To ride in a hilly area, I'd like to replace a weak BR-IM45 front brake with something stronger (e.g. IM70, IM81 or IM85).
I have noticed that Shimano has two different lineups for their drum brakes: BR-IM and BR-C.
What's the difference?
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The BR-IM roller brakes are older models that are discontinued. The current models are the BR-C (e.g. BR-C6000).
From the Hub roller brake Dealer's Manual (dated 05/31/2021), we can see that the BR-IM and BR-C roller brakes target different series of internal gear hubs. The BR-IM are for the INTER-3 and INTER-5 series (3 and 5-speed) and the BR-C are for the INTER-7 and INTER-8 series (7 and 8-speed).
As a side note, the bigger the model number, the better the braking power and/or the heat dissipation.