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Tech FAQ: More on fast-rolling tires

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Dear Lennard,
After reviewing the various information regarding tire size and rolling efficiencies, it would appear that if you ride “fast,” the use of a 700×23 front tire and a 25mm rear tire would be optimal. The 23mm on the front minimizes aerodynamic drag while the 25mm rear has increased rolling efficiency. My question is that since the larger tire is in the rear, wouldn’t it be less exposed to aerodynamic drag factors than the front tire, making it a slightly “faster” option? Or would this tire combination still have greater aero drag than the traditional 23-23 combination at speed? It would seem that this approach could be especially useful for TT applications.
— Mark

Dear Mark,
I agree. I think that would be an ideal way to approach the problem. Yes, the rear 25mm would have slightly more wind drag than a 23mm, but on the rear, the air passing by is already so “dirty” aerodynamically that I would be surprised if it were even a measurable effect. And the rolling resistance savings would be higher on the rear where you have more weight concentrated.

I think that is partly what Continental had in mind years ago when it offered a pair of tires in which the front one was smaller than the rear.
— Lennard

Dear Lennard,
I have found your tire pressure discussions fascinating and informative — thank you for educating us!

I am 130lbs and took your advice to go wider — I bought a beautiful pair of locally made I9 i25 wheels.  They had the HED C2 rim, which is nice and fat.  I’m running 23c Continental 4000 tires on the bike.  What tire pressure do you suggest?  I had been running 110psi on previous wheels and tried 90psi on these and it felt amazing.  Can/should I go lower??

Is there a rule of thumb for psi based on weight, like Stan’s does for his MTB wheels?  I think his is Rider Weight (in lbs) / 7 + 2 (for rear) and Rider Weight (in lbs) / 7 – 1 for front.
— Dave

Dear Dave,
I don’t have any hard and fast data on this, and I don’t know how Stan came up with his.

One way would be to experiment by performing your own roll-down tests alongside a buddy who always uses the same control equipment. Use the same hill, same position on the bikes, and same initial speeds each time. Be enough behind and to the side of your partner that drafting is not a factor, but close enough that your relative distance apart can be gauged fairly accurately as it widens or narrows depending on your relative speeds. Do multiple runs with each pressure change to eliminate factors like wind changes, passing cars, etc. I recommend changing pressure in only one of your tires at a time. I would be very surprised if you didn’t discover that the optimal pressure for the front tire is considerably below that of the rear tire.

Another way to start would be with the “15 percent tire drop” method Frank Berto wrote about years ago in Bicycling. His idea was that the optimum inflation pressure for comfort and rolling resistance is the one at which, when the rider sits on the bike, the tire “drops” (i.e., the rim moves down) 15 percent of the tire width. For a clincher, Berto says that this will be around 20 percent of the height of the rim above the ground.

So if you weigh 130 pounds, your shoes and clothes weigh three pounds, and your bike weighs 17 pounds, you have 150 pounds sitting on your wheels. It is worth it to check and see what percentage of that weight sits on each wheel. Put a bathroom scale under one wheel and a stack of wood or books the same thickness as the scale under the other wheel. Sit on the bike, clipped into your pedals, while touching the wall minimally for balance. Have somebody else read off the scale reading. Turn the bike around and do it again, so that the scale is now under the other wheel. Hopefully, the two measurements will add up to 150 pounds, and you’ll know how much is on each wheel when you ride on level ground.

For the sake of argument, let’s say that the weight on your rear tire is 100 pounds, which is not far-fetched. That would mean that 67 percent of the weight of you and the bike is on the rear tire, and 33 percent is on the front. If you were to pump your rear tire up to 100 psi, then your contact patch would be exactly one square inch in area. (This is because the tire would push down on the ground with 100 pounds of force while the ground would push up against the tire with the equal and opposite 100 pounds of force, and since there is a pressure of 100 pounds per square inch in your tire, then the area of contact is one square inch.)

