How to switch from 105 triple crank to 105 double crank?

Question

I know this has been asked a few times prior, but I couldn't find a straight answer. I was wondering what would be required to make a complete switch from a Shimano 105 triple crank to a Shimano 105 compact double crank - 10-speed rear cassettes. Here is how I understand it: The cassette, rear deraileur, and shifters should work perfectly for both a triple and a double crank. The front derailleur and bottom bracket, however, will need to be replaced. Is this correct?

I am well aware that the differences between the two cranks' performances will be very minimal, so I don't need to hear that lecture ;).

Answer 1

Mate, You can get it to work just by changing the crank arm with the chainrings on it. The chainline will wind up a bit further out, but not so bad it can't be used.

The bottom bracket for a triple is 118 mm and a double is 109 mm. So changing the bottom bracket will bring the rings in 4.5 mm which will put it back on a normal chain line.

A triple front derailleur will change two cogs fine but you can change that too if you want. this link will take you through setting it up.(scroll down to "front derailleur adjustment")

http://www.sheldonbrown.com/derailer-adj...

If you roll back the outside of your front shifter hood you will find the part number. ST-5510 is designed to do both doubles and triples. With a bit of fiddling any triple shifter can be set up for two chainrings.

Your rear derailleur will be a long cage, normally a double chainring only requires a short cage. A long cage can  take up more slack allowing for a greater range of gears. A short cage is a tiny bit lighter.

My tip would be change crank and bottom bracket then adjust front derailleur. If your short a few bucks leave the bottom bracket for a while. If you like to buy heaps of bits you don't really need change the lot.

Answer 2

you can do it in phases

it must be done in this order though

1 at the least all you need to do is change the crank arms.

2 next would be to get the 109mm bottom bracket.

(for both of these 2 options you could use the same derailleur but you will have to adjust the limit  screws.)

3 next step would be replacing the derailleur

4 the last step would be getting a double front shifter

Answer 3

Front Deraileur won't need to be replaced I think. You can bodge it by raising the position of it by a few mm. I've just changed from Tiagra double compact, to a double 52/39 and the derailleur seems fine with it. You will need to adjust the derailleur limit stops though, otherwise it will throw the chain onto the BB shelf.

You will need a shorter BB length, but if you go for a new Hollowtech 2 one, the axle is built onto the right-hand crank, the left hand crank simply bolts on (albeit a rubbish bolt system). If you go down this route, you'll also need a Hollowtech 2 BB, which are cheap. Only about $35 approx.

Answer 4

Yes,

you will only need to change the front derailleur and bottom bracket.

However you may find that only the front derailleur may need replacing as the difference between the BB axle length is a mere 2 to 3 mm. Adjustment of the front derailleur may be all that is required. Try changing the front derailleur first before buying a new BB.