Last edited by A.Winthrop; 09-01-10 at 06:58 AM.
The 7s index spacing is 5.0mm, where the 6s index spacing is probably around 5.3mm (Sheldon doesn't list Shimano's index 6 on his site, just a generic spacing). So you can probably make it work, it just won't work as well as designed.
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This can work quite well as in index mode the 6 speed shifter will still pull cable after it's last downshift to put you in your lowest gear without a *click*... the cog spacing is not so great between a 6 and 7 to cause significant problems in most cases.
Friction mode is a no brainer as you can run as many gears as your rear deraileur will allow.
It is quite likely that shifters do not have a 'hard stop' (physically stopping the shifter from going past the last click), so if the bike is in the 6th cog, simply pulling the shifter past the click will move the derailleur into the 7th cog. In this case, the end-point adjuster screws on the derailluer will serve as a hard stop and a final index position.
The real problem, alluded to above, is that the spacing between the gears is different. If the spacing on the 6 speed is 0.3 mm larger than the spacing on the 7 speed, the derailleur will be 1.8mm off after the sixth shift.... this may or may not be enough to make it index badly.
Another option is to try to change the cable actuation ratio by moving the anchor point on the derailleur. If the cable is clamped between two washers a few mm away from the normal pinch bolt, it could make the derailleur move a little less for a given amount of cable pull.
Use the rd limit screw for the top gear and the shifter index positions for the other 6 gears
Or buy some 7 speed shifter levers- they're not particularly expensive these days.
Just do it and find out for yourself how it works. That's not sarcasm. It's what I would do.
Standard 6 speed freewheels had the cogs 5.5mm c to c. 7 speed freewheels are 5.0 mm. So, 6 speed shifters won't work well with 7 speed spacing except in friction mode. Visa versa, too. Campy 8 speed was 5.0mm also so if you have Camp 8 speed shifters with a Campy 8 speed or early 9 speed RD, it will work fine with a 7 speed freewheel.