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8f.17 Electronic Shifting




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This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.

8f.17 Electronic Shifting

From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Wed, 23 Aug 2000 17:08:29 PDT

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 that 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.



 

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