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Achieving Chain Tension
One of the first issues that you'll need to address in building a singlespeed bike is
the method by which your bike achieves and maintains proper chain tension. The reason for
this is that even though you will have shortened your chain to accomodate the new single gearing,
there's a high probability that there will be some amount of slack in the chain's tension (unless you're
extremely lucky). Without proper tension, the chain will not stay secured while riding,
especially when under stress of climbs.
On a geared bike, the rear derailleur provides tensioning as well as shifting the chain
from gear to gear. If you aren't really sure that you'll be into singlespeeding for the long haul
and want to avoid spending money, you can continue to use your derailleur as a tensioner
by removing the shift-cable and adjusting the two travel-limiting screws to lock the positioning
of the derailleur to the gear. This isn't very ideal, but serves well enough as a stepping stone.
When you're ready to ditch the complex derailleur entirely and go with something a more
simple such as a singlespeed-specific a tensioning device, there are a couple of choices
to choose from. The original tensioning device is from the guys at Surly, and it's called
the Singleator. The alternative device is from Paul's, and for reasons unknown to me it's
called the "Melvin".
* The images below enlarge (and can be dragged) if you click them. Click the image a second time to close the enlargement.
If you're working with a very limited budget and don't mind a little bit of elbow grease, a low-tech
tensioner can be fabricated from the cage and a single pulley of an old derailleur. This method basically
requires that you dismantle the derailleur to take off one side of the pulley cage then bend it as
appropriate to line up with your cog. Excellent details of doing this can be found on the
Dragon Tongue Singlespeeds site.
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The traditional derailleur can be used for tensioning the chain for short-term purposes.
If you go this route, keep your chain somewhat longer than a normal singlespeed chain length
because the derailleur won't be happy "wound up" so tightly.
Get a satisfactory chainline by positioning the derailleur to line up with the cog or gear of your choice,
then just screw in the two travel limiting screws to lock it in place. In some instances it may be
necessary to remove a plastic plate under the limiting screws in order to allow the screws to tighten enough.
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Surly's Singleator is the simplest-looking tensioning device available. The initial setup can be a little
tricky, but once properly installed (I recommend pushing UP on the chain) it works very elegantly to keep
your chain on.
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Paul's Melvin tensioner is machined from aluminum and somewhat resembles a derailleur with its two
pulley design. This more sophisticated/complicated design allows the tensioner to take up a wider range
of chain slack, which normally isn't an issue. However, if you want to run TWO speeds up front the Melvin
enables you to do that.
I've previously run a two-speed setup by having both a 24-tooth granny and 34-tooth middle ring up front.
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The ideal way to achieve chain tension is with horizontal dropouts, which are normally only found on
dedicated singlespeed frames. Horizontal dropouts allow you to slide the wheel axle fore and aft to adjust
the chain tension without the use of any additional hardware. This method should seem familiar since
this is how BMX frames have done it for years.
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One of the latest innovations in singlespeed frame design is the adaptation of tandem-style eccentric
bottom brackets for chain tensioning, as seen on this nice Seven closeup. The bottom bracket
shell rotates to adjust the chain tension and thus alleviates the need for special treatment such
as horizontal loading to the rear dropout. This in turn makes it possible to accomodate disc-brake
setups since the rear wheel is always mounted in the same position. If you need to run discs, this is
a very elegant way to go.
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Special thanks to those who have contributed feedback and/or photos.
Photo credits can be seen by leaving the mouse cursor over selected images.
Manufacturers of Components in Topic
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