What is a "wheel factor" with respect to calibratin a bike speedo?

Question

And why is it necessary to know the wheel width?? (seems irrelevent to tracking distance)

In my bike computer manual it says that if your wheel size is not listed in the chart, you just need to convert wheel size to mm, then multiply by pie.
But how does wheel width factor into this?

Answer 1

not true. the wheel size has everything to do with it. A smaller tire will spin faster than a larger wheel at the same speed, because the circumference of the wheel differs that much. The speed has to be correct on the speedo in order to track the correct distance. Same as on your car. there is a setting that is in your cars computer that tells it exactly what size tires are on your car, and by that it can determine weather 100 rotations of your tires equal 100 Meters or 1 Kilometer.

The rate that your tire spins will basically be the total distance around your tire. and since a tire is supposed to be the same distance from the center at all points.

In turn, the wheel factor is what directly determines the accuracy of the speedometer!

Answer 2

Bicycle wheels are a component of 2 parts, a rim and a tire. Tire sizes (such as 700c, 27", 650c, 26" etc.) is the size of specific tires that fits onto the appropriate sized (700c, 27", 650c, 26" etc.) rim, and has nothing, per se, to do with the diameter of the final product – the wheel when installed and fully inflated. Width of the tire plays a role in wheel diameter because tires are essentially a ½ circle; thus, if you increase the diameter (width of the tire) you correspondingly increase the radius (distance of the outer edge of the tire from the rim): radius being ½ diameter.

Therefore, with increased width, one increases the radius of the tire, and one correspondingly increases the diameter of the wheel (tire and rim), thereby increasing the circumference of the wheel. Although this increase is small (on the wheel) it adds up with every rotation.

Width is an important factor only when mathematically calculating the wheel diameter. Width is automatically factored into any actual measurement of wheel circumference.

So, if you measure the actuial diameter of the wheel (tire plus rim) you have a measurement, and thus width factor is accounted for.