Tire width considerations

[genedoc]

I badly cut my rear Vittoria Open Corsa EVO tires on my Fuji Team Pro on a
training ride and went to buy a replacement. The tires on the bike were 700x23, but the replacement the LBS had turned out to be 700x20. I didn't see that at the time and now I have one 23 on the front and one 20 on the rear. I can't say I've noticed any difference. What are the considerations for going with the narrower or wider tire. Does it really matter? I'm 6'2" and 180#, and I run them both with 120# (they're supposed to have 120-140#).

 

[John M]

There probably won't be much of a difference in ride if you keep the tires properly inflated. The 23 will actually have a slightly lower rolling resistance than the 20 and may be more puncture resistant and more resistant to pinch flats.

 

[gclark8]

Put the 20 on the front and the 23 on the rear.

 

[Strid]

Put the 20 on the front and the 23 on the rear.

Yep, that would be the smart thing to do in this case!

 

[genedoc]

There probably won't be much of a difference in ride if you keep the tires properly inflated. The 23 will actually have a slightly lower rolling resistance than the 20 and may be more puncture resistant and more resistant to pinch flats.

OK. How does a wider tire at the same pressure give less rolling resistance? Seems counter intuitive. Also, I can understand the pinch flat benefit, but why the puncture resistance?

 

[bobbyOCR]

OK. How does a wider tire at the same pressure give less rolling resistance? Seems counter intuitive. Also, I can understand the pinch flat benefit, but why the puncture resistance?

I do know that 23 will be more stable when cornering. Pinch flats are punctures as well. All I can think of is less surface area in contact with the ground. Also, a well designed 23c tyre can have a lower rolling resistance than a 20c tyre.

But do put the 20 on the front, it would look better and also be more aero.

 

[genedoc]

I do know that 23 will be more stable when cornering. Pinch flats are punctures as well. All I can think of is less surface area in contact with the ground. Also, a well designed 23c tyre can have a lower rolling resistance than a 20c tyre.

But do put the 20 on the front, it would look better and also be more aero.

No question about the 20 in front and 23 in the rear (if I don't buy another tire to match one or the other, which is sort of my plan), but I'm still confused by your reasoning and maybe I'm just missing something, but the 20 should have less surface in contact with the ground and therefore would have less rolling resistance. In this case we're talking about identical quality tires - just different diameters.

 

[Retro Grouch]

OK. How does a wider tire at the same pressure give less rolling resistance? Seems counter intuitive. Also, I can understand the pinch flat benefit, but why the puncture resistance?

Assuming that everything else is equal, at equal air pressure and load all tires will have the same contact patch area with the ground. Although the area will be equal, the shapes will differ. The narrower tire will have a longer and narrower contact patch. Now take a tire and try to flatten a portion of it with your hands. The effort that it takes to flatten the tire from it's normal round shape is the major component of rolling resistance. Since a narrower tire will have a longer contact patch, it has to be flattened more from it's normal round shape. Consequently it will have more rolling resistance.

There are, however, some other factors that at least partially mitigate this advantage:

First of all, most riders use more air pressure with narrower tires. This factor alone might be enough to equalize the shape of the contact patch and negate the whole rolling resistance argument. Harder tires may have some other drawbacks, but that's another whole issue.

Narrower tires have less frontal area and consequently are more aero.

Narrower tires will weigh less.

Truthfully, all of these factors combined are still tiny. Compared with the effort that's required to push your torso through the air, everything else is small potatoes. If your goal is to ride faster, work on your position on the bike.

 

[DiabloScott]

No question about the 20 in front and 23 in the rear (if I don't buy another tire to match one or the other, which is sort of my plan), but I'm still confused by your reasoning and maybe I'm just missing something, but the 20 should have less surface in contact with the ground and therefore would have less rolling resistance. In this case we're talking about identical quality tires - just different diameters.

The 20 should go in front but for another reason: it's new; you always want good tread on your front tire, that's why you shouldn't rotate them. Rears wear out faster so it's OK to put your front on the back and a new one in front.

