dw@yale.ARPA (David Wittenberg) (04/09/85)
Summary: Expires: Sender: Followup-To: Distribution: Keywords: In article <56@uw-june> wagner@uw-june (Dave Wagner) writes: >> (From mupmalis@watarts.UUCP (mike upmalis)) >> Shimano has come out with the bipace chainring set. The rings are >> slightly oval and is supposed to optimize pedalling by improving >> or increasin the torque. The pedals are least efficient at 12/6 oclock >> and most efficient at 3/9. >> >There was an article in Bicycling magazine sometime last year about >them. Basically, they felt that these rings were most suitable for >the tourist, whose RPMs are not likely to exceed 80-90. In this case, >the rings seem to decrease the strain on the knee. Most racers, however, >found them ackward for spinning. > > Dave Wagner I talked to the people at Houdaille Industries. They are a company in Texas that makes the "PowerCam". It is a set of two cams that sit (logically) between the Crank and the Chainwheels. In fact it sits near the cups. It gives one revolution of the chainwheel for each revolution of te crank, but the cranks turn more slowly from 1-5 o'clock, and more quickly near vertical. They claim that their uneven gearing is closer to optimal than Biopace (PowerCam is somewhat adjustable as to where the speed changes, and how much) and Biopace puts too much stress on the derailleur (which has to move back and forth twice per revolution of the cranks). Both Houdaille and a review in Bicycling state that the recommended cadence for PowerCam is 45-55 instead of te 80-100 for normal cranks. The thing that makes this work is that VERY high gear ratios are used. (The one I've seen has a 62 tooth chainwheel and an 11 tooth freewheel cog for a ratio around 160 inches). PowerCams have been used in races quite succesfully. The problems that I know about are: 1. Acceleration isn't very good 2. You must replace your gears with higher ones 3. It takes some getting used to (Houdaille suggests 100-150 miles before making a decision) 4. It is expensive. The powercam costs ~$200 This is partially because they are almost custom. I asked "can I get triple chain rings?" the response was "sure, we can make any you want" 5. (racers only) You can't spin, so for very fast descents ( >35 mph) it is hard to get gears that are high enough. 6. It makes a slight clicking noise twice per revolution. Advantages: 1. Several people said it took strain off their knees 2. It seems to allow better speed on flats and up hills. 3. The push-rest cadence is said to be less tiring. I was planning to get a biopace granny ring to see if I liked it, and if so to convert to PowerCam, but my spider doesn't fit Biopace, so going to Biopace would cost ~$80, so I am going to try to borrow a bike with PowerCam and try it. If it is really as easy on the knees as the maker and Bicycling say it is I'll get it despite the expense ( I'd like to have functional knees for at least the next 50 or 60 years). David Wittenberg Wittenberg@yale Yale University decvax!yale!wittenberg
neal@druny.UUCP (Neal D. McBurnett) (04/12/85)
As I understand it, the Biopace has an ellipse which is effectively
90 degrees out of phase with the others.
This article suggests that "the crank spins more slowly
from 1-5 o'clock,..." for the PowerCam. The Biopace is exactly the opposite:
during the power stroke (say at about 4 o'clock) the crank spins
more easily ("faster", "more efficiently"). Their objective
is to reduce the knee strain during that part of
the stroke when some crucial muscle groups are changing from contracting
to relaxing, and vice versa (see one of the "Bike Tech" back issues for
LOTS of details).
Thus, it makes no sense to compare the two, or to say that one is
"closer to optimal" than the other: they depart in fundamentaly different
directions from the traditional circular chain ring.
-Neal McBurnett, ihnp4!druny!neal, 303-538-4852