Does a 25mm wide tire have less contact area than a 23?

[dasho]

I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?

[johnny99]

Originally Posted by dasho

I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?

Depends on the tire pressure. Wider tires should generally be run at a lower tire pressure, which increases the contact area.

[Spoke Wrench]

Originally Posted by dasho

I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?

Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.

At equal pressure, the shape of the contact patch on a 23mm tire will be longer from front to back and the 23's contact patch will be shorter and wider. A long, narrow contact patch will generate more rolling resistance because the tire has to deform more from it's natural round shape to make the flat patch where it contacts the road.

[Fredrico]

Originally Posted by Spoke Wrench

Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.

At equal pressure, the shape of the contact patch on a 23mm tire will be longer from front to back and the 23's contact patch will be shorter and wider. A long, narrow contact patch will generate more rolling resistance because the tire has to deform more from it's natural round shape to make the flat patch where it contacts the road.

You mean the 25C's contact path will be shorter and wider, right?

I've heard that a long, narrrow contact patch provides more rolling resistance from the tire deforming, too. Its quite noticeable at 95-110 psi. So you have to air up the 23Cs really hard, and then they ride harshly.

Besides better pinch flat protection and providing a more comfortable ride, that's another good reason to run 25Cs. Too bad LBSs don't stock more of them. Heck, the 28Cs on my commuter bike are about as fast as the 25s, same tire, Conti Ultra 2000. Nobody stocks those. Tyranny of the weight weenies.

[Cory]

In my speed-obsessed youth, I did the traditional experimenting with all kinds of tires, down to 19mm. I weighed about 220 then, and skinny tires were just unpleasant. I moved up to 25s despite the then-common belief that skinnier was always faster, and I was a lot more comfortable and didn't go any slower. Eventually I settled on 32s for most riding, 35s for rides when I know the pavement will be bad. Last summer I put on a set of 23s (apparently what most people on this board use), and I HATED 'em.
As one of the other posts mentioned, its hard to find anything bigger than 25mm that isn't a big, clunky touring tire. Two I've used a lot, and really like, are Panaracer Paselas (come in 28, 32, 35 and 37mm) and Rivendell's Ruffy Tuffy, which I think is 27.
The labels, incidentally, aren't worth much. I just measured the Paselas on my bike and my wife's, and the 35s are actually 30.5 mm, the 32s about 25mm.
All right, I've got too much time on my hands. I've been shoveling snow for six hours, and there's another storm due tonight.

[Fredrico]

Originally Posted by Cory

In my speed-obsessed youth, I did the traditional experimenting with all kinds of tires, down to 19mm. I weighed about 220 then, and skinny tires were just unpleasant. I moved up to 25s despite the then-common belief that skinnier was always faster, and I was a lot more comfortable and didn't go any slower. Eventually I settled on 32s for most riding, 35s for rides when I know the pavement will be bad. Last summer I put on a set of 23s (apparently what most people on this board use), and I HATED 'em.
As one of the other posts mentioned, its hard to find anything bigger than 25mm that isn't a big, clunky touring tire. Two I've used a lot, and really like, are Panaracer Paselas (come in 28, 32, 35 and 37mm) and Rivendell's Ruffy Tuffy, which I think is 27.
The labels, incidentally, aren't worth much. I just measured the Paselas on my bike and my wife's, and the 35s are actually 30.5 mm, the 32s about 25mm.
All right, I've got too much time on my hands. I've been shoveling snow for six hours, and there's another storm due tonight.

My search is over. I'll check out those Panaracer Paselas down at the shop. 30.5 cross section would be a great commuter/winter tire.

Watch your back with all that shoveling. Can you believe that? Snow! And more coming? Outside Reno, Nevada? Heck, put some more logs on the fire, make a hot chocolate, and read that book you've been putting off.

[cxwrench]

Originally Posted by Spoke Wrench

Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.

huh? how can pressure determine contact patch. enlighten me. what if it's a 26" tire. or a 650c. or a 27". what if i don't go by psi, but use bar instead...that means my contact patch would be in sq/cm, right? what about the difference between 20mm tires and 25mm tires. or what about difference in brands. they all have different shapes. this sounds like a crock to me, but i'm open to any science that'll prove it.

