650 B on 700c computer

[Owlie]

What sort of computer is it?

My guess would be that it'll be somewhere in the range of 5-10%, depending on tire size, and it'll show a higher speed than you're actually travelling.

[LovelyGirl]

What sort of computer is it?

My guess would be that it'll be somewhere in the range of 5-10%, depending on tire size, and it'll show a higher speed than you're actually travelling.

Some Bontrager thing. It came on the previous bike, which I bought used. So, 5-10% inaccurate or ONLY that accurate?

[Owlie]

Not sure I did the math entirely right in calculating the diameter of the different wheel sizes, then converting to inches. (I think in imperial units for distance traveled, and metric for bike parts. Argh!) But according to the math I did for the speed, it comes out to be just over 5% too high at 15 mph on the smaller wheels. It comes out as 15.8 as it would be read by the computer. (Assuming same tire width on each.) Don't know what the effect on distance would be, but again it would read too high, and I imagine the odometer output would be increasingly inaccurate at longer distances. (It's 2:30AM and I don't want to do more math.)

(The smaller wheel has to spin faster at a given speed, so the magnet passes the sensor more often. Except that the computer thinks the wheel diameter is larger, and the rpm when multiplied by a larger wheel circumference gives a speed higher than that at which "real wheel" is actually travelling.) I imagine the difference will hold at other speeds, since the difference in the wheel diameter is fixed, again assuming the same tire width.