Bicycle Mechanics - Biopace II chainring with non Biopace-specific crankarm

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fatsoforgotso
03-31-09, 05:32 PM
Hi all,

Just tried to put a Biopace II chainring on a Sugino XD500 crankarm.
To my surprise, it's impossible to set up the chainring on the crankarm so that the pin that usually goes behing the crankarm is correctly placed.

It turns out the spider for the Biopace chainring is designed to have the crankarm positioned between two chainring bolts (see pic below)

http://img5.imageshack.us/img5/8195/deoreredim.jpg





The Sugino XD500 spider is designed so that the crankarm is positioned in front of one of the 5 chainring bolts (pic below of the way I mounted it). Notice that one of the 5 crankbolts is behind the crankarm, and the chain retention pin is 180º away from where it should be.

http://img5.imageshack.us/img5/8775/suginoredim.jpg

I can't get the pin behind the crankarm because the spider on the XD500 is different from the one on the Deore Biopace II crankset.

I guess that the most correct way of mounting the Biopace II chainring on this crankset is to bolt it flipped 180º from the way is was designed (pin standing 180º away from the crankarm, instead of directly behind it).

Am I right?. Will I still reap the benefits of Biopace this way? (I'm guessing that the chainring can be flipped 180º because it is symmetrical)

AEO
03-31-09, 05:44 PM
yeah, I'd do it that way too.

they're shaped like 2 halfs of an egg joined with the long side facing away from each other, so you're correct.
LarDasse74
03-31-09, 05:45 PM
You are right - the pin should be 180 deg. from the arm if it cannot be behind the arm - if you want the Biopace to work as Shimano originally intended. There is some debate as to whether it might be improved by aligning the ovals differently, but I cannot comment on this.

Make sure you drill or file the pin off because it will tear a new arsehole into the middle of your ankle. :D

However, the Sugino design with one spider arm in line with the crank arm is actually the cause of the confusion - the vast majority of 5-arm spiders (non just Biopace) have the crank arm between two spider arms, not lined up with one.

fatsoforgotso
03-31-09, 05:53 PM
Make sure you drill or file the pin off because it will tear a new arsehole into the middle of your ankle.

Scheisse, should've thought about that.
Time to buy or borrow a Dremel.
TallRider
03-31-09, 06:39 PM
You did it right. This way you have the same orientation of the rings as they are supposed to have.

I did a similar thing using Campy Record chainrings (which are designed for a Record crank, which has a hidden 5th spider-arm like the Sugino XD) on a Campy Mirage crank. In this case, the orientation of the ramps and pins for the sake of improved shifting are what I wanted to maintain.
davidad
03-31-09, 07:28 PM
Put regular rings on it.
Gonzo Bob
04-01-09, 04:33 PM
Make sure you drill or file the pin off because it will tear a new arsehole into the middle of your ankle.

Scheisse, should've thought about that.
Time to buy or borrow a Dremel.

It's probably just pressed in (or perhaps threaded). Pull it out or unthread it with a pliers.

Yes, 180 will align the ring correctly. No worries.