Wladyslaw Hura Wrote:
>>I would like to interest you in seeing a comparison between
>>parallelogram derailleur and my conception of derailleur (an endless
>>cable derailleur) at the website http://www.derailleur.republika.pl.
>>This comparison is available in FAQ section on this website.
>>
A shy person replied:
>
> The looping cable means double the cabling, so extra weight aerodrag
> and twice as many opportunities to snag. Major manufacturers have
> tried this linkage in the past and abandoned it.
It's true about the weight, but the aero drag and snaggability are less
clear cut, since the cables generally follow a parallel path. My
Rohloff shifter works this way, and it doesn't seem to present a
problem. Indeed, the fact that a dual-cable system doesn't rely on a
return spring to move it in one direction makes it more tolerant of
frictional losses than a single-cable system.
>
> The demonstrated derailleur follows a linear path.
> Parallelogram rear derailleurs follow an arc arcate path.
> The contour of most rear sprocket clusters is arcate in contour,
> especially non road bikes. Having the rder near the sprockets gives a
> smoother faster shift as there is less chain slop.
Right. This is one of the features that Hura touts for his system.
> An arcuate path
> deraileur facilitates placement of the rder wheels in close proximity
> to the sprockets it is trying to lead the chain onto.
> When a rder is
> further from the sprocket, there is more chain slop, more hysterisis in
> the shifting trim.
Actually, the arcuate* path of the rear derailer's parallelogram is
roughly orthogonal to the concave curvature of a well-designed cluster's
profile. The exception to this is the old Hurét Alvit and it's
variants, but those are not known as paragons of shifting precision.
Modern conventional derailers do track the cluster pretty well despite
the undesirable direction of the paralellogram's motion, by dint of
clever spring balancing and careful location of the cage pivot.
> But the linear movement rder does simplify construction and would be
> more robust than a parallelogram.
I can't agree there. The simple pivots of a parallelogram are much
easier to make, have less friction and more reliable than anything the
sliding rack shown in Hura's design.
A modern conventional derailer can be made almost entirely from sheet
metal stampings. Hura's design would require a precision cast/forged
rack and pinion, and whatever would be used to guide the rack back and
forth would have to be pretty special to combine low friction and
positive location.
Sheldon "Cyclo" Brown
* (thanks for that word, never ran into it before...always glad of an
opportunity to enlarge my vocabulary!)
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