edmandsd Posted January 15, 2004 Share Posted January 15, 2004 Best tyres I ever had for my drag racing were a pair of 6.0/20.0/13 Hoosier Asphalt slicks but they don't make them any more. When I couldn't get those any more I imported some drag tyres from the States with a 9.5 inch tread (as opposed to section) width, but I honestly believe the car was overtired with these on. The fact that my rear end is super weatherweight obviously doesn't help matters, but as far as staightlining goes wider ain't necessarily faster on a Caterham or equivalent. Home of BDR700 Link to comment Share on other sites More sharing options...
smurfy Posted January 15, 2004 Share Posted January 15, 2004 Paul Thanks for your response ! I thought the same but took the time to listen, research and talk to the real knowledge holders and ultimately changed my view. I think I may agree "wide tyres with sticky rubber and low pressures have more grip" but unless I'm wrong thats not what was being debated. Ash Link to comment Share on other sites More sharing options...
smurfy Posted January 15, 2004 Share Posted January 15, 2004 Here I've found a link that although long is a good read http://www.scoobynet.co.uk/bbs/thread.asp?ThreadID=32479&Page=1 I can also share the names of the books I have referenced on the subject for those that are interested. Ash Link to comment Share on other sites More sharing options...
Tight fart Posted January 15, 2004 Share Posted January 15, 2004 As with many motoring facts. There is theory and there is bu!!sh!t. It only counts when your out there. You can't over simplify it with 2 or 3 sums. Bottom line is the chemical bond between the tyre and the surface. You also have to take into account rolling resistance, drag, weight, downforce and tyre life. F1 make a tyre for 5kms, others make tyres for 60k kms. You pays your money and you takes your chance. Any way less than 200bhp I would go for 7" all round and over 200bhp 7" fronts and 9" rears (tyres) T.F@O.F. A7 RDP Pics here Link to comment Share on other sites More sharing options...
Paul Ranson Posted January 15, 2004 Share Posted January 15, 2004 There are no assumptions, huge or otherwise. It's just simple arithmetic. Based on a huge assumption. All we know is the pressure inside the tyre and that the integral of the pressure between the road and rubber over the contact patch must equal the down force. You've offered no evidence that the average pressure between road/tyre is equal to the inflation pressure. And the key question was - even if you have a bigger contact area, you have less pressure per square inch, therefore the pressure/area ratio is the same as a smaller contact area. This should therefore generate the same amount of grip (according to Newtonian physics). 'Newtonian Physics' is about actions/reactions, inertia and rate of change of momentum being proportional to accelerating force. There is no universal truth in his friction model. In the case of tyres the grip available is non-linear with applied down force, and increases with increased contact patch for a given down force. Proper tyre data is very hard to get hold of, the best I know that is free is from www.avonracing.com, go to downloads and pick youf favourite racing formula. Inside the 'stab front' and 'stab rear' files are graphs of cornering force against slip angle for various down forces. What's clear is that the force doesn't double with a doubling of downforce. Paul Link to comment Share on other sites More sharing options...
Chris W Posted January 16, 2004 Share Posted January 16, 2004 Paul You've offered no evidence that the average pressure between road/tyre is equal to the inflation pressure. 🤔 I don't understand what you mean by this statement - can you explain please? Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
Paul Ranson Posted January 16, 2004 Share Posted January 16, 2004 Your assertion is that the contact patch area is equal to down force divided by tyre pressure and constant for different tyre geometries. But it's obvious that the pressure between the tread and the road surface isn't constant, this implies that your claim is the average tyre/road pressure being equal to the internal tyre pressure. I suppose this may tend towards trueness with heavy vehicles like Subarus and Land Rovers, but not with a Caterham at 1/4 the mass. Paul Link to comment Share on other sites More sharing options...
