"David L. Johnson" <
[email protected]> wrote in message
news:[email protected]...
> On Mon, 21 Apr 2003 22:14:43 +0000, BillX wrote:
>
> > On a new Shimano Ultegra transmission with 53,39 up front and 12-25 9
speed
> > rear how many gear combinations should I expect to be possible without derailer/chain rub? Right
> > now the 39 will only run 4-5 of the 9 rear
gears
> > without rubbing and the bike is brand new. Suspect the shop hasn't
adjusted
> > the derailerst beyond factory settings and that a greater gear range is possible.
>
> Several things. Yeah, if the derailleur rubs on the chain with half the cogs in the back, it's not
> adjusted well. Some shifters have a "trim" capability, so you can click it a little to the right,
> to take care of this. This is good, since the alternative is a more complicated derailleur cage
> that IMO will not shift as well. Check your dealer on that.
>
> Secondly, how many gears do you think you have? Here are the gear ratios for a standard 700c wheel
> and 23mm tire, in "gear-inches".
>
> 53: 115.4 106.6 99.0 92.4 81.5 72.9 66.0 60.2 55.4
>
> 39: 85.0 78.4 72.8 68.0 60.0 53.7 48.6 44.3 40.8
>
> So, you see that the largest 4-5 combinations with the 39, the ones that are rubbing, are all
> duplicated with the other chainring, and without using the 53/25, which you should avoid. OK, the
> 85 and 78 are not
available, but
> the 81.5 is close enough, and no one can use the 39/12 without it rubbing on the outer ring.
> That's another combination to avoid.
>
> Really you have a 12-speed, 13 if you could use the 39/13. This is not a set-up problem, but a
> design decision that you can write Shimano about.
>
>
It's not a design problem. It's easier to shift (especially when under load) the rear cogs. When in
one chainring it is preferable to stay in that ring until you run out of rear gears for that ring.
For the 39 ring you should be able to use all the cogs except the two smallest ones (from 14 to 25),
maybe even the second last one (the 13) but if you couldn't that wouldn't be unreasonable. Only
being able to use 4-5 cogs is not a normal situation.
For the rear you should be able to use all but the largest one (from the 12 to the 23).