pinnah <
[email protected]> wrote in message news:<
[email protected]>...
> [email protected] (Russell Seaton) wrote:
> >You will have a very hard time getting your cassette cogs to work in a
> >half step pattern if you go with 42-39 chainrings.
>
> Interesting. I'll have re-run my numbers tonight. My first cut at it
> showed a perfect interleave with the stock 12-28 7 speed cassettes.
> I'll take a second look. Glancing at my numbers, my suggested 42/39
> combo isn't very happy as it doesn't give me enough high gear choices.
> My prefered choice for a 7-speed cassettes would be a 44/41 and I
> think it will work for most common cassettes. But, I can't seem to
> find 41t middles for a 110mm bolt pattern.
I've run the numbers on my spreadsheet using my existing
14-16-18-21-24-28-34 7 speed cassette and a 42-39 did not work well.
42-40 worked better. The best chainring setup I've found with the
above 7 speed setup is the 48-45 half step. The 42-40 setup just did
not produce enough jump between successive gears. Your 12-28 7 speed
cassette is the same as the above cassette except you have a 12
instead of a 34 or 32.
Harris Cyclery carries 110mm bcd chainrings in every size. Including
41 and 45 teeth. Both T.A. brand, expensive, and a cheap BMX brand.
Also look at your local bike shop's Quality Bicycle Products catalog.
They will list every tooth in the 110mm bcd pattern. Or a tandem
shop. Tandem shops carry lots of chainrings.
>
> >One of the reasons I am using 48-45 chainrings is because
> >they work wonderfully with the very common 14-16-18-21-24-28-32 7
> >speed cassettes.
>
> Where are you finding a 45t middle ring? My current cranks have a
> 110/74 bolt pattern and 39 is one of the few odd numbered chain rings
> I've been able to find (Salsa and Sugino both make 'em).
>
> >My low gear on the middle ring will be 45x30 or 45x34, assuming I get
> >the 9 speed cassette to work. 41 or 36 gear inches. My next lowest
> >gear on the granny ring would be 20x18. 30 gear inches. I really
> >need the front derailleur to work with the top 5 cogs on the cassette
> >without dragging on the connecting bolt.
>
> Hmm.... What I like for loaded touring is to have big enough jump
> that I can sit and spin on a granny during a climb and then shift to
> the middle to climb out of the saddle (to stretch the legs). When the
> jump is just right, this can be done with only a front shift no rear
> shifts. For this I only needed the top 2 or 3 rear cogs with my
> granny, which resulted in a higher chain line.
I'm not a stand up and pedal person when touring loaded. I sit and
spin. Or grind depending. With the 20 tooth granny, its spin. I
like to have a good progression with the granny ring when I hit a
slightly less steep section. I can shift in back and not have to jump
from the granny to the middle ring. When the granny is a 20 tooth, it
takes quite a shift to get it from the 20 tooth granny up to the 45
middle ring.
>
> I'll look in my notes but I'm pretty certain my old (6 speed) set up
> was a 44/40/24. I think I would find your proposed 45 middle to create
> too high of a gear, even on the larger rear cogs. That's what I was
> trying to say earlier... if you can go lower on your middle, you don't
> need to go as far into your rear cassette with your granny.
My 48x14 high gear is 93 gear inches. High enough for a loaded
touring bike. With the 45 middle ring, I have 12 nicely spaced gears
between 93 and 43 gear inches. Just right for loaded touring.
>
> I'm struggling to see the advantage of a half-step with 9 speeds
> though. At that point, doesn't wide range alpine gearing make more
> sense with fewer double shifts?
There is no advantage going to a 9 speed half step gearing pattern.
But I'm getting a new loaded touring frame so I thought it would be
nice to change over to 9 speed. And the 12-14-16-18-20-23-26-30-34
Shimano 9 speed cassette is actually a very good full step cassette
when coupled with 48-45 half step chainrings. And if it does not work
out, I'll convert back to 7 speed. The 9 speed cassette just adds the
12 on the bottom and an extra cog at the top end. Not much different
than my current 7 speed cassette. Except it will be new to go with my
new frame. It will add a little extra fun to the parts swap since I
will have to redish the rear wheel and change hub bodies.
>
>
> -- Dave
> ==============================================
> "It is impossible, or not easy, to do noble acts
> without the proper equipment."
> Aristotle, <<Politics>>, 1323a-b, trans Jowett
> ==============================================