Originally Posted by
The Big Wheel
I have a Sigma BC 8.12 hard wired bike computer that I just installed on my two wheel 'bent which has a 20" front tire and 26" rear tire.
According to the wheel chart that came with the computer it only has one setting for a 20" tire and the problem is that the wheel calculation is for a 20" 1.75" tire and my tire is a 20" 1.5" tire. The manual states to enter "1580" for the 20" 1.75" wheel. Since my wheel is a 1.5" how much off will it be? Are we talking about less than 1 mph at say 25mph?
Is anyone here really good at math and can do the calculation for the correct setting for the 1.5" tire?
I spent long hours to define the circumference of the wheels of my bicycles. Both that of the front ones and the rear ones, since I use some Powertap hubs which behaves like a speed sensor, too.
I tended to measure not just the wheel circumferences of loaded bicycles, but also those of unloaded ones, because I use my bicycles as a sort of calibration tool to measure the length of the non-standard running tracks I use for jogging and running.
My verdict is that as regards 700C tyres Cateye wheel size chart data are pretty close to my data of loaded bicycles, while Sigma wheel size chart data is quite close to the my data of unloaded bicycles.
And when I realized it I was a bit shocked, how the hell the German guys who are told to be precise are distributing data related to unloaded bicycles??? My weight is in the range of 75-80 kg, and my effect on the real circumference is around 1.2-1.5% depending on the tyre pressures.