# Bicycle wheel size question.

1. Nov 25, 2009

### Nashvis

Could someone aid my understanding of the factors involved here?

The question that has been rolling (pun intended) around my mind is in regards to a trend in mountain biking. The new thing in MTBing is the 29" wheel. Standard wheel diameter is 26". Advocates of the 29er say there is less rolling resistance with the greater diameter wheel which enables them to more easily maintain speeds on trails with varying terrain (rocks, roots, logs, etc..). The thing is, as I understand it, a larger diameter wheel is heavier as are the bike components and bike frame (so as to accommodate a larger diameter wheel). This heavier wheel should create more resistance, requiring more power to accelerate it.

The "lever" of the wheel is also longer on the 29" wheel which would make greater friction.

So is the perceived benefit of the 29"wheel derived from the reduced angular momentum of a larger diameter wheel?

Thanks for any replies.

2. Nov 25, 2009

### rcgldr

The main advantage of the larger diameter wheel and tire is the decreased horizontal component of force when rolling over moderately sized objects, rocks, tree roots, small logs, ..., because the angle between vertical and the line between contact point with a small object and the center of the wheel is less with a larger diameter wheel. This angle is = cos-1((radius of wheel - height of object)/(radius of wheel)). The horizontal force is related to the sin of that angle.

Assuming that parameters other than diameter are the same, the wheel weighs more and the angular momentum is higher because of the weight (diameter doesn't directly affect angular inerita, just the radial mass distribution and the amount of mass). The net affect on acceleration is about double the weight of the wheel and tire (if all the mass were at the outer diameter of the tire, then total inertia of such a wheel is double that of of a non rotating mass). A 1 kg tire would have about the same affect on acceleration as a 2 kg mass on the frame.

Rolling resistance would be slightly less with a larger diameter tire, but would be offset by aerodyanmic drag at higher speed.

Last edited: Nov 25, 2009