Alias Posted January 13, 2004 Share Posted January 13, 2004 Having seen a couple of threads on this, thought I would ask the question here rather than hijacking someone elses thread. The amount of friction generated by a surface is dependent on three aspects 1) The type of surface 2) The pressure per square inch on that surface 3) The area of that surface. If you have 185 tyres and 205 tyres of the same compound on the same car, surely they will generate the same amount of friction? The wider tyres have more surface area which will generate more friction, but have less pressure per square inch. The narrower tyres have less surface area but the pressure per square inch is higher. If 185s and 205s have an identical co-efficient of friction, what's the advantages of 205s? (apologies if I'm demonstrating an abysmal ignorance of physics/mechanics) One car - 1400 Supersport with 6 gears and clamshell wings. Link to comment Share on other sites More sharing options...
Paul McKenzie Posted January 13, 2004 Share Posted January 13, 2004 Larger contact patch Link to comment Share on other sites More sharing options...
Alias Posted January 13, 2004 Author Share Posted January 13, 2004 Yes, but less pressure over that larger contact patch thus negating its benefits. One car - 1400 Supersport with 6 gears and clamshell wings. Link to comment Share on other sites More sharing options...
Chelspeed Posted January 13, 2004 Share Posted January 13, 2004 I can see your logic but common sense says you're wrong. If you were right then we would all be driving round on push bike width tyres which would have the same grip but less weight and less drag. But we don't, so you must be wrong. Just need to wait for someone with some intellignce to explain why..... Link to comment Share on other sites More sharing options...
Blatman Posted January 13, 2004 Share Posted January 13, 2004 And to further muddy the waters........ I'd have though that, all things being equal, a wider tyre will run slightly cooler than a narrower one....... Link to comment Share on other sites More sharing options...
paul w Posted January 13, 2004 Share Posted January 13, 2004 I think you are basically right, as illustrated by the inferior performance of the wider tyres in the wet. I think the issue at hand is tyre temperature, you can probably still get a 205 up to a good working temperature as easily as you can a 185 so therefore there is more grip. Personally though, I think there is a far amount of 🙆🏻 on this subject and that many people over-tyre their cars for cosmetic/misguided reasons. Any extra grip/friction than is required is just slowing you down! Link to comment Share on other sites More sharing options...
MikeK Posted January 13, 2004 Share Posted January 13, 2004 I thought the problem with wide tyres in the wet was a lack of contact pressure resulting in an inability to clear water leading to aquaplaning.... MikeK Link to comment Share on other sites More sharing options...
Irrelevant Posted January 13, 2004 Share Posted January 13, 2004 RobF: I'm sure you are correct in what you're saying - a wide tyre gives the same theoretical grip as a narrow tyre. However, when the tyres are compared by being put on a car and driven round a bend, the wider tyres will take the lateral load better due to the wider shape of their contact patches - or something along those lines. I think it was Peter Carmichael that explained it; in his normal eloquent way, in a post I read a while back - can't find it now though. J351 TPE . . . battered old X/Flow Link to comment Share on other sites More sharing options...
paul w Posted January 14, 2004 Share Posted January 14, 2004 MikeK, I thought contact pressure was the main point of the original posting? Edited by - paul w on 14 Jan 2004 00:13:50 Link to comment Share on other sites More sharing options...
Alias Posted January 14, 2004 Author Share Posted January 14, 2004 I certainly concur that a narrower tyre should get warmer as the heat is concentrated in a smaller area. Perhaps the advantages of 205s is that they don't get too hot (I presume there comes a point when the tyre gets too hot its co-efficient of friction starts dropping again). Can definitely see that narrower tyres are more aerodynamic and better in standing water. One car - 1400 Supersport with 6 gears and clamshell wings. Link to comment Share on other sites More sharing options...