[curlybike]

Originally Posted by cxwrench

huh? how can pressure determine contact patch. enlighten me. what if it's a 26" tire. or a 650c. or a 27". what if i don't go by psi, but use bar instead...that means my contact patch would be in sq/cm, right? what about the difference between 20mm tires and 25mm tires. or what about difference in brands. they all have different shapes. this sounds like a crock to me, but i'm open to any science that'll prove it.

For example, if you weighed 200 lb and the weight on your wheels was distributed 50%/front/50%rear. That would give you 100 lb per wheel, the bike weighs 20 lb, for a total of 220 and 110 per wheel. A tire that has 110 psi inflation pressure will have a contact patch of 1 sq. in. Becuse the down pressure is matched by the tire pressure you get a round # as the combined bike weight goes up you divide the individual loaded weight of each tire by the inflation pressure to derive the area of the contact patch. Works for any size tire and any tire pressure. You will have to do the metric conversion. 1 bar =14.7psi, 1 square inch =6.4516 sq. cm. I hope that this is not too muddy a concept, but that is the way it works.

[Spoke Wrench]

Originally Posted by cxwrench

huh? how can pressure determine contact patch. enlighten me. what if it's a 26" tire. or a 650c. or a 27".

OK, take a look at a friend's bike just standing by itself. Rim diameter and tire width don't matter. The normally inflated tires will look virtually round even at the bottom. Now have your friend sit on the bike and check out the contact patch. The tire will be squished down somewhat and have a flat contact patch area from front to back. With your friend still sitting on his bike, let some air out of his tire. The contact patch will elongate and become bigger because he will have less air pressure to hold him up.

Now I'm curious. What did you think determined the size of your tire's contact patch?

[AFS]

The narrower tires may buy you something aerodynamically, if you go fast enough. The aerodynamic drag reduction would have to out weigh the increase in rolling resistance. The necessary average speeds maybe out of reach for most riders on a given course . I run 28-mm tires.

[cxwrench]

Originally Posted by curlybike

For example, if you weighed 200 lb and the weight on your wheels was distributed 50%/front/50%rear. That would give you 100 lb per wheel, the bike weighs 20 lb, for a total of 220 and 110 per wheel. A tire that has 110 psi inflation pressure will have a contact patch of 1 sq. in. Becuse the down pressure is matched by the tire pressure you get a round # as the combined bike weight goes up you divide the individual loaded weight of each tire by the inflation pressure to derive the area of the contact patch. Works for any size tire and any tire pressure. You will have to do the metric conversion. 1 bar =14.7psi, 1 square inch =6.4516 sq. cm. I hope that this is not too muddy a concept, but that is the way it works.

this is sounding ridiculous. the tire has 110psi in it. that 110 pounds per sq inch pressure is exerting itself outwards against the tire casing. i understand that. but who came up w/ the formula that says this pressure equals this size tire patch? the amount of air pressure inside a tire versus the amount of weight riding on it can't strictly determine the size of the contact patch.at this point you're not even looking at the size of the tire, you're basing this solely on pressure vs. weight supported. obviously it is going to have an effect on the size, but make that work for me w/ a car tire. c'mon, i dare ya...what effect does speed have on this? what about flat profiled car tires vs round profile motorcycle tires. there are just too many variables that you aren't taking into account.
why are we even arguing about this anyway? this is why the term "bike geek" exists.

[cxwrench]

Originally Posted by Spoke Wrench

OK, take a look at a friend's bike just standing by itself. Rim diameter and tire width don't matter. The normally inflated tires will look virtually round even at the bottom. Now have your friend sit on the bike and check out the contact patch. The tire will be squished down somewhat and have a flat contact patch area from front to back. With your friend still sitting on his bike, let some air out of his tire. The contact patch will elongate and become bigger because he will have less air pressure to hold him up.

Now I'm curious. What did you think determined the size of your tire's contact patch?

i had never bothered wasting time thinking about it. you think i don't realize that a tire w/ no rider weight on it will have a smaller contact patch that one w/ the weight of a rider on it? c'mon, i did go to school some of the time, and i'm not blind. i just don't understand how you can "formulate" this w/o taking the size of the tire into consideration. get somebody w/ "dr." after his name or an engineer to say that this is the way things are, and i'll believe it, but 'til then, i don't.

