Is it possible to use a 3x7 set of shift levers with a 2x10 drivetrain?

Question

Basically just wondering what's going to happen. I have some old seven speed shifters and I'm wondering if I can use them for a build instead of buying new ones.

Answer 1

No - not as written, and assuming indexed shifters.

6/7/8 speed drive trains all use the same spacing between cogs. So one could use an 8 speed indexed shifter on a 6 or 7 speed setup, and simply have a "ghost click" at one end of travel.

Conversely, an indexed shifter with fewer detents/positions will not be able to get to all gears.

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Things changed when 9 speed came out - the cassette was the same physical width, but the cogs were closer together, which mandated a thinner chain. So the derailleur has to move slightly less distance to get to the next chain position.

So your 7 speed shifter will line up fine on exactly one cog, and maybe it will work noisily on the adjacent cogs, but after that the chain will be slipping up/down between two cogs, making noise and causing wear, and you're not going to put all your power through.

Solutions: Depend on exactly what gear you already have.

• Bite the bullet and buy a 2x10 set of shifters and do it as a 2x10 speed properly. This would be my preference.
• Buy a 7 speed cassette, and run that on your wheel along with a 6/7/8 speed chain and your existing shifters. You will certainly need spacers behind the cassette to make sure the lockring engages properly.

7 speed cassettes are rare, but fortunately cheap. I have one on my wet-day road bike, sourced from China because no local bike shop had anything.

Answer 2

Depends on the shifters.

If the shifter is a thumb or bar-end shifter, which are probably the most useful shifter types out there, you can simply set the rear shifter from "index" to "friction". Then it will work with any number of speeds. The front shifters in these are friction anyway to allow trimming the front derailleur. Of course you lose the luxury of rear indexing, but people rode bicycles for a long time without any indexing.

For index-only shifters such as Shimano rapidfire or Shimano STI shifters, don't even try. Since you are limited to indexed mode only, the indexing won't match on the rear. Whether it matches on the front is dependent on the pull ratio which may differ between "road" and "MTB", or "double" and "triple", or different group generations.

What could work is a 2x8 drivetrain. For 8 speeds, cassette pitch is 4.8mm, for 7 speeds it's 5mm. Since you are always at most 3 speeds away from the middle gear on a 7-speed system, you have an error of at most 0.6mm, or 12.5% of a gearshift on the 8-speed systems. This should be a tolerable error. A triple front shifter will always work on a double drivetrain, but depending on the pull ratio compatibility, you may need to do two front shifts every time instead of one. This shouldn't be a dealbreaker since it isn't guaranteed to happen, and if it happens, you shift on the front rarely anyway.