• Howdy! we're looking for donations to finish custom knowledgebase software for this forum. Please see our Funding drive thread

Chain Drive Reduction --- Does Chainring Size Only Affect Gear Selection to Maintain Appropriate Torque

richj8990

100 W
Joined
Jan 21, 2020
Messages
277
The mid-drive stock motor-driven chainring in this 'kit' (don't ask which one) was 46T. Due to serious/critical chainline considerations for the cassette chain, I need to move the drive chain into what would be the middle chainring area. It looks like 46T will fit next to the frame, but if I reduced it to for example 32T, it would fit even better, then I can move it inward even more, and I can use more lower gears (or hopefully all of them) in the rear cassette so it doesn't cross-chain and rub against the drive chain in low gears.

The question is: will going from 46T to 32T up front immediately lose almost 1/3 of the torque, or is the torque curve simply shifted up in rpm and/or cadence, so I just need to use gearing that's a couple cogs lower to get the same torque that 46T up front would provide? Remember this is not a Bafang single chainring setup, it's a dual chainring with one motor-driven and one driving the cassette chain.

For example, with 46T as the motor-driven chainring, if I had enough torque in 18T in the back (which is cog 8 of 11), in order to get the same amount of torque with a 32T motor-driven chainring, I would downshift twice more to the 10th cog which is 13T? Since that's about 1/3 less teeth than 18 and therefore matches the tooth drop from 46 to 32 in the front?
 
To calculate the total gear reduction you just multiply the two ratios, so say it's currently 46T on the motor driven sprocket and divide that by the motor sprocket and that's the gear ratio from motor to cranks. Do the same with the cranks to cassette gear, say 18T on the cassette and a 34T chain ring so 18T/34T. Then multiply the two numbers you got that's the total gear ratio. Do it again for your new cog change and you can find what gear would be the same torque. So yes it is linear and your general assumption is correct but it's often easier to just calculate the actual drive ratios.
 
Thanks that's what I thought. It's only around two cogs of difference for torque if I went down from 46T to 32T drive chainring. 34T cog to 46T cog. That's not bad at all since I have 11 speeds. I think I'll do that, still need to see how bad the chains overlap first.
 
Back
Top