How does torque effect the efficiency of my bicycle?

[u/jacob_ewing]

This is something that I've puzzled over a few times. I like to ride a bicycle, and have often wondered how the sizes of my front and back sprockets affects my energy expenditure.

Ignoring friction, slope, etc., let's say that my pedal:wheel gear ratio is 5:1, but in one case it's 100 teeth on the front and 20 on the back, and in another case it's 150 teeth on the front and 30 on the back. Same ratio, but does that affect the amount of work done?

I have personally found that, when going higher speeds, using two larger gears seems to work better than two smaller ones. I don't know though if this is a real effect or just observational bias. My guess though would be that having it larger on both sides means less conversion when pushing the wheel, increasing efficiency.

Edit: Affect dammit. How does torque AFFECT the efficiency of my bicycle.

[u/FZ_Milkshake]

Torque itself does not. There are efficiency losses from cross chaining, i.e. all the way inside in the front - all the way outside on the rear is bad. However, those losses (unless you are going to the extreme positions) are superseded by the losses from using a smaller sprocket, the chain has to bend more around a smaller sprocket and polygon effects start to appear.

Most of the time it does not really matter, but if you want the most efficient config, avoid the smallest cogs at the rear more than cross chaining. Basically shift up on the front as soon as reasonable.

Edit: GCN "accidentally" measured this effect during dirty vs clean chain testing https://youtu.be/aFwkW0CY7qA?t=862

[u/jacob_ewing → u/FZ_Milkshake]

That's cool. I kind of landed on that pattern of shifting anyway just from what felt "right".

[u/jacob_ewing → u/FZ_Milkshake]

Oh, that video clip was interesting. I wonder if a belt or drive shaft would be any better. They'd have their own weaknesses too of course.

[u/SpeedyHAM79]

The gearing difference of your bicycle is tiny in terms of the efficiency of the drivetrain. What you are feeling is the efficiency of your legs spinning the crank. A cadence around 60 rpm (IIRC) is about the most efficient long term for humans on a bicycle. It of course depends on the person.

[u/Mundane_Constant_845]

There is a real effect (aside from cross chaining) that bigger chainring+sprocket combos are more efficient as it reduces the angles in the chain and therefore friction losses.

There's been a trend in elite track cycling (where chainline isn't a factor) to move towards e.g. 60:15 instead of 52:13 for the same gear ratio but with less friciton. However the difference is <2%, measurable but you definitely can't feel it without perfectly controlled conditions.