rudypoochris Posted November 27, 2006 Share Posted November 27, 2006 So basically my friend and I are having a debate whether or not the size of a tires contact patch affects friction when not sliding. At full grip is friction more with a larger contact area than with a smaller one? wikipedia says that contact patch does matter when not sliding, but can someone please verify this or add some insight, thanks! Quote Link to comment Share on other sites More sharing options...
blueovalz Posted November 27, 2006 Share Posted November 27, 2006 I would think that with alignment perfectly set at zero camber and toe, there would be no difference in drag. Introduce any changes to this and the drag increases for straight line motion. One comment I'd like to add to this query is the contact patch. For a given weight on the tire, and the pressure in that tire, the contact patch area will be the same for a 185 wide tire as it will be for a 315 wide tire. The difference between the two will be the shape of the contact patch. The 185 will have one longer (front to back) and narrower patch than the 315's short (front to back), but wide pattern. Quote Link to comment Share on other sites More sharing options...
johnc Posted November 27, 2006 Share Posted November 27, 2006 For a given weight on the tire, and the pressure in that tire, the contact patch area will be the same for a 185 wide tire as it will be for a 315 wide tire. That's true in a very broad sense, but tire construction plays a part in contact patch size. The most extreme example: a 315 run flat tire vs. a 315 wrinkle wall slick. The run flat will have more pressure per square inch of contact patch because of the stiff sidewalls and a smaller area while the wrinkle wall slick will have more square inches of contact patch with less pressure per square inch. So basically my friend and I are having a debate whether or not the size of a tires contact patch affects friction when not sliding. At full grip is friction more with a larger contact area than with a smaller one? Maybe. A lot depends on a tire's construction and tread compund. It also depends on the slip angles that tire works best at. When a tire is working nears its "best" slip angle the contact patch is not a static thing that is stuck to the road. There's a wave pattern moving through the contact patch itself with the rear of the contact patch providing the least amount of friction. Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted November 27, 2006 Author Share Posted November 27, 2006 Maybe. A lot depends on a tire's construction and tread compund. It also depends on the slip angles that tire works best at. When a tire is working nears its "best" slip angle the contact patch is not a static thing that is stuck to the road. There's a wave pattern moving through the contact patch itself with the rear of the contact patch providing the least amount of friction. Thats some good insight. The majority of the debate comes from the approximation for friction where: F = mu*R Where F is the frictional force, mu is the coefficent, and then R is the reaction force normal (I believe its called). Basically one argument is that contact area does not matter because there is no area in that equation. The other argument is that that function only works for non static friction and that that formula is an inaccurate example when it comes to tires. If one car pulls 1.0g's with large tires, and then pulls .8g's with smaller tires, can it be stated that larger tires create more friction with the road surface since they resist more force? Quote Link to comment Share on other sites More sharing options...
Guest magnadyne Posted November 27, 2006 Share Posted November 27, 2006 This is a tricky one and I can see where all the rumors came from. First off you learn in physics that surface area has nothing to do with friction - and they are right. If you picture a wooden block sitting on a wooden surface you learn that it will take a certain amount of force to push the block because there is a resisting force fighting back (this is friction). Friction comes from the amount of force pushing down on the contacting surfaces. This is called the normal force. Then you have to factor in the coefficient of friction. This is a measure of how easy 2 surfaces rub across each other. A low CoF would be a hockey puck on ice. A higher one would be sandpaper on wood. Knowing both the normal force and Cof you can find the resisting friction force. Notice that nowhere in here did we talk about surface area. However, this does not carry over to tires. The way tires work when you corner isn't as simple as a block sliding on a surface. You can think of tires as a bunch of tiny rubber balls that do work. Each little rubber ball's job is to grip the road. Now, you can do things to make it harder or easier on each rubber ball because they can only do so much. For instance, braking, accelerating, cornering, and the weight of your car all makes the little rubber balls work. So, try not to do 2 of those things at once. OR you employ more rubber balls - aka get bigger tires! The amount of work each rubber ball can do is directly related to the CoF of the tire. However unlike simple objects on a physics test, tires can get "stickier" by getting bigger. Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 27, 2006 Share Posted November 27, 2006 So what you're saying is bigger tires are "stickier"? Don't they have more "friction" then? Quote Link to comment Share on other sites More sharing options...
johnc Posted November 27, 2006 Share Posted November 27, 2006 Let's stop using the word FRICTION when we are really talking about GRIP. The above friction calculations are only somewhat approximate when a tire is spinning or sliding, they are completely irrelevant when a tire is not. A tire generates grip through "gearing" of the tread to the road surface irregularities and through molecular adhesion to the road surface. This grip process is the least understood aspect of tire behavior and is closely related to a tire's slip angle while generating lateral force. Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 27, 2006 Share Posted November 27, 2006 Why do land speed racers use the narrowest tires possible? It must be some other reason than power loss due to rubber meeting the ground. Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted November 27, 2006 Author Share Posted November 27, 2006 This is a harry one. So does having a larger tire and pulling 1.0g's compared to smaller and pulling .8g's mean there is more friction? Or is it the same? Thats basically what it comes down to. Or is that just a question that us mere mortals cannot solve at the moment? Quote Link to comment Share on other sites More sharing options...
