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speedub.nate

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There is a bit of a debate in the world of mountain bikes, concerning the conventional 26" wheel diameter versus the rising popularity of the 29" wheel size (nominal diameters).

One of the popular beliefs is that the 29" wheel requires a larger brake rotor than a 26" wheel.

At face value, this appears true, given that the larger wheel has greater leverage over the brake pads clamping the rotor. We're talking roughly 10% difference in diameters.

However, for the most part, rider weights are not changed, bike weights are roughly comparable, and average speeds are equal (many riders will say they are faster on one size or the other, but no study has clearly concluded that either is a "faster" wheel size).

My speculation is as follows: Two bikes rolling downhill (riders attached), at similar speeds and weights. It appears to me we have a similar amount of kinetic energy in each mass as we begin to apply the brakes to slow and/or stop them.

If the braking system's job is to convert kinetic energy to heat, then my impression is we're looking at two identical tasks.

While the 29" wheel has ~10% greater leverage over the pads clamping the rotor, the 29" rotor is also spinning ~10% more slowly at any given ground speed (compared to the 26" wheel).

For what it's worth, a typical rotor diameter is 160mm / 15mm brake track width, your average bike may weigh around 30 pounds, and an average downhill speed may be 30 MPH.

Is my approach too simplistic? Should I take other factors into consideration?

One area a 29" bike will always be heavier than a 26" bike is in the rim and tire weight -- this typically accounts for 150g increased tire weight, and 50g rim weight (per wheel).

I'm more interested in a discussion of the forces at play and am not (yet) looking to work the problem to "solve" it in the form of a numerical answer.