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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 05-17-07, 10:47 PM   #1
idcruiserman
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38/15 to 42/16, new chain?

I'm currently running the stock 38/15 on my CRB, and I want to go to 42/16. How much extra chain will I need? The axle is about 3/4" back from the front of the fork end, so I have room to move it forward.

Just wondering if I should order a new chain with the ring and cog. This chain is essentially new.

thx, eric
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Old 05-18-07, 05:10 AM   #2
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Kind of depends on chainstay length. If I run the numbers on my bike, 42/16 would need two more links than 38/15. I suppose the only real way to find out is to try it.
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Old 05-18-07, 06:36 AM   #3
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Here, figure it out...

http://www.parktool.com/repair/readhowto.asp?id=26
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Old 05-18-07, 06:40 AM   #4
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Quote:
Originally Posted by idcruiserman
I'm currently running the stock 38/15 on my CRB, and I want to go to 42/16. How much extra chain will I need? The axle is about 3/4" back from the front of the fork end, so I have room to move it forward.

Just wondering if I should order a new chain with the ring and cog. This chain is essentially new.

thx, eric

i don't know, but i do know how to find out...and it doesn't involve a message board...it involves a chain breaker and a few minutes at most.
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Old 05-18-07, 07:07 AM   #5
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42 x 16 will need 5 more links that 38 x 15. One extra tooth at the back, four at the front. Not rocket science.

The question is, have you got 2.5 links worth of dropout space?
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Old 05-18-07, 07:18 AM   #6
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Quote:
Originally Posted by idcruiserman
I'm currently running the stock 38/15 on my CRB, and I want to go to 42/16. How much extra chain will I need? The axle is about 3/4" back from the front of the fork end, so I have room to move it forward.

Just wondering if I should order a new chain with the ring and cog. This chain is essentially new.

thx, eric
Throwing some numbers into FixMeUp! for a setup that's close to yours.. I assumed that your current axle for 38/15 is at 16.785" (corresponds to 47 full chain links). From there, it appears that you could add a full link to put the 42/16 at 16.635" (0.15" forward), or even leave the chain as-is and have the 42/16 at 16.125" (0.66" forward). If you've got track ends, then I guess either is fine; with dropouts, you might be close to the front. Try it first and see.. if not, remove a link!
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Old 05-18-07, 09:08 AM   #7
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I've got track ends. Thanks for the chain length link.

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i don't know, but i do know how to find out...and it doesn't involve a message board...it involves a chain breaker and a few minutes at most.
Gee thanks. I don't have the cog nor the chainring nor extra links, and I don't know the relationship between number of teeth and ring/cog diameter. I do have a chain tool. Sorry I pissed in your cornflakes this morning. It wasn't my intention.
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Old 05-18-07, 10:11 AM   #8
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Quote:
Originally Posted by CF4L
The "rigorous" equation mentions that adding 1 is for derailleurs. Does that go for the simple equation as well?

The bike's at home, but using VD's chainstay length of 17.7", I get 49" for stock chain and 51" for 42/16. So add 2 links to keep the axle in the same location.
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Old 05-18-07, 12:10 PM   #9
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Quote:
Originally Posted by idcruiserman
The "rigorous" equation mentions that adding 1 is for derailleurs. Does that go for the simple equation as well?

The bike's at home, but using VD's chainstay length of 17.7", I get 49" for stock chain and 51" for 42/16. So add 2 links to keep the axle in the same location.
1) Looking at the construction of the equations, I suggest the 1 added for derailleur compensation is applicable to both.

2) I got single-speed chain lengths of 48.84 and 50.14 inches, or 49 and 50 links.
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Old 05-18-07, 12:44 PM   #10
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Sammyboy almost nailed it, I think. The problem with how he put it is that the chain only goes over half the added links at once. So, Extra # of teeth/2 = how many added links you'll need for exactly the same length chain. The fact that chainrings are measured in units congruent to chain links makes it simple. Right?? Maybe there is some weird geometry im missing here... I guess the slight increase in diameter would add a little fudge factor to the length of the chain that is not in contact with the ring and cog.

Last edited by mander; 05-18-07 at 12:56 PM.
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Old 05-20-07, 10:19 PM   #11
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With actual bb to rear axle measurement of 17.375", I get 48" chain length. Assuming same chain length with 42/16, new bb to rear axle length is 16.75", which I can do. Sweet.

LBS had a 16T freewheel, but no 42T chainring nor tools. Ordered the rest. I flipped the rear wheel today and road fixed around the neighborhood for a while. Felt strange but interesting. I may give a fixed commute a try on Tues.
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