Help with compact dbl

[Jeff5]

I have a 53/39 standard double. I want to get a 50/34 compact double, and need help identifying which parts are compatible. I have a TruVatia Elita, Isis drive, with a Shimano 105 front derailer. The center of the crank arm has "68/108 BB Only" on it. I've seen some compact dbls with the bottom bkt. Should I get one of those. My bike is a 2004 Cannondale R700.

[Painkiller]

Is your front derailleur a clamp on or braze on ? if it is braze on that may impede your crank selection. Your crank would determine your BB spindle length. so if your spline type and length differ from Isis/108 replace it. just make sure you get one for your 68mm shell width
I think you have to have a clamp-on style front derailleur if you cannot move it down the tube. check the specs of the FD to make sure it can handle the 16t difference also

[AndrewB]

You might also need a new chain due to the length changes with any gear changes.

[Painkiller]

(01-22-2012, 11:12 PM)AndrewB Wrote:  You might also need a new chain due to the length changes with any gear changes.

at most you would shorten the chain you have for this mod. if your chain is good leave it and shorten to proper length.

[Jeff5]

(01-22-2012, 08:48 PM)painkiller Wrote:  Is your front derailleur a clamp on or braze on ? if it is braze on that may impede your crank selection. Your crank would determine your BB spindle length. so if your spline type and length differ from Isis/108 replace it. just make sure you get one for your 68mm shell width
I think you have to have a clamp-on style front derailleur if you cannot move it down the tube. check the specs of the FD to make sure it can handle the 16t difference also

So the 68 is the diameter of the bottom bracket and the 108 is the length from side to side? How do I calc the 16t difference and how do I ck it.
Tx- Jeff

[Jeff5]

(01-23-2012, 12:15 PM)Jeff5 Wrote:  

(01-22-2012, 08:48 PM)painkiller Wrote:  Is your front derailleur a clamp on or braze on ? if it is braze on that may impede your crank selection. Your crank would determine your BB spindle length. so if your spline type and length differ from Isis/108 replace it. just make sure you get one for your 68mm shell width
I think you have to have a clamp-on style front derailleur if you cannot move it down the tube. check the specs of the FD to make sure it can handle the 16t difference also

So the 68 is the diameter of the bottom bracket and the 108 is the length from side to side? OK-I got it (50-34=16) How do I ck if my FD will be OK with the 16t diff.
Tx- Jeff

[nfmisso]

(01-23-2012, 12:15 PM)Jeff5 Wrote:  ......

So the 68 is the diameter of the bottom bracket and the 108 is the length from side to side? How do I calc the 16t difference and how do I ck it.
Tx- Jeff

Yes 68mm is diameter of the BB, and 108mm is the length.

16T difference - is the difference between the largest and smallest cogs on the chain ring: 50-34=16, vs 53-19=14. Front derailleurs are rated for this difference in number of teeth. Should not be an issue. You can check the tech docs that Shimano has on the web.