Monday, September 17, 2007

It's About the Bike

David Clay said... I know this is bad form to hijack a comment on old topic, but I had a question about converting a 70s vintage Raleigh Grand Prix to fixed gear. I came across your post earlier in the year on this topic. What kind of hub do you recommend for this (am I correct that the spacing is 120mm?) Also, what do you recommend for a SS crank? Were you able to change out the cranks only, or did you have to change out the entire BB?

No worries, David, changes of subject move conversations forward.

In the 1970s, when frame spacing was 120, available hubs were as well. Using a road hub and a BB lock ring, a mechanic could affix a cog to the wheel the Raleigh originally had. I've mentioned before that one might flip the axle around and re-dish the wheel to provide a straighter chain line, but I rode my early versions before I knew that trick.

A current track hub, one- or two-sided, will have 120mm spacing. You may choose, as I do, to put in a quick-release axle. That complicates things ever so slightly, because you have to obtain or modify a q/r axle to work in 120 spacing.

Old steel frames are very accommodating. A 120 stretches easily to 126. Many 120 frames made that transition accidentally. With a bit more care one could probably even sprawl it out to 130, but that's not necessary for single speeds.

The very first Raleigh GP fixie I rode may well have had the original cottered crank. A chain doesn't care if the chainrings are narrow, only if they are too wide. Running an eighth-inch chain you could use anything.

Later fixed gears I built used various cotterless cranks I had lying around. A road crank offers the choice of mounting chain rings in the inner or outer position to further adjust chain line. With Surly's Dingle Cog you can run two chain rings to give two gears to match the Dingle's two cogs. They recommend a 9-speed chain, but if you're not shoving it through a front derailleur it shouldn't perish too quickly.

I prefer to use a two-sided hub with one cog on each side, and continue to use chunky eighth-inch chains.

Road cranks with 130 bolt circles give you the full range of chain ring sizes from 38 to ungodly big. Single speed cranks may have 110, 135 or 144 bolt circles. Take note of the bolt circle on whatever you choose. Remember that no component choice has to be permanent.

I don't recall now whether the BB shell of the old Grand Prix was an odd width by current standards. My 1978 Super Course takes a 68mm Shimano BB-UN 52 (which is now a UN 54). I have an old Dura Ace or pre-Ultegra 600 crank on there at the moment. Cotterless cranks give you the best chance of finding replacement rings, unless you happen to get something weird like an old Stronglight or Miche.

Square tapered BB axles open up the largest selection of salvaged cranks. You don't have to use a sealed cartridge unit, but they're widely available, cheap and basically maintenance free until they completely croak.

Hope this helps. Have fun out there!

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