Resistance is futile: How tire pressure and width affect rolling resistance
Editor’s note: This tech article ran in the November 2012 issue of Velo magazine.
Whether on tarmac or singletrack, a tire with lower rolling resistance reduces the power required to move forward while also providing a better quality ride. The tire absorbs small bumps by not transferring them into the bicycle and rider, resulting in a smoother ride, faster speeds, and better cornering. How could you pass that up?
During a February, 2012 visit to Wheel Energy, an independent tire-testing lab in Finland, I was fascinated by the tests the lab was doing for tire brands like Vittoria, Specialized, Bontrager, and many others. As an independent lab, Wheel Energy will not disclose any of the results of a test it has performed for its clients, so the only way to obtain results was to pay for our own test. Many of us at Velo have our own opinions about what contributes to tire rolling resistance and we wanted to see if they held up when the rubber hit the road. Rather than which brand or model is fastest, we were much more interested in how tire width and tire pressure affect rolling resistance.
We sent a Zipp 303 wheel and five pairs of Challenge tires to Wheel Energy. We chose Challenge because the company makes a number of tubulars with similar casings and tread patterns in different tire widths. We tested the following tires:
Forté, 700 X 22mm, diamond tread pattern, 240 grams, 30mm tread width
Forté, 700 X 24mm, diamond tread pattern, 255 grams, 30mm tread width
Strada, 700 X 24mm, herringbone tread pattern, 250 grams, 28mm tread width
Strada, 700 X 25mm, herringbone tread pattern, 260 grams, 29mm tread width
Paris-Roubaix, 700 X 27mm, herringbone tread pattern, 320 grams, 37mm tread width
Editor’s note: Challenge no longer offers some of the tested sizes. The Forté tubular is now only available in 24mm size and 300tpi casing. The Strada tubular is available in the 25mm size with 300tpi casing, but not the 260tpi casing. The Paris-Roubaix tubular tire is now only sold with 300tpi casing, 27mm size. Challenge’s Criterium tubulars were not tested for this article, but they are available in 23mm and 25mm sizes with 320tpi casings or the Seta silk casing.
Wheel Energy warms up each tire for 30 minutes on a drum with a hydraulic load cylinder pushing the tire down against it. Besides getting the tires up to normal operating temperatures, this also serves to remove wax and mold-release lubricants from the tire.
The wheel is then transferred to a much larger, 1,200-millimeter diameter drum, which has bolt-on removable texture plates. To represent a road with cracks or chip seal, we chose a diamond plate on the roller similar to industrial stair treads. Each diamond-shaped bump is about 40mm long, 4mm high, and 3mm wide.
Each tire was run on the drum at 40kph with a 50-kilogram mass pushing down on the Zipp wheel’s axle. Wheel Energy ran two rolling-resistance tests with each tire — at 112 psi and 84 psi.
Once the power stabilizes, software records the power it takes to drive the wheel at that speed on the roller. The higher the power required to drive the wheel, the higher the rolling resistance of the tire.
When asked how he measures the power to drive the wheel, Wheel Energy director Petri Hankiola says, “It is secret information. We used about 1,000 hours to plan this device.” It almost certainly has something to do with measuring the current the motor draws to hold the speed at 40kph.
For rolling resistance coefficient (rr), the tires finished in the same order at both tire pressures, with the 24mm Stradas having the lowest coefficient (i.e., fastest rolling), followed by the 25mm Stradas, the 27mm Paris-Roubaix, the 24mm Forté, and the 22mm Forté.
All of the tires had lower rolling resistance at 112 psi than at 84 psi, with the difference being very slight with the higher-rr tires — the two Fortés — and the difference being greater with all of the herringbone-tread tires, the Stradas and Paris-Roubaix.
Based upon experience, especially with cyclocross and mountain bike tires, and from data shared with me by Specialized, Schwalbe, and Vittoria (much of which came from testing done at Wheel Energy), I had expected the wider tires to have lower rolling resistance than the narrower ones. I had also expected the rolling resistance to be less at lower pressure. While at first glance the former seems to be in doubt, a closer look reveals that that assumption is probably sound. The latter was proven not to be true, however.
Width versus rolling resistance
Tire rolling resistance on the road is caused by (1) internal friction and hysteresis (deformation over time) within the tire’s materials, and (2) on rough roads, small bumps lifting the bike and rider slightly (uphill) on each little impact.
By definition, wider tires will have shorter contact patches and, hence, less tire deflection; if the tire pressure is the same, the area of the contact patch must be the same to support the same load. The shorter the contact patch, the lower the vertical depth of tire deflection; the internal friction and hysteresis within the tire’s materials is lower.
If a wider tire is made of the same materials in the same thickness as a narrower one, it will roll faster, because (1) the internal friction and hysteresis within the tire’s materials will be lower, and (2) because the surface imperfections in the road will be absorbed into the tire more easily (since it has more deflection available), thus lifting the bike and rider slightly less with each little impact.
