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Technical FAQ: Tire widths and pressures

  • By Lennard Zinn
  • Published Aug. 12, 2014
  • Updated Oct. 31, 2014 at 6:13 PM EST
Determining which tire pressure to use is mostly a trial-and-error process. Photo: Lennard Zinn | VeloNews.com

Tire width and pressure vs. traction

Dear Lennard,
I am a Physics teacher, and I am writing in response to your recent column regarding running different tire widths. In your response to Doug you said, “The 25mm tire also gives you more traction, assuming you’re running lower pressure in it than in the 23mm one.”

I am writing for clarification as to why a 25mm tire would provide better traction at lower pressure. My assumption is that you are basing your answer on the fact that a lower pressure will provide a larger area contact patch. However, friction does not depend on contact area (for non-interlocking surfaces). Friction only depends on normal force and coefficient of friction. It is a common misconception that friction depends on surface area. It is also a common misconception that a wider tire will have a larger area contact. As you understand, contact area depends only on tire pressure and rider plus bike weight. The only other possibilities I can think of as to why lower pressure would increase traction is that on a bumpy road, the lower pressure tire might bounce less, and thereby maintain physical contact with the road. Or, a lower pressure tire on a rough surface might create an interlocking effect as the low pressure tire bends to conform to the road surface.
— Michael

Dear Michael,
Great question. First of all, on a bicycle (particularly a mountain bike and a cyclocross bike), you do have interlocking surfaces much of the time. Secondly, as you suggested, a tire at low pressure bounces up less with each little bump than the same tire with more pressure in it. The reduced pressure in the tire not only increases the surface area of contact and the interlocking of the surfaces, but with each bump it encounters, the normal force doesn’t drop as much as it would with higher pressure.

Anyone who has experimented with tire pressures while riding a mountain bike or a cyclocross bike on a side hill, particularly on mud or wet grass, knows very well that running a tire at lower pressure increases traction. Same goes with a road bike when the coefficient of friction is low — like on wet, oily, or icy roads, there is a noticeable increase in traction, both cornering and driving traction, when you let some air out of the tires. The rougher the surface, the more this is true, because, the coefficient of static friction between the tire and its rolling surface is higher than the coefficient of kinetic (sliding) friction; if the tire bounces up enough on a deviation in the surface to drop the normal force enough that the tire starts to slide, it’s not likely to stop sliding when full weight once again comes back onto the tire. (When the tire is rolling and not sliding, static friction applies, because the contact patch of the tire on the ground is stationary relative to the ground.)

You’re right; a wider tire will not have a larger contact patch if it is run at the same pressure as a skinnier tire. (That’s why I specified that the rider was using lower pressure with the wider tire.) A tire inflated to 100psi sitting on a flat, smooth surface and holding up 100 pounds of weight will have one square inch of contact with the surface. If the tire is wider, the contact patch will be wider and shorter, but its area will be the same as that of the longer, narrower contact patch of the skinnier tire.
― Lennard

Mixing tire sizes

Dear Lennard,
I just read your most recent column about using different sized road tires. As a mountain biker, I am used to running the larger tire in front. As a tubeless road user, I was ecstatic about the Hutchinson Secteur and ran them in the front and rear until I had some frame rub in the rear. Since then, I’ve replaced the rear with a 25mm Hutchinson at 90. I run the front at 80. I’ve have found excellent performance on paved and dirt roads and have yet to be cited for breaking any of Boulder’s unwritten rules.
— Jeff

Dear Jeff,
Way to evade the Boulder PC police!
― Lennard

Deflating tires after a ride

Dear Lennard,
Some folks deflate their tires (tubulars or clinchers) after each ride, but I don’t. I would like to know if it makes a difference or not on the durability or performance of tires. I ride Michelin Pro Race clinchers and Continental tubulars.
— Denis

Dear Denis,
Deflating tires after every ride makes no sense to me. While I can understand the thought that perhaps the strain on the tire casing is high just sitting with the tires inflated, I doubt that deflating it will increase longevity, unless the rider is using excessive pressure in the tires or leaving them in a hot car or other hot area.

If the tire is fully deflated, there is no doubt in my mind that the casing and the tube are more compromised by being folded flat with the weight of the bike on them than they would have been if inflated. If they are only partially deflated, or if the bike is hung from a hook with the tires deflated completely or partially, then, yes, that reduces the strain on the casing. (As long as it’s not completely deflated, I don’t think it matters one way or another for the inner tube how much pressure it has in it, as long the tire is properly seated.)

