Fixies - finding gear combo giving right tension on vertical dropoutframe

Robert said:

Hi,

I am about to convert one of my road bikes to fixed-wheel. Frame (an
aluminium Colnago) has vertical dropouts.

Nonetheless, I believe that I will be able to get satisfactory chain
tension provided that I find the right chainring - back cog combination.

I have all equipment required but I have not yet bought chainring and
back cog. Before I do this, I want to ask those of you having made the
conversion, if someone somewhere has drawn up a chart, for each of the
popular gear combinations, that says what distances, between BB centre,
and back axle centre, would be possible using the given combination
("distances", since the chain length can of course be altered).

As an example, I have 403 mm from BB centre to back axle centre. I
believe that I can alter the distance +/- 3 mm by shifting the axle.
Suppose I wanted an 81 inch gear on this frame. The chart that I hope
exists, may tell me that e.g. 45/15 does not work but that 48/16 does.

Of course I could go out and buy lots of chainrings and lots of cogs and
test all combinations for the desired ratios, to see which of these
gives the right chain tension but I would prefer to buy one chairing and
max 3 cogs.

BTW my preferred ratio would be around 75 in (e.g. 48/17).

I'm aware that I can calculate the resulting distance for each
combination, adding appropriate pi * r for cogs and rings, but honestly
I'd prefer the chart if it exists . . .

If you google "magic gear calculator" you should find at least one.

An ENO hub is another option.

Zog The Undeniable said:

Robert said:

Hi,

I am about to convert one of my road bikes to fixed-wheel. Frame (an
aluminium Colnago) has vertical dropouts.

Nonetheless, I believe that I will be able to get satisfactory chain
tension provided that I find the right chainring - back cog combination.

I have all equipment required but I have not yet bought chainring and
back cog. Before I do this, I want to ask those of you having made the
conversion, if someone somewhere has drawn up a chart, for each of the
popular gear combinations, that says what distances, between BB
centre, and back axle centre, would be possible using the given
combination ("distances", since the chain length can of course be
altered).

As an example, I have 403 mm from BB centre to back axle centre. I
believe that I can alter the distance +/- 3 mm by shifting the axle.
Suppose I wanted an 81 inch gear on this frame. The chart that I hope
exists, may tell me that e.g. 45/15 does not work but that 48/16 does.

Of course I could go out and buy lots of chainrings and lots of cogs
and test all combinations for the desired ratios, to see which of
these gives the right chain tension but I would prefer to buy one
chairing and max 3 cogs.

BTW my preferred ratio would be around 75 in (e.g. 48/17).

I'm aware that I can calculate the resulting distance for each
combination, adding appropriate pi * r for cogs and rings, but
honestly I'd prefer the chart if it exists . . .

If you google "magic gear calculator" you should find at least one.

An ENO hub is another option.

it's /the/ option if you want to ride more than a few hundred miles and
thus encounter chain wear.

Robert said:

I am about to convert one of my road bikes to fixed-wheel. Frame (an
aluminium Colnago) has vertical dropouts.

Nonetheless, I believe that I will be able to get satisfactory chain
tension provided that I find the right chainring - back cog combination.

I have all equipment required but I have not yet bought chainring and
back cog. Before I do this, I want to ask those of you having made the
conversion, if someone somewhere has drawn up a chart, for each of the
popular gear combinations, that says what distances, between BB centre,
and back axle centre, would be possible using the given combination
("distances", since the chain length can of course be altered).

As an example, I have 403 mm from BB centre to back axle centre. I
believe that I can alter the distance +/- 3 mm by shifting the axle.
Suppose I wanted an 81 inch gear on this frame. The chart that I hope
exists, may tell me that e.g. 45/15 does not work but that 48/16 does.

Of course I could go out and buy lots of chainrings and lots of cogs and
test all combinations for the desired ratios, to see which of these
gives the right chain tension but I would prefer to buy one chairing and
max 3 cogs.

BTW my preferred ratio would be around 75 in (e.g. 48/17).

I'm aware that I can calculate the resulting distance for each
combination, adding appropriate pi * r for cogs and rings, but honestly
I'd prefer the chart if it exists . . .

Although riders with plenty of time and moderate expectations have done
that, I would recommend either an ENO hub, a frame with an inch of
horizontal travel or a frame end replacement on your frame.

I've never seen such a chart but as you note the idealized numbers are
obvious. A chart may not cover all the small variables- chain tension is
optimized over a smaller range that you might at first imagine.
'Sensitive' setups with verticals may require frequent new chains (cheap
at $10). Not to say you can't do that but alternates exist.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Robert said:

I am about to convert one of my road bikes to fixed-wheel. Frame (an
aluminium Colnago) has vertical dropouts.

Nonetheless, I believe that I will be able to get satisfactory chain
tension provided that I find the right chainring - back cog combination.

Here's what you need:

http://www.eehouse.org/fixin/fixmeup.php

It really works. Chain wear will be an annoying issue, though, with
no way to take up slack.

Chalo

jim beam said:

[re ENO hub]

Quoted message said:

it's /the/ option if you want to ride more than a few hundred miles and
thus encounter chain wear.

$5 chains make frequent replacement pretty tolerable. Especially if
you have another use for single speed chains with minimal wear.

Chalo

Zog The Undeniable said:

Robert said:

I am about to convert one of my road bikes to fixed-wheel. Frame (an
aluminium Colnago) has vertical dropouts.

