Chris W Posted January 14, 2004 Share Posted January 14, 2004 Paul Please note I always stated "at the same pressure". For sure, if you can run a particular tyre at a lower pressure you will get a bigger area. However.... second can of worms....... a larger AREA will also NOT give you more friction 😳 To use the values in our last example: If an area of 16.5 sq in has a pressure over it of 20 lbs/sq in (a downforce of 330lbs) then if you lower the pressure to say 10lbs/sq in in the tyre the area will double to 33 sq in but the total downforce is still 330lbs. The only way to increase grip is to significantly increase the downforce either by adding weight (obviously not good for performance) or by adding a lightweight downforce generator, ie: a tail fin (upside down aircraft wing) which can give huge downforce for very little weight impact. That's why racing cars employ these devices. Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
ImALoserBaby Posted January 14, 2004 Share Posted January 14, 2004 Quite enjoying all the above; So I would be right in saying that with a wider tyre (for the same car weight / tyre pressures) you'd get a 'longer' rectangle of contact with the road ( in the direction of the axis of ratation) Is this better in terms of performance in that more of the area of contact is directly under the centre of the wheel ( ie the road makes a tangent with the tyre ) and thus there is less torque (from the tyre) pushing downwards / upwards onto the road? Yes? ... or No? Link to comment Share on other sites More sharing options...
Paul McKenzie Posted January 14, 2004 Share Posted January 14, 2004 Chris, I don't know the equations but I'm willing to bet that a greater area of footprint, with the same downforce will transmit power more effectively via friction or whatever. There is a major problem somewhere with your logic, otherwise everyone would use bycycle width wheels on the rear to transmit maximum power with minimum weight and minimum aero drag. I think the answer lies in the wheel diameter, the larger the wheel, the larger the footprint for the same pressure. What was the original question? Paul Link to comment Share on other sites More sharing options...
Sheds Moderator Posted January 14, 2004 Share Posted January 14, 2004 ChrisW, I believe your reasoning is simplistic as it does not take into account the construction of the tyre and the fact that it is supported by the steel bands when inflated and is hence a structure in its own right. If I have a correctly inflated tyre at 20psi (minimal distortion) and then double the pressure the contact patch cannot double in area because the tyre can't change shape to that extent. Your reasoning only works with (say)an inner tube at very low pressures and an assumption of complete elasticity over the inflated range. Link to comment Share on other sites More sharing options...
Super_Rich_Bernie Posted January 14, 2004 Share Posted January 14, 2004 Ken 185 front, 205 back. I think I was parked next to you at MIRA - I have Nick Potter's old car. Sadly the Toyos on it are now past their best. Jonathan Link to comment Share on other sites More sharing options...
Paul McKenzie Posted January 14, 2004 Share Posted January 14, 2004 You're right BOSS...although I'm sure you meant halve, not double the pressure A limitation is definitely, tyre diameter. Paul Link to comment Share on other sites More sharing options...
neil.cavanagh Posted January 14, 2004 Share Posted January 14, 2004 my two pence worth... remembering those A-level physics lessons... Ignoring the shape of contact patch and the arguments over different tyre size/structure affecting this: If you are just talking about FRICTION then the contact area does NOT matter. It is the total force being applied and the co-efficient of friction. This is because if you double the area you half the PSI, so each 'inch' on contact produces half the amount of friction, but with twice as many 'inches' in contact you get the same total friction. I believe however that tyres are move complex than this in how they produce grip. The rubber actually 'sticks' to the road, forming glue like bonds with the road surface. So the bigger the contact patch the more rubber in contact with road to create these sticky bonds. So putting the two together, if you increase your contact patch you won't be getting any more 'friction' but you will be getting more 'stickiness'. So you run wider tyres, at a lower pressure ('cos their shape and reduced PSI load of car can handle this) and get more grip due to extra stickiness... Link to comment Share on other sites More sharing options...
R2D2 Posted January 14, 2004 Share Posted January 14, 2004 The Physics that everyone seems to be arguing about is only valid to describe the Limiting Friction condition and is based on a block of wood sliding along a plank. It is only the friction needed to cause the block to slide. As a tyre is always slipping relative to the road at the point of maximum grip I don't think limit friction conditions models are entirely valid. A chap called Amhurst Villiers described the basic manner in which tyres gripped in the 1920's or 30's and this inolved some means of estimating the mechanical grip as well as friction. This is the same Villiers that worked on the Blower Bentleys and ended up at NASA. There must be some record of the papers that he published Link to comment Share on other sites More sharing options...
