Biopace is a tradename for a type of ovoid bicycle chain ring manufactured by Shimano from 1983 to 1993 The design was intended to help overcome the "dead zone" where the crank arms are vertical and riders have little mechanical advantage. 
The intention of most oval style chainrings is that at top and bottom dead centre (TDC and BDC) the effective diameter of the chainring is small, thus making the crank easier for the rider to turn through BDC for a constant chain tension. By having the chainring at its peak effective diameter with the cranks level, that is, where the rider has maximum leverage over the crank during the power stroke, better use can be made of the rider's power output.
Biopace chainrings do almost exactly the opposite to this, having the reduced chainring diameter coinciding with the cranks being horizontal. The reason this is beneficial is that it tends to smooth the pedalling action, allowing the rider's feet to carry a lot of momentum through the power stroke, having it smoothly removed at the bottom of the stroke rather than encouraging riders to push bigger gears and risk knee damage due to higher knee joint loadings.
Some cyclists find their benefits worthwhile, including well-respected bike mechanic Sheldon Brown. Some riders may also value a Biopace crankset for its historical accuracy on a vintage bike, for its novelty value, or even retro-cachet.
See also 
- Berto, Frank; Ron Shepherd, et al. (2005). The dancing chain : history and development of the derailleur bicycle. San Francisco, CA, USA: Van der Plas Publications/Cycle Publications. p. 297. ISBN 1-892495-41-4.
- "Sheldon Brown: Biopace". Retrieved 2011-08-10.
- "Rotor Bike Components: QRINGS GENERAL INFORMATION". Retrieved 2011-08-10.