Shifters will work okay going to double but you may want to change the rear der. Using the triple left shifter on a double works he'll just have to shift a little more with the front. ie. 2nd click is used for the bottom half of the cassette when in the small ring to prevent the front der from rubbing. I did the same thing on my road bike 4 years ago and still ride it this way today. Rear der can be found cheap on ebay.

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Human Motor Works Racing

Do you need to change the bottom bracket I think you do going
the other way.

correct, it will move the crank arms and chainline in a bit, which is a good thing. The rear derailleur is fine.

I seem to recall on some of the sti shifters there is an adjustment screw or something to change from double-triple, but maybe i mis remembered.

Your son might like one of the new compact cranks, they allow for a little easier climbing. I like shimano crank-bb very much, way more that the fsa products. The 105 is a really decent crankset

I swapped a triple for a compact (then later a double) and all I had to do was readjust the front der, nothing else, still using the triple shifter and FR with the double fine.

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Chain line will be better with a smaller BB but it will work with one it came with.



jaretj

I bought a used Trek 5200 that had a triple ring many years ago. It was 9 speed. I changed the crankset to a double. I changed nothing else and everything works just fine.

I didn't change the bb until I changed to a newer Blackwell crankset a few months ago. I'm still running the long cage rear der and it works great.

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Paul