Biopace is a tradename for a type of ovoid bicycle chain ring manufactured by Shimano

Shimano

Shimano, Inc. is a Japanese multinational manufacturer of cycling components, fishing tackle, and rowing equipment.In 2005, the company had net sales of US $1.4 billion. Bicycle components provided 75% of its sales income...

 from 1983 to 1993 The design was intended to help overcome the "dead zone" where the crank arms

Crankset

The crankset or chainset , is the component of a bicycle drivetrain that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain, which in turn drives the rear wheel...

 are vertical and riders have little mechanical advantage

Mechanical advantage

Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...

.

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.

Biopace chainrings have been cited as the epitome of a solution looking for a problem among the biking press, though there is little evidence to suggest they were particularly bad, just unnecessary. However some cyclists find their benefits worthwhile, including well-respected bike mechanic Sheldon Brown

Sheldon Brown (bicycle mechanic)

Sheldon Brown was an American bicycle mechanic and technical authority on bicycles. He contributed to numerous print and online sources related to bicycling, bicycle mechanics and maintenance, including his own website — and received numerous awards for his contributions.-Biography: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. Increased mechanical complexity of using these chainrings with a front derailleur

Derailleur gears

Derailleur gears are a variable-ratio transmission system commonly used on bicycles, consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another...

is one possible disadvantage.