Nonetheless, I believe that I will be able to get satisfactory chain
tension provided that I find the right chainring - back cog combination.

Quoted message said:

If you google "magic gear calculator" you should find at least one.

An ENO hub is another option.

And a half-link for your chain is another.

--

John ([email hidden])

Robert said:

I have all equipment required but I have not yet bought chainring and
back cog. Before I do this, I want to ask those of you having made the
conversion, if someone somewhere has drawn up a chart, for each of the
popular gear combinations, that says what distances, between BB centre,
and back axle centre, would be possible using the given combination
("distances", since the chain length can of course be altered).

I doubt that there is a chart, since it will be a very finicky fit. You
really do need to have several sprockets, and at least a couple chainrings.

For each tooth added on the rear sprocket, the distance around the back
half of the sprocket where the chain wraps is increased by 1/4", so the
wheel will move forward 1/8". Well, approximately, since the change in
the angle of the chain will also change the distance ever so slightly.

If you have a bit of play in the rear dropouts that will help. Also,
getting a half-link for the chain (only available in 1/8"😉 will help.

Quoted message said:

As an example, I have 403 mm from BB centre to back axle centre. I
believe that I can alter the distance +/- 3 mm by shifting the axle.

3mm? That's quite a bit. Most bikes have more material in those
dropouts than is essential; you might take a grinder and widen the slots
a bit, too. I have seen some vertical dropouts that actually had a good
1/4" adjustment to them -- they were probably early vertical dropouts
(Raleigh, I think), which still had some adjustment potential designed
into them.

Quoted message said:

Of course I could go out and buy lots of chainrings and lots of cogs and
test all combinations for the desired ratios, to see which of these
gives the right chain tension but I would prefer to buy one chairing and
max 3 cogs.

Well, you also have to take into account chain wear, which will change
the amount of slack over time. Having more cogs/chainrings might help
in that case, too.

--

David L. Johnson

And though I have the gift of prophecy, and understand all mysteries,
and all knowledge; and though I have all faith, so that I could remove
mountains, and have not charity, I am nothing. [1 Corinth. 13:2]

David L. Johnson said:

Robert said:

I have all equipment required but I have not yet bought chainring and
back cog. Before I do this, I want to ask those of you having made the
conversion, if someone somewhere has drawn up a chart, for each of the
popular gear combinations, that says what distances, between BB
centre, and back axle centre, would be possible using the given
combination ("distances", since the chain length can of course be
altered).

I doubt that there is a chart, since it will be a very finicky fit. You
really do need to have several sprockets, and at least a couple chainrings.

For each tooth added on the rear sprocket, the distance around the back
half of the sprocket where the chain wraps is increased by 1/4", so the
wheel will move forward 1/8". Well, approximately, since the change in
the angle of the chain will also change the distance ever so slightly.

If you have a bit of play in the rear dropouts that will help. Also,
getting a half-link for the chain (only available in 1/8"😉 will help.

Quoted message said:

As an example, I have 403 mm from BB centre to back axle centre. I
believe that I can alter the distance +/- 3 mm by shifting the axle.

3mm? That's quite a bit. Most bikes have more material in those
dropouts than is essential; you might take a grinder and widen the slots
a bit, too. I have seen some vertical dropouts that actually had a good
1/4" adjustment to them -- they were probably early vertical dropouts
(Raleigh, I think), which still had some adjustment potential designed
into them.

Quoted message said:

Of course I could go out and buy lots of chainrings and lots of cogs
and test all combinations for the desired ratios, to see which of
these gives the right chain tension but I would prefer to buy one
chairing and max 3 cogs.

Well, you also have to take into account chain wear, which will change
the amount of slack over time. Having more cogs/chainrings might help
in that case, too.

The ENO hub sounds like the best way to go, given the adjustment
possibilities, even if I'm likely to have to sacrifice back brake due to
its use.

Of course the cleanest solution is to find a frame with horizontal
dropouts but that would be cheating of course.

Thanks for your advice and comments folks.

Cheers, Robert

Robert said:

The ENO hub sounds like the best way to go, given the adjustment
possibilities, even if I'm likely to have to sacrifice back brake due to
its use.

If you have yet to build the wheel, then, yes, it could well be the best
solution. But is it cheaper than a frame with horizontal dropouts?

--

David L. Johnson

The lottery is a tax on those who fail to understand mathematics.

Robert said:

Quoted message said:

The ENO hub sounds like the best way to go, given the adjustment
possibilities, even if I'm likely to have to sacrifice back brake due to
its use.

David L. Johnson replied:

Quoted message said:

If you have yet to build the wheel, then, yes, it could well be the best
solution. But is it cheaper than a frame with horizontal dropouts?

Actually, that question isn't quite to the point, 'cause if you have
horizontal dropouts (or, heaven forbid, old-fashioned rear-opening
forkends) installed you _still_ need to buy a hub.

The White Industries hub is a very high quality track hub, close to
Phil Wood level, so more to the point is wether the price _difference_
between whatever hub you would otherwise buy is more expensive than
the frame modification.

Note, getting frame parts replaced wrecks the paint job. A decent
frame paint job is going to cost more than an ENO eccentric hub!

Sheldon "ENOphile" Brown
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com
Useful articles about bicycles and cycling
http://sheldonbrown.com