Compact Crank Gear Ratios

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing.

I'm going to assume you have a standard Shimano 12-27 9 speed cassette
with cogs of 12-13-14-15-17-19-21-24-27.

I have ordered a 50/34 front. Can I use my current cogs or

Quoted message said:

must I switch to a 25 or 23 rear?

Yes you can use your current cassette. Might be able to take one link
out of your chain since you are now using a smaller big chainring and
the same size 27 rear cog.

Will a 34/27 combo work?

Yes your derailleurs will shift into this gear just fine.

Will I spin too

Quoted message said:

much?

Depends on how fast you are trying to go. If you use the 34x27 at 15
mph you will be spinning 150 rpm. If you use the 34x27 at 10 mph you
will be spinning 101 rpm. If you use the 34x27 at 5 mph you will be
spinning 50 rpm. The formula to use is Gear Inches * RPM / 336.3 =
MPH. Gear Inches = Front Ring / Rear Cog * Wheel Diameter. Wheel
Diameter is 26.5 inches for a 700x23 tire.

Will it mechanically line up etc?

Yes.

Is a 34/23 combo better for climbing

Quoted message said:

than a 42/27?

A 34x23 is 39 gear inches. A 42x27 is 41 gear inches. You might be
able to tell the difference. At 10 MPH you are spinning 86 rpm in the
34x23. At 10 MPH you are spinning 82 rpm in the 42x27. So the 34x23
is a bit better at climbing because it is a lower gear than the 42x27.

Thanks for your help?

Depending on how much lower of a low you need, you could just swap out
your 42 chainring for a 39, or 38 if using 130 mm bcd crankset. $20
from many places. The 38 is hard to find but I know Sugino makes one.
A 39x27 is a low of 38 gear inches. This is 3 gear inches lower than
your current low of 42x27. The 34x27 will get you down to 33 gear
inches. Definitely lower than the 42x27 and noticeably lower than the
39x27. But maybe lower than you really need. In which case its much,
much, much cheaper and simpler to put a 39 or 38 ring on your current
crankset than to switch to a compact crankset. A 38x27 is a low of 37
gear inches.

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing. I have ordered a 50/34 front. Can I use my current cogs or
must I switch to a 25 or 23 rear? Will a 34/27 combo work? Will I spin too
much? Will it mechanically line up etc? Is a 34/23 combo better for climbing
than a 42/27? Thanks for your help?

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing. I have ordered a 50/34 front. Can I use my current cogs or
must I switch to a 25 or 23 rear? Will a 34/27 combo work? Will I spin too
much? Will it mechanically line up etc? Is a 34/23 combo better for climbing
than a 42/27? Thanks for your help?

Let's just do the math.

Existing ratios:

top : 4.82
bottom : 1.56

Proposed:

top : 4.54
bottom:
a. 27 = 1.26
b. 25 = 1.36
c. 23 = 1.47

Tooth ranges:

Existing Proposed 11-27
proposed 11-23
front 11 16
16
rear 16 16
12
total 27 32
28

The tooth ranges are important for your derailleurs...the "front" range
affects the front drailleur, and the "total" range is a specification
for rear deraillers. I can tell you right now that you will not be able
to use a Nuovo Record with this set-up :-)

As you can see, if you keep your existing block, your total tooth range
will go up dramatically (from 27 to 32) and you will have a
ridiculously low bottom gear of 1.26.

If you switch to an 11-23 (I'd prefer a 13-25, or a 14-26 if you could
find one) you will have almost the same tooth range, and you will still
have a low gear somewhat lower than your existing low.

Can you run a 36 or 38 tooth small chainring on a "compact" or is that
not an option? That would be better with the ranges, and also with the
13-25.

The only thing you give up in any case is the high gear, but anything
over a 4.0 is ridiculous unless you ride 35 mph in a peloton.

Why do I prefer the smallest cog to be bigger? Simple: it gives
noticably narrower gear ratios. The 11-12 ratio jump is severe---makes
it difficult to use it without "blowing up" making the effort. And the
smaller your chainrig is, the more this difference grows...

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing. I have ordered a 50/34 front. Can I use my current cogs or
must I switch to a 25 or 23 rear?

