I prefer chains over belts because of reliability issues and because I already have a chain on the motorcycle. I tend to trust automotive engineers about longevity: if they don't use belts on the drive it's probably because they don't last long enough. However, one point to be made is that the oft-quoted statistic of 98.6% efficiency is probably untrue. While it might or might not have occurred in a laboratory setting, it certainly isn't true for real-world situations where lubrication comes once a year, if that often. I can't find an updated source for real-world efficiency, so I'm going to assume 90%. If my motorcycle chain transmitting 100kW is only 90% efficient, that's a lot of friction energy (10kW!) burning up the joints of my chain. (And since this motorcycle is 20 years old, I wonder if it's not time to do some chain maintenance.)

The chain itself can come in many different forms, but there are two major categories: roller chain and silent-- or inverted tooth-- chain. I can't speak much about the silent chain, but it seems that they're not as hardy as the roller chain variety, nor as cheap and readily available. Furthermore, chain-guide.com claims that the silent chains are not suited to reverse drive, which is more or less what happens with regenerative braking. Therefore, I think classic roller chain is best suited to the task.


There are a number of criteria for choosing a chain and sprockets.

In real short form (click for my more in-depth explanation):

1) Gear ration should never exceed 7.
2) Sprocket center distance should be between 30 times and 50 times chain pitch. In no case should it ever exceed 80 times.
3) Try to choose sprockets with an even number of teeth.
4) Design the sprockets to have 110 teeth or less.
5) Design chain length to have an even number of links.
6) The optimum chain speed is around 12.5m/s.
7) Chain length is given by 2*Cm+(N1+N2)/2 +(N2-N1)^2/(4*pi^2*Cm), where Cm=C/p, N1 is the number of teeth on the first sprocket, N2 the # on the second, C is the center length, and p the chain pitch.


Chains should only be installed in a certain positions, as you can see in the following image. Also note that it's better to have the smaller pinion driving the larger one.
chain_layout

It should go without saying that you want the sprockets to be parallel and on the same plane. While chains still work when twisted a bit-- as on a bicyle, efficiency and longevity start suffering greatly so keep your sprockets lined up at all costs.