Chris W Posted January 16, 2004 Share Posted January 16, 2004 Paul Of course the pressure between tyre and road isn't constant. I don't disagree with that (in a dynamic situation). The static downforce is, of course, the downforce due to the weight of the car and passengers acting on that particular tyre and, as it's static, it's constant. However, when actually driving, cornering or braking etc the dynamic downforce will be all over the place.... sometimes more than the static downforce and sometimes less. I don't have any argument about that. But you can't get away from the fact that whatever the actual downforce is at any moment in time the tyre will resist it in an equal and opposite way by increasing or decreasing its contact patch. Since of course it takes a finite time for the tyre to distort and take up a new shape, the force which caused it to distort may have already changed before the tyre has had time to adopt its new shape and therefore there will always be a situation where the contact patch is lagging or leading where it wants to be in a dynamic sitiuation. I don't have any disagreement with that. So I go back to my original assertion (and only assertion) that wider tyres do not of themselves increase the contact patch area. There may be all kinds of other good reasons to have wider tyres due to stickiness etc but increasing contact patch isn't one of them. Go sit different weight people on a pedal bike, as someone suggested, with known tyre pressures in a static situation. You will find the contact patch area is very predictable and easily calculable. Those people who say above that maths doesn't matter and it's all obvious and a fact had better hope that there cars weren't built without maths. Chris 2003 1.8K SV 140hp see it here Edited by - Chris W on 16 Jan 2004 12:10:53 Link to comment Share on other sites More sharing options...
smurfy Posted January 16, 2004 Share Posted January 16, 2004 Guys I'm listening to all arguments with interest and its a good thread. Take a look at 'Car suspension and handling' written by Donald Bastow and Geoffrey Howard I wont quote bits as that may be unfair/out of context so read it and form your own opinions, because I did think bigger meant more grip (all other items constant) and now I dont. Trying to be a scientific as you can on a testing day, swapping to the same type of wider tyre did not appear to improve the grip levels (accepting variables)- straight line or cornering. And I wanted to believe it was better because I wanted an advantage for sprinting and hillclimbing. There are advantages for bigger and wider and without giving an exhaustive list I think it includes, better life, better ride, better sidewalls equally there are disadvantages and these are for example weight, water clearing etc. Ash Link to comment Share on other sites More sharing options...
Paul Ranson Posted January 16, 2004 Share Posted January 16, 2004 Of course the pressure between tyre and road isn't constant. I don't disagree with that (in a dynamic situation). In a static situation the pressure between the road and the tyre isn't constant across the contact patch. All we can say for sure is that the integral of pressure over the contact patch will equal the weight pushing down on the tyre. Paul Link to comment Share on other sites More sharing options...
Chris W Posted January 16, 2004 Share Posted January 16, 2004 yawn ❗ 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
DanB Posted January 16, 2004 Share Posted January 16, 2004 But you can't get away from the fact that whatever the actual downforce is at any moment in time the tyre will resist it in an equal and opposite way by increasing or decreasing its contact patch. True. But where's the evidence to suggest that the relationship is linear? If I have a 500kg car on 13" rims with a certain footprint area, are you suggesting that an 4 ton car will have 8 times the contact area? Cos it won't... So I go back to my original assertion (and only assertion) that wider tyres do not of themselves increase the contact patch area. Only if the relationship between weight/tyre width is linear. Which it isn't. Can't really see what the issue is... Link to comment Share on other sites More sharing options...
Slipper man Posted January 16, 2004 Share Posted January 16, 2004 I thinkwhat you are demonstrating here is that tyres are complex, and that simple linear models do not work in this application. There are a number of factors that are related. The first is the simple friction equation: Force (friction) = Coefficient of friction*normal reaction (i.e. weight) if that were the only factor then wide tyres would make no difference. With a tyre, however it forms a weak covalent bond with the road surface, and the larger the area of the bond the better it will stick. However, the strength of that bond is dependand on temperature, which is depenant on size (among other things) - if the tyre is too large it will not heat up enough to create the bond. The final factor is the 'rack and pinion' effect. The tyre tread distorts to 'lock on' to nooks and crannies on the road surface. Again, larger contact patch means this becomes more significant. My maths isn't up to modelling all this, together with the distortion effects from different sidewalls, tyre pressures and so on. Mind you, even if it was, you would need a Cray like the tyre companies have to run the program. Link to comment Share on other sites More sharing options...
Alias Posted January 17, 2004 Author Share Posted January 17, 2004 Hi Keith, thanks - that does answer my original question. One car - 1400 Supersport with 6 gears and clamshell wings. Link to comment Share on other sites More sharing options...
Tight fart Posted January 17, 2004 Share Posted January 17, 2004 Bottom line then is this, wider tyres are capable of generating more grip but you may not be able to generate that grip with the car. I think. T.F@O.F. A7 RDP Pics here Link to comment Share on other sites More sharing options...
Slipper man Posted January 17, 2004 Share Posted January 17, 2004 Correct! Link to comment Share on other sites More sharing options...
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