8 speeds on a 9-speed cassette?
I have a well-used Trek 520 that is in need of a drivetrain overhaul. The older RSX 7-speed STI shifters are showing their wear (the right one slips on downshifts occasionally) so I'd like to replace them with bar-ends. I've noticed that 9-speed bar-ends are much easier to find than older 7-speeds. Since I have a 7-speed freehub on the bike now that's in good shape (just the cassette needs replacement), I'd like to keep it. I just read the following on Sheldon Brown's site for a description on a 7-speed freehub for sale:
"You can use a 7-speed cassette, or, if you want more gears, you can install 8 sprockets of a 9-speed cassette, and use 9-speed chain and shifters to get 8-speed shifting without exceeding the 126 mm spacing."
Has anyone ever done this? This would solve several problems for me: it would allow me to use the current wheel and dropout spacing and replace the worn shifters with new bar-ends. Since I plan on replacing the cassette anyway, the extra expenses would be for shifters and brake levers, which I'd need to get once my current STI's complete their disintegration. Once the wheel wears out I could replace it with a 9-speed and be on my way. If not, I run the risk of spending the money for a new 7-speed drivetrain which would be rendered obsolete if my shifters give up.
So, have any forum members ever attempted the 8-speeds-from-a-9-speed-on-a-7-speed-hub trick mentioned above?
This trick should work properly, since you would be using 9-speed spacing between cogs with a 9-speed indexing system. With barcons, you also have the fallback of operating in friction (non-indexed) mode, which many of us prefer.
cycles per second
Doing a little math...
(7) 7-speed cogs and (6) 7-speed spacers gives a total stack of 7*1.85mm + 6*3.15mm = 31.85mm
(8) 9-speed cogs and (7) 9-speed spacers gives a total stack of 8*1.78mm + 7*2.56mm = 32.16mm
So the (8) 9-speed cogs is just a bit wider. Most bikes can handle this without a problem. There is a small chance that a 1mm spacer would need to be added to the freehub side of the axle to get enough clearance.
Also, your freehub body needs to be HyperGlide compatible. Really old stuff is only Uniglide.
How do I tell the difference
Thanks for doing the math! I didn't know the specifics of figuring dimensions. There might be enough clearance at the dropout, but I know how to add spacers so that wouldn't be a problem.
How do I tell the difference? The bike is about 10 years old, give or take.
Originally Posted by Gonzo Bob
cycles per second
lockring = HyperGlide (threads inside the freehub body)
Originally Posted by Bikesalot
smallest cog threads on = Uniglide (threads outside the freehub body)
Some freehub bodies are threaded for both. I would bet that yours is one of those given the age.