YellowSeven Posted January 14, 2004 Share Posted January 14, 2004 I cant offer an answer to your question, but look at F1, its the pinical of motorsport and the tyres they use are WIDE!! So, someone somewhere has obviously sat down and thought about it, done all calcalations (sums) and decided wide is best!! Link to comment Share on other sites More sharing options...
ozcrossflow Posted January 14, 2004 Share Posted January 14, 2004 But 205 will need a wider rim to keep the wall firm. The thought seems to be that you need this extra width only from 150 Hp and up. Maybe this can balance the understeer coming from more grip at the rear? But why is there more grip from a different shaped contact patch? It's all beyond my ( current!) knowledge, ability and power output. Peter Link to comment Share on other sites More sharing options...
Hector Posted January 14, 2004 Share Posted January 14, 2004 Just a thought - I think it's more to do with deformation of the tyre and the maintenance of a contact patch during cornering. If you think about a motorcycle tyre the contact patch is small but when you lean the bike you can maintain that contact area because you have a curved tyre surface. On a road car you have to keep the surface on the road without leaning. A smaller tyre may deform more in shear and so you lose the contact patch more quickly. A wider tyre can be made to resist the shear deformation and so you maintain the contact under higher shear loads. Does that make sense? Link to comment Share on other sites More sharing options...
Paul Bowden Posted January 14, 2004 Share Posted January 14, 2004 To add more questions and no answers, why do "performance" cars tend to have large diameter wheels/tyres. I am aware that high power Caterhams tend to use 13" , but alot of the sprint boys with high power cars use larger diameter wheels/tyres, is this the availablity of list 1A tyre sizes or is there a different reason Link to comment Share on other sites More sharing options...
Graham Perry Posted January 14, 2004 Share Posted January 14, 2004 You can't easily go above 205 inch with a standard road tyre at 13 inch. with the Softer 1B tyres nearly anything is possible. Now someone on here must have tried their 220 hp monster on 185's and then tried 225's or similar, can anyone explain what the difference they found was ? Edited by - Graham Perry on 14 Jan 2004 11:21:17 Link to comment Share on other sites More sharing options...
KenEvans Posted January 14, 2004 Share Posted January 14, 2004 Paul - the use of 15 inch wheels in sprints by the serious guys, especially at national level, is as you say down to greater choice of list 1A tyres. There doesn't seem to be any 195 list 1A tyres at all for 13 inch so it's one extreme or the other 205/215 or 185. Ken Link to comment Share on other sites More sharing options...
KenEvans Posted January 14, 2004 Share Posted January 14, 2004 I sprinted last year on 195/15 Bridgestone SO2 and then tried a couple of events on 185/13 RE720 basically to get the benefit of the shorter gearing and lighter weight. I was amazed with the grip, braking and turn in of the 185 tyres (buffed), not forgetting I wasn't only losing rubber on the width but also on the circumference. My times were slightly slower, only by tenths, and I believe this entirely due to not being able to put the power down out of bends in my very competitive car which incidentally is in the "for sale" section 😬 If I was running a car of 150bhp or less then I'd certainly go for the narrower tyre but on 13 inch of course. Ken Link to comment Share on other sites More sharing options...
JimB Posted January 14, 2004 Share Posted January 14, 2004 I remember doing this friction thing in a physics lesson at school (a long time ago) and I asked exactly the same question. The reply I got was that the theory was based around two sheets of metal sliding over each other and that, as tyres were actually a bit sticky, they would not necessarily behave like the theory. IMHO, a rotten answer (why did teacher not just admit to not knowing?). I am still waiting for a plausible explanation as to why tyre sizes seem to contradict Isaac Newton & Co. Link to comment Share on other sites More sharing options...
