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As above. What width are the alloys?
Width of Trophy wheels please
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The contact patch of both tyres will be similar, if not the same,.
OliS said:
tut tut, don't you know how tyres work?
a 165 also has the same contact patch as a 185. interestingly, a 145 is also the same as the 165. It also goes the other way, so a 225 is the same as a 205. Basically all tyres have the same contact patch size.
but only on a clio williams
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Firstly a wider tyre will not increase contact patch:
Let's take four standard 185/55R15 road tyres, as supplied by Michelin, and inflate them to 30psi (2.25Kg/cm²). Now lets fit them to our Clio that weighs 990 kg. Roughly speaking, each tyre is taking about a quarter of your car's weight (247.5Kg). So using the following equation:
Pressure = weight / area
We can calculate the area of your contact patch as follows:
Contact patch (area) = 247.5 (weight) / 2.25 (Pressure) = 110 cm²
So the contact patch is 110 cm². If the width of the tyre is 185 mm then we can say the contact patch length is:
110 (contact patch area) / 18.5 (width of tyre) = 5.95 cm
So our contact patch is a rectangle of tyre measuring 18.5 cm wide by 5.95 cm long.
Now lets suppose that we fit a 205 tyre. Lets plug the same numbers into our equation
Contact patch (area) = 247.5 (weight) / 2.25 (Pressure) = 110 cm²
The weight of the car hasn’t changed an the tyre pressures remain at 30 psi, so the contact patch cant change. Its that simple. But here is the real twist in the tail. Lets work out the dimensions of the contact patch:
110 (contact patch area) / 20.5 (width of tyre) = 5.37 cm
So now our contact patch is a rectangle of tyre measuring 20.5 cm wide by 5.37 cm long. In other words fitting a wider tyre hasn’t increased the contact patch area, but simple changed the shape of that footprint.
Wider tyres do not therefore increase contact patch area.
Secondly, grip is related to the coefficient of friction. The coefficient of friction is is the ratio of the force required to slide the surfaces to the force perpendicular to the surfaces. A lower friction coefficient indictates that the surfaces are slicker - there is less resistance to the sliding motion.
Coefficient of friction is unrelated to surface area (which is irrelevant as we've just proved surface area doesnt go up with tyre width anyway). Coeffiecient of friction is however related to the softness of the rubber.Sifter rubber = more grip.
In short for those who cant be bothered to understand that:
Wider tyres do not have a larger contact patch. They dont increase grip. Wider tyres appear to offer more grip because wider tyres are usually made of softer rubber (because manufacturers can get away with softer rubber because the sidewall is lower; a high sidewall tyre wouldnt support itself on soft rubber). So if your 205 tyres are made of the same rubber as a 185 tyre they will offer no more total grip. However they will aquaplane you into the nearest field in standing water.
The main reason race cars run wide tyres is because wide tyres = bigger total surface area = more cooling. Narrow tyres would simple over heat quicker. Thats one reason the rear tyres are wider on F1 cars. Rear wheel drive = more heat in rear tyres = more need for cooling = need for wider tyres.
Tyre science isnt as simple as this in reality, but i guarantee that wide tyres wont increase grip or handling.
If you want more front end grip, you should be asking 'where can i find some softer tyres'?
And i think that my friend, nicely blows you out of the water.
Taken from an article i wrote on www.f7power.com. This is not a cut and paste from a web site.
Let's take four standard 185/55R15 road tyres, as supplied by Michelin, and inflate them to 30psi (2.25Kg/cm²). Now lets fit them to our Clio that weighs 990 kg. Roughly speaking, each tyre is taking about a quarter of your car's weight (247.5Kg). So using the following equation:
Pressure = weight / area
We can calculate the area of your contact patch as follows:
Contact patch (area) = 247.5 (weight) / 2.25 (Pressure) = 110 cm²
So the contact patch is 110 cm². If the width of the tyre is 185 mm then we can say the contact patch length is:
110 (contact patch area) / 18.5 (width of tyre) = 5.95 cm
So our contact patch is a rectangle of tyre measuring 18.5 cm wide by 5.95 cm long.