Ken elle Posted January 14, 2004 Share Posted January 14, 2004 You all seem to miss the fact that as lateral loads increase the physics change. To try and work this out with a static example is pointless, we are not talking about a wheel on the kitchen table but a car going round a track. Your 150kg per wheel in the car park is irelevent when going round Gerrards with your foot planted. If you want to know what sticks best have a look at the tyres on a open class racing Caterham (such as JCC) that run on road legal tyres. Link to comment Share on other sites More sharing options...
Chris W Posted January 14, 2004 Share Posted January 14, 2004 Well guys... I know my equations are correct and my basic theory is correct. I'm not arguing about dynamic testing. I was ONLY countering the original statement that wider tyres mean a larger contact area at the same pressure. THEY DON'T! A great article on this and tyres in general can be found at "The Tyre Bible" here The piece about contact patch and friction etc is about half way down the article. This guy has all the equations you need. and then some. Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
Alias Posted January 14, 2004 Author Share Posted January 14, 2004 Chris W has articulated the very problem. If your contact area is doubled, the pressure on that contact area is halved, thus negating it's benefits. I understand that there are other forces coming into play here (this has to be the case otherwise we would all be driving on light-weight 10mm tyres), the question is, does anyone know exactly what these forces are? One car - 1400 Supersport with 6 gears and clamshell wings. Link to comment Share on other sites More sharing options...
Ken elle Posted January 15, 2004 Share Posted January 15, 2004 Gravity and acceleration. Link to comment Share on other sites More sharing options...
Alias Posted January 15, 2004 Author Share Posted January 15, 2004 😬 One car - 1400 Supersport with 6 gears and clamshell wings. Link to comment Share on other sites More sharing options...
Paul Ranson Posted January 15, 2004 Share Posted January 15, 2004 There are a whole bunch of assumptions being made here that don't seem plausible. Tyres are not balloons. The grip that can be generated by a tyre isn't linearly related to the vertical force on it, so ideas based on Newtonian friction coefficients will be misleading. The pressure over the contact patch isn't constant. The area of rubber on the ground matters as well as the pressure on that area. Consider Velcro. The force required to separate stuck Velcro strongly depends on the area in contact. Tyres interact with the road surface in a slightly analagous way. Consider camber. When we tilt a tyre does the contact patch stay the same size? Does the grip available from the tyre increase or decrease? In general the more rubber on the road the better, and wider or taller tyres put more rubber on the road. But the wider or lower profile your tyres are the more important it is to control the tyres presentation to the road. In practice for most users the suspension geometry of the Caterham is pretty fixed, so controlling the camber in roll or pitch is compromised. Turning the car into a kart with very high spring rates is one approach. But softer rubber overrides almost any other criteria. Small soft beats big hard (for once...) Paul Link to comment Share on other sites More sharing options...
Chris W Posted January 15, 2004 Share Posted January 15, 2004 Paul ".......... wider or taller tyres put more rubber on the road." No they don't!!!! 😬 You're still not grasping the fundamentals here. A tyre will squash down till the upward pressure it exerts exactly balances the static downforce (that part of the weight of the car acting down upon it - to a first approximation 1/4 of the total weight per wheel). The result will be a certain area of contact whose pressure per sq in integrated over the whole area exactly matches this downforce. Ergo, the shape of the tyre doesn't change the area in contact with the ground, it only changes the shape (aspect ratio) of the contact area. Note... I am not debating dynamic forces only static fundamentals. Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
Paul Ranson Posted January 15, 2004 Share Posted January 15, 2004 As I said, tyres are not balloons. The result will be a certain area of contact whose pressure per sq in integrated over the whole area exactly matches this downforce. Right. Ergo, the shape of the tyre doesn't change the area in contact with the ground, it only changes the shape (aspect ratio) of the contact area. Huge assumption. For a thought experiment try filling your tyre with concrete and letting it set. Would the resulting contact patch have length? Paul Link to comment Share on other sites More sharing options...
Michael Posted January 15, 2004 Share Posted January 15, 2004 From this it sounds like you base it all on the tyre distortion being linear. If you lay a can on it side you have a contact patch, a can twice as tall when layed over has twice the contact patch. maybe if your tyre distorts to the same scale that you've dreamt up it may be true, but a 6" wide tyre with 50psi will have half the contact patch as one 12" wide. (as near as dam it) and a light load will not alter that.. IMPORTANT: This email is intended for the use of the individual addressee(s) named above and may contain information that is confidential, privileged, or unsuitable for overly sensitive persons with low self-esteem, no sense of humor, or irrational religious beliefs. If you are not the intended recipient, any dissemination, distribution or copying of this email is not authorized (either explicitly or implicitly) and constitutes an irritating social faux pas. Unless the word absquatulation has been used in its correct context somewhere other than in this warning, it does not have any legal or grammatical use and may be ignored. No animals were harmed in the transmission of this email, although the Collie next door is living on borrowed time, let me tell you. Those of you with an overwhelming fear of the unknown will be gratified to learn that there is no hidden message revealed by reading this warning backwards, so just ignore that Alert. Notice from Microsoft: However, by pouring a complete circle of salt around yourself and your computer you can ensure that no harm befalls you and your pets. If you have received this email in error, please add some nutmeg and egg whites, whisk, and place in a warm oven for 40 minutes. Link to comment Share on other sites More sharing options...
