Originally Posted by
MrPolak
And what is the scientific basis for this opinion?
Scenario: I'm on my cyclocross bike with 700x28c slick tires. The bike weighs 20lbs with all the doodads I put on it. I weigh 180lbs. My friend is on his GT mountain bike which weighs 26 lbs and is equipped with 26x1'' (25c) slicks. He weighs 215lbs. When starting to roll down hill at the same speed I continually accelerate ahead of him although neither of us is pedaling and there's no aerodynamic advantage. Our tires are inflated to within 5 psi of each other. He has a 41 lb advantage. WHAT IS GOING ON?
As for tire choices, the popular and good touring tires are available in both 26'' and 700c sizes.
Well, that is not scientific either, though I don't think you meant it to be. The difference in speed is caused by many factors. You and your bike weighing less is actually to your advantage. Because of the smaller amount of mass, you have less inertia. This means it is easier to change, in this case it would be your speed. The smaller wheel will also have less rotational inertia, allowing it to start faster, but there are other factors, like the rolling resistance of the tires. Since the tires are not the same, then the rolling resistance is not the same. Even tires that look the same can have different rolling resistance. The friction of the hubs also needs to be overcome, your bike might be tuned better.
700c wheels have a greater rotational inertia than 26" wheels. 26" wheels are easier to get up to speed whereas 700c wheels want to stay at that speed better.
Originally Posted by fueled by boh
among two wheels with similarly wide tires, the one that is smaller in diameter will have a smaller contact patch. a smaller contact patch generally equals less rolling resistance, all other things being equal.
This is not true. If a 700c tire and a 26" tire of the same width and equal pressure will have the same contact patch. The contact patch is determined by the air pressure and the total weight. When the contact patch x air pressure = weight, the tire stops deforming.
Wide tires have less rolling resistance than thinner tires at the same pressure. Though the contact patch will be equal, the thinner tire has a longer patch. This means more sidewall length needs to deform, this causes a greater rolling resistance. Though wider tires at the same pressure have less rolling resistance, most wide tires are of smaller air pressures, which means that a longer patch is needed to equal air pressure and total weight. This causes greater sidewall deformation causing greater rolling resistance. The sidewall strength is also a factor in rolling resistance. If a tire's sidewall is made to easily deform, there is less rolling resistance.
Rolling resistance is not a function of wheel size, it is a function of tire width, air pressure and sidewall strength.