All else being equal, tire width has no impact on patch size, only shape. - Discuss please. (Updtd)
11-22-2005, 07:59 PM
#31
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2nd picture
Source: An Introduction to Race Car Engineering. Warren J. Rowley.
I bought it a while ago, it is time to read it. Found these pictures there, as well a discussion on this topic.<ul><li><a href="http://www.startupracing.com/Other/Pictures/20PSI.jpg">http://www.startupracing.com/Other/Pictures/20PSI.jpg</a</li></ul>
I bought it a while ago, it is time to read it. Found these pictures there, as well a discussion on this topic.<ul><li><a href="http://www.startupracing.com/Other/Pictures/20PSI.jpg">http://www.startupracing.com/Other/Pictures/20PSI.jpg</a</li></ul>
11-23-2005, 10:08 AM
#32
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Maybe i'm missing something
but if pressure stays the same as well as the overall diameter of the tire when you go wider, why would the length of the contact patch decrease.
11-23-2005, 12:03 PM
#33
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Because the width increases.
For now just agree with my premise that the total area of the patch remains the same given weight on the tire and the amount of air. So if the patch gets wider, and the total area remains the same, somethign must get smaller - that is the length of the patch.
Or
X = A * B, X is 40 (for example), A goes up, what happens to B?
Or
X = A * B, X is 40 (for example), A goes up, what happens to B?
11-23-2005, 03:53 PM
#34
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Which begs the question...
A RWD drag car switches to a wider rear tire... why will it run a quicker E.T. yet lose MPH - due to increased mechanical drag. If the rear contact patch is the same, the MPH should not change, right?
11-23-2005, 05:02 PM
#35
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Well, interesting.
But if you look at the real fast drag cars their tires are skinny when comapred to how large they are. Talking the funny looking cars.
I am not sure if wider tire will lower ETs.... unless somehow wider tire can run less air.... maybe this is it. If you throw wider tire on, can you also reduce the pressure inside therefore increase the patch?
Did you check out the pics above?
I am not sure if wider tire will lower ETs.... unless somehow wider tire can run less air.... maybe this is it. If you throw wider tire on, can you also reduce the pressure inside therefore increase the patch?
Did you check out the pics above?
11-24-2005, 06:20 AM
#36
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Don't mistake a tall tire for a narrow tire.
I don't know what Funny cars run, but it's probably in the neighborhood of a 32X13.50 which is as wide as the widest tire on any road race car. The reason it's tall is so the sidewall can deflect and therefore give the tire a much larger contact patch in the first couple hundred feet.
I wasn't really referencing funny cars in my post above, more or less "wild street" or a slower class where competitors are still trying out different equipment in an attempt to maximize the car. The point I was trying to make is that if the contact patch really did stay the same for all tires, then a drag car wouldn't experience the mechanical drag of a wider tire. I'd venture to say that road race tires with extremely rigid sidewalls might not have the same extreme results as a drag tire. If the contact patch really did stay the same then I wonder why NASCAR doesn't run narrower tires when they race at tracks like Talledega and Daytona... it'd reduce the mechanical and aero drag that the tires create.
I wasn't really referencing funny cars in my post above, more or less "wild street" or a slower class where competitors are still trying out different equipment in an attempt to maximize the car. The point I was trying to make is that if the contact patch really did stay the same for all tires, then a drag car wouldn't experience the mechanical drag of a wider tire. I'd venture to say that road race tires with extremely rigid sidewalls might not have the same extreme results as a drag tire. If the contact patch really did stay the same then I wonder why NASCAR doesn't run narrower tires when they race at tracks like Talledega and Daytona... it'd reduce the mechanical and aero drag that the tires create.
11-24-2005, 06:28 AM
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I have some time this morning, so let me try to clarify my points...
Let's start with some background. I have a degree in Mechancial Engineering (though I am long gone from that profession) and worked in automotive development for a decade. Some of that was in tire testing (testing not design so I don't have huge depth there).
The Rowley book is, IMO, not very useful. It is a theory book, full of math. It is focused on designing a race car, not an area I care that much about. I am interested in tuning and driver development to go faster. Rowley seems to have added some very, very basic background info to make the book appear more for a general audience. His tire contact patch model is just flat wrong in so many dimensions.
I think I will do a couple of posts, each adressing an issue with the model.
The Rowley book is, IMO, not very useful. It is a theory book, full of math. It is focused on designing a race car, not an area I care that much about. I am interested in tuning and driver development to go faster. Rowley seems to have added some very, very basic background info to make the book appear more for a general audience. His tire contact patch model is just flat wrong in so many dimensions.
I think I will do a couple of posts, each adressing an issue with the model.
11-24-2005, 06:43 AM
#38
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#1 How does a tire worK?
Let's look at the extreme model of how a tire might work.
Case 1 - it is like an infinitley stretchable baloon. There is zero mechanical strength. Any amount of weight is going to settle on the floor. A real ballon is not that stretchable but it does have near zero mechanical strength. A bollon inflated to 14.7 psi (atmospheric) can hold almost no weight at all. Even sealed, it collapes on itself. The model of contact patch vs pressure and weight is reasonable.
