get, and how much is potential tread cooling reduced? Let's take a theoretical example, and take a 155-width tyre compared with a 225 tyre of the same circumference. Agreed, this is an extreme example, but it will suit our point very well. Assume that the wheel/tyre-unloaded circumference is 60cm. Assume the tyre pressure is 30 psi, and that the weight on the wheel is 600lb, giving an area of 20 square inches (or 129 square cm). Assuming that the contact patch is rectangular, with the wider (225) tyre, the patch will be 5.73cm long, and with the 155 tyre, the patch will be 8.32cm long. Now, the circumference of the wheel-tyre combination is 188cm, so the 225 is heating for 3% of its cycle, and cooling 97%, whereas the 155 is heating for 4.5% of the cycle and cooling for 95.5%. So, you can see that the narrower tyre is generating heat 50% longer than the 225, and is not spending so much of its cycle cooling.
Now, as far as heating of the tyre is concerned, simple geometry shows us that the 155 tyre bends by 0.29cm, and the 225 bends by 0.14cm. Now, assuming that the heating of the tyre is roughly proportional to the deformation, let's find out the results of all of this. We will multiply the deformation by the percentage of time the tyre sidewall is under stress, and divide this number by the percentage of time that the tyre is being cooled. Multiplying the resulting numbers by 100, we get a figure of 1.37 for the 155 tyre, and 0.43 for the 225. Dividing the 155 tyre's number by that of the 225, we find that the heat generation of the 155 is 3.2 times that of the 225! This is quite an amazing result, given that the 225 is only 45% wider than the 155.
As a result on this increased generation of heat, and the reduced capacity for self cooling, the tyres need to be made of a harder rubber compound that is more able to resist heat. This harder compound will, of necessity, have a reduced coefficient of friction, particularly when cold. The tyres that are wider can have a softer compound with better frictional properties. Due to the reduced bending stresses, and greater cooling opportunities, the tyre will tend to stay within a narrow temperature range quite consistently, giving greater cold grip, while managing to have a reduced propensity for overheating. Obviously, this makes for a better performance tyre.
On the issue of wheel size (the diameter, not the width), it is therefore clear that increasing the wheel/tyre diameter combination is beneficial. The reason for this is that the tyre will not have to deform so much to get the required contact patch length, and the percentage of the tyre tread in contact with the road will be less than for a smaller diameter combination.
So, what about tyre pressure? Obviously, tyre pressure plays a very important part, but there are clearly limits on both sides of the tyre pressure equation. At the higher end, there is the maximum tyre pressure that can be sustained before there is damage to the carcass. At the low end, you don't want the sidewall almost collapsing, generating massive heat, and have the tyre slipping on the rim. So, you can play around with tyre pressures to optimise your set-up, but there are limitations.
A simple way to find out approximately what pressure is optimal for your combination is to draw a chalkline across the width of the tyre, drive for a bit, and look at the wear pattern of the chalkmark. Wearing more quickly in the centre indicates pressure that is too high, and wear on the edges indicates too low a pressure.
One issue to consider is that, for wet weather driving, despite what you may have heard, it is better to increase your tyre pressure, not reduce it. The reason is that there is a relationship between tyre pressure and the speed at which there is the onset of aquaplaning. In the Imperial system, the equation is 9 times the square root of the tyre pressure. So, if your tyres are at 25 psi, if you drive into a puddle that is deeper than your tread depth, you will aquaplane at 45 mph (72 km/h), whereas if your tyre pressure was 36psi, you would aquaplane at 54 mph (87 km/h). The advantages are obvious.
As far as tyre profile is concerned, the main benefit is one of handling - the lower sidewalls give reduced sidewall deformation under lateral loading, which results in improved steering response and a more stable contact patch.
Conclusion
Summarizing, what factors are important in terms of tyre grip? Tyre width has no direct relation to the amount of grip generated; it is a secondary factor, and the width basically relates to cooling potential and so the tyre compound that can be used. The size of the contact patch has no bearing on the amount of grip generated at all, apart from the extreme of where the compound is getting so hot that it no longer acts as a solid (and therefore doesn't follow Amonton's Law). The tyre pressure has no direct bearing on the level of grip (apart from aquaplaning), but it does have a bearing on the heating and cooling characteristics of the tyre. Having a lower tyre profile gives improved handling through reduced sidewall stress and improved contact patch shape stability."
not quite what you expected !
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