johnc Posted November 27, 2006 Share Posted November 27, 2006 Please be clearer in what you're asking. A larger tire? Larger in OD, wider in width? LSR cars are concerned with aerodynamics more then anything else. A narrower tire has less frontal area given the same OD. Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 28, 2006 Share Posted November 28, 2006 I think this is simpler than that. Take a cue ball and put thirty pounds of pressure on it with the palm of your hand... roll it back and forth on a hard surface. Then do the same thing with a really squishy rubber-surfaced ball. Umm... the later is much harder to roll. But ya' know... this is just a simplistic observation based on empirical data. I'll leave the arguing of the physics to the experts here. I don't know nuthin'. Quote Link to comment Share on other sites More sharing options...
roninjiro Posted November 28, 2006 Share Posted November 28, 2006 never would have thought about this in that sense mike Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 28, 2006 Share Posted November 28, 2006 Well Ron... I'm just a simple guy. What else can I say;) Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted November 28, 2006 Author Share Posted November 28, 2006 I think this is simpler than that. Take a cue ball and put thirty pounds of pressure on it with the palm of your hand... roll it back and forth on a hard surface. Then do the same thing with a really squishy rubber-surfaced ball. Umm... the later is much harder to roll. But ya' know... this is just a simplistic observation based on empirical data. I'll leave the arguing of the physics to the experts here. I don't know nuthin'. You are talking about modifying the mu of the two materials. I am talking about widening the area of contact between the two materials. John, sorry I should have been more clear. I am refering to tire width only. Same OD. The tires for this debate are meant to have no effect on aerodynamics. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted November 28, 2006 Share Posted November 28, 2006 Let's stop using the word FRICTION when we are really talking about GRIP. The above friction calculations are only somewhat approximate when a tire is spinning or sliding, they are completely irrelevant when a tire is not. A tire generates grip through "gearing" of the tread to the road surface irregularities and through molecular adhesion to the road surface. This grip process is the least understood aspect of tire behavior and is closely related to a tire's slip angle while generating lateral force. I believe this is the heart of why it was believed for so long (decades?) that "theoritically", no tire could grip better that 1g. Once it was understood about how adhesion works, the 1g barrier no longer existed. Quote Link to comment Share on other sites More sharing options...
Guest magnadyne Posted November 28, 2006 Share Posted November 28, 2006 Yes, I suppose I should have been clearer to answer your question. In an effort to dumb everything down I think I missed some good points. As stated before you have to be careful using the word friction. There is friction between your tires and the road, but that is only part of the story. What you are really looking for is grip. I mentioned how none of the wooden block analogy applies to tires, but it does in some ways. That is, when we raised the normal force, we raised friction. You can do the same to your tires, only to raise the normal force on your tires means you have to add weight to your car. (the weight of the car pushes down on the tires more - aerodynamics can to this too)However, remember what we said about how your tires can only do so much work? With a heavier car your tires are under more stress, and less work is now being done in other areas like braking and cornering. So, while you raised the amount of friction between your tires and the road, you didn't gain anything in overall grip. I should also note (as pointed out earlier) that when I was talking about larger tires I am really talking about your contact patch. A wider (what I meant earlier about larger) tire will give you a larger contact patch. So, to sum up, the best way to get more grip is to get a larger contact patch, or raise your CoF. (as talked about earlier) Does this help? Quote Link to comment Share on other sites More sharing options...
Guest magnadyne Posted November 28, 2006 Share Posted November 28, 2006 This is a harry one. So does having a larger tire and pulling 1.0g's compared to smaller and pulling .8g's mean there is more friction? Or is it the same? Thats basically what it comes down to. Or is that just a question that us mere mortals cannot solve at the moment? Not more friction, but more grip. A tire that can pull 1g has more GRIP than a tire that can pull .8g's. Why does it have more grip? It could be because it has a better Cof (aka mu) or it could have a wider contact patch. Maybe the car that is pulling .8g's is too heavy for the tires used? Many things can go into why a tire will perform better than another. If a car ran a 1g then changed tires of the exact same size and ran a .8g my guess it would be the tire compound and resulting Cof (or mu) that gave the advantage. Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 28, 2006 Share Posted November 28, 2006 The only real input I can offer is that, when I installed wider tires with lighter rims (same make/model tires), the total weight per wheel was less. However I still felt a power decrease. This can be attributed to only two things that I can think of... 1. more friction due to slightly different compound, 2. more friction due to more rubber contacting the ground. Quote Link to comment Share on other sites More sharing options...
Guest magnadyne Posted November 28, 2006 Share Posted November 28, 2006 I suppose you could have lost power due to the larger amount of rolling resistance (inertia) you added. (yes, even with the lighter rims , but this is another conversation) However we are talking about grip. Did the wider tires grip any better? Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 28, 2006 Share Posted November 28, 2006 Yes, they did grip better. In particular, launch was better. Cornering was only marginally better. But the car wasn't that well designed... 1972 Formula Firebird with a 455. The rear was just too light and the suspension sucked. Quote Link to comment Share on other sites More sharing options...
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