|
|
|
Tweet
Follow @sheldonbrowncom |
Subject: Electronic Shifting
From: Jobst Brandt
Date: August 23, 2000
A reader asks whether the Mavic Mektronic is any better than the earlier Mavic Zap electronic shifting.
New styling didn't fix the basic problems of this device, although it has an elegant speedometer and controls. The same basic problems remain in the derailleur mechanism that shifts by means of a ratchet pushrod that moves in and out with each idler wheel rotation. The faster the chain moves the faster it pumps. A shift occurs during 1/2 revolution but primarily in 1/4 revolution considering the profile of sinusoidal motion. The stroke takes place in about 35 milliseconds when pedaling a 52t chainwheel at 100rpm. This heavily loads the small electrically activated ratchet pawls, one for up and one for down, that engage one of the sides of the pushrod. The opposing ratchets of the pushrod have teeth space exactly one gear apart with little overshoot.
Besides the ratchet problem, the upper idler must lie on axis with the derailleur pivot, a feature that reduces chain slack take-up. Today derailleurs have the pivot offset from and between the two idler wheels, and use a slant parallelogram (low friction) movement. The Mektronic uses a sliding post (like early Simplex derailleurs) that resists motion when chain tension loads it with torque. Moving it is similar to pulling a socket wrench off a nut while tightening it. A rubber boot covers the mechanism, which must run in an oil bath.
Drawing power to shift from the chain is both the novelty and the fault of this design. The novelty is that only control power is drawn from a battery while power for shifting comes from the chain and only while shifting. The fault is that to make this possible the function of the derailleur is compromised. Because it can support only a short tensioning arm due its sliding post, it cannot take up large chain differences typical of large-to-small chainwheel shifts. Most seriously, pushrod velocity is too great to be reliable at speed.
More Articles by Jobst Brandt
Next: SRAM (formerly SACHS) Power-links
Previous: Chain Care, Wear and Skipping
Last Updated: by John Allen