Why are bicycle gear ratios computed as front/rear and not the opposite? I recently realized that when people talk about bicycle gear ratios, it's calculated as number of teeth on the (front) chainring divided by the number of teeth on the (rear) sprocket/cog.
This made me a bit confused, because I was thinking that gear ratios are usually the opposite - driven gear/sprocket/pulley divided by the driver (e.g. motor).
When talking about motorcycle final drive, the formula is rear/front sprocket teeth. When reading motorcycle/car specs, the lowest gears - 1st gear and reverse - have the highest numbers.
I tried to find why it's the opposite with bicycles, but so far couldn't...
Can someone shed some light on this topic? How this came to be historically?
I think it's simpler than what many think.
Car and motorcycle drivetrain is a reductor: their ratio (driver to wheel) &lt; 1 in most cases. Bicycles are the opposite: the wheel normally turns faster than the cranks, and the drivetrain is a multiplier.
It is somehow easier to remember and 'feel' ratios that are greater than 1: compare 4.3, 3.7, 2.9 with 0.23, 0.27, 0.34. So that's how it's done.
In the end, this is the same argument: tradition. But at least this is an attempt to explain how such tradition came about.
Besides, cyclists rarely operate ratios per se. We may fancy with 'proper' units like gain ratios etc. mentioned in other answers, when selecting a new cassette, but in practice we just know it by the teeth :) Tell me any standard ratio expressed in teeth (say, 39/17), and I'll immediately say at which speed I will use it and how 'hard' it is. Yet I don't even know what ratio it is without a calculator.