Standard chain rings to semi compact chain rings = new crank?

Back by popular demand, the general all-things Road forum!

Moderator: robbosmans

Post Reply
User avatar
stockae92
Posts: 295
Joined: Fri Sep 25, 2015 5:13 pm

by stockae92

From what I read seems like going from standard to semi compact chain rings means a new crank?

Or there is a way to do it without a completely new crank?

mattr
Posts: 4671
Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Depends on the crank. What crank do you have.

by Weenie


Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓    Broad Selection ✓    Worldwide Delivery ✓

www.starbike.com



User avatar
stockae92
Posts: 295
Joined: Fri Sep 25, 2015 5:13 pm

by stockae92

I have a FSA K-Force Light BB386 53/39.

I read the smallest chain rings for standard crank is 52/38

jrmynthn
Posts: 109
Joined: Thu Dec 18, 2014 3:27 pm

by jrmynthn

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.

User avatar
BRM
Posts: 817
Joined: Sun Mar 16, 2014 3:43 pm

by BRM

stockae92 wrote:I have a FSA K-Force Light BB386 53/39.

I read the smallest chain rings for standard crank is 52/38


Give proper specs of your stuff, like BCD

mattr
Posts: 4671
Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr


RussellS
Posts: 916
Joined: Wed Feb 03, 2010 1:31 am

by RussellS

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.

User avatar
stockae92
Posts: 295
Joined: Fri Sep 25, 2015 5:13 pm

by stockae92

Well, I guess I got the older version of it, which has 130 BCD. I looked online and seem like 38T is smallest I can go. There were some 37T talk way back but I don't see any product available. :(

Image

wpccrunner
in the industry
Posts: 429
Joined: Fri Dec 07, 2012 8:34 pm
Location: CO

by wpccrunner

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)

User avatar
stockae92
Posts: 295
Joined: Fri Sep 25, 2015 5:13 pm

by stockae92

Yep, BH Ultralight RC it is, great bike. :)

I am thinking if I should get a 38T to have marginal higher cadence on steep climb (rear is 11-28) for not a lot of money?

nathanong87
Resident master of GIF
Posts: 3405
Joined: Mon Jul 18, 2011 2:44 am
Contact:

by nathanong87

get a long cage RD..... u can do 52/38 if u want, but 11-30 or 11-32 would help some.

topflightpro
Posts: 829
Joined: Tue Jan 13, 2009 2:35 am

by topflightpro

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.

by Weenie


Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓    Broad Selection ✓    Worldwide Delivery ✓

www.starbike.com



Post Reply