Gear re-sizing question

On Sun, 06 May 2007 23:11:44 GMT, "john_mcrae" <[email hidden]>

Quoted message said:

(I hope this makes sense.)

It's a little small, I want to re-size the (only) chainring on my bike to
better suit the commute to work.

If my current fave gear is 13 teeth on the rear, and 39 teeth on the
front... will 15 teeth on the rear, and 45 teeth on the front chainring, be
the same?

Thanks in advance,
JM

Dear JM,

Yes, 39x13 and 45x15 will be the same, 3:1.

That is, the same cadence will produce the same speed.

The larger gears are slightly more efficient at transmitting power,
but it's not likely that any rider can tell.

The larger gears will also last slightly longer, but again the
difference is likely to be too small to be noticed.

Darned if I can see how 45x15 would suit anything better or worse than
39x13. What do you have in mind?

Cheers,

Carl Fogel

<[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On Sun, 06 May 2007 23:11:44 GMT, "john_mcrae" <[email hidden]>
wrote:

Quoted message said:

Darned if I can see how 45x15 would suit anything better or worse than
39x13. What do you have in mind?

Cheers,

Carl Fogel

Thanks for the reply Carl,

I ride the 13 tooth cog for most of my commute, it's right in the middle of
where I'd like my gearing to be (because of the terrain/grades on my
commute), but it's the 2nd last cog on the freewheel.
Swapping the chainring out for a 45 tooth one will put that "perfect gear"
right in the middle of my available gears.

Thx,

On Mon, 07 May 2007 00:22:21 GMT, "john_mcrae" <[email hidden]>

Quoted message said:

<[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On Sun, 06 May 2007 23:11:44 GMT, "john_mcrae" <[email hidden]>
wrote:

Quoted message said:

Darned if I can see how 45x15 would suit anything better or worse than
39x13. What do you have in mind?

Cheers,

Carl Fogel

Thanks for the reply Carl,

I ride the 13 tooth cog for most of my commute, it's right in the middle of
where I'd like my gearing to be (because of the terrain/grades on my
commute), but it's the 2nd last cog on the freewheel.
Swapping the chainring out for a 45 tooth one will put that "perfect gear"
right in the middle of my available gears.

Thx,

Dear John,

Aha! My mistake--I assumed a single-gear fixie.

It sounds as if you have something like an x-x-x-15-x-13-12 rear
cluster and spend most of your time in the 13, with only one smaller
cog.

With a 45 front, you'll spend most of your time on the 15.

That will give you a few more higher gears for faster riding downhill
or with a tailwind.

But a 45-tooth will cost you some low gears for hills.

If you use your largest rear sprocket (whatever it is) to climb a hill
on your commute with your current 39 tooth front sprocket, then don't
switch to a larger 45-tooth front sprocket.

If you do, the hill will become 45/39ths as hard to climb.

(If you're only concerned about getting the chain to run between gears
as straight as possible, don't bother changing your sprocket. Tests
show that transmission power losses due to normal chain-line angles
are negligible.)

Cheers,

Carl Fogel

john_mcrae said:

It's a little small, I want to re-size the (only) chainring on my bike to
better suit the commute to work.

If my current fave gear is 13 teeth on the rear, and 39 teeth on the
front... will 15 teeth on the rear, and 45 teeth on the front chainring, be
the same?

Yes, 39-13 is an 81, 45-15 the same
You can set up a spreadsheet quickly to do that for all combinations.
(you may find you want a 13-1/2 tooth cog!)
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

On 2007-05-07, [email hidden] <[email hidden]> wrote:
[...]

Quoted message said:

(If you're only concerned about getting the chain to run between gears
as straight as possible, don't bother changing your sprocket. Tests
show that transmission power losses due to normal chain-line angles
are negligible.)

Although it is said that chain wear is accelerated by poor chainline.
But who cares, it's not like chains really cost that much.

<[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On Mon, 07 May 2007 00:22:21 GMT, "john_mcrae" <[email hidden]>
wrote:

Quoted message said:

Aha! My mistake--I assumed a single-gear fixie.

It sounds as if you have something like an x-x-x-15-x-13-12 rear
cluster and spend most of your time in the 13, with only one smaller
cog.

Exactly.

Quoted message said:

With a 45 front, you'll spend most of your time on the 15.

That will give you a few more higher gears for faster riding downhill
or with a tailwind.

But a 45-tooth will cost you some low gears for hills.

There aren't any hills on my commute, I only use this bike to commute to
work and back.
Thanks again,

Quoted message said:

If you use your largest rear sprocket (whatever it is) to climb a hill
on your commute with your current 39 tooth front sprocket, then don't
switch to a larger 45-tooth front sprocket.

If you do, the hill will become 45/39ths as hard to climb.

(If you're only concerned about getting the chain to run between gears
as straight as possible, don't bother changing your sprocket. Tests
show that transmission power losses due to normal chain-line angles
are negligible.)

Cheers,

Carl Fogel

A Muzi said:

john_mcrae said:

It's a little small, I want to re-size the (only) chainring on my bike to
better suit the commute to work.

Quoted message said:

If my current fave gear is 13 teeth on the rear, and 39 teeth on the
front... will 15 teeth on the rear, and 45 teeth on the front chainring, be
the same?

Yes, 39-13 is an 81, 45-15 the same
You can set up a spreadsheet quickly to do that for all combinations.
(you may find you want a 13-1/2 tooth cog!)
--
Andrew Muziwww.yellowjersey.org
Open every day since 1 April, 1971

I think Sheldon offered one for sale a week after spring started a few
years back. On the anniversary of YJ's opening.

Technically a 27 on a double-pitched drivetrain, IIRC.