Depends on the capacity of your rear derailer. You're asking for 32t
capacity. You might be pushing some derailers past their limit. Capacity
is the difference between chainring teeth (50-34=16) plus the difference
between biggest and smallest cogs (27-11=16). What kind of rear derailer
do you have?

Quoted message said:

Will a 34/27 combo work?

Yes.

Quoted message said:

Will I spin too much?

Only you can answer that question.

Quoted message said:

Will it mechanically line up etc?

Yes.

Quoted message said:

Is a 34/23 combo better for climbing
than a 42/27? Thanks for your help?

You've written the equation, now do the math: 34/23 = 1.48. 42/27 =
1.55. The lower number is a lower gear, so the 34/23 gear is lower.

--
Dave
dvt at psu dot edu

On 24 Mar 2006 09:51:13 -0800, bill <[email hidden]> wrote:
[...]

Quoted message said:

As you can see, if you keep your existing block, your total tooth range
will go up dramatically (from 27 to 32) and you will have a
ridiculously low bottom gear of 1.26.

Well, it depend on climbing.
I am pretty sure that if you are climbing Mortirolo, you will feel a
34/27 a bit hard yet...!

Cheers.

--
Massimo Bacilieri AKA Crononauta
ICQ:4005815

Crononauta said:

On 24 Mar 2006 09:51:13 -0800, bill <[email hidden]> wrote:
[...]

Quoted message said:

As you can see, if you keep your existing block, your total tooth range
will go up dramatically (from 27 to 32) and you will have a
ridiculously low bottom gear of 1.26.

Well, it depend on climbing.
I am pretty sure that if you are climbing Mortirolo, you will feel a
34/27 a bit hard yet...!

:-{~~ (tongue hanging out in exhaustion)

I am so strong and tough that I consider anything easier than 42 x 19
to be whimpy ;-)

crononauta = time + navigate?

bill said:

I am so strong and tough that I consider anything easier than 42 x 19
to be whimpy ;-)

I think that Mortirolo with 42/19 would be more close to a God's
Punishment than to cycling... Something like to SYSIPHUS...!!!
;-)

Quoted message said:

crononauta = time + navigate?

Yes, something like that. Correct translation should be "time
navigator", I suppose.

Cheers.

--
Massimo Bacilieri AKA Crononauta
ICQ:4005815

Quoted message said:

As you can see, if you keep your existing block, your total tooth range
will go up dramatically (from 27 to 32) and you will have a
ridiculously low bottom gear of 1.26.

I recently switched to compact cranks and have a 50/34 on front and
12/27 cassette.
I routinely climb hills in excess of 20% grade and 34/27 is not
ridiculously low, but it makes it easier to climb steep grades. If you
ride in an area where it is relatively flat, then you might consider
12/23 or 12/25. With the 50/12 I have no problem keeping up on fast
group rides.

Joel

Quoted message said:

As you can see, if you keep your existing block, your total tooth range
will go up dramatically (from 27 to 32) and you will have a
ridiculously low bottom gear of 1.26.

I recently switched to compact cranks and have a 50/34 on front and
12/27 cassette.
I routinely climb hills in excess of 20% grade and 34/27 is not
ridiculously low, but it makes it easier to climb steep grades. If you
ride in an area where it is relatively flat, then you might consider
12/23 or 12/25. With the 50/12 I have no problem keeping up on fast
group rides.

Joel

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing. I have ordered a 50/34 front. Can I use my current cogs or
must I switch to a 25 or 23 rear? Will a 34/27 combo work? Will I spin too
much? Will it mechanically line up etc? Is a 34/23 combo better for climbing
than a 42/27? Thanks for your help?

You can use the cassette you have, but you might want one with tighter
ratios. It all depends on how low a gear you feel you need. Obviously a
standard cassette was not doing it for you with the 52/42. But the 34
small ring will make that 27 a lot lower. Try it as it is, and see
whether you want to change the cassette, or whether you want/need/like the
really low gears.

--

David L. Johnson

__o | Accept risk. Accept responsibility. Put a lawyer out of
_`\(,_ | business.
(_)/ (_) |

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing. I have ordered a 50/34 front. Can I use my current cogs

yes.

or

Quoted message said:

must I switch to a 25 or 23 rear?

No

Will a 34/27 combo work?