Contact patch is probably the same for a 20 and 23 on the same wheel at the same pressure, but contact patch in the rear is higher than the front because of the weight distribution.

 

[alienator]

OK. How does a wider tire at the same pressure give less rolling resistance? Seems counter intuitive. Also, I can understand the pinch flat benefit, but why the puncture resistance?

Because the narrow tire's casing has to deflect/deform more to form the contact patch. Rolling resistance is all about energy loss in tire flexion.

 

[John M]

OK. How does a wider tire at the same pressure give less rolling resistance? Seems counter intuitive. Also, I can understand the pinch flat benefit, but why the puncture resistance?

It is counter intuitive. This subject has been debated alot and it has been scientifically studied. Do a google and you can find several discussions on this.

 

[bobbyOCR]

That is what I had heard, just needed reminding (abut the equal contact, longer narrower etc.). Running to high a pressure in your tyres also increases rolling resistance on poor quality roads.

 

[Akadat]

OK. How does a wider tire at the same pressure give less rolling resistance? QUOTE]

The theory is that a narrow tyre will have a long narrow contact spot, a wide tyre has a more round contact spot. The length of the contact spot determines rolling resistance, not the width; therefore wider tyres have less RR. That is assuming they have equal pressure, and ignoring wind resistance, and that narrow tyres can go to higher pressures. The difference in RR between 20mm and 23mm must be very small, perhaps 1 more pound of pressure will cancel it out?

 

[Strid]

OK. How does a wider tire at the same pressure give less rolling resistance? QUOTE]

The theory is that a narrow tyre will have a long narrow contact spot, a wide tyre has a more round contact spot. The length of the contact spot determines rolling resistance, not the width; therefore wider tyres have less RR. That is assuming they have equal pressure, and ignoring wind resistance, and that narrow tyres can go to higher pressures. The difference in RR between 20mm and 23mm must be very small, perhaps 1 more pound of pressure will cancel it out?

I don't completely follow this reasoning, but anyways, just to sum it up... so you would say that 23c is the "sweet spot" in tire width? If it gets wider, it gets worse, and if it gets less wide it also gets worse? Otherwise everyone would be riding MTB slicks at insane pressures.

 

[Retro Grouch]

I don't completely follow this reasoning, but anyways, just to sum it up... so you would say that 23c is the "sweet spot" in tire width? If it gets wider, it gets worse, and if it gets less wide it also gets worse? Otherwise everyone would be riding MTB slicks at insane pressures.

Another interesting topic.

Air pressure is constantly trying to blow your tires off of the rim. Air pressure is, of course, measured in pounds per square inch. Since a larger tire has more square inches of sidewall area there's more total force trying to push the beads sideways past the rim flanges.

 

[alienator]

OK. How does a wider tire at the same pressure give less rolling resistance? QUOTE]

The theory is that a narrow tyre will have a long narrow contact spot, a wide tyre has a more round contact spot. The length of the contact spot determines rolling resistance, not the width; therefore wider tyres have less RR. That is assuming they have equal pressure, and ignoring wind resistance, and that narrow tyres can go to higher pressures. The difference in RR between 20mm and 23mm must be very small, perhaps 1 more pound of pressure will cancel it out?

This is an incorrect line of thought. Increasing tire pressue in a narrower tire will not necessarily improve performance. It is true that strictly speaking, rolling resistance does go down with increasing inflation pressure for a given tire. However, as tire pressure goes up other losses start to manifest quickly. One such loss occurs when a tire, inlfated to high pressure hops over road imperfections instead of conforming to them. These perfections can impart a force on the tire that has components that are in the opposite direction of travel. It is because of this that high pressures aren't beneficial except on extremely smooth surfaces, like velodrome surfaces.

Worse yet is the decrease in contact patch size with increasing tire pressure. Smaller contact patch means less friction and less traction.

23's are what might be called the best compromise between low rolling resistance and minimized drag.

 

[threaded]

Actually I'd leave the fat one on the front and the thinner one on the back, but that's 'cause my hands need the extra cushion, and I only use the seat to help keep the bike upright.