[Lifelover]

Originally Posted by Spoke Wrench

Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.

WOW there must not be much contact area when the 20 lb bikes does not have anyone sitting on it.

Theoreticallly, any statement that starts with the word "Theoreticallly" is wrong.

[asgelle]

Originally Posted by cxwrench

i had never bothered wasting time thinking about it. you think i don't realize that a tire w/ no rider weight on it will have a smaller contact patch that one w/ the weight of a rider on it? c'mon, i did go to school some of the time, and i'm not blind. i just don't understand how you can "formulate" this w/o taking the size of the tire into consideration. get somebody w/ "dr." after his name or an engineer to say that this is the way things are, and i'll believe it, but 'til then, i don't.

O.K. I'm an engineer and have Dr. before my name, is that good enough? Gravity provides a force based on the mass of an object. On earth, that force is equal to the object's weight directed downward. For an object to be at rest, the forces on it must be in balance. So for a bicycle standing on it's tires (rather than the rims) there must be an upward force equal to the weight of the bicycle. That force is provided by the pressure in the tire acting over the contact area. Pressure is force per unit area directed perpendicular to the surface of the object; so the force from the tires is inflation pressure times contact area directed perpendicular to the ground. Setting the downward force equal to the upward force gives: weight of bicycle (and rider) = pressure in tires X tire contact area. Think of lowering the bicycle to the ground. At first the tire will be undeformed and there will be no upward force. The bicycle will move down and the tire will deform untill the up and down forces are equal. At that point, the contact area, A, will be A=weight on the tire/tire pressure. Size and shape of the tire will determine the shape of the contact patch, but the contact area is set only by the weight on the tire and the pressure in the tire.

[Spoke Wrench]

Originally Posted by AFS

The narrower tires may buy you something aerodynamically, if you go fast enough. The aerodynamic drag reduction would have to out weigh the increase in rolling resistance. The necessary average speeds maybe out of reach for most riders on a given course . I run 28-mm tires.

Yeah, there are some other factors that impact tire performance also.

Generally you have to use higher air pressure with a narrower tire or you'll get pinch flats. That pretty much negates the whole tire patch rolling resistance thing that we were just talking about.
A tire with a thin tread layer and nice supple sidewalls will deform more easily and consequently have less rolling resistance.

All of those things have an effect and, when added together, can make a difference, but everything combined is still small potatoes when compared with the amount energy that has to be used to push your torso through the air.

[RCA]

I am big and I ride a bike (those are my qualifications for my expertise) I have tried 23s,25,and 32s I now put 95lbs of air in Conti 2000sx32 and ride. If they are slowing me down then I will work harder and lose more weight. If as some of you allege they are faster then I will enjoy the speed. They allow me to head out as a roadie then scurry home ahead of a storm on an unpaved rail trail.
Don't ever stop arguing boys and girls here in the land of ice and snow you are my entertainment. I'll go out and shovel now so the forum can build up again
Rick

[AFS]

Originally Posted by Spoke Wrench

Yeah, there are some other factors that impact tire performance also.

Generally you have to use higher air pressure with a narrower tire or you'll get pinch flats. That pretty much negates the whole tire patch rolling resistance thing that we were just talking about.
A tire with a thin tread layer and nice supple sidewalls will deform more easily and consequently have less rolling resistance.

All of those things have an effect and, when added together, can make a difference, but everything combined is still small potatoes when compared with the amount energy that has to be used to push your torso through the air.

It's funny how we tend to worry about the small potatoes.

[MisterMo]

Originally Posted by asgelle

Size and shape of the tire will determine the shape of the contact patch, but the contact area is set only by the weight on the tire and the pressure in the tire.

I should know better to stick my nose in this but... You are correct in the 'textbook' sense, one where the tire materials and resistance to shape deformation are assumed to have little enough effect to be ignored. The stiffer the tire materials involved, the less effect the pneumatic properties have...the ride of course goes to hell as well.

