Are 406mm Bicycle Wheels As Fast As Standard 700C Wheels?
By: Karl T. Ulrich
When riding at constant speed on level ground, the power delivered to the pedals goes three places:
- Friction in the pedals, cranks, chain, and gears.
- Rolling resistance of the tires.
- Wind resistance (i.e., "air drag") of the bicycle and rider.
For a conventional chain and derailleur drive, the efficiency of the drive system is about 98 percent, and this number doesn't vary much across bikes (except for those with internal hub gears, which are much worse.) So, for simplicity, let's focus on wind resistance and rolling resistance.
The power required to overcome wind resistance increases as the cube (power of three) with speed. So, a doubling of speed results in an eight-fold increase in the power consumed by rolling resistance. The power required to overcome rolling resistance, in contrast, increases in direct proportion to speed, so a doubling of speed results in just a doubling in the power consumed by rolling resistance. This means that as speed increases, wind resistance becomes increasing significant until it consumes a dominant fraction of the rider's power. At speeds over about 15 mph (25 kph), wind resistance matters a lot.
It is true that a 406mm tire will exhibit higher rolling resistance than a 700C ("full size") tire for an equivalent tire and inflation pressure. At first glance this is a nasty penalty to pay for a smaller tire. However, in reality this difference won't matter much for most of us. This is because rolling resistance just isn't a very significant portion of the power requirement in most day-to-day riding situations.
At 15 mph (25 kph) an average rider on a standard "hybrid" bike will have to deliver about 150 watts to the pedals, with almost all of it going to wind resistance. The same rider on a bike with 406mm tires might have to deliver an extra 10-15 watts of power to go 15 mph. However, this amounts to just a few percent of the total power required. In practice, this usually means riding a fraction of a mile per hour more slowly. A much more significant factor is the riding position of the rider (with an upright riding position increasing the power requirement by as much as 25 percent over a crouched riding position).
I've run a lot of experiments over the last year with 406mm tires. The rolling resistance is definitely higher than for 700C tires, as theory predicts. However, this difference is not apparent to the rider except in highly competitive situations at peak power output.