Shimano Biopace ® Kettenblätter: Unterschied zwischen den Versionen

Around the turn of the century, shortly after the development of the chain-driven bicycle, someone came up with the bright idea of elliptical chainwheels. The idea was that the large radius of the chainwheel would drive the chain when the cranks are horizontal and the small radius would pull the chain when the cranks are vertical.

The major problem is that this design tends to hurt people's knees. The high gear when the cranks are horizontal encourages the rider to push too hard, and we all know that pushing too high a gear is a common cause of knee problems. In addition, the low gear when the cranks are vertical means that the knees are moving extra fast when they are changing direction from going up to down and vice versa. This "whiplash" effect caused most users to abandon elliptical chainwheels. Ever since the chain driven bicycle was invented, elliptical chainwheels have been re-invented and re-abandoned for the same reason every ten or fifteen years.

Biopace stickerBiopace is a patented non-round chainwheel design made and licensed by Shimano. To a casual glance, Biopace chainwheels resemble elliptical chainwheels, but on closer examination they turn out to be the opposite of the classical elliptical design. The product of extensive research and computer-aided design, Biopace chainwheels have the small radius engaged when the cranks are horizontal, the large when they are vertical. This is because the Biopace design is based on a dynamic analysis of the motion and momentum of moving cranks and legs, unlike the static, geometric analysis that produced classical ellipticals.

The image above shows 48-38-28 Biopace chainrings on a triple crankset. Note that the smallest diameter is at a right angle to the crank, so the crank turns fastest when horizontal -- in the middle of the power stroke.

Biopace chainwheels are particularly suitable for touring cyclists and time trialists, or any application that involves a steady, fairly constant cadence. They allow healthy, efficient pedaling at slower cadences than is possible with round chainwheels. They are especially suitable for triathletes and mountain bikers. The triathlete benefits because the motion is a little bit closer to that of running, making the transition easier.

(A bicyclist may achieve a similar effect by leaning back to accelerate the bicycle when the cranks are vertical, then leaning forward during the power stroke, but this "pigeon walk" technique has a cost in efficiency and is not possible when spinning at high RPMs.)

The marketers of Biopace made a crucial error of judgment: too much information. In particular, they mentioned that the Biopace design was optimized for cadences of about 90 rpm and slower. Many readers interpreted this as an indication that Biopace chainrings would somehow interfere with pedaling faster than that. This perception caused a bit of a self-fulfilling prophecy, and the Conventional Wisdom arose that Biopace was bad for high-performance cyclists for this reason. I [Sheldon] used to believe this myself, but I rode them anyway because I appreciated their other virtues.

The slower motion at top and bottom means that your leg changes direction from upward to downward, or downward to upward motion at a slightly slower, more gentle speed. The increased leg speed near the middle of the stroke is the result of a more gradual acceleration/deceleration with the leg moving in the same direction. Bouncing in the saddle generally results from difficulty with changes in leg direction, not speed in the middle of the stroke. Thus, as Biopace makes it easier on your knees, it also can help you spin faster without bouncing!

Cranksets of Sheldon's all-Biopace Piccio fixed-gear tandem.

[I've tried this too, but I'm not as happy with the change in tension. The sprocket is smaller than the chainring, so the chain doesn't meet the chainring at two points precisely opposite one another -- John Allen.]

Biopace chainrings are no longer made, but they are found on many bicycles from the 1980s and early 1990s. Biopace chainrings turn up in substantial numbers on eBay.

Sizes 53, 52T, 42T and 40T (that one only in steel) were made in the 130mm bolt circle and were generally used in racing doubles, though some cranksets have a 130mm outer and 74mm inner bolt circle. Few front derailers could handle a 52T outer and 28T inner Biopace chainwheel, though.

The usual stock combinations with Biopace chainrings were 48-38-28 or 46-36-26 for a wide-step triple; 52-42 for a racing double. A half-step-plus-grannies combination is possible with 50T-46T or 48T-44T for the two outer chainwheels and a 28T or 26T inner chainwheel. This works well with an 11-13-16-19-23-28-34 cassette -- a custom combination, to be sure.

Biopace chainrings and cranksets were made for 7-speed and 8-speed systems, but will work with 9-speed chain.

For installing Biopace chainrings, the orientation should generally be preserved. Each Biopace chainring has an "indexing mark." This is a small tab pointing inward toward the middle of the chainring.

For recumbents, the chainring should be rotated forward by 1 bolt position.

It is possible to mix Biopace and round chainwheels on the same crankset, but I [Sheldon] would not recommend it. When chainwheels are mixed, it is usually a small Biopace with a large round ring. The idea is to take advantage of the Biopace's superior climbing performance at low RPM's, and have the big round ring for descending and spinning fast.

You can usually lower the bottom gear by installing a larger rear sprocket instead.

I [John Allen] bought a Bike Friday travel bike at the factory in Eugene, Oregon in September, 2005. At Portland International Airport, the TSA's x-ray machine ejected the bicycle in its carrying case like a baseball from a pitching machine, to smash into into a rigid barrier at the bottom of a chute.

A couple of years ago, I added a 28-tooth Biopace chainring, using the previously unoccupied 74-mm bolt circle of the Bike Friday's crankset. I haven't contrived a front derailer yet to work with the Bike Friday's unusual frame, so I have to shift the chain manually. That's OK because, thanks to the bicycle's small wheels, I only need to use the 28T chainring for very steep, long climbs. It has taken me to the summit of Mt. Wachusett, here in Massachusetts, at age 65, without walking, spinning all the way!

[Sheldon here.] Biopace chainwheels were made by Shimano and were also made under license by some other manufacturers. The shape of genuine Biopace chainwheels is not a simple ellipse, but a more complicated shape which Shimano described as a "point-symmetric egg curve." Other crankset makers have made similar designs, generally of simpler shape (pure elliptical) but oriented in a similar manner to Biopace chainwheels. Examples are SR's "Ovaltech" and Sugino's "Cycloid."