Originally Posted by
clarkbre
Yes, scientifically skinny, highpsi tires may roll slower…
However, where the rubber meets the road (very literally),is a different story. It’s all aboutavailable energy and resistance. Take tiresA and B (A=26x2.0@40 PSI B=26x1.5@80 PSI) mounted on a bike that weighs30 pounds with a 170 pound rider. Withthe weight of the bike and rider, Tires A may have a contact patch of 10 squareinches each. However, Tires B have acontact patch of 6 square inches. Inturn, Tires B have a 40% smaller contact patch on the ground allowing 40% lessenergy from the cyclist to turn the tires at the same speed of Tires A. The 40% of energy saved from not having topush Tires A can be converted into more efficiency. This allows the cyclist to move the bikeequipped with Tires B at the same speed as Tires A using much less energy. The saved energy also allows the cyclist touse different gearing which can help create higher speeds with Tires B asopposed to A.
So…yes, a skinnier, higher psi tire may roll slower but takeinto account all parts of the equation.
That just isn't how it works. First, barring some (minor) losses of contact patch size caused by sidewall stiffness, 40PSI tires (regardless of width) will have a contact patch of 10sqin TOTAL. Bike + rider = 200lbs, 200lbs / 2, since there's two tires = 100lbs per tire (a bit more in rear, depending on bike, usually). 100lbs / 40lbs/sqin = 5 sqin of contact per tire. An 80PSI tire would be 2.5sqin each. Thankfully, under normal circumstances, the addition weight per sqin on the higher PSI tires cause them to have approx the same grip on the road. However, as they have half the area on the road, they have half the chance of maintaining that grip on slick areas. If 3sqin of tire contact slips on the skinny tire, you go down. If 3sqin of the fatter tire are on something without grip, you still don't slip since there's 2 more sqin to hold. Fatter tires also don't get caught in rail tracks, grooves, bridges, etc.
Now, here's the kicker, the fatter tire will have to have significantly less drop (deform to flatten against the road) to deform the tire at the contact patch, and if all else is equal (casing, construction, rubber compound, etc), the fatter tire will lose less energy from this deformation. Combine this with the fact that a fatter, softer tire will deform more easily on road bumps, rather than sending the shock of road bumps and holes to the bike and rider and you end up having less total energy losses at normal cruising speeds. Every time you feel a bump or even vibration on your bike, that's energy that USED to be carrying you forward, but is now being lost as shock and vibration. Fatter tires can simply roll over a lot of smaller bumps, have the tire deform around the bump and continue forward with minimal losses. A skinny tire will buck the bike and rider upward on the same bumps, losing a LOT more energy (and being annoying/painful). Because fatter tires roll over so much of the road irregularities, they feel much smoother. Skinny tires transmit all of that as vibration and shock to the bike and rider, which make them FEEL faster, since the road is beating you up more.
Fatter tires weigh more, which is important in road races with LOTS (not more some, but basically constant) attacking and acceleration. Constant, rapid, all-out acceleration doesn't occur on anything approaching a normal ride. Fatter tires are also wider and have more air resistance, however due to the exponential way speed affects air resistance, this only matter above about 30MPH or so...as the difference between a 21mm tire and a 23mm tire and a 25mm tire is almost un-measurable. The difference between a 26x1.25 and a 26x2 is more, but still not a factor at anything under 30mph, at least not one that's measurably significant.
I ride on 26x1.85 tires (ISO 559x47) and my commute speeds average around 18-21MPH depending on direction, wind, and temp. If I ride without really trying, I still average 15-18, and that's at a "leisurely" pace. My 47mm wide tires definitely don't slow me down. They are also pretty much flat-proof, comfortable, and safe. Bottom line, there's no way in hell I'd be going 40% faster if I had different tires.