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I do not know if rapid fire shifters can be used with a derailleur, but I matched my Boardwalk three speed internal hub with a simple thumb shifter which I am far more happy than those awful grip shifters the bike industry has been pushing on the comsumer for several years. I wish flexibility rules rather than profits.

I put a seven speed freewheel on my Boardwalk 6, and then got a 7 speed rapidfire shifter and installed it.

It's great. Go for it.

My Speed 8 has a Shimano rear derailleur(don't know the model but it's all black and only says "Shimano" )and I switched to LX rapidfire shifters. No problems. Shifts great and I agree, I hate the gripshifters that came with the bike