[asgelle]

Originally Posted by MisterMo

I should know better to stick my nose in this but... You are correct in the 'textbook' sense, one where the tire materials and resistance to shape deformation are assumed to have little enough effect to be ignored. The stiffer the tire materials involved, the less effect the pneumatic properties have...the ride of course goes to hell as well.

And which model bicycle tire would that be where the sidewall stiffness is significant? We're not talking runflat car tires here. The bike tires I use aren't stiff enough to support their own weight but I'll grant you, if you use total weight to calculate contact area your error may be as great as 0.03% (20 gm sidewall support for a bike+rider weight of 150 lbs). Now tell me on what other aspects of your life do you impose that rigor? Do you measure your ride distance to 3 parts in 10,000? What computer do you use and how often do you correct roll out to account for changes in weight and tire pressure (don't forget your weight and the air temp in the tire changes during a ride).

Oh my God, I just realized I also forgot to include the van der Waals attraction of the tire surface to the roadway. That would pull the tire to the road and increase contact area. It get's even worse since the area will increase with time the longer the tire rests on the road.

[dasho]

Originally Posted by AFS

It's funny how we tend to worry about the small potatoes.

I wasn't really worrying about it but I found it hard to believe.

[C-40]

"Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in."

I don't think so, Spoke Wrench. Your statement might sound logical, but the contact area would NOT be one square inch, just because the air pressure and load are the same.

[asgelle]

Originally Posted by C-40

I don't think so, Spoke Wrench. Your statement might sound logical, but the contact area would NOT be one square inch, just because the air pressure and load are the same.

What would the area be then, and how would you determine it?

[bigrider]

Originally Posted by C-40

"Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in."

I don't think so, Spoke Wrench. Your statement might sound logical, but the contact area would NOT be one square inch, just because the air pressure and load are the same.

Copied and pasted from the internet on HOW A TIRE WORKS

The next time you get in your car, take a close look at the tires. You will notice that they are not really round. They are squished at the bottom. The flat spot on the bottom where the tire meets the road is called the contact patch.
If you were looking up at a car through a glass road, you could measure the size of the contact patch. You would multiply the length of the contact patch by its width to get the area, then add up area for all four tires to get the total area of the contact patch.



A tire showing the side and bottom view of the contact patch.
For your 2-ton (4,000 lb) car, you will find that the area of the contact patch is about equal to the weight of the car divided by the tire pressure. In this case 4,000 pounds divided by 30 pounds per square inch equals 133 square inches. That may seem like a lot, but your car's tires are probably about 7 inches wide. That means that the contact patch for each tire will be about 4.75 inches long.

[Bill70J]

Originally Posted by asgelle

What would the area be then, and how would you determine it?

It's very difficult to refute what asgelle is saying - it's proven theory. Infinitesimal as they would be, he has even offered to take into consideration differences between ideality and a physical measurement.

[Reynolds531]

Originally Posted by asgelle

And which model bicycle tire would that be where the sidewall stiffness is significant? We're not talking runflat car tires here. The bike tires I use aren't stiff enough to support their own weight but I'll grant you, if you use total weight to calculate contact area your error may be as great as 0.03% (20 gm sidewall support for a bike+rider weight of 150 lbs). Now tell me on what other aspects of your life do you impose that rigor? Do you measure your ride distance to 3 parts in 10,000? What computer do you use and how often do you correct roll out to account for changes in weight and tire pressure (don't forget your weight and the air temp in the tire changes during a ride).

Oh my God, I just realized I also forgot to include the van der Waals attraction of the tire surface to the roadway. That would pull the tire to the road and increase contact area. It get's even worse since the area will increase with time the longer the tire rests on the road.

When the weight is applied to the tire the tire deforms and reduces the volume in the tube, thereby increasing the pressure and reducing the contact area. This might reduce the contact area by another 0.1%or so, however, this error can be eliminated if you inflate the tires while sitting on your bike.

And what about the angular acceleration which effectively pushes outward on the tire, tube, and the air molecules within the tube? Doesn't this also reduce the contact area? All these errors reduce the contact area for 125 psig tires loaded with 125 lbs of weight to 0.99873 square inches. That's my number, prove me wrong.