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Old 05-17-09, 01:15 PM   #1
cblaster
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Biopace Chainrings usable on a single speed?

I posted this a little earlier on the SS/FG forum, but I figured you guys would have more experience with Biopace, so:

Yesterday I went and picked up this Nishiki Modulus off of craigslist for my girlfriend's birthday; its got Biopace chainrings on it.





I'm going to be converting this bike to singlespeed, and I was curious to see if anyone had any experience with these chainrings. I checked Sheldon's website, and he says they work fine with a fixed gear. The thing is, on a fixed gear, the chainring will be constantly rotating, so the chain will never be at it's loosest point in the rotation for more than a split second. Because this bike will be coasting on some relatively crappy roads, I was wondering; if the bike coasts when the chain is the least tense in the rotation and hits a bump, will the chain be loose enough to come off the chainring? Is the change in tension that Biopace creates going to be a problem at all? I just want to make sure that this bike is mechanically sound before it hits the street.
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Old 05-17-09, 01:29 PM   #2
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I did pretty much explain exactly what you just said to me in the first sentence of my post. I realized that I wanted more informed answers so I reposted here.
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Old 05-17-09, 02:01 PM   #3
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sheldon brown covers it.

there was a thread about this about a month back. if you used search...
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Old 05-17-09, 02:12 PM   #4
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sheldon brown covers it.

there was a thread about this about a month back. if you used search...
Just found that thread when I re-searched. I skipped over it the first time because it was just titled "Oval Chinrings". Thanks.
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Old 05-17-09, 07:16 PM   #5
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Shimano Biopace Chainrings

Stop crossposting. If you want uninformed, bull**** answers post in the SSFG forum.
Glad you could be of some help...
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Old 05-17-09, 07:18 PM   #6
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I used a 52t Biopace with a single 20t cog and spacers on a freehub. No problems in maybe a hundred road and trail miles. Then I went fixed & smaller ratio.

Last edited by garage sale GT; 05-17-09 at 07:57 PM.
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Old 05-17-09, 07:49 PM   #7
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I have a fixed gear with a biopace crank. The only thing to note is the chain tension is not constant throughout the crank cycle so you need to make sure the chain is not overstressed at any point in the cycle before riding off. Other than that no problems.
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Old 05-17-09, 08:18 PM   #8
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Try it on a shaky trail! The less hard you pedal the more likely the chain is to fall off, which I believe was your point...so it's safe to try because if you lose the chain while coasting with the cranks at the lowest tension, (horizontal, I believe?) you won't have a big problem.

If it is a problem, you can always get a standard 130mm ring, or you can put a sprung chain tensioner in your derailleur mount just to take up the slack.
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Old 05-17-09, 11:12 PM   #9
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Take a ruler and lay it across the center of the biopace ring, now measure the circumference along one side of the ruler -ie the half of the ring the chain will lie against.

Now rotate the chainring and repeat; notice anything about the circumference?

It should be pretty much the same regardless of orientation - always 1/2 of the total number of teeth.

If the circumference of chainring is always constant length, what does this tell you about the amount of chain length it will take up at any given time? What does this imply about the tension of the chain at any given time?

hint: also constant
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Old 05-18-09, 07:11 AM   #10
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I would be comfortable with singlespeed using Biopace, less so fixed. If you throw your chain on a singlespeed, you probably won't faceplant.
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Old 05-18-09, 07:11 AM   #11
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Another thing to keep in mind is that the smaller BioPace rings are less round than the large ones, so using a large ring and appropriate size rear cog would minimize the variable chain tension issue.
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Old 05-18-09, 07:49 AM   #12
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Thanks for the input everyone. I was going to use the smaller 42t ring for it but, now I'll go with the larger 52.
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Old 05-18-09, 08:00 AM   #13
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Quote:
Originally Posted by xenologer View Post
Take a ruler and lay it across the center of the biopace ring, now measure the circumference along one side of the ruler -ie the half of the ring the chain will lie against.

Now rotate the chainring and repeat; notice anything about the circumference?

It should be pretty much the same regardless of orientation - always 1/2 of the total number of teeth.

If the circumference of chainring is always constant length, what does this tell you about the amount of chain length it will take up at any given time? What does this imply about the tension of the chain at any given time?

hint: also constant
The chain tension does vary.
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Old 05-18-09, 03:27 PM   #14
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On paper, where the math is continuous, the chain length is constant. With actual gears the math is discrete, and the results are that the chain length is not constant. They designed the shape of the gear to minimize the amount of chain stretch but it is there.
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Old 05-19-09, 03:40 PM   #15
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If you're worried about throwing the chain, just press the chain to the side while turning the crank and see if it falls off. I use an open-end wrench for easy chain pushing without finger greasing.
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