8 speed derailleur with 10 speed system.

raldat

So the above is my question. Why? I am building an old bike into a hack/commuter. I have a complete 3 x 8 system for it which would work just fine. But I also have a spare 10 speed shifter, spare 10 speed cassette and chain so thought a 1 x 10 might be good just because I can. It would cover my riding needs and would do away with a front shifter.

So it occurred to me, why couldn't I use my 8 speed derailleur? The total cassette width is the same so the derailleur should be able to move from one side to the other. It currently handles a 22 to 42 difference up front and 11 to 32 up back so it should have enough capacity for a 11 - 36 up back and single up front.

My logic may well be wrong, but I am really curious on people's thoughts.

Chunkers1980

Nope

raldat

Chunkers1980 wrote:

Nope

Thanks, why?

cooldad

Different pull ratios. Read the FAQS

bikaholic

The correct answer is maybe - because the shifter model was not specified.

If you use the Shimano Tiagra SL-4600 or Ultegra SL-R770 flatbar trigger shifter, it may work.

I say "may work" because all 8-speed RDs had a stated max cog size at most 34T. So 36T compatibility would depend on frame hanger geometry being slightly outside the Shimano spec.

The pulley wheels are a little thick for 10-speed chains but are still within working tolerances.

cooldad

OK, the correct short answer is 99.452% no.

raldat

Shifter is slx 10 speed. Hadn't considered the pull ratio issue, that is why I asked the question. The technical issue I had considered when thinking about it didnt, at least to me, adequately explain why it wouldn't work.