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RE: compact cranks:should I
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John Harritt
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May 14, 2007 15:55 PDT
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Here are the gear inches associated with chain ring, cassette combinations
for 53/50/39/34 chain rings (middle cassette gears are omitted),(calculated
with a 700x23 rim/tire). If you use a 50/34 which is common with compacts,
and an 11-21 10 speed cassette, there is only one two tooth shift (19-21),
the rest are all one tooth shifts. In order to get to the lower end gear
inches associated with a 34/21, a 39/25 needs to be used which will end up
with more two tooth shifts (17-19, 19-21, 21-23, 23-25). The difference
between a 34/21 and a 39/25 are negligible (<4%). There is a larger
difference between the 50/21 and the 53/25. Both are "cross chaining".
Disregarding friction, number of teeth contact, etc. a 50/34 used with an
11-21 is nearly identical to a 53/39 with a 12-25. The compact advantage
seems to me is it's closer to the old "straight" block with no corresponding
loss of either upper end or lower end gear inches.
34
39
50
53
11
81.2
93.2
119.5
126.6
12
74.5
85.4
109.5
116.1
21
42.5
48.8
62.6
66.3
23
38.8
44.6
57.1
60.6
25
35.7
41.0
52.6
55.7
27
33.1
38.0
48.7
51.6
John Harritt
n-tieractive | Bike Doctor-Waldorf Cycling Team
euro-@comcast.net
-----Original Message-----
From: Will's Account [mailto:willka-@comcast.net]
Sent: Saturday, May 12, 2007 2:54 PM
To: D20
Subject: Re: D20: compact cranks:should I
I liked Vicki's matrix. It shows you how you can conceivably cover
the same final drive ratio with a compact, which I believe to be the
case. Not to discount the friction aspect, as I am sure it is
relevant, but another add to the thread:
As I am told, one tooth on the cassette is "about" 4 teeth on the
chain ring. So, 53/12 is very close to 50/11. I am sure this basic
rule varies as the ring gets smaller. Until I made an upgrade
yesterday, I was running compact 50/36 x 11/23. I am not a great
climber, yet never had a problem climbing anything in northern VA.
On fast sprints and downhills, my cadence was never noticably
different then those next to me (more like in front of me) running
53/12.
Further, my upgrade has 53-39 vise the old 50/36 (3 tooth difference
on each ring for about the same gap). With a 12-27 cassette instead
of 11-23, I have gained lower options in both rings, a lower possible
final drive ratio, with the sacrifice of a slightly steeper
progression on the low half of the cassette. So, if I went to 12-25
on the upgrade, once again I would have "nearly" the same final drive
ratio, with "nearly" the same progression in the cassette.
If I am screwing up the math, someone please set me straight, but I
think I have it correct. I also think, if you have been running
bigger rings for years, you are not likely to go compact and be
happy. If you are experienced, and what you know and like works,
stick with it. If you are a new guy, like me, you will like your
compact fine, then realize you are the minority, and likely upgrade
to what the pros and cool kids are doing, because that is what makes
them pro or cool.
Will
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