posted 2 days ago
measuring your current rear hub would work, assuming it’s the right size for your frame (is there a chance somebody swapped out the rear wheel at some point in the bike’s life? it’s pretty easy to force the wrong size in.) I haven’t ever found measuring a hub to be very easy, though. could be I just don’t know the right trick or have the right tools.
it is easy to measure the spacing between the rear dropouts with the wheel removed, though. the distance you want is between the inside flats of the dropouts. also called the O.L.D., short for over locknut distance. the locknuts in question are the part on the wheel hub that seats against the dropouts on your frame.
sometimes the distance you measure will be halfway between two common hub spacings, e.g. 132 mm. in that case it’s safe to use the size on either side; 130mm or 135mm in this example. you could also re-space your frame or find some axle shims to adjust a hub to fit exactly if you wanted a little project, but neither is necessary.
what definitely isn’t a good idea is to force a hub into a frame when the spacing differs by more than 3 mm in either direction. so if you have your heart set on that 120 mm hub but your frame measures 125 mm or more, you’ll need to make an adjustment to one or the other. how difficult that is will depend on what your frame’s made of and how the hub is fixed to its axle. I haven’t tried to make that adjustment on an internal hub, but I’m guessing it’s more complicated than it would be on an external hub.
some other things to think about:
-gearing: will the cog that comes with the hub give you a gear range that works well for you with the chainring(s) you currently have? if not, would you rather get a different cog or a different chainring?
-front drivetrain: do you plan to use a front derailer? not common with internal hubs, but certainly doable with some additional considerations.
-chainline: you want the chain to be as straight as possible. you might get lucky and not need to make any adjustments. I’ve never found it easy to know in advance. if you’ve got a cog that adjusts by flipping it over and/or shims for the cog, those are simple and easy. failing that, the easiest way I know of to adjust the chainline if you need to is to move the drive-side crank in or out by either replacing (or flipping if it’s asymmetrical) the bottom bracket spindle (larger adjustment) or using bottom bracket cup shims (smaller adjustment). I’m not familiar with that hub you’re looking at, so I can’t comment on what will be necessary (if anything).
getting the chain perfectly straight isn’t really necessary if you’re happy using a normal derailer chain because those are made to bend a fair amount. but if you do get it real straight, you can use single-speed/internal/fixed-gear chains that last a lot longer. you’ll also reduce wear on the whole drive train and gain some very small amount of efficiency. could also use 1/8” cogs and chainrings with an appropriate chain, which will also make everything last longer.
-chain tension: do you have horizontal dropouts? how long are they? you’ll need to have some way of adjusting how far from the cranks the hub is so that your chain will have the right amount of tension. the right spot for the hub will change as your chain wears, so the longer your dropouts the better.
alternately, a chain tensioner will solve that problem (as will converting to belt drive, but that’s not commonly an option for frames that aren’t designed for it).
-shifter location: where do you want it? will what comes with the hub work there? sometimes there’s a mismatch between the shifter clamp diameter and your handlebars if that’s where the shifter’s going. if you want the shifter on the downtube, do you have a boss for that on your frame? do you have the mounting hardware you’ll need?
I like internal hubs. apologies if anything above isn’t clear.
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