Yes

Will I spin too

Quoted message said:

much? Will it mechanically line up etc? Is a 34/23 combo better for climbing
than a 42/27? Thanks for your help

Chainline will be the same, just install, lower the FD and go ride. You
may have to take one link of the chain out, depending how well it's
sized now. A 53/11 is gigantic, a 50/11 is like a 53/12.

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing. I have ordered a 50/34 front. Can I use my current cogs or
must I switch to a 25 or 23 rear? Will a 34/27 combo work? Will I spin too
much? Will it mechanically line up etc? Is a 34/23 combo better for climbing
than a 42/27? Thanks for your help?

I'll let the mechanics here answer the compatibility questions. But to
compare gear combinations you can use Sheldon Brown's online gear
calculator:

http://sheldonbrown.com/gears/

Matt O.

David L. Johnson said:

You can use the cassette you have, but you might want one with tighter
ratios. It all depends on how low a gear you feel you need.

Without going into the gory details, a 34/50 with a close-ratio
cassette can be annoying... because with a big jump in ratios between
chainrings you end up having little overlap in gear ratios... and you
have to double shift a lot. For most people their normal cruising speed
is right where this double shifting takes place.

So... I'd advise keeping the wide ratio cassette, or going with tighter
ratios on the rings too if you want to make the cassette ratios
tighter.

In article
<[email hidden]>,

Ron Ruff said:

David L. Johnson said:

You can use the cassette you have, but you might want one with tighter
ratios. It all depends on how low a gear you feel you need.

Without going into the gory details, a 34/50 with a close-ratio
cassette can be annoying... because with a big jump in ratios between
chainrings you end up having little overlap in gear ratios... and you
have to double shift a lot. For most people their normal cruising speed
is right where this double shifting takes place.

So... I'd advise keeping the wide ratio cassette, or going with tighter
ratios on the rings too if you want to make the cassette ratios
tighter.

There were two forces driving the popularity of compact
spiders.

* Small chain rings for more ground clearance.

* Lower overall gearing for the rest of us.

Hence the 34-42 double. Running a 50 tooth chain ring
indicates that the rider does not seek either of these
goals. What other goal does a 34-50 chain ring
configuration meet?

When someone wants to run big gears, and wants a low gear
the best options are

* 38-52; 13 -> 24 and a 32.

* A triple. 26-38-52; 13-23. (and obvious variations)

--
Michael Press

In article
<[email hidden]>,

Matt O'Toole said:

HGL said:

My current bike's chain rings are the standard 53/42 with a rear 27/11 cog
for climbing. I have ordered a 50/34 front. Can I use my current cogs or
must I switch to a 25 or 23 rear? Will a 34/27 combo work? Will I spin too
much? Will it mechanically line up etc? Is a 34/23 combo better for climbing
than a 42/27? Thanks for your help?

I'll let the mechanics here answer the compatibility questions. But to
compare gear combinations you can use Sheldon Brown's online gear
calculator:

http://sheldonbrown.com/gears/

or a slide rule. 🙂

--
Michael Press

Ron Ruff said:

David L. Johnson said:

You can use the cassette you have, but you might want one with tighter
ratios. It all depends on how low a gear you feel you need.

Without going into the gory details, a 34/50 with a close-ratio
cassette can be annoying... because with a big jump in ratios between
chainrings you end up having little overlap in gear ratios... and you
have to double shift a lot. For most people their normal cruising speed
is right where this double shifting takes place.

It's really odd that lots of gear overlap is considered a good thing. Why
bother with bikes with 20 gears if only 12 are really different? Double
shifting is not a problem. For me, the usual downshift when I come to a
long hill means pressing down with both thumbs. Not too challenging, with
modern shifters. The advantages of compact cranks, having a lower low end
while still having small steps between gears, beats the heck out of the
"problem" of shifting down on the cassette at the same time that you shift
down on the chainring.

--

David L. Johnson

__o | Deserves death! I daresay he does. Many that live deserve
_`\(,_ | death. And some that die deserve life. Can you give it to
(_)/ (_) | them? Then do not be too eager to deal out death in judgement.
-- J. R. R. Tolkein

David L. Johnson said:

It's really odd that lots of gear overlap is considered a good thing. Why
bother with bikes with 20 gears if only 12 are really different? Double
shifting is not a problem.

