This morning I read a post on Bike Forums about a custom Shimano freewheel made by sciencemonster in a close ratio plus bailout style. Inspired, I decided to see what sort of gearing you could get out of just two different freewheels we had here in the store, the HG37 13-28 7s Freewheel, and the HG37 14-28 7s Freewheel. Shimano hasn’t made a freewheel with a large cog smaller than 28 teeth in a long time, and with some cogs from these two freewheels, you can put together a freewheel with a low gear of 24 teeth.
The newer Shimano 7 speed freewheels use a right-threaded lockring to hold all of the splined cogs on. The easiest way to get the lockring off is using a cog vise to hold the freewheel in place, and go at it with the lockring tool of your choice. The notches on these lockrings are pretty narrow, and they both have a larger radius than a track or bottom bracket lockring, so I had the best luck getting them off with a plier-type lockring tool. I used Stein’s locking lockring wrench, Hozan makes lockring pliers that would work ok, or you can make one yourself by grinding teeth into a pair of slip-joint pliers.
When you take both freewheel lockrings off, you can see the difference in diameter between the 14 tooth and 13 tooth freewheel cogs. The different diameter splines limit the changes you can make to the gearing on these freewheels without some serious grinding.
The cogs and spacers should slide right off. The 14 tooth freewheel body has two different splines, with outer diameters of about 43mm and 50mm. The upper spline holds four of the cogs, and the lower spline holds three. The thirteen tooth has an additional 41mm spline pattern, which holds the top two cogs, leaving two on the middle spline, and three on the bottom. Your new inventory of cogs is as follows:
- Two 28T
- Two 24T
- One 22T
- One 21T
- One 20T
- One 19T
- One 18T
- One 17T
- One 16T
- One 14T with spacer
- One 15T with relief
- One 14T with spacer
For a reason obvious only to Shimano, the two freewheels use a slightly different large spline pattern. One of the splines opposite the square spline on the 14 tooth body is offset clockwise slightly. You’ll need to grind or file a small amount of material off of any large spline cog you want to install on the other freewheel body.
The first custom freewheel uses all cogs but the 28T from the 13T freewheel, substituting the modified 22T cog from the 14T freewheel. Keep the spacers in the same order and orientation as the original freewheel, just remove the 28T, and install the 22T from the other freewheel after the original 24T.
This gives you a not unreasonable 13-15-17-19-21-22-24 range. Just reinstall the lockring as tight as you can reasonably make it.
Another option makes use of the two spline body to make a 14-24T freewheel with a little bit different spacing.
Use the 24T and 22T cogs from the original freewheel, then add the silver 21T, 19T, and 17T, then the original 16T and 14T.
This gives you a 14-24 freewheel with a spread of 14-16-17-19-21-22-24. Again, tighten the lockring down before using. There are a few other combinations you can make with these parts, but the splines are pretty limiting. If you ever wanted a 7 speed freewheel with a range of 13-15-19-20-21-22-28, this is probably the only way to get it with new parts.
Both freewheels needed for this project are were available at Harris Cyclery. We can’t make a custom freewheel for you, nor can we get any of the Shimano freewheel cogs separately, but we do did sell the 13-28 7 speed freewheel and the 14-28 7 speed freewheel necessary to make your own.
Update 5/2/12: The 14-28 7 speed freewheel has been discontinued by Shimano. The 13-28 7 speed freewheel is still available and is in stock.