Case 2 - It is infinitely strong (mechanically). This is the model of a cylinder and piston. As long as we are within the structural limits of the materials the piston will never collapse to the floor. The stiff sidewall (and total shape) simple forces the air into compression until it can support the weight. Contact patch never changes.
BTW - The pressure that determines how much weight can be supported is not gauge pressure (off a tire gauge), it is absolute pressure. So you need to add 14.7 to Rowleys data to see the realtionship he is really claiming.
What is a real tire? It is a mix of the two - More like the piston model with flexable parts. The sides can stretch and the piston can bend. You can not assume that a real world tire follows the model of either extreme. Your assertion that it is like a baloon would tell us that at atmospheric pressure in the tire, it can support ZERO weight. Well, take any mounted tire and let all the air out. Stand it up and it is supporting the weight of the wheel as well as some of it's own weight. Does it collapes? None that I have ever seen will collapse under that light weight. Put 2000 lbs on it and it will collapes.
What can we conclude from that little experiment? If the weight is high enough for a given tire, then we can ignore mechanical strength. Rowley used one example of a very weak tire with a heavy load. That does not mean it applies to all cars and all tires. It doesn't. I have two cars sitting in my garage and they will not behave the same way with dropping pressures. The race car has about 150 lbs on each fron tire. I can drop the pressure to near zero and there is little change in contact patch. The front engined passenger car at ~1200 lbs on each fron tire will certainly drop near zero pressure.
Conclusion - You can not ignore mechanical strength of a tire unless you are limiting your model to only those combinations where it can be ignored and that is not for all car/tire combinations.
Case 1 - it is like an infinitley stretchable baloon. There is zero mechanical strength. Any amount of weight is going to settle on the floor. A real ballon is not that stretchable but it does have near zero mechanical strength. A bollon inflated to 14.7 psi (atmospheric) can hold almost no weight at all. Even sealed, it collapes on itself. The model of contact patch vs pressure and weight is reasonable.
Case 2 - It is infinitely strong (mechanically). This is the model of a cylinder and piston. As long as we are within the structural limits of the materials the piston will never collapse to the floor. The stiff sidewall (and total shape) simple forces the air into compression until it can support the weight. Contact patch never changes.
BTW - The pressure that determines how much weight can be supported is not gauge pressure (off a tire gauge), it is absolute pressure. So you need to add 14.7 to Rowleys data to see the realtionship he is really claiming.
What is a real tire? It is a mix of the two - More like the piston model with flexable parts. The sides can stretch and the piston can bend. You can not assume that a real world tire follows the model of either extreme. Your assertion that it is like a baloon would tell us that at atmospheric pressure in the tire, it can support ZERO weight. Well, take any mounted tire and let all the air out. Stand it up and it is supporting the weight of the wheel as well as some of it's own weight. Does it collapes? None that I have ever seen will collapse under that light weight. Put 2000 lbs on it and it will collapes.
What can we conclude from that little experiment? If the weight is high enough for a given tire, then we can ignore mechanical strength. Rowley used one example of a very weak tire with a heavy load. That does not mean it applies to all cars and all tires. It doesn't. I have two cars sitting in my garage and they will not behave the same way with dropping pressures. The race car has about 150 lbs on each fron tire. I can drop the pressure to near zero and there is little change in contact patch. The front engined passenger car at ~1200 lbs on each fron tire will certainly drop near zero pressure.
Conclusion - You can not ignore mechanical strength of a tire unless you are limiting your model to only those combinations where it can be ignored and that is not for all car/tire combinations.
11-24-2005, 06:47 AM
#39
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#2 - What is the contact patch?
Rowley shows photos of a tire at different pressure, loads and camber, illustrating the contact patch. But clearly from his photos the tire is not evenly loaded. There are places of high contact force and low contact force.
Ooops - there are even rocks and areas where the tire is not touching the floor. What exactly is the contact patch? If 1 square inch of the tire is just barely touching, does that count? It carries near zero weight. If it was .001 inch higher it is not in contact and carries zero wieght.
Tires conform to the surface. Was his measurement surface flat? We know the tire was not consistent with the rocks. I would guess there was also rubber build up in some places.
His measurement of contact patch is suspect.
Ooops - there are even rocks and areas where the tire is not touching the floor. What exactly is the contact patch? If 1 square inch of the tire is just barely touching, does that count? It carries near zero weight. If it was .001 inch higher it is not in contact and carries zero wieght.
Tires conform to the surface. Was his measurement surface flat? We know the tire was not consistent with the rocks. I would guess there was also rubber build up in some places.
His measurement of contact patch is suspect.
11-24-2005, 06:50 AM
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#3 - sidewall strength
This adds to #1. Let's consider my very lightweight front tire load on my race car. If I put a tire on there with strong sidewalls, I can go to zero pressure and it will still hold weight. OTOH, if I put a very weak sidewall tire on there, it is going to tip the sidewall and the wheel will contact the floor (tire layers in between).
The sidewall strength matters. In the real world, the air pressure helps to support the sidewall. I get support from the air cushion that you are focusing on, but I also get added mechanical strength from the sidewall.
The sidewall strength matters. In the real world, the air pressure helps to support the sidewall. I get support from the air cushion that you are focusing on, but I also get added mechanical strength from the sidewall.