switching compact to standard double

[Miranda]

What mechanically would need to be done to my bike to switch from a comact double to a standard double?

Current: Shimano Ultergra SL 50/34

Considering: Shimano Ultergra 6700 or DuraAce 7900 53/39

Current cassette Shimano Ultergra 12-25.

FWIW...
One lbs wrench went on something about it couldn't be done without getting a whole new crank set, something about "bolt pattern compatibilty" of the rings etc. A differnt lbs wrench thouhgt I could just switch my 50/34 rings to 53/39... but something about adjustments (to the front derailer I assume) and longer chain. Um okay, boys ... (mechanical help at the lbs makes my head hurt usually, gheez lol)

So... I decided to come to my expert bike friends here on TE for help.

TYIA

[indysteel]

I don't actually know the answer to your question, but it might be easier and cheaper to just go with an 11-23 or 11-21 cassette than to swap out the chainrings (if you even can), but that's assuming you're just after some bigger gears.

[HillSlugger]

I'm not sure if the chainrings can be switched out. If not, you're looking at a new crankset, new chain, and an adjustment to the position of the front derraileur. I've done it myself, but in the other direction.

[dianne_1234]

Current: Shimano Ultergra SL 50/34
Considering: Shimano Ultergra 6700 or DuraAce 7900 53/39

There are exceptions, but in general, and specifically for the cranks you listed:
Compact cranks have 110mm bolt circle diameter.
Standard cranks have 130mm bolt circle diameter.
So yes, in general, you'd get a new crank.
Except... You can buy 39 and 53 tooth chain rings (from FSA I think?) in 110mm which will bolt onto your existing crank. However in my experience nothing beats the shifting performance of Shimano rings, and these are not available from Shimano. So my personal choice would be a new crank.

The new cranks you're considering are good. However with this current generation, Shimano's moved the two chainrings farther apart by about 1 millimeter, making those cranks tecnically incompatible with your current shift levers. I haven't tried it to see just what happens, but you can avoid the issue by using a crank of the last generation: Ultegra 6600 or Dura-Ace 7800.

With 6600 or 7800 cranks and a longer chain (maybe not; 3 teeth more means only 1.5 links longer and your current chain *might* be on the long side already?), you should be good to go as far as parts are concerned, only adjusting needed to get back on the road. Oh yeah, maybe a new front derailleur cable, as your FD will need to go 6mm higher to fit the new 53 tooth ring.

Have fun!

[Miranda]

Thanks for the replies thus far! Hmm, ok... going with the short answer for now... that helps and gives me some things to think about. Always appreciate the help I receive here.

[ridebikeme]

Your first mechanic is correct. You would definitely need to buy a new crank if you'd like a 53/39 set up. The bolt pattern on compact cranks and a double are entirely different.. hence not compatible. The other issue is that you definitely should change your chain to accomodate the bigger gearing. Front derailleur will need to be adjusted as well. If you decide to upgrade to another crank, have your mechanic check the length of your current bottom bracket with the new crank... it also might be two different lengths.

Good luck with your decision!



http;//chasecyclery.blogspot.com

[Miranda]

Your first mechanic is correct. You would definitely need to buy a new crank if you'd like a 53/39 set up. The bolt pattern on compact cranks and a double are entirely different.. hence not compatible. The other issue is that you definitely should change your chain to accomodate the bigger gearing. Front derailleur will need to be adjusted as well. If you decide to upgrade to another crank, have your mechanic check the length of your current bottom bracket with the new crank... it also might be two different lengths.

Good luck with your decision!



http;//chasecyclery.blogspot.com

Thanks for adding on! Didn't think about the bottom bracket. OK, something else to check into.

[Velocivixen]

Forgive me for asking this question, as I am new and am trying to understand gearing. Why do you want to switch from compact to standard double? I know it would give different gear ratios but not sure about the differences between the two. For example, do you want to be able to go up hills better or ???
Thanks.

[ridebikeme]

I forgot to ask whether you had a regular bottom bracket or outboard bearings.. more than likely they are outboard bearings, so perhaps the switch will be a bit easier for you. At any rate, have fun in your search and good luck with the new crank.


http://chasecyclery.blogspot.com

[aicabsolut]

I think the biggest you can go with 110mm BCD is 38/52 from a few brands like FSA and SRAM. to go with a 39/53 you will need a new crankset with 130mm BCD.

Also, yes, a longer chain.

You may need to raise the FD up slightly higher to clear the larger outer ring. Limits and cable tension will also need to be checked/adjusted, but all of the rest of your existing equipment will be compatible.