105 Double Chainrings

[auswi2]

Good Morning,

I have a Standard 53/39 and I was going to upgrade to a compact 50/34. Is it possible to keep the 53T and change the smaller sprocket to a 34T or even buy a 50/34 double instead of compact as it is cheaper.

Are there any pros or cons to this

thanks

auswi2

[darkhorse75]

A 34t chainring wont fit on the crank spider, that's why compact cranks have a PCD of 110mm instead of 130mm. A 50/34 double is a compact crankset if you want 50/34 then you should buy a compact crankset.

[usernameforme]

The 130BCD (bolt circle diameter) of a standard is too big for a 34t sprocket, the smallest you can get on 130BCD is 38 teeth. Compacts have a 110BCD so they are slightly smaller. You can get Standard (53/39) ratios for compacts (110BCD) but you can't get compact (50/34) ratios on a standard (130BCD)

[antipodean]

The 130BCD (bolt circle diameter) of a standard is too big for a 34t sprocket, the smallest you can get on 130BCD is 38 teeth. Compacts have a 110BCD so they are slightly smaller. You can get Standard (53/39) ratios for compacts (110BCD) but you can't get compact (50/34) ratios on a standard (130BCD)

Not quite true Shimano, do have a 50T chainring with 130BCD.

http://www.chainreactioncycles.com/Mode ... elID=18491

[usernameforme]

Not quite true Shimano, do have a 50T chainring with 130BCD.

http://www.chainreactioncycles.com/Mode ... elID=18491

Yes, but the 34 cog is too small for the 130BCD. The smallest chainring you can run on 130BCD is a 38 (I think)