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I've replaced the rear wheel on my bicycle. The new wheel is different in two respects:

  • I got a 8-speed freehub because 7-speed freewheel hubs seem to be difficult to find these days.
  • I had a 7-speed cassette with 14-28 teeth and replaced it with 12-32, using an extra 12T cog as a spacer on the distal side.

Two problems emerged:

  • The previous cassette had a lot of space between the smallest cog and the dropouts. Not so with the freehub cassette. I had to put the spacer cog on the distal side rather than the proximal one because otherwise the chain had no clearance when on the smallest cog. Even then the lock ring touched some sort of round screw head used to secure the derailleur. With a wrench, I was able to bend the steel (more exactly, unbend it; without wheel, I measure 133 mm rather than 135 between the drop outs and they still bend a little to the inside in the back) on the drive side and now it doesn't touch anymore. However, the clearance is very small when the QR skewer is tight (less than 1/2 mm). The question is: is it a good idea to file the screw head a bit or bend the drop out more so as to get more clearance? I'm worried that under heavy load the frame will bend in a way that there is contact with the lock ring.

  • The derailleur can't handle the chain being the two largest cogs. This crossed configuration is not something that I would ever use, at least on purpose. Is there anything I should be concerned about here?

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Yeah you're entering a world of hurt by changing one thing, and the slippery slope of knock-on upgrades start...

  • 6/7/8 speed cassettes all use about the same spacing between gears, so they get wider as you add cogs.

  • 8/9/10 speed cassettes all use the same width of cassette, so the cogs get closer together as you add more cogs but all fit on the same width of freehub.

Bending the dropout more sounds like a bad idea. It should be parallel to the other dropout, and have the correct OLD spacing for your hub.

I've had to file off the backside of the derailleur hanger on a couple different bikes, but that bolt is anti-rotation and isn't doing much to hold the derailleur on. So it can be thinned down successfully. Its not hardened steel so a normal file should do the job fine.

Your other option is to adjust any spacers you have on your wheel to re-center it, and possibly adjust every spoke nipple to move the rim over a bit and keep it centered.

7 speed casssettes are totally available from a decent LBS, or you can order them from wiggle or CRC online. There's not a huge amount of choice, but 12-28 is not uncommon.

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    Shimano still sell a 7-speed cassette CS-HG 41 (range 11-28)
    – Carel
    Commented May 5, 2018 at 18:02
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    The problem was not sourcing the cassette, but a new 7-speed freewheel hub. Anyway, it's good to know that I can file the hanger. About the "adjust the spacers" option: what do you mean? adding a spacer after the nut on the drive side? With a quick release axle I have little margin there: I measure only 5.5 mm of axle left. And I guess this would make the wheel weaker, because the difference of spoke tension between the two sides would increase.
    – Artefacto
    Commented May 7, 2018 at 22:50
  • @Artefacto I had a wheel to move over, so I undid the locknuts and swapped a single spacer for others with a higher total width, then reassembled and tweaked bearings again. Worked very well.
    – Criggie ♦
    Commented May 8, 2018 at 7:18

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