The Sora 9 speed STI levers on my road bike are borked but the rest of the kit is fine. So I have a plan.

Buy some new 10 spd 105 STIs, a 10 speed cassette and chain. My research tells me this is fine, the 9 speed tiagra rear mech has the same cable pull ratio as the 10 speed so it will work OK. Won’t it?

I can’t see any reason the 105 Double STI won’t work with my Sora compact front mech.

So my issue is with the spare back wheel I use on the turbo trainer. It has a 9 speed cassette. I don’t want to buy another 10 speed cassette just for the turbo. What will happen if I run the 10 spd set up with the 9 speed cassette? Obviously it won’t shift properly but thats not really an issue on the turbo. Will any other mishaps befall me or my bike?

I don’t know the answer to any of your questions, but these may help…

http://www.merlincycles.com/bike-shop/road-parts/transmission/gear-sti-ergo-levers/shimano-105-5700-gear-kit.html

http://www.merlincycles.com/bike-shop/road-parts/transmission/gear-sti-ergo-levers/shimano-105-5700-gear-kit-black.html

You will die, yes, but not because of that.

billytinkle – Thanks, looks interesting but still £40 more than my option and the FD in those kits is braze on when I need a clamp. I’m sure Merlin would switch for the correct one but I’d still prefer not to go that far if I don’t have to.

If you want a 10spd cassette just for a turbo, I have one you can have for £10+postage. It’s a 12-27 105 cassette, the top 3-4 cogs are quite worn but that won’t be of any consequence on a turbo, as the smaller 5-6 cogs are all in good condition and this is where you will be on the turbo anyway.

You may find it cheaper/better/easier in the long run just to buy a cheap 10spd group set off the likes of merlin than go down the route you describe, that said, I can’t see any reason why what you describe shouldn’t work.