MattyK wrote:
GreenPlease wrote:
Many manufacturers say go with 25mm for "crr" but that's mostly a red herring driven by an incorrect understanding of air pressure, casing tension, and crr. A 25mm tire does not roll measurably faster than a 23mm tire of the same construction when adjusted for air pressure (proper crr measurements would compare the 23c at a higher, not equivalent, air pressure to the 25c).
Please tell me if I'm wrong, but my understanding is that there's more to going fast than just a Crr test and an aero test.
(Also, who says you can't compare a 23 and 25 at the same pressure?)
The higher pressure 23 (compared to the lower pressure 25) will as far as I can see transmit more vibrations/forces to the rider, where they will have to be absorbed, creating a power cost to the body that subtracts from the power you can put through the pedals.
A secondary issue would be less suspension travel, meaning it's more likely to bottom out on a big hole / snakebite puncture.
Finally, I would imagine that a 23 vs 25 (or even a 28) on the rear wheel would make SFA difference since it's already sheltered. But the comfort and Crr difference might be beneficial.
You are incorrect. 100 psi is not 100 psi across all tire widths. If you put 100psi into a 29x2.2" mtb tire it will be significantly "harder" than a 23c tire at the same pressure. Accordingly, if you were to run a 29x2.2" tire at 50psi and a 23c tire at 50 psi, the 23c tire would be significantly more comfortable (assuming you don't bottom the tire out!) The number that determines rider comfort is really casing tension which I consider to be analogous to spring rate in suspension systems. Read the two links below:
http://flocycling.blogspot.com/...u-use-less-tire.html http://flocycling.blogspot.com/...sion-study-with.html So why does everyone say "wider tires roll faster"? It's because they are improperly being compared at equal an equal psi. If you look at TomA's or BRR's data, it's all standardized around pressure even though wider tires at higher pressures have a higher spring rate (or are harder... however people want to deem it). You have to realize that tires are basically springs: they absorb energy when they deform and they return some of that energy when their shape returns. This is primarily driven by the tire's construction: the rubber compound, the casing, and the presence of any other materials that might absorb energy. A larger steel spring is not more energy efficient than a smaller steel spring made of the same material (both will be >99% efficient). Likewise, tires of equivalent construction will not be more or less energy efficient across different sizes*.
What larger tires allow you to do is to run a lower
relative pressure with less risk of bottoming out the rim.
You are largely correct on rear tire choice: modern frames fair the rear wheel effectively and the air is pretty darn turbulent by the time it gets back there anyways.
*the caveat in this is rolling diameter. As you change rolling diameter you change how much the tire deforms and this will impact rolling resistance. However, going from a 23c to a 25c doesn't drastically change the rolling diameter and the change in rolling resistance is a % of this % such as to be so small I doubt we can measure it with commonly available tools in the cycling industry.
