C
On Sat, 16 Dec 2006 15:36:42 -0600, John Thompson
<[email protected]> wrote:
>On 2006-12-15, [email protected] <[email protected]> wrote:
>
>> On Fri, 15 Dec 2006 14:19:26 -0600, John Thompson
>><[email protected]> wrote:
>>>
>>>You can't use a chain tensioner with a fixed gear; it would be destroyed
>>>the first time you tried to push back on the pedals to slow down.
>
>> Dear John,
>>
>> True, a chain-tensioner based on a foolishly cannibalized double-lever
>> derailleur won't work on a fixed gear.
>>
>> But single-lever trailing-arm chain-tensioners have been used for over
>> forty years on much heavier and more powerful "fixies" like this one:
>>
>> http://i13.tinypic.com/479xidi.jpg
>
>Very nice. Are similar items readily available for bicycles? It would be
>fairly trivial to cobble one up, but a secure mount would have to be
>welded/brazed/whatever onto the chainstay. This makes it less than ideal
>for a quick 'n' easy home fixie conversion unless you happen to already
>have welding equipment (which I do, but that's beside the point).
Dear John,
As far as I've been able to tell, the simple trailing-arm
chain-tensioner is as unknown to RBT as the wheel was in the New
World.
It's been standard on trials motorcycles since the early 1970's on
Bultaco, Montesa, Ossa, Honda, Yamaha, Kawasaki, Suzuki, GasGas,
Sherco, Beta, Scorpa, Fantic, and other brands.
Usually when I mention the trailing-arm chain-tensioner, the reaction
is either blank incomprehension or fierce denial, so it's nice that
someone else finally sees how it works.
It could be welded or brazed onto the chainstay, but a pair of
hose-clamps holding a short length of half-pipe will serve as a mount.
Indeed, that's how chain-tensioners were added to a lot of pre-1972
trials machines. The stress on the lever amounts to little more than
the resistance of the coil spring, and the chain promptly drags the
lever back into alignment whenever it tries to bounce to either side.
It works fine for "fixie" trials machines with several inches of rear
suspension travel and heavy motorcycle chains, so it would seem likely
to work on rigid-frame fixie bicycles with light chains.
Of course, there may be some detail that makes it inefficient or even
pointless, but the usual objection that braking would instantly rip
the chain-tensioner off only shows how the simple trailing-arm is a
dark and bloody mystery in these parts.
Curiously, lots of old derailleur bicycles used trailing-arm
chain-tensioners with small idler sprockets on the ends of huge arms
before the compact modern two-lever tensioner-and-derailleur design
triumped.
The difference was that they pushed the chain away from the chainstay,
not toward it like a modern trials motorcycle chain-tensioner. The
reason is that the old bicycles were dealing with the enormous amount
of chain slack needed for derailleur gearing--a trials-type
chain-tensioner would have hit the chainstay before it gathered up all
the extra chain. That's one reason why the double-arm triumphed--all
that extra slack is gathered up into the S-bend.
A modern fixie bicycle has scarcely any chain slack compared to a
derailleur, so the trials-style tension-toward-the-chainstay design
would work.
Berto's "The Dancing Chain" includes numerous examples of bicycles
with these single-lever trailing-arm chain-tensioners from the 1920's
through the 1950's:
http://i17.tinypic.com/2lxf9k6.jpg
Click on the lower right in Explorer for full-size. Berto notes that
it was produced from 1936 to 1954 and was based on an earlier French
design. Similar models in Berto's book are sometimes mounted further
back on the chainstay. You can see the evolution from this monster
single-arm chain-tensioner pushing the chain away from the chainstay,
to more complicated two-arm tensioners that gather the chain into the
neater S-bend, to the modern two-arm tensioner integerated on the
parallelogram that moves the chain from side to side while taking up
the slack.
In all likelihood, no one uses such chain tensioners on fixie bicycles
because it's reasonably easy to adjust the chain tension by moving the
axle on horizontal dropouts, just as chain tension is adjusted on
typical motorcycles, which lack chain-tensioners.
