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I have a trek 1500 with a Bontrager Race triple crankset. is there a way I could simply replace the 42 and the 30 with a single 39 tooth ring? Of course I would have to limit the range of the front DR or replace it with a double, but is there something important here that I am missing? any advice would be greatly appreciated. thanks. tim
 

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timmyc said:
I have a trek 1500 with a Bontrager Race triple crankset. is there a way I could simply replace the 42 and the 30 with a single 39 tooth ring? Of course I would have to limit the range of the front DR or replace it with a double, but is there something important here that I am missing? any advice would be greatly appreciated. thanks. tim
It's going to be more economical in the long run to ebay that bad boy and buy a double as a replacement.
 

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chain line...

The only problem with just replacing the middle ring is the incorrect chainline. The middle ring on a triple is 6mm further to the right than it is on a double. This makes the middle ring and largest cog chainline quite extreme. If you replace the bottom bracket with the model made for a double, it will move the middle ring 5mm to the left and reduce the tread width between the pedals (Q factor). This setup will be very close to a normal double crank.
 
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