Double vs. Compact

On Wed, 10 Nov 2004 11:50:24 -0700, Drew Eckhardt wrote:

Is a compact a good solution or would a triple be better.

It depends on how high you want, how low you want, how many cogs you have,
and what sort of gear spacing you like.

Play with the numbers.

Exactly. Also, don't neglect to think about the amount of overlap. A
typical 53/39 with a 12-23 9-speed has three ratios that are repeated:
The 53/19 is the same gear (72") as the 39/14, the 53/21 is essentially
the same as the 39/15, and the 53/23 is the same as the 39/17. Now, I
would not use the 53/23 because it is an extreme crossover, but there is
no reason for this much overlap except that the 53/39 is essentially the
only choice for road cranks. With a compact crank you have a lower low
with no loss of the close shifts you would want in between.

YMMV. 53-39 really only makes sense when have a
6 speed corncob and
want tight gears with minimal overlap. With 9 or 10 cogs it's a bit
silly.

Exactly.

--

David L. Johnson

__o | What is objectionable, and what is dangerous about extremists is
_`\(,_ | not that they are extreme, but that they are intolerant. --Robert
(_)/ (_) | F. Kennedy

On Wed, 10 Nov 2004 09:28:08 -0800, Russell Seaton wrote:

Going from a 39 to a 34 gets you one lower gear roughly. About 4-5
gear inches. A 39x26 is identical to a 34x23. Both right at 39 gear
inches. A 34x26 is 35 gear inches. One lower gear roughly. I've
ridden in the mountains and I wanted more than just one lower gear.
The triple provides many lower gears. A 30x26 is 31 gear inches.

But then the difference between the 50/34 double and the 50/40/30 triple
is, again, one gear. On the other hand, the triple tends to shift less
reliably than the double.

The 110 mm bcd double with the usual configuration of 50/34 chainrings
gives terrible gaps between gears.

No, it doesn't. With a 12/23 9-speed there is no jump more than 8
gear-inches.

Unless you change to a cassette such
as 11-21. If you use a 13-26 cassette for instance, the 34 ring
produces too low of gears to use unless you are riding about 12 mph.

Depending on the hill, 12mph might be pretty fast.

And
the large ring has too big of gaps between gears when riding around 20
mph.

Um, no.

In my everyday riding I use a 42 inner ring with a 13-26 cassette.
The 42 with 13 to 17 straight block gives nice 5 gear inch changes that
are just right for riding between 15 to 29 mph.

Well, the 42/39 gives a poor chainline, and will likely rub the chain on
the large ring. You can get the same gears with the large ring on a 50/34
with better chainlines and better durability.

With the compact 110 mm bcd double crank, the outer ring of 50 or 48
gives too large 8 to 10 gear inch changes between cogs.

The only range in which you get 8-inch changes is in the larger gears
(50/12 to 50/13 is an 8.4" change). The 53 gives the same gaps.

--

David L. Johnson

__o | Arguing with an engineer is like mud wrestling with a pig... You
_`\(,_ | soon find out the pig likes it!
(_)/ (_) |

Drew Eckhardt wrote:

53-39 really only makes sense when have a 6 speed corncob
and want tight gears with minimal overlap. With 9 or 10 cogs it's a
bit silly.

Using 39/53 (or 52) as a "dual range" setup, I find the overlap between ranges
is really useful. I'm not put off by duplicate gears between chainrings; to the
contrary, with a 39/53 crank the only useful gear* on a 9-sp 12-23 that might
be worth a double shift is the 81- inch 39/13. That lies in the gap between the
53/19 (75.3") and 53/17 (84.2"). What a nice place for an 18t cog (79.5")...
yes, more cogs are *good*.

The only problem I have with 39/53 is that I rode a 42t inner for so long that
I don't get on the 52-3 soon enough sometimes (when group speed goes up or for
known or obvious terrain changes), and I have to double shift at inconvenient
times. Even with brifters, and otherwise modern chain moving mechanisms, I'm
not much on double shifts. To each his own. No, really. Some people like double
shifting, and thinking about their gears, etc. I don't.

* Well there is the 53/23, giving a 62-in. gear that splits the 39/17 - 39/16
gap (58.5 in.- 65.8-in.). My chain is long enough, I've grabbed the gear and
used it briefly, but it seems noisy and unhappy. And that would be a double
shift anyhow. --TP

writes:

Dear Donald,

A smaller front chain ring may encourage a higher cadence,
but dropping from 39 to 34 teeth won't raise many all-day
climbing cadences from less than 70 to 90-110 rpm.

I agree completely. Probably the most widely sold campy gear setup is
a 39/53 and 12/25 setup. This gives a low gear of 39:25 which is 42
inches.

If you instead change to a 36/50 and 13/29, then your low gear would be
36:29 which is 33.5 inches.

If you cadence is 70 rpm at 42 inches, your cadence would be about ~88
rpm - which is close enough for government work.

- Don Gillies
San Diego, CA

Wasatch- Double tooth count- 53/39
Compact tooth count- 50/34

How big of a difference with the 3 or 4 tooth difference make? I live in
the rockies and tend to climb a lot of canyons. BRBR

Look at a gear chart. With an appropriate rear cluster, a 50/34 will have a
higher 'high' gear, and a lower 'low' gear. Like with a 11-23..Plus if ya have
a 12-25 or so, putting that on will give you a really low low.
I think they are a great idea. The only negative is if you have a 50-13, it may
be a little tall, but that's what I have for the flats here in the rockies w/o
any problem.

Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl St.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
"Ruote convenzionali costruite eccezionalmente bene"