Standard chain rings to semi compact chain rings = new crank?
Moderator: robbosmans
Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓ Broad Selection ✓ Worldwide Delivery ✓
www.starbike.com
Not necessarily. Depends on the crank and the BCD of said crank. In the shimano world the older cranks use a 130 BCD for 53/39 chainrings and 11 BCD for the 50/34. The newer DA 9000 and Ultegra 6800 use the same 110 BCE for all sizes so you can swap up and down.
For my Ultegra 6800 groupset I have a set of 52/36 and 50/34 as well as a set of AbsoluteBlack 50/34 ovals that I swap around depending.
For my Ultegra 6800 groupset I have a set of 52/36 and 50/34 as well as a set of AbsoluteBlack 50/34 ovals that I swap around depending.
http://www.fullspeedahead.com/fsa-k-for ... kset-ever/
According to this your crank has 110 mm bolt circle diameter chainrings. So you can run any chainring from 33 teeth up to about 60 on either or both positions. You can run 34-33 chainrings if you want. Or 53-52 if you want. Or 50-36, 50-38, 50-34, 52-36, etc.
The bolt circle diameter, bcd, is important because it defines the minimum size of chainring. 110mm bcd is the diameter of the chainring bolts. Chainrings have 1/2 inch between teeth. Using the geometry formula of 2*Pi*Radius. We get about 345mm diameter. About 13.5 inches. Times two for the 1/2 inch spacing on teeth, we get 27 teeth possible with 110mm bcd. But the 110mm bcd is to the center of the chainring bolts. So add 5mm to the outside on both ends, and 5 extra millimeters for the material of the ring itself. Reverse calculating we end up with 133mm between teeth gullets diameter for 33 tooth chainrings. 110mm bcd + 11mm for half the bolt and chainring material on either side.
According to this your crank has 110 mm bolt circle diameter chainrings. So you can run any chainring from 33 teeth up to about 60 on either or both positions. You can run 34-33 chainrings if you want. Or 53-52 if you want. Or 50-36, 50-38, 50-34, 52-36, etc.
The bolt circle diameter, bcd, is important because it defines the minimum size of chainring. 110mm bcd is the diameter of the chainring bolts. Chainrings have 1/2 inch between teeth. Using the geometry formula of 2*Pi*Radius. We get about 345mm diameter. About 13.5 inches. Times two for the 1/2 inch spacing on teeth, we get 27 teeth possible with 110mm bcd. But the 110mm bcd is to the center of the chainring bolts. So add 5mm to the outside on both ends, and 5 extra millimeters for the material of the ring itself. Reverse calculating we end up with 133mm between teeth gullets diameter for 33 tooth chainrings. 110mm bcd + 11mm for half the bolt and chainring material on either side.
-
- in the industry
- Posts: 429
- Joined: Fri Dec 07, 2012 8:34 pm
- Location: CO
yes, you will need a new crank.
Looks like a BH Ultralight RC, eh?
I thought they shipped all those with a semi compact 52/36 chainring combo (or maybe semi compact is US spec only)
Looks like a BH Ultralight RC, eh?
I thought they shipped all those with a semi compact 52/36 chainring combo (or maybe semi compact is US spec only)
-
- Resident master of GIF
- Posts: 3405
- Joined: Mon Jul 18, 2011 2:44 am
- Contact:
get a long cage RD..... u can do 52/38 if u want, but 11-30 or 11-32 would help some.
-
- Posts: 829
- Joined: Tue Jan 13, 2009 2:35 am
nathanong87 wrote:get a long cage RD..... u can do 52/38 if u want, but 11-30 or 11-32 would help some.
This is what I do. I have a 53/39 up front, and when I head out to the mountains, I put on a long cage rear derailleur and an 11-32 cassette.
The 39-32 combo is pretty much equal to a 34-27.
Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓ Broad Selection ✓ Worldwide Delivery ✓
www.starbike.com