However, I am willing to bet that damage to the casing from flexing it while riding, from abrasion, from soaking, and subsequent drying while riding in the rain and through puddles, and from the sun’s rays is so much higher than from storage between rides as to make it irrelevant whether you deflate them or not after riding.

I would ask yourself, “What is the cause of most of my tire failures?” My guess is that it’s not the casing failing at a point not damaged from riding, but rather, it’s the tread wearing out or being punctured. If so, the whole thing seems like a waste of time to me. Furthermore, pumping the tires every single day (which yes, it does need to be done anyway if you’re training on high-end tubulars or other tires with latex tubes) often results in prematurely breakage or bending of the valve stem with some pump heads and increases the wear of the valve gasket inside of the pump head. In the long run, these hassles would seem to far outweigh any trivial gains in longevity that might be attained through daily tire deflation.
― Lennard

The ideal tire pressure

Dear Lennard,
After much research and asking, I am still completely mystified as to what tire pressures I should be running front and back for my bikes.

Me: 144 pounds

My bikes:
— S-Works Epic Full Suspension Cross Country Bike 2.0 Fast Trak Tires
— S-Works Venge Road Bike with 24c Turbo Tires
— S-Works Shiv Time Trial Bike with 24c Turbo Tires
— Kris

Dear Kris,
This is a good place to start on a road bike at least: 15 percent tire drop. If you have 60 percent of the weight of you and the bike on the rear wheel, you would have around 95 pounds of weight on the rear tire and 66 pounds of weight on the front tire. With a 24mm tire, you can see on the graph that a 15-percent tire drop would correspond to around 90psi of pressure in the rear tire and 60psi in the front.

You’d want to actually measure your weight distribution, of course, and I’m sure that there will be a big difference in weight distribution between the Shiv and the Venge.

Then you’d experiment from there. I generally run lower pressure in the front than the rear, and it has taken some experimentation for me to find what kind of percentage difference I prefer between them. I find that a rear/front tire-pressure ratio on the order of 3 to 2, like that 90/60psi I just mentioned, is too much for me. That’s because I care most about front tire pressure when I have more weight on it than when static and level on a scale and a block. For instance, if I’m descending fast down a mountain and encounter a hard switchback, the combination of the downward angle of the bike and the shift of weight to the front tire when braking gives me much more weight on the front tire than I have in a static and level situation, and that’s a time when I really care about how my front tire responds and tracks. So I like more pressure in the front than the graph would indicate in order to compensate for that; I don’t worry about slightly higher rolling resistance because of it.
― Lennard

Minimizing tubular flats

Dear Lennard,
I recently bought a pair of Bontrager Aeolus 5 tubular wheels for use during ’cross season, but I also don’t want to only use them in the fall. So I glued on some Bontrager XXX road tubular tires and added 1.5-2 ounces of Stan’s Sealant in order to run them on fast group rides. Sadly, I’ve suffered two rear flats in 20 rides, both of which looked like pinches. One sealed well enough to get me home when I added CO2, but the other refused to hold air and forced me to rip off the tire and install a pre-glued spare on the roadside. It wasn’t a horrible process, but I’d rather not make a habit out of it.

I’m 6-foot-2 and 200 pounds. Do you have any tire selection and/or pressure advice for me that I can use to minimize flats while riding these wheels? I love the way they corner and feel, but dislike worrying about getting stranded.
— Eric

Dear Eric,
Using Frank Berto’s table from Bicycle Quarterly, you can see that you need higher pressure or higher tire section or both than probably most of the guys on your fast group rides. If you have the same 60/40 weight distribution described above, you and your bike, with bottles, spare tire, and your clothes, shoes, food, etc., will have around 140 pounds of weight on the rear tire. That corresponds to around 150psi on the Berto graph with a 23mm tire (!) and around 130psi with a 25mm tire.

Having sufficient pressure and/or sufficient tire volume is the way to avoid pinch flats, even though the latex tubes inside high-end tubulars, combined with the more rounded rim edges on tubular rims, already are highly resistant to pinch flats. Since you’ve specified neither the tire size nor the pressure you were using, that’s all I can say at this point.

Since it’s happening on the rear, it may only be your rear tire that needs to have more air pressure or more volume (bigger tire size).
― Lennard

Switching sealants

Dear Lennard,
If I want to change the sealant type/brand (i.e. from Stan’s to Caffelatex) in my tubeless tires, what do I need to do so there is no conflict?
— Valerie

Dear Valerie,
Those sealants are water soluble. All you have to do is wash the inside of the tire with water to remove any trace of the “old” sealant liquid; don’t sweat trying to get all of the hardened stuff out. Dry the inside of the tire with a clean rag, and you’re ready to put in the new sealant.
― Lennard

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Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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