I have had success with an Ultegra long cage rear derailleur and a 12-32 coupled with a 39-53 front.
It is an inexpensive quick change conversion for a double road bike. Screw it on for a trip to the
mountains. Pop it off when you get home. Much simpler than dealing with a triple. I believe the 105
long has the same specs. Bill Brannon Please remove the nospam stuff to reply
"Sheldon Brown" <
[email protected]> wrote in message
news:[email protected]...
> Max Quoted from Shimano's european website:
> > (105 rear derailleur capacity is 37 teeth) // How to calculate the front difference, rear
> > difference and total
capacity of
> > your shifting system. Each front and rear derailleur has its limitations to the number of
teeth
> > that can be shifted. Mixing components and not obeying this "rule" may
lead
> > to improper shift [function]. In order to make the right choice of components please find below
> > a sample calculation of front and rear difference and total capacity:
> >
> > Take for example a bike with a front gear ratio of 42/32/22 teeth and a cassette ratio of 11 -
> > 28 teeth. To calculate the front difference you
have
> > to take the number of teeth of the biggest chainring minus the number of teeth of the smallest
> > chainring. So calculation for front difference
will be
> > 42 minus 22 is 20 teeth.
> >
> > To calculate the rear difference you basically do the same: You take the number of teeth of the
> > biggest sprocket of the cassette
minus
> > the number of teeth of the smallest sprocket. So calculation for rear difference will be 28
> > minus 11 is 17 teeth .
> >
> > To calculate the total capacity of the system we take above values: Front difference (20) plus
> > rear difference (17) is 37 teeth. An indication of the capacity of your system is mentioned in
> > the service instructions that can be found on this website. As mentioned before, components that
> > are mixed and are outside the total capacity may lead to improper shifting, either front, rear
> > or both.
>
> That is the Official Party Line. Here's some more accurate info from my Bicycle Glossary:
>
> "Capacity (of a derailer) "The "capacity" of a particular derailer model is the largest range of
> sprocket sizes it can handle:
>
> "For front derailers, when the derailer is mounted high enough to clear the largest
> chainwheel, there is a certain minimum size that you need for the smallest ring so that the
> chain won't be dragging over the bottom of the front cages. Different front derailers have
> different capacities depending on how tall their cages are. It is expressed in a number of
> teeth, which is the difference between the largest and smallest chainwheel. For instance, a
> 52/42/30 crank set would call for a front derailer with a minimum 22 tooth (52-30) capacity.
>
> "Front derailers are also designed to be used with a certain size for the largest chain ring.
> The curvature of the outer cage plate is matched to this size. If you use a different size
> big ring, capacity may be reduced. If the big ring is substantially smaller than the derailer
> is designed for, shifting precision will suffer. If the big ring is much smaller than the
> derailer is designed for, it may shift OK, but you are likely to have to "trim" the front
> derailer as you shift the rear derailer to the extremes.
>
> "For rear derailers, the capacity relates to the amount of chain slack the derailer can take
> up, and is equal to the front range (22 in the example above) plus the rear range. Thus, if
> you have a 52/42/30 crank set, and a 12-28 (16 tooth difference) cluster, the total capacity
> required would theoretically be 38 teeth (22 front difference + 16 rear difference).
>
> "Rear derailers are also commonly designed for a particular maximum size rear sprocket. If
> you exceed this size, by too much, the jockey pulley may rub against the sprocket when using
> the lowest gear.
>
> "Manufacturers specify this fairly conservatively. They must do so, because they have to
> assume that some of their derailers will be sold to incompetent cyclists, who will abuse
> their drive trains by using the smallest chainwheel with the smaller rear sprockets.
>
> "Competent riders can considerably exceed the official rated capacity, since they will not
> misuse the granny ring by running it with the smaller rear sprockets, so it doesn't matter
> if the chain hangs slack in those gears."
>
> That said, in my experience, Shimano's "road" rear derailers generally work OK for a cassette with
> a 30 tooth cog, maybe for a 32, not for a 34.
>
> I have a fair amount of experience with this, 'cause I sell a lot of custom wide-range touring
> cassettes, such as 13-30 and 13-34.
>
> See:
http://sheldonbrown.com/harris/k7.html#9
>
> Sheldon "Century Special" Brown +---------------------------------------------+
> | Television: | A medium. So called because it is | neither rare nor well done. | --Ernie
> | Kovacs |
> +---------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
> 617-244-9772 FAX 617-244-1041
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>
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