Biopace chainrings

Discussion in 'Bicycle Mechanics and Repairs' started by Yellow Fang, 8 Jan 2008.

  1. Yellow Fang

    Yellow Fang Guru

    I bought a biopace chainring for my road bike after my outer chain-ring bent over while out cycling. It was cheap, the right number of teeth and fitted. However, I didn't realise it was eliptical. I haven't really noticed any difference, but now I'm concerned I put it on wrong, because I suppose you'd want either the long or short end to be at the top of the stroke. How can I tell?
  2. Will1985

    Will1985 Über Member

    South Norfolk
    Sheldon Brown has a bit about them:

    "Biopace chainwheels have the small radius engaged when the cranks are horizontal, the large when they are vertical."

    Jan Ullrich used elliptical chainwheels IIRC. That might make sense with what I have just read on another website which says it is harder to pedal good circles with them as he was a bit of a grinder.

    Does that help?
  3. OP
    Yellow Fang

    Yellow Fang Guru

    Yes, thanks Will. If it's good for knees and good for triathletes, then perhaps I won't replace it like I was thinking of doing.

    Out of interest, that stud that comes out the side of the the chain-ring, where should that go? Does it indicate something?
  4. Smokin Joe

    Smokin Joe Legendary Member

    The pin should line up with the crank as it is there to stop the chain getting jammed if it comes off.
  5. Theres a timing mark on the inside edge of the ring which should line up with the RH crank. According to Shi**no that is.

    The general consensus is that the long axis of an oval ring should be at 90 degrees to the crank. Worth having a play to see what suits you.
  6. MartDavis

    MartDavis Senior Member

    "Biopace chainwheels have the small radius engaged when the cranks are horizontal, the large when they are vertical."

    Am I missing something here? When the cranks are vertical, there is less torque due to the force being partially transmitted along the length of the crank, thus making it harder to pedal. So surely it would seem to make sense to have a smaller radius at that position thereby making it easier to pedal.

    BTW, I have biopace on my tourer and can't tell and difference, but maybe that's because the large radius operates with vertical cranks, as in the OP.
  7. Graham O

    Graham O New Member

    There are two schools of thought with elliptical chainrings. The mechanical advantage approach says that you want the maximum effect when the cranks and greatest diameter of the chain ring are horizontal, but that makes the wear and tear on the knees and ankles the greatest as they have to change direction at the greatest speed.

    Alternatively, and the biopace rings use this theory, the cranks are 90 degrees away from the maximum diameter which does not significantly affect mechanical advantange, but it does mean that the joints change direction at a slower speed and hence suffer less damage.

    This is the reason why some people say that Shimano got them exactly wrong.

    Of course, I could be wrong! :rolleyes:
  8. Smokin Joe

    Smokin Joe Legendary Member

    A solution looking for a problem.
  9. wafflycat

    wafflycat New Member

    middle of Norfolk
    I've got my 1989 hybrid with a Shimano Biopace chainset on it (triple). Can't say I've ever noticed any difference concerning 'feel' from the eliptical shape as opposed to the 'feel' of a circular chainset.
  10. andygates

    andygates New Member

    Try the granny ring, if you've got a biopace granny. They're so small they've got corners.
  11. Dave5N

    Dave5N Über Member

    I've got one on my turbo bike. Probably exactly the wrong place for one.
  12. col

    col Veteran

    I thought there was actualy no difference when all comes to all with these rings?Im sure there was something about them a long time ago?
  13. andygates

    andygates New Member

    There's bugger-all difference in how fast you actually go. But the sharper-shaped weirdnesses do feel different when you pedal: a biopace granny ring feels very stomp-stomp-stompy, it's really hard to pedal in circles.
  14. bobg

    bobg Über Member

    Crosby Merseyside
    Me neither - I particularly like 'em 'cos they're usually dirt cheap on E Bay following their demise due to experts suggesting that they caused knee damage. :biggrin:
  15. Dave5N

    Dave5N Über Member

    Invest the money you've saved. You might just need it for some reconstructive surgery!

    (Only kidding, as Andy said they feel a bit weird but I can't see how they could damage the joints.)
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