For a one-square-inch section of the tire to flatten out on the road, the tire has to squish down some percentage of its height. To achieve a one-square-inch contact patch, Berto wrote that a 20mm tire drops about 0.25 inch, or 10 percent of its width, and a 32mm tire drops about 0.2 inch, or 18 percent of its width. For fun, you could calculate what width of tire would be required to have this amount of squish be equal to 15 percent of the tire width, and that would tell you what size tire to run at 100 psi. Roughly extrapolating from Berto’s estimates, that looks like around maybe 27-28mm…

In any case, Berto actually did the work. He inflated 700c tires from 18mm to 37mm in width to pressures ranging from 30psi to 140psi. He set the wheel on a scale, pushed down on the wheel with a hydraulic jack, and measured with a dial caliper how much the rim dropped down. He then drew lines on a graph of “Optimum Inflation Pressure in PSI” vs. “Total Weight of Rider and Bicycle + Load” and got a linear plot for each within the load range of 80-280 pounds and pressure range of 30psi to 140psi.

You could just read off on his graph what pressure to put in. But the unfortunate thing is that his graph assumes that the load is borne 50-50 on the two tires, which I have never found to be the case (or even close to it) with the bathroom scale test I described above. Figuring for your rear tire loaded at 100 pounds, I’d need to use the 200-pound mark on his horizontal axis, and for your front tire loaded at 50 pounds, I’d need to find the intercept with the 23C line above 100 pounds on his horizontal axis.

Looking at his hand-drawn graph on unlined paper, it looks like roughly 50psi for the front and 105psi for the rear. Since that sounds like close to 100psi for a 23mm tire will be optimal with a 100-pound load on it, there’s something fishy either in Berto’s graph or in his assertion that a 20mm tire drops about 0.25 inch, or 10 percent of its width, and a 32mm tire drops about 0.2 inch, or 18 percent of its width, to achieve a one-square-inch contact patch. (Rather than 23mm, we saw that his assertion looked like it would take a 27-28mm tire to achieve a one-square-inch contact patch at 100psi with a 100-pound load on it.)

Obviously, experimentation from some of us is needed!

You could sit on the scale on your bike with your tires at various pressures and see what you get! Since the tolerance in some tire factories on tire width is as much as plus/minus 4mm, you have to first measure your tire width with a caliper. If your 23C tires actually measure 23mm in width, you’re looking for the rim to drop down 3.5mm closer to the ground when you’re on your bike than when it is completely unloaded. Then further roll-down tests could tell you whether Berto’s 15 percent rule is actually optimal…
— Lennard

Regarding the comments and data from Zipp on tire aerodynamics:
Dear Lennard,
If you are interested I can pull some more data on this, but the point I was mainly trying to make is that Crr (coefficient of rolling resistance) is a big deal, and there are many tire choices where the less aero tire with lower Crr is still the better choice, but the data has to be understood, as aero and low Crr is going to be the better option.  The Al Morrison data is really great. Kraig Willet sells some great data on his site discussing different shapes of tires and their affect on aero and Crr.
— Josh Poertner
Zipp Lead Engineer

Dear Lennard,
Your response to Charlie in the Midwest doesn’t seem entirely correct. Or it may be correct, but for the wrong reasons. The friction that keeps a bicyclist from losing traction in a corner is static friction, not sliding (a.k.a. kinetic) friction. Either way, the friction is the same regardless of tire pressure (and by extension, contact patch or surface area). The benefits of lower tire pressure come not by increasing friction, but by minimizing the upward forces of bumps in the road. When you hit a bump, your weight on the pavement is reduced, which decreases friction (friction = mass*coefficient of friction). By lowering tire pressure, you minimize the degree to which bumps change the downward force of your weight (in this case equivalent to mass) on the pavement, and the friction holding you upright is less likely to decrease suddenly. In short, lower tire pressures/increased contact patches don’t “increase grip,” they decrease the likelihood of losing it.
— Ethan