A friend of mine sent me this article Tech FAQ: Seriously, wider tires have lower rolling resistance than their narrower brethren, which states:
‘Wider is faster’ holds for clincher and tubular tires under the condition of the same air pressure.
If a wider tire is made of the same materials in the same thicknesses as a narrower one, it will roll faster, because the internal friction and hysteresis within the tire’s materials will be lower, and 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 on each little impact.
The claim is about rolling resistance (only!) using the same tire design constructed with the same materials in the same thickness with the same tire pressure. The risk I see is for a casual reader is to forget that all those factors are taken together when making a tire choice and actually riding it (at the same tire pressure with the same tubes on the same wheels).
But sometimes one should look at the forest, not the trees.
As far as I can tell, the Schwalbe Ultremo ZX 700 X 28C is not quite the same thickness as the Schwalbe 700 X 25C. Those two tires are exactly the same model, and vary in weight only by 14g (but with a much heavier tube for 28C). Right away the tube weight and possible tire/tube thickness difference goes outside the bounds of the rolling resistance claim. A 25C vs 22C would probably match in this regard for the tube at least.
The quoted wattage differences are 1-2 watts. Significant to be sure, but there are other considerations which might undermine the article’s claim:
- A 50kg /110 pound load is absurd. . That would be a 9-10 year old girl on a 15 pound race bike. My riding load is 175 + 22 = 197 pounds = ~90 kg. A quoted tire pressure of 8 bar / 116 psi for a 50kg load is absurd. Unless it’s 50kg per tire (220 pounds total), which is way beyond most rider weights. So neither figure seems rational.
- The test tire pressure of 116 psi is dangerously high for rim-blowoff on clincher wheels (personal experience) and beyond the max pressure for 28C tires, so one cannot extrapolate to 28C tires. Nor do most riders run that high a pressure, for reasons of comfort.
- The tire selection is extremely limited. These results are indefensible given the very few number of tires used.
- Is a Specialized tubular the same quality as an Italian-made tubular? Might the resistance properties be superior in some tires?
- The range of tire pressures is not quoted. Again, single context makes no sense at all— maybe slightly different tire pressures for 22/23C vs 25C would even out the differences.
- Running 92 psi in a 700 X 28C and the same 92 psi in a 700 X 25C or 700 X 22C is not what I’d expect to do; I'd run 105-110 psi in a 700 X 25C.
- A wider tire has a wider cross section. I don’t see wind resistance accounted for, straight on or with side winds. A tire on a roller in a lab is not taking wind resistance (including vortex drag) into account.
- Tire drag measured on a drum is not a real world situation. What about the tire squish from changes in rider position, sprinting, climbing?
- The tubes need to be different to accommodate 700 X 28C vs 700 X 25C/22C. The 28C tubes are almost twice the weight of the 25C tubes.
- The "same material" does not exist between clinchers/tubulars. The Veloflex Record tubular is not the same as a Veloflex Record clincher + butyl tube.
- It bothers me to see some factors omitted: the table doesn't say if clincher weight is with a tube is the same (a 700 X 28C tube is almost twice the weight of a 700 X 25C tube), nor does it count wheel weight aerodynamics. Clincher rims could be less aero than tubular rim/tire combinations (maybe, maybe not), and a lower weight total when climbing is not mentioned at all.
- If a 25C tire has lower resistance than a 22C tire, this does not prove that a 28C tire lower resistance than a 25C or 22C tire, especially once the tube weight and thickness and tire pressures are considered.
- Ride quality might matter more, especially on rough roads; this could cause a change in rider position (standing vs sitting). A rider might thus do better with a wider tire for that reason alone.
Charles J writes:
Here's a picture. Of course there is less rolling resistance in a larger tire at the same pressure. But you always have higher pressure in smaller tires for reasons clear in the picture above. It is not the pressure that determines the resistance, it is the tension on the tire wall. That determines how much it will deflect under load. More deflection means more resistance, not less. You can tell Mr. Zinn is confused as he gets that wrong from the second table.
As you can see, this is a principle that you empirically figured out in kindergarten.
This is called LaPlace's law - he was French. Perhaps this can be my second favorite French concept.
I know you know something about tubeless tires. The tension principle also accounts for why you can have a tubeless 2 inch mountain bike tire with 25 lb of pressure, but if you try to run a 35c cyclocross tubeless tire at low pressure, it will come off the rim. The small diameter tire with low pressure does not generate enough tension to keep from deforming under load.
DIGLLOYD: matches my intuitive sense. Maybe one can run a 23C tire at 116 psi and get away with it— or maybe one ends up in an ambulance because the extra stress of a larger tire (at the same pressure) rolls it off the rim— 116 psi is right near the limit for a 22C tire on many clincher rims. Running a 23C or 25C... well that’s a nightmare waiting to happen.
I abandonded clinchers that twice blew off the rim at 120 psi, and those were 22C tires. So the “same pressure” assumption is in my view a dangerious assumption that might lead to death or serious injury— running my 28C tires at 116 psi would mean an almost certain tire blow-off (larger tires exert more lateral force to pull off a clincher rim). See Why I Ride Tubular Tires, and remember, that same article shows tubulars to be much more efficient in the first place.