Thread count and thread pattern
Clearly on the Forté tires, the wider tire has lower rr. These two tires have identical construction and tread; they differ in casing diameter, which is a clear demonstration that wider is faster. The casing is 300 threads per inch (TPI) of Super Poly threads, the same thread material (but not the same thread thickness) used in all five pairs of tires tested.
The thinner the threads, the higher the thread count in TPI, which is simply the number of thread wraps lined up next to each other in one inch of the casing fabric. On tires like this, the casing is made by wrapping thread around a long, spinning spool and then wetting it with liquid latex. Then it is cut on the bias and folded over to make a two-ply casing with the threads in each ply crossing each other at right angles.
Generally, the higher the thread count, the more supple the casing. That’s because the threads are thinner in high thread-count tires, making the casing thinner, lighter, and more flexible. Also, when the tire encounters an object, thinner, lower-mass threads will be able to move and absorb the object into the casing more quickly than will thicker threads.
The only other tire in the test with a 300 TPI casing is the 24mm Strada, the tire that came out with the lowest rr of any of the tires. The wider (25mm) Strada and the Paris-Roubaix (27mm) both have thicker, and hence stiffer, 260 TPI casings than the other three tire models. Otherwise, there is no difference between the black casing of the Forté and the tan casing of the Strada and Paris-Roubaix. “It is just a few drops of yellow or black color in the latex,” according to Challenge Tech president Alex Brauns.
The herringbone tread on the Strada, however, is noticeably more supple than the Forté tread (a “diamond with S-shape sides” pattern, as Challenge calls it). The herringbone tread (one might also call it a file pattern) on the Strada and the Paris-Roubaix lies flat when deflated, whereas the diamond casing on the Forté seems much stiffer and holds its curved shape when deflated.
The rubber compound in the tread of the Forté and Strada is the same, but Brauns says that the “Paris-Roubaix has a slightly harder compound to increase protection from cuts and stones. The difference is small — from 50 to 55 (durometer) — to avoid losing grip on wet surfaces, but enough to give a more resistant compound.” The tread thickness on the Forté is the same as the Strada, but the Paris-Roubaix has thicker tread and its anti-puncture protection belt is wider.
Since the 25mm Strada has a stiffer casing, a wider tread, and a wider puncture-protection belt than the 24mm Strada, it is not surprising that it has higher rr than does the narrower Strada. And while the 27mm Paris-Roubaix has the same casing fabric as the 25mm Strada, it has a harder tread compound and a wider puncture-protection belt than the Strada, so it is also not surprising that its rr is a bit higher. Despite these extra stiffening measures in them, the 25mm Strada and the 27mm Paris-Roubaix both have lower rr than either of the narrower Forté tires, which also have suppler casings. So, I think that the contention that a wider tire rolls faster, all other things remaining equal, remains intact.
Inflation pressure versus rolling resistance
Lower tire pressure means fewer pounds of pressure on each square inch of the tire. Since the tire supports the same load at lower pressure, the contact patch will be larger. For example, Wheel Energy measured the footprint area of a 700 X 23mm tire inflated to 112 psi with 50kg of weight on it to be 75mm long and 15mm wide. The footprint of the same tire with the same weight on it, but inflated to 84 psi, became 82mm long and 14mm wide. The longer contact patch means that the tire deflection will be deeper, resulting in more internal friction and hysteresis within the tire’s layers. That makes the case that higher pressure reduces rolling resistance.
On rough surfaces, however, a tire at lower pressure is able to absorb more of the bumps than a tire at higher pressure, with less deflection of the bike and its rider. This is the same “sprung vs. un-sprung weight” argument that demonstrates why suspension makes a bicycle faster on rough terrain — it takes less energy to keep the bike rolling if only a small amount of weight is lifted (like a small section of the tire) than if the entire bike and rider is lifted by the bump.
If the bike were rolling on smooth glass, it’s clear that higher pressure would be faster. The question is, what is the ideal pressure for the surface you’ll be riding on?
For mountain bikes and cyclocross bikes, this is a lot more clear and shows why top riders tend to run quite low tire pressures on rough courses or where traction on sidehills is needed. On the road, there will be a point with every surface where, above a certain pressure, rolling resistance will increase.
Interestingly, the tires with the highest rolling resistance, the two Fortés, have almost no difference in rolling resistance at 84 psi versus 112 psi. The fastest rolling tire, the 300 TPI Strada 24mm and the widest tire, the 27mm Paris-Roubaix, had the biggest increase in rolling resistance (1.6 Watts) when dropping pressure from 112 psi to 84 psi. This indicates that the more supple tires (whether due to more supple materials or to more width) have the biggest change in shape with reduction in pressure and thus the greatest amount of corresponding internal friction and hysteresis due to the materials in the tire layers moving around more.
Clearly, the fastest tire here is the 300 TPI 700 X 24mm Challenge Strada. That’s the tire we would choose, even though it weighs more than the 22mm Forté.
We would also pick a wider tire when presented with two tires that are otherwise identical.