Cheers,
Carl Fogel
<[email protected]> wrote:
>On 2006-12-15, [email protected] <[email protected]> wrote:
>
>> On Fri, 15 Dec 2006 14:19:26 -0600, John Thompson
>><[email protected]> wrote:
>>>
>>>You can't use a chain tensioner with a fixed gear; it would be destroyed
>>>the first time you tried to push back on the pedals to slow down.
>
>> Dear John,
>>
>> True, a chain-tensioner based on a foolishly cannibalized double-lever
>> derailleur won't work on a fixed gear.
>>
>> But single-lever trailing-arm chain-tensioners have been used for over
>> forty years on much heavier and more powerful "fixies" like this one:
>>
>> http://i13.tinypic.com/479xidi.jpg
>
>Very nice. Are similar items readily available for bicycles? It would be
>fairly trivial to cobble one up, but a secure mount would have to be
>welded/brazed/whatever onto the chainstay. This makes it less than ideal
>for a quick 'n' easy home fixie conversion unless you happen to already
>have welding equipment (which I do, but that's beside the point).
Dear John,
As far as I've been able to tell, the simple trailing-arm
chain-tensioner is as unknown to RBT as the wheel was in the New
World.
It's been standard on trials motorcycles since the early 1970's on
Bultaco, Montesa, Ossa, Honda, Yamaha, Kawasaki, Suzuki, GasGas,
Sherco, Beta, Scorpa, Fantic, and other brands.
Usually when I mention the trailing-arm chain-tensioner, the reaction
is either blank incomprehension or fierce denial, so it's nice that
someone else finally sees how it works.
It could be welded or brazed onto the chainstay, but a pair of
hose-clamps holding a short length of half-pipe will serve as a mount.
Indeed, that's how chain-tensioners were added to a lot of pre-1972
trials machines. The stress on the lever amounts to little more than
the resistance of the coil spring, and the chain promptly drags the
lever back into alignment whenever it tries to bounce to either side.
It works fine for "fixie" trials machines with several inches of rear
suspension travel and heavy motorcycle chains, so it would seem likely
to work on rigid-frame fixie bicycles with light chains.
Of course, there may be some detail that makes it inefficient or even
pointless, but the usual objection that braking would instantly rip
the chain-tensioner off only shows how the simple trailing-arm is a
dark and bloody mystery in these parts.
Curiously, lots of old derailleur bicycles used trailing-arm
chain-tensioners with small idler sprockets on the ends of huge arms
before the compact modern two-lever tensioner-and-derailleur design
triumped.
The difference was that they pushed the chain away from the chainstay,
not toward it like a modern trials motorcycle chain-tensioner. The
reason is that the old bicycles were dealing with the enormous amount
of chain slack needed for derailleur gearing--a trials-type
chain-tensioner would have hit the chainstay before it gathered up all
the extra chain. That's one reason why the double-arm triumphed--all
that extra slack is gathered up into the S-bend.
A modern fixie bicycle has scarcely any chain slack compared to a
derailleur, so the trials-style tension-toward-the-chainstay design
would work.
Berto's "The Dancing Chain" includes numerous examples of bicycles
with these single-lever trailing-arm chain-tensioners from the 1920's
through the 1950's:
http://i17.tinypic.com/2lxf9k6.jpg
Click on the lower right in Explorer for full-size. Berto notes that
it was produced from 1936 to 1954 and was based on an earlier French
design. Similar models in Berto's book are sometimes mounted further
back on the chainstay. You can see the evolution from this monster
single-arm chain-tensioner pushing the chain away from the chainstay,
to more complicated two-arm tensioners that gather the chain into the
neater S-bend, to the modern two-arm tensioner integerated on the
parallelogram that moves the chain from side to side while taking up
the slack.
In all likelihood, no one uses such chain tensioners on fixie bicycles
because it's reasonably easy to adjust the chain tension by moving the
axle on horizontal dropouts, just as chain tension is adjusted on
typical motorcycles, which lack chain-tensioners.
Cheers,
Carl Fogel
