Bicycle Mechanics - RX100 7-speed w/ 8-speed cassette?

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thundercleese
08-13-09, 06:00 PM
I'm trying to kludge a commuter together from the remains of an early-90s Miyata 914 and some spare parts. I've got the following to work with:

- Shimano RX100 7-speed downtube shifters
- Shimano "Light Action" rear derailleur (7-speed I presume)
- Shimano Ultegra 8-speed cassette (11-23 I think)
- Formula 8/9 speed hub
- 4mm spacer


First, will the RX100 7-speed accomodate an 8-speed cassette in friction mode? Will the Light Action RD have enough range to do 8-speed? (If not, I have a 8/9 speed Sora RD that should do the trick.)

Secondly, if I wanted to run indexed, I'd have to get a 7-speed cassette-- because the 8-speed cassette has the wrong spacing, correct? I couldn't just add the spacer and omit the 11t cog?

HillRider
08-13-09, 06:27 PM
Both the shifters and rear dertailleur should work fine. The shifters may work in indexing but probably not too well. They will certainly work in friction mode.
Retro Grouch
08-13-09, 07:46 PM
I agree with Hillrider.

I've tried mixing and matching various combinations of 7-speed and 8-speed components for years. The actual cog spacing is very close and, in index mode, I can get it to kind of work (just shift past the last shifter detent to the derailleur limit stop).

There are folks who claim they can get the shifting to index perfectly but I'm not one of them. They are either better bike mechanics than me or they're less picky than me. If it was my bike I'd try for a while to get it to index and convert it to friction if I couldn't.

MWnyc
08-13-09, 08:27 PM
Just today I put 105 7 speed DT shifters on a bike with an 8 speed cassette and got it to work smoothly in index mode. Since rx100 is supposedly the same as 105 with a different finish, you should be able to make it work. 7 and 8 speed spacing is nearly the same, they only added 4mm to the freehub body to accept one more cog. The only issue is that the drivetrain will be very sensitive to cable tension. It took a little barrle adjuster fiddling, but it works fine.
Also, you would have to lose one of the other cogs instead of the 11t lockring cog if you wanted to go down to 7. I rode an 12-23 8 speed cassette minus one cog on a 7 speed system w/o any problems, but I chose to drop the lagest cog. You will have to run such a cassette "loose" or get 7 speed pins that lock the cassette together. I wound up filing down the pin on my Sram 8 speed cassette in order to run it as a "solid" 7 speed. Hope this is helpful.