[tobu]

BB drop is the distance the BB is below the imaginary line connecting the front and back drop outs. A smaller drop helps increase pedal clearance at the expense of a higher center of gravity. Keirin lore also contends that smaller drops favor acceleration while bigger drops favor top end speed.

 

[tobu]

It used to be that pretty much everyone ran 165mm on track bikes, but that's changing. Pedal clearance was one of the issues on steep tracks at low speeds on bikes with greater BB drop. The modern trend to push bigger gears has convinced some riders that running longer cranks give you more leverage when out of the saddle. I don't think running 170 is a bad thing, and as long as you don't get a bike with the BB drop in the 70's you should be all right.