Now lets suppose that we fit a 205 tyre. Lets plug the same numbers into our equation
Contact patch (area) = 247.5 (weight) / 2.25 (Pressure) = 110 cm²
The weight of the car hasn’t changed an the tyre pressures remain at 30 psi, so the contact patch cant change. Its that simple. But here is the real twist in the tail. Lets work out the dimensions of the contact patch:
110 (contact patch area) / 20.5 (width of tyre) = 5.37 cm
So now our contact patch is a rectangle of tyre measuring 20.5 cm wide by 5.37 cm long. In other words fitting a wider tyre hasn’t increased the contact patch area, but simple changed the shape of that footprint.
Wider tyres do not therefore increase contact patch area.
Secondly, grip is related to the coefficient of friction. The coefficient of friction is is the ratio of the force required to slide the surfaces to the force perpendicular to the surfaces. A lower friction coefficient indictates that the surfaces are slicker - there is less resistance to the sliding motion.
Coefficient of friction is unrelated to surface area (which is irrelevant as we've just proved surface area doesnt go up with tyre width anyway). Coeffiecient of friction is however related to the softness of the rubber.Sifter rubber = more grip.
In short for those who cant be bothered to understand that:
Wider tyres do not have a larger contact patch. They dont increase grip. Wider tyres appear to offer more grip because wider tyres are usually made of softer rubber (because manufacturers can get away with softer rubber because the sidewall is lower; a high sidewall tyre wouldnt support itself on soft rubber). So if your 205 tyres are made of the same rubber as a 185 tyre they will offer no more total grip. However they will aquaplane you into the nearest field in standing water.
The main reason race cars run wide tyres is because wide tyres = bigger total surface area = more cooling. Narrow tyres would simple over heat quicker. Thats one reason the rear tyres are wider on F1 cars. Rear wheel drive = more heat in rear tyres = more need for cooling = need for wider tyres.
Tyre science isnt as simple as this in reality, but i guarantee that wide tyres wont increase grip or handling.
If you want more front end grip, you should be asking 'where can i find some softer tyres'?
And i think that my friend, nicely blows you out of the water.
Taken from an article i wrote on www.f7power.com. This is not a cut and paste from a web site.
interesting
so if you were making a hill climb car you would put bicycle width tyres on it? i say this because you can get the same grip as with any width tyre, you dont have to worry so much about cooling in a short race, and the enourmously decreased unsprung and rotating masses would be very useful indeed for acceleration and handling?
so if you were making a hill climb car you would put bicycle width tyres on it? i say this because you can get the same grip as with any width tyre, you dont have to worry so much about cooling in a short race, and the enourmously decreased unsprung and rotating masses would be very useful indeed for acceleration and handling?
The fallacy in Stromba's argument is assuming that the pressure inside the tyre is the same as that seen by the road. It is also a function of the carcass and tread stiffness. The tyre pressure tends to have more to do with the wall stiffness
He is right that the shape of the contact patch will change, but not to the extent that he postulates. The other problem with changing tyre width on the same rim is that the radial stiffness of the tyre may change - so again one is not comparing like with like.
The argument that friction is independent of area is well known in scientific circles, but is a first approximation and relies on many other factors - in particular an infinitely smooth surface.
Stromba, do you have an active technical involvment in motorsport, for as you will see I am not convinced that your arguments are entirely valid. Oh and while I am at it, dare I say that 'for those of you cant be bothered to understand that' is un-necessarily arrogant and patronising.
He is right that the shape of the contact patch will change, but not to the extent that he postulates. The other problem with changing tyre width on the same rim is that the radial stiffness of the tyre may change - so again one is not comparing like with like.
The argument that friction is independent of area is well known in scientific circles, but is a first approximation and relies on many other factors - in particular an infinitely smooth surface.
Stromba, do you have an active technical involvment in motorsport, for as you will see I am not convinced that your arguments are entirely valid. Oh and while I am at it, dare I say that 'for those of you cant be bothered to understand that' is un-necessarily arrogant and patronising.
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'Tyre science isnt as simple as this in reality'
I did say this at the end. Yes in reality it is more complicated that this. However, the argument about buying wider tyres to increase grip in inherently flawed.
Id rather have skinny tyres in a monsson for example.
I did say this at the end. Yes in reality it is more complicated that this. However, the argument about buying wider tyres to increase grip in inherently flawed.
Id rather have skinny tyres in a monsson for example.
