ccollins <
[email protected]> wrote in message news:<
[email protected]>...
> Originally posted by Gwhite
>
> > No problem. If you are still insistant on the setup, I can give a few
> > more tips.
>
> Need you have asked? With my hard-head and affinity for using old stuff.
> Yes, this is my intent.
>
> So, on E-bay (for $10.00) I just bought a NOS front derailuer. It is a
> FD-MT62 (half-step) "Deore II"
I don't know that model in particular. If it is a 1/2+granny front,
then it will be
1. "deep," so that the granny can be used. (A "shallow" one would be
okay for just a 1/2-step w/ no granny, but that is not what you're
doing.)
2. the inner plate "hangs" only scarcely lower than the outer plate.
> I am making a fully loaded touring bike.
>
> The 110/74 cranks are my pallette on which I'll place good gears.
The right choice for nearly any bike in my opinion.
> From
> this thread it seems I am going to have the outer two be half-step
> distance and a bailout (For that big hill just south of Molera State
> Park in Big Sur).
By saying "bailout," in reference to the crank and rings, I assume you
are talking about the granny chainring. Anywhere from 24t to 28t is
fine, depending on your low gear requirements.
> The 170mm cranks need, I guess, 50/44 as the outers and something small
> like a 26.
Hey now. No "guessing" is needed or desired. The gears are defined
by a balancing of mathematical relationship, as the 1/2-step is
defined, and actual parts available. Don't guess. Design or re-use
that already designed. A 50-44 is unlikely to meet the "half"
criteria. A 50-46 is more likely to be mathematically "correct."
> What thought process goes into these decisions?
I think this has been noted in this and countless other threads to
date. The mathematical definition of a half-step is that the *step*
between the front chainrings is *one-half* that of the *step* between
the rear cogs. Thus a shift in the back is a "step," and that in the
front is a "one-half step." The relationship of steps is a so-called
"geometric" one. Another absolutely equivalent way of saying this is
that we strive to have certain "percent difference" between gears.
Tooth counts are obviously integers (no 15.23 tooth cogs, hah hah).
The impact of this that one can only approximate the ideal
mathematical numbers representing the steps when constrained by
integers and the actual cogs those integers represent. This
approximation is practically unimportant: the approximations are
easily "close enough."
So the system can only be designed by understanding the mathematical
foundation. You don't have to do that because it has already been
done.
> For the rear freewheel,
> shouldn't I go with the Mega 7 (11,13,15,18,21,24,24)?.
I presume (11,13,15,18,21,24,_28_)?.
We seek _equal_ geometric steps for the rear cog tooth gradation.
This is because we have only one geometric step in the front (the
granny is dismissed from design consideration at this point because it
is not a part of the half-step system).
Computing the steps:
>> [meanStep, compavggeo] = geomeanstep([11,13,15,18,21,24,28])
meanStep =
0.1672
0.1432
0.1826
0.1543
0.1336
0.1543
compavggeo =
0.1559
So we would seek a chainring step of about half "compavggeo," which is
0.1559/2 = 0.078 (7.8%).
Choosing some common big rings (50,48, & 46), we have
For 50 big ring
middleRing = 46.2
round to 46
For 48 big ring
middleRing = 44.4
round to 44
For 46 big ring
middleRing = 42.5
round to 44 or 45
So let's arbitrarily test the result for the 48-44 combo:
44 48
11 106.000 115.636
13 89.692 97.846
15 77.733 84.800
18 64.778 70.667
21 55.524 60.571
24 48.583 53.000
28 41.643 45.429
For this we obtain (from low to high):
meanStep =
0.0870
0.0672
0.0870
0.0465
0.0870
0.0672
0.0870
0.0953
0.0870
0.0561
0.0870
0.0801
0.0870
I don't like it. Note the step variance can get high. There is a
4.65% and a 9.53% step.
Now let's look at the middle six cogs of my design and a 46-42 ring
set.
42 46
12 92.750 101.583
14 79.500 87.071
17 65.471 71.706
20 55.650 60.950
24 46.375 50.792
29 38.379 42.034
For this we obtain (from low to high):
meanStep =
0.0910
0.0983
0.0910
0.0914
0.0910
0.0716
0.0910
0.1032
0.0910
0.0632
0.0910
The step variance is a bit lower in this design. I think it is
superior.
If you are stuck with a 7sp freewheel, I would go with a
12-14-17-20-24-29-34 rather than the 11-13-15-18-21-24-28. The "need"
for a heavily loaded touring gear above 100 inches is dubious -- it is
easily sacrificed. On the other hand, a 34 tooth cog will get used on
tough climbs.
(12-14-17-20-24-28-34 & 12-14-17-20-24-28-32 are fine too.)
> sheldonbrown.com
> writes of it as good technology and solid. The gearing is unique.
Unique itself doesn't count.
> Assuming it works with my shifters, and framespacing, how does this
> current proposed combo effect my choice of rear deraillier?
It will be MTB and any modern 43t wrapup Shimano is fine.
> I have 7-SIS downtube shifters and the framespacing *is* 126mm.
>
> So, I am going to jump through hoops, and search for odd old equiptment,
> and the result will be a good drivetrain for loaded touring.
The only "odd" old part truly required is the front derailleur. Most
everything else can be had brand new.
I should note another "trouble" with this setup. The middle and inner
rings should be closer together than they are on any other modern
setup. Failure to "make this happen" will result in occasional "bad"
big-ring to middle-ring shifts in the front. The chain will sometimes
"freewheel" on the middle ring if spaced with "normal" rings. How
does one get them closer? This is not easily done. What I did was
buy a _middle_ 46t ring and use it as the outer of my 46-42 combo.
This is the only simple way I found to solve this problem. Rivendell
did for a time have 46t "middles" for 110 cranks. I know of no other
easy and cheap solution. The "pickup section/ramp" of "normal" outer
rings places the tooth centers too far apart. This is one of those
subtleties that makes me advise against the system.
You could make life easier you know. I still think you should
question your direction. I have some stuff you can buy if you still
insist. I have a Sachs 7sp 12-14-17-20-24-28-32 freewheel (shimano
7sp SIS compatible) and a 126 mm touring wheelset with a 40 spoke rear
and 36 spoke front in excellent condition.
> This is
> esoteric, but I appreciate the feedback. (and lessons).