Moulton406

Hi erstwhile contributors

On a Trek 1.2 (2009) I have fitted the original cranks with 50 39 24. Using the existing Sora Brifters FD and RD all works well together to the 11-28 cassette. I am thinking to change the set up to 10 speed keeping the crank, FD, RD. and changing the Brifters to Shimano 10 speed 105 ST 5703, the chain to 10 speed Shimano and the rear cassette to something such as 12-32.

I hope someone can give me some good information and advice on the potential of this conversion. Thank you

maddog34

mid cage, or long cage Sora? that's a 46 total wrap change at 50-24, and 12-32... that's a LOT!

you'd have to avoid big front/big rear, or small/small, for sure.... and if that Sora is a mid cage, i wouldn't even try it.

i've seen, and heard of, way too many damaged bikes from too little wrap issues... think broken derailleurs, bent/broken der. hangers, and messed up spokes...

you're jumping from at(or actually ABOVE) the limit, to roughly 7%+ over the limit, with that extra 3 teeth in the rear....... Max capacity for an XT Shadow extra long cage is like 45 teeth change..... kinda doubt a Sora has more available range, eh?
Whatever you do, size the chain as slack as possible with no rubbing at small/small, ok?

cny-bikeman

If you don't have a pressing need for a 50-12 you could a 48 tooth front, which avoids the wrap issue.

dsbrantjr

You must always size the chain long enough to go into the big/big combination, without exception.

Retro Grouch 

Also, think about how you really use your gears:

I only use the granny while grinding up big hills with the largest 2 or 3 rear cogs. As soon as I crest the hill, I immediately shift out of the granny. I can't remember ever accidentally getting even close to shifting into the little/little. I don't worry anymore about having enough slack take up for that gear combination.

I DO worry about my chain being long enough to cover the big/big. That can cause a lot of damage and there's no warning.

HillRider

I'm running similar but not as extreme setups as the OP proposes on two bikes. Shimano 5703 triple cranks geared 50/39/26 and 12x27 10-speed cassettes (39 total teeth) with a 105GS or Ultegra GS rear derailleur. The chains are sized to allow big-big which I consider essential. Small-small has no appreciable slack but I never use it either.

I would not be as concerned as you are about some slack in small-small as long as it isn't extreme and the OP doesn't use it except as a mistake. I agree that if his rear derailleur is a short cage, gearing that wide is inadvisable.

maddog34

and how does one get enough chain length as needed for this extreme setup?

by going as loose as possible on the small/small.... after getting/installing an XT/XTR shadow extra long cage derailleur, if it was my task.... but it sure sounds like the OP wants to attempt making his setup work as is....

longer chain, loose as possible at small/small.... THEN carefully check big/big.

he or she was already past max wrap of even a long cage Sora.

turkey9186

You might run into issues with the rear derailleur going from eight to ten speed chain. When I converted my 9 speed DA triple to 10 speed I ended up replacing the rear derailleur. The narrower ten speed chain was hanging up on the jockey wheels of the DA7703. No problems with a DA7803.

bradtx

Moulton406, You may need a 1 mm spacer behind the 10s cassette. If the cage is too short to wrap the small-small combo, a 9s mountain bike RD can be substituted.

Brad

Slash5

Crank may or may not work. I've had to replace a 8 speed crank before but also got other cranks to work.