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dcw@pr said:
you can't have it both ways[/quote:2ar5xb2s]
Yes you can. What i said was generally accepted science. However, there are so many variables in physics that reality is always more complicated. George is right, in some circumstances i will be wrong. But the theory 'generally' is sound. A general rule might be:
'If you want more grip get stickier tyres, not wider ones'
The trouble with this is that to get a stickier tyre, you have to have a softer tyre. A soft tyre cannot support itself on a high sidewall (without lots of roll), so it has to be a low profile tyre. Therefore if you want a super sticky tyre, you are forced to look at wide low profile tyres.
So why cant you have a low profile tyre that isnt wider? Well again, the physics dictates the tyre dimensions. If the tyre is softer it will heat up more quickly (or more correctly the same temp that doesnt affect a harder compound, will affect a softer compound). So sticky tyres tend to have as bigger surface area as possible to disperse heat.
Having said all this the speed of a car isnt solely related to grip IMO. Just because a car has more grip doesn make it 'better'. Im no expert, im sure some people know alot more about them than me on this forum. I do find the tyre science facsinating though.
I thought about Stromba’s theories overnight and how best to analyse them and check their validity.
As far as the contact patch area being a constant, I broke the hypothesis down into two elements; an air jack to represent the air inside the tyre and a piece of material on the foot of the air jack to represent the tyre contact patch. Using the corner weight on a front wheel of 800 lbs, then at a pressure of 30 psi, the area of the piston inside the jack cylinder would be 26 square inches. Stromba quite correctly says that for a given pressure the area of the piston is fixed.
However where his theory fails is that where the piston sticks out of the cylinder and makes contact with the ground, the footplate can be any size from less than the 26 to far greater. Thus it is entirely feasible to increase the area of the contact patch for a given pressure.
There are other arguments which become quite complex, but reduced to its most basic I do believe that this is a valid first approximation.
As far as friction being independent of area, this is a known scientific fact in theoretically ideal conditions. However in the case of tyres the ability of the rubber to lock into the minor surface irregularities (which all road surfaces have) changes the picture completely. I remember a couple of examples from F1 which might help illustrate this.
In the late 90’s I was involved in a team where we decided to exploit an anomaly in the tyre regulations. We felt that on a certain high speed circuit the reduction in drag from running smaller rear (narrower and I think smaller diameter) tyres would more than outweigh the possible loss of cornering speed. As we intended to try this in the days when qualifying tyres were allowed we felt that we could chose a compound which would last for a few qualifying laps, and then if it worked see if we could find a suitable compound for the race tyre. We did all the necessary simulations, including re-mapping the aero set-up and it looked as though it would work. We were disappointed to find that it did not. Top speed did improve, and wear was not a problem even on a soft compound, but the cornering speeds were significantly down, so it was not worth pursuing. A definite case of the effect of the reduced contact patch area, even with like for like compounds.
More recently it was found that the Michelin fronts were marginally too wide. Once this was rectified it is significant that most of the Michelin teams lost ground to the Bridgestone runners.
If life were as simple as this all you would need to do is to run wider and wider tyres. The law of diminishing returns will quickly apply, and if fitting wider tyres to the same rim the loss sidewall stability could well be counter-productive. The old adage of all things in moderation seems to apply quite well.
As far as the contact patch area being a constant, I broke the hypothesis down into two elements; an air jack to represent the air inside the tyre and a piece of material on the foot of the air jack to represent the tyre contact patch. Using the corner weight on a front wheel of 800 lbs, then at a pressure of 30 psi, the area of the piston inside the jack cylinder would be 26 square inches. Stromba quite correctly says that for a given pressure the area of the piston is fixed.
However where his theory fails is that where the piston sticks out of the cylinder and makes contact with the ground, the footplate can be any size from less than the 26 to far greater. Thus it is entirely feasible to increase the area of the contact patch for a given pressure.
There are other arguments which become quite complex, but reduced to its most basic I do believe that this is a valid first approximation.
As far as friction being independent of area, this is a known scientific fact in theoretically ideal conditions. However in the case of tyres the ability of the rubber to lock into the minor surface irregularities (which all road surfaces have) changes the picture completely. I remember a couple of examples from F1 which might help illustrate this.
