# Biopace Crank Replacement?



## Pimpride (Nov 14, 2005)

Hello, I have a old 1984 or so, mystery frame (Lotus?) with biopace cranks on it and was wondering if I can replace them with something else that works better. I don't like the bob of the biopace cranks and noticed regular MTB cranks don't fit. :madman: Do road cranks fit? Here's a few pics --->


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## laffeaux (Jan 4, 2004)

Pimpride said:


> Hello, I have a old 1984 or so, mystery frame (Lotus?) with biopace cranks on it and was wondering if I can replace them with something else that works better. I don't like the bob of the biopace cranks and noticed regular MTB cranks don't fit. :madman: Do road cranks fit? Here's a few pics --->


You should be able to put on a different crankset, but it may reguire a different sized bottom bracket.

However... the easy solution is to just replace the rings. Chainrings of a similar vintage should go on without any problems - newer rings have a different bolt diamater, and often a different ring thickness.


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## DeeEight (Jan 13, 2004)

Biopace chainrings may make the chain appear to bob, but the only reason your body would is if you did it yourself. Many a person mentally assumes they have to bounce up and down when they use biopace rings and then start doing just that. Your feet are attached to pedals which attach to straight crank arms which turn thru a perfect circle around the BB spindle. Any "bobbing" that occurs is your own fault, not the cranks or the chain or the chainring.


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## laffeaux (Jan 4, 2004)

DeeEight said:


> Biopace chainrings may make the chain appear to bob, but the only reason your body would is if you did it yourself. Many a person mentally assumes they have to bounce up and down when they use biopace rings and then start doing just that. Your feet are attached to pedals which attach to straight crank arms which turn thru a perfect circle around the BB spindle. Any "bobbing" that occurs is your own fault, not the cranks or the chain or the chainring.


I tend to agree. I ride on bio-pace rings pretty regularly and don't notice anything unusual when I ride them. Setting up the front derailler is a bit harder (if you're dumb like me), but the ride is fine.

On the other hand, I have a bike with a bent crank arm and I can definitely feel it "bio pace" when I ride it. I do not recommend riding with bent cranks as it feels very odd.


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## stan4bikes (May 24, 2006)

laffeaux said:


> .
> 
> On the other hand, I have a bike with a bent crank arm and I can definitely feel it "bio pace" when I ride it. I do not recommend riding with bent cranks as it feels very odd.


all you have to do is bend the straight arm to match the bent one..kinda ends up like Carumba Double Shots


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## laffeaux (Jan 4, 2004)

stan4bikes said:


> all you have to do is bend the straight arm to match the bent one..kinda ends up like Carumba Double Shots


That's not a bad idea. 

I keep hoping that a blue non-driveside Cook's crank comes up on eBay. In the mean time, pedaling with a bent crank is better than pedaling with one leg.


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## PCC (Sep 5, 2005)

Pimpride said:


> Hello, I have a old 1984 or so, mystery frame (Lotus?) with biopace cranks on it and was wondering if I can replace them with something else that works better. I don't like the bob of the biopace cranks and noticed regular MTB cranks don't fit. :madman: Do road cranks fit? Here's a few pics --->


What do you mean "regular MTB cranks don't fit"? It's a mountain bike. It's got a 68mm bottom bracket with English threads. Any modern crankset will bolt right up after you remove your old bottom bracket. Even the wider 73mm bottom brackets bolt up with a pair of 2.5mm spacers.

Road bike cranks may not work as the chainrings or the crank arms may hit the chainstay because they are designed to fit sleaker road bikes with thinner tires (the chainstays don't need to bow out as much to clear fat tires like on an MTB). I know that when I tried to install a set of compact double cranks on to my '89 Specialized Stumpjumper that the inner chainring was rubbing on the chainstay.


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