Chris W Posted January 14, 2004 Share Posted January 14, 2004 Rob The wider tyres have more surface area which will generate more friction, but have less pressure per square inch. The narrower tyres have less surface area but the pressure per square inch is higher. If 185s and 205s have an identical co-efficient of friction, what's the advantages of 205s? The width of the tyre does NOT affect the surface area nor the pressure per unit area. This may sound non-intuitive but let's run the maths and you will see why. First some assumptions: 1. Let's say the car weighs 600kg 2. Let's say this is distributed evenly over the 4 wheels, ergo 150kg downforce per wheel. Neither of these is exactly true but it doesn't change the maths only the numerical values. Therefore the weight of the car will squash the tyres down until the internal pressure of the tyre exactly matches the downforce. So the area of the tyre in contact with the ground will be the weight pushing down on it (150kg or 330lbs) divided by 20lb/sq in = 16.5sq in. The width of the tyre will only determine the shape of the rubber in contact with the ground but NOT its area. So a 185mm (7.3") width tyre will have a footprint of 7.3" x 2.3" =16.5 sq in while the 205mm (8.1") width tyre will have a footprint of 8.1" x 2" = 16.5 sq in. Of course in reality the footprint is not exactly rectangular but the principle is the same. The advantage of a wider tyre is that it usually has a softer compound and therefore more "stickiness". I also own a Jag XKR which has low profile 20" diameter wheels running 285's on the rear but again these enormous tyre do NOT give me a larger surface area in contact with the ground compared with narrower tryes. They are however very soft and stick like sh*t to a blanket. Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
Paul McKenzie Posted January 14, 2004 Share Posted January 14, 2004 Chris, You're wrong 😳 Pressure alters only the length of the footprint, not the width. The width of the footprint is determined by the width of the tyre, hence wider tyres give a greater area of footprint for the same pressure ...as I said right at the start 😬 Paul Link to comment Share on other sites More sharing options...
Michael_E Posted January 14, 2004 Share Posted January 14, 2004 Not sure if this covers everything you ever wanted to now about tyre physics but where too afraid to ask, but worth a look. http://www.performancearticles.com/topics.php?topicId=3 scroll down and start looking at the bits following "Introduction to The Physics of Racing article series" parts 21 to 25 appear to cover several aspects of how to measure the relative performance and more maths than is health for you. C7 MLE The Furry Fast Car Currently wondering what colour fur for 2004? Link to comment Share on other sites More sharing options...
Super_Rich_Bernie Posted January 14, 2004 Share Posted January 14, 2004 I don't particularly want to change the wheels on my car, but if the choice is limited to list 1A tyres 205 width has a serious disadvantage - availability of suitable tyres. The RE720 is not availble in 205 width, the NCT5 similarly. The choice after much shopping around was Falken ZE502, Kumho 711 or Toyo F5. I have just ordered a set of Falkens. The Kumho sounded good, but are something of an unknown since I haven't seen anyone sprinting on them. Jonathan Link to comment Share on other sites More sharing options...
Chris W Posted January 14, 2004 Share Posted January 14, 2004 Sorry Paul... you're wrong. A wider tyre will of course have a WIDER footprint - whatever its width is...... BUT the AREA of the tyre on the ground will be identical to any other width tyre at the same pressure carrying the same downforce. This area will change in length (as you correctly state) depending on the width of the tyre, but the arithmetic result of Width x Length = Area will be the same. So using wider tyres will NOT give you any greater AREA (footprint) on the ground. Only the aspect ratio (shape) of the footprint will change, as my calculations showed. If the tyre is supporting the same weight at a particular pressure, the area will give a particular lbs/sq in upforce to exactly cancel the downforce from the weight of the car. So wider tyres will have a shorter length footprint and narrower tyres will have a longer length footprint but the areas the same in both cases. cheers Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
KenEvans Posted January 14, 2004 Share Posted January 14, 2004 Jonathan - what are you planning to out on the front? 205 or 185? Ken Link to comment Share on other sites More sharing options...
Paul McKenzie Posted January 14, 2004 Share Posted January 14, 2004 The answer Wider tyres can be run at lower pressures hence larger footprint and greater grip. Top fuel dragsters, which are the ultimate in terms of power to be transmitted through the wheels, run extremely wide slicks at less than 10 psi, IIRC around 6-8 psi. Paul Link to comment Share on other sites More sharing options...
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