In the late 90’s I was involved in a team where we decided to exploit an anomaly in the tyre regulations. We felt that on a certain high speed circuit the reduction in drag from running smaller rear (narrower and I think smaller diameter) tyres would more than outweigh the possible loss of cornering speed. As we intended to try this in the days when qualifying tyres were allowed we felt that we could chose a compound which would last for a few qualifying laps, and then if it worked see if we could find a suitable compound for the race tyre. We did all the necessary simulations, including re-mapping the aero set-up and it looked as though it would work. We were disappointed to find that it did not. Top speed did improve, and wear was not a problem even on a soft compound, but the cornering speeds were significantly down, so it was not worth pursuing. A definite case of the effect of the reduced contact patch area, even with like for like compounds.
More recently it was found that the Michelin fronts were marginally too wide. Once this was rectified it is significant that most of the Michelin teams lost ground to the Bridgestone runners.
If life were as simple as this all you would need to do is to run wider and wider tyres. The law of diminishing returns will quickly apply, and if fitting wider tyres to the same rim the loss sidewall stability could well be counter-productive. The old adage of all things in moderation seems to apply quite well.
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The trouble with using race cars as an alalogy for normal road cars is that they produce downforce that tyre grip is rarely a problem. What is a problem is tyre consistency and tyre degradation over a number of laps.
The best tyres in racing are usually the ones that remain most consistent over a run. Set up a car one way based on tyre at time zero, and if the tyre has altered its characteristics by lap 10 the set up of the car is wrong and the lap times increase.
Road cars dont have this problem (to the same extent).
But im not arguing with George, he seems to have some valuable and cogent points.
The best tyres in racing are usually the ones that remain most consistent over a run. Set up a car one way based on tyre at time zero, and if the tyre has altered its characteristics by lap 10 the set up of the car is wrong and the lap times increase.
Road cars dont have this problem (to the same extent).
But im not arguing with George, he seems to have some valuable and cogent points.
King Stromba said:
you can't have it both ways[/quote:3864xzlz]
Yes you can. What i said was generally accepted science. However, there are so many variables in physics that reality is always more complicated. George is right, in some circumstances i will be wrong. But the theory 'generally' is sound. A general rule might be:
'If you want more grip get stickier tyres, not wider ones'
The trouble with this is that to get a stickier tyre, you have to have a softer tyre. A soft tyre cannot support itself on a high sidewall (without lots of roll), so it has to be a low profile tyre. Therefore if you want a super sticky tyre, you are forced to look at wide low profile tyres.
So why cant you have a low profile tyre that isnt wider? Well again, the physics dictates the tyre dimensions. If the tyre is softer it will heat up more quickly (or more correctly the same temp that doesnt affect a harder compound, will affect a softer compound). So sticky tyres tend to have as bigger surface area as possible to disperse heat.
Having said all this the speed of a car isnt solely related to grip IMO. Just because a car has more grip doesn make it 'better'. Im no expert, im sure some people know alot more about them than me on this forum. I do find the tyre science facsinating though.[/quote:3864xzlz]
so in that case you can't "guarantee that wide tyres wont increase grip"?
time to remove yourself from the high horse
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I guaratee that the 205's on a trophy wont provide more grip than the 185's on a williams.
What youll find is that the 205s will provide grip like this:
Grip > Grip > Grip > Grip > Grip > Grip > Grip > Slide > Break away
The 185's on the williams will provide grip like this:
Grip > Grip > Grip > Grip > Grip > Slide > Slide > Slide > Break away.
So it 'feels' like the wider tyre is providing more grip because the point of slide is delayed. In a skinnier tyre will slide earlier, but that slide is what makes the tyre more fun to use. Its still gripping, but its grip begining to eb away. This sliding gives you a warning as to when you will lose control. Wider tyres give you less warning.
This is assuming that the two cars have the same compound. The Williams was actually released on a hard touring tyre, not a soft sport tyre.
What youll find is that the 205s will provide grip like this:
Grip > Grip > Grip > Grip > Grip > Grip > Grip > Slide > Break away
The 185's on the williams will provide grip like this:
Grip > Grip > Grip > Grip > Grip > Slide > Slide > Slide > Break away.
So it 'feels' like the wider tyre is providing more grip because the point of slide is delayed. In a skinnier tyre will slide earlier, but that slide is what makes the tyre more fun to use. Its still gripping, but its grip begining to eb away. This sliding gives you a warning as to when you will lose control. Wider tyres give you less warning.
This is assuming that the two cars have the same compound. The Williams was actually released on a hard touring tyre, not a soft sport tyre.