105 Double Chainrings

105 Double Chainrings

Postby auswi2 » Wed Oct 03, 2012 9:27 am

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

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by BNA » Wed Oct 03, 2012 9:55 am

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Re: 105 Double Chainrings

Postby darkhorse75 » Wed Oct 03, 2012 9:55 am

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.
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Re: 105 Double Chainrings

Postby usernameforme » Wed Oct 03, 2012 9:57 am

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)
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Re: 105 Double Chainrings

Postby antipodean » Wed Oct 03, 2012 7:48 pm

usernameforme wrote: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.

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Re: 105 Double Chainrings

Postby usernameforme » Wed Oct 03, 2012 8:15 pm

antipodean wrote: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)
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