Electric road bike - Compatibility questions I am building an ebike on a road frame. I am using a Bafang G310 rear hub motor &amp; 8 Speed Shimano cassette. I plan to use a long MTB rear 8 spd derailleur (either Acera or Alivio). For my crankset I have been considering the 105 Shimano FC 5700 Crankset OR the SHIMANO 2018 FC-5800 105 172.5 50/34 11-SPD BLK CT CRANKS. Can these be compatible with an 8 speed cassette? My frame is a Gosforth 931. Is there a set for 2x8 spd flat bar shifters that is compatible with both the rear mtb derailleur and 105s?
is an 11 speed 105 crank/chainrings going to work if everything else is 8 speed?

Yes that should work fine.  Bicycle chain has a fairly consistent inner width, its the outer width that varies between 7+8/9/10/11/12 chain.  7 and 8 speed chain is the same width.

If you're using an 11 speed Front Derailleur then the cage could be quite tight for 8 speed chain, so using the 8 speed FD should fit better or you could spread the cage with leverage.

Are there shifters that will work?

Sure - the rear shifter only depends on the cassette spacing and not its width.  Since you have a MTB rear derailleur, you want an 8 speed shifter which could be indexed pod/trigger shifter, or a non-indexed friction shifter.

There is no need to have identical shifters either.  You can mix and match whatever you like.

As for the front shifter, you'd want a double that supports the 11 speed chainring spacing.  This might be different to the 8 speed spacing on the front.  

So the dirty hack for front shifting is a friction shifter.  It may not be as pretty as a pod shifter, but you can adjust both position and trim while riding.



I've had an ebike, and it was quite heavy.  Don't discount a triple on the front if you ever ride up significant hills.

Your choice of 8 speed rear is good - there are a lot of parts readily available for reasonable prices, and the chain is robust compared to thinner chains.

A long cage derailleur may not be required - but will allow a larger rear cog than a medium cage.  Long cage hangs lower than medium, so can be more exposed to grasses or rocks.