What I was addressing is something like rolling hills or a slightly
varying grade. If you ride for long periods in one gear, then it isn't
much of an issue... but if you find yourself double shifting a lot,
then you'll probably want to change things so you don't have to. It is
a lot more trouble to shift the front plus 3 gears on the back than
just shifting one gear on the back.

I personally don't need big gears for everyday riding so I have a 36/48
and a 13/25 9 spd. I like it that I can leave it in the big ring unless
I'm on a good climb.

Ron Ruff said:

What I was addressing is something like rolling hills or a slightly
varying grade.

Well, what gear you need to shift into depends on the amount of roll in
the rolling hills. Around here (Eastern PA), the inclines can be short,
but steep, which either requires no shift at all (very short), or a
significant one.

Quoted message said:

If you ride for long periods in one gear, then it isn't
much of an issue... but if you find yourself double shifting a lot,
then you'll probably want to change things so you don't have to. It is
a lot more trouble to shift the front plus 3 gears on the back than
just shifting one gear on the back.

But it is not that big a deal any more, with brifters. Click, click,
versus just one click.

Quoted message said:

I personally don't need big gears for everyday riding so I have a 36/48
and a 13/25 9 spd. I like it that I can leave it in the big ring unless
I'm on a good climb.

Imagine a slightly different front: mine is a 46/30. I also usually use
a 12-23 rather than a 13-25, though I have one wheel with that cassette.
You still stay on the big ring for mild climbs through downhills, but have
a lower gear for big hills. It works for me. BTW, these are 94mm bcd
mountain bike chainrings; I leave the granny gear off except for loaded
tours.

--

David L. Johnson

__o | The lottery is a tax on those who fail to understand
_`\(,_ | mathematics.
(_)/ (_) |

David L. Johnson said:

But it is not that big a deal any more, with brifters. Click, click,
versus just one click.

Isn't it click and (maybe) trim with the left, and click click click
with the right to move one gear? I don't have brifters... just barends
in a strange configuration. When I had those trigger shifters on my MTB
I still thought it was annoying to double shift, though...

I think your gears are pretty sensible. A 36/25 is low enough for where
I ride, so I really don't have an incentive to decrease the overlap...
well maybe it would be nice to have an 18, but I'll wait until I go 10
spd. If I was racing I might want a bigger gear than a 48/13... but
generally if a tailwind or downhill is pushing me faster than 35 mph
I'm happy to tuck and coast.

Ron Ruff said:

David L. Johnson said:

But it is not that big a deal any more, with brifters. Click, click,
versus just one click.

Isn't it click and (maybe) trim with the left, and click click click
with the right to move one gear? I don't have brifters... just barends
in a strange configuration. When I had those trigger shifters on my MTB
I still thought it was annoying to double shift, though...

With the 46-30 and 12-23 gears mentioned by the previous poster, its
click on the left, and then click, click, click, click (4 times) to get
to the next linear gear. He would go to a 46x21 (58 gear inches) and
then shift to the 30 ring and drop four cogs in back to the 30x15 (53
gear inches). A fair amount of double shifting involved.

The bike I am currently building up has a 94mm bcd crank. Outer ring
is 48 and inner will usually be 40 but I also have a 30. Cassette will
be 12-23 10 speed. With the 48-40 setup, the double shifting is
minimal, and will be the ring choice for the vast mojority of local
riding. When going on a trip with unknown climbs, the inner ring can
be changed to 48-30 easily. Then I will have a bad double shift.
48x21 (61 gear inches) to 30x14 (57 gear inches). A downshift of 5
cogs in back. I will also have a spare cassette of 13-29 to use with
the 48-30 ring setup. Almost a 1:1 ratio with the 30x29 gear. Yet I
will have very close ratios in the normally used cogs.

Quoted message said:

I think your gears are pretty sensible. A 36/25 is low enough for where
I ride, so I really don't have an incentive to decrease the overlap...
well maybe it would be nice to have an 18, but I'll wait until I go 10
spd. If I was racing I might want a bigger gear than a 48/13... but
generally if a tailwind or downhill is pushing me faster than 35 mph
I'm happy to tuck and coast.