in message <
[email protected]>, Tom
Kunich ('cyclintom@yahoo. com') wrote:
> "Simon Brooke" <[email protected]> wrote in message
> news:[email protected]...
>> in message <[email protected]>, Tom
>> Kunich ('cyclintom@yahoo. com') wrote:
>>>
>>> Bret - what is the power costs to run a stepping motor mechanism strong
>>> enough to shift and maintain positioning of a chain on a bicycle for
>>> the length of one day?
>>
>> What is the need to run a stepper motor when you're sitting on top of a
>> jockey wheel being powered by 0.4Kw of cyclist, and which power you can
>> tap just by releasing a clutch?
>
> Maybe you missed the resident genius Kveck telling us that there wasn't
> any clutches in the Mektronic.
>
>> The power is there for the asking. All you need to do is to control when
>> to
>> ask it.
>
> Indeed but the group was talking about using a stepping motor to
> precisely position the RD so that they wouldn't have to turn the
> adjusting screw a quarter turn once a year between tune-ups.
>
> Simon, it is apparent that you have some education in mechanical design.
> This whole argument began when I said that there wasn't anything to GAIN
> by going to electronic shifting. It's only another failure point in an
> otherwise highly reliable machine. But jim beam (named apparently from
> what he is under the influence of ) seems to believe that derailleurs
> which are almost the perfect mechanism, can be markedly improved with
> electronics.
OK, I hear what you're saying and I'm not sure I agree. It's partly because
I'm a geek and like playing with toys. But also, one of my bikes - the one
I ride to work most days - is an old steel Raleigh. It suffers from frame
flex. And one of the problems that frame flex causes is that because the
parallelogram is attached to the back of the frame, and controlled by a
bit of wire that is under tension from the front of the frame, as the
front of the frame and the back of the frame move relative to one another
you get ghost shifts. This is exacerbated on my Raleigh, of course, by the
fact that it was designed for a five speed freewheel and now has a nine
speed cassette, so smaller distortions of the frame cause ghostshifts than
would have been the case when it was new...
With modern, closer tolerance derailleur trains the derailleur cage is
still attached to the back of the frame but the detents which control its
position are in the shifter mechanism at the front of the frame (or,
actually, on the handlebar...). Modern frames are not made out of wet
spaghetti, of course, but they still flex - and there are reasons why it's
good that they flex a bit in controlled ways. And the cables bend through
tighter and more relaxed radii as the steering moves, and so on. And this
causes the pantograph to move, and this causes the cage to move... And
from an engineering point of view this is just wrong.
The detents really ought to be in the derailleur mechanism itself.
[As an aside a pantograph is not really the ideal mechanism for something
that wants to track across a cassette at a fixed distance from the cogs -
particularly on a machine on which different sized cassettes may be used.
The optimum trajectory for a 12-21 cassette is quite different from that
for a 13-26 cassette. Mind you, I'm not saying I could design a better]
If the detents are going to be in the derailleur mechanism, then an
electronically triggered movement is quite a good idea. I can imagine a
mechanical system where a short tug on the cable released the mechanism
one detent, and a longer tug on the cable lifted the mechanism one detent,
with a user interface much like the SRAM 'double tap' (the cable being
slack between actuations). But an advantage of electronic actuation, as
Mektronic demonstrated, is you can have multiple switch positions so that
it becomes easy to change gear from the tops as well as the hoods and the
drops. And if you have electronic actuation, then taking the energy to
lift the mechanism from the chain seems to me clever and cool.
Also, it's easy to build an electrical wiring harness into a carbon
composite structure. Thus exposed cables could be a thing of the past, and
I'd see that as a positive thing.
>> The power needed by the control electronics can be in terms of
>> fractions of a watt, and that too can be derived directly from the chain
>> without need for anything more than a capacitor to buffer the power.
>
> Well, not quite but I agree that you don't need a lot of power if you're
> willing to rob most of it from the drive train. But then you're either
> stuck with the Mektronic mechanism which has fixed stops or complex
> sensors and micro-adjustable position sensors which in the end would be
> quite a pain in the neck without adding anything to reliability, reducing
> costs or weight.
I certainly think that if you're going to have electronic gear actuation on
a racing bike in a sporting context then the energy used to lift the
mechanism ought to come from the competitor's muscular effort in near real
time - if you're using stored power from a battery charged before the
event that ought to be seen as cheating.
> The modern bicycle is the end result of a hundred and fifty years of
> evolution. It achieved it's peak in the 1960's and everything added since
> then has been only for performance on smooth roads.
I'm not at all sure I agree with that. I have two road bikes I ride
regularly, a modern carbon Dolan and the fifteen-year-old steel Raleigh.
The Dolan performs better than the Raleigh in every department. The slant
parallelogram was a significant improvement; the indexed shifter was a
significant improvement; integrating the shifter and the brake lever was a
significant improvement. All these things have happened in the past twenty
years. And none of them is as significant as the development of the carbon
monocoque frame.
But what's held cycling development back has been the luddite sabutage by
the UCI of any significant technical improvement. It's ludicrous, for
example, that we're still not riding bikes with monoblades front and
back - aerodynamics would be better, and changing wheels in race
conditions would be enormously faster (and you wouldn't need different
spare wheels for front and rear).
> It is possible to build reliable carbon bikes but not with a significant
> reduction in weight.
True. But the benefits of carbon aren't mainly about weight, in my opinion;
they're mainly about how precisely you can design the stiffness and
compliance of different parts of the structure. A good carbon frame may
weigh only a little less than a good steel one, but it can be much stiffer
laterally while being even more compliant vertically.
> That isn't to say that there aren't certain
> advantages to carbon bikes but there are significant disadvantages as
> well. It is pretty difficult to beat a good well designed steel bike from
> Bob Jackson or Waterford. (Queue in Donnelly's calculation showing that
> the difference in weight can save 2 seconds on the Stelvio.)
Yup, but the difference in not ghost-shifting when you're out of the saddle
and stomping can make far more than that!
--
[email protected] (Simon Brooke)
http://www.jasmine.org.uk/~simon/
;; I'd rather live in sybar-space