Chris W Posted January 15, 2004 Share Posted January 15, 2004 Paul There are no assumptions, huge or otherwise. It's just simple arithmetic. If the contact patch is a certain area at a given pressure and you, say, increase the width of the tyre, then the length of the contact patch will decrease to compensate so that the overall area of the contact patch (length x width) remains the same. The AREA will only vary in size if the downforce is changed, whereas, for the same downforce and tyre pressure the area will remain constant but the aspect ratio of that patch (length vs width) will change as the tyre width changes but always so that the area remains the same. Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
Paul McKenzie Posted January 15, 2004 Share Posted January 15, 2004 Here we go again ☹️ Chris, Michael is right, it's all about tyre diameter Consider, for a moment, a wheel and tyre of infinite diameter, inflated to the same pressure as a 13" tyre......are you seriously going to tell us that the contact patch would be the same for both 🤔 Paul Link to comment Share on other sites More sharing options...
Pelico Posted January 15, 2004 Share Posted January 15, 2004 Can I just say that I for one am loving this thread. Keep going guys. My new Caterham owner's site Link to comment Share on other sites More sharing options...
Alias Posted January 15, 2004 Author Share Posted January 15, 2004 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). I do accept the point that when tyres start melting the friction they generate is probably different than when they are dry due to liquids having surface tension and so forth. One car - 1400 Supersport with 6 gears and clamshell wings. Link to comment Share on other sites More sharing options...
Ken elle Posted January 15, 2004 Share Posted January 15, 2004 But in cornering you generate more force, an narrow tyre wil slip before a wider tyre. Edgar Jessops law 1961. Link to comment Share on other sites More sharing options...
Sheds Moderator Posted January 15, 2004 Share Posted January 15, 2004 Chris, you said "If the contact patch is a certain area at a given pressure and you, say, increase the width of the tyre, then the length of the contact patch will decrease to compensate so that the overall area of the contact patch (length x width) remains the same." No it won't because tyres are not balloons. (Thanks Paul ranson). There is a false assumption. The force holding the corner of the car up does *not* come solely from the area of the tyre in contact with the ground multiplied by the air pressure in PSI but from the strength of the tyre which is a function of its structure (the air in it is part of its structure). Practical illustrations come as ever from studying extremes. If I have a racing bike with tyres at 120psi they are solid, they feel like stone. If I could double this pressure (I know I can't) then they would still feel like stone. The contact patch in use would not halve. Equally my bike unladen weighs 25lb. With me on it it increases to about 170lb, c.7 times the original. The tyre patch doesn't become 7x the size. The opposite goes for offroaders who drop tyre pressure to 2psi in snow. Therefore you can vary the size of the contact patch independently of vehicle weight. I could after all fit 285/40x20s to the Caterham. There would obviously be more rubber on the deck than with skinny tyres. What doesn't change is the corner weight so a broad tyre will exert a lower force per unit area betwen it and the road. Clearly rubber which is being pushed harder onto the road per unit area will stick more , so there will be a tradeoff as widths increase. Link to comment Share on other sites More sharing options...
smurfy Posted January 15, 2004 Share Posted January 15, 2004 This has been done to death on various other car forums and references taken for everyones learning from tyre manufacturers and chassis and suspension text books. The 'wide tyre equals more contact patch equals more grip' theory did not hold. Please search on the Subaru forum also, there are letters there from 2 major tyre manufacturers confirming the wide tyre greater footprint was not true. But they did confirm wider tyres and lower profile and different side wall construction could offer more grip (along with other variables). Link to comment Share on other sites More sharing options...
Paul McKenzie Posted January 15, 2004 Share Posted January 15, 2004 Ash, Boll*cks. 😳 Forget contact patches, forget all the other b*ll****, wide tyres with sticky rubber and low pressures have more grip, and I don't care about the maths. or about even what any tyre manufacturers say...this is fact Paul Link to comment Share on other sites More sharing options...
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