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Friday, April 6, 2012

Product Review: Convert your geared bike to singlespeed (DMR STS chain tensioner)

The simple tension seeker (STS) by DMR Bikes should not really be called a chain tensioner, but rather a slack-remover. It's an incredibly simple solution for those looking to convert a bike with vertical dropouts for single speed use. DMR is a UK-based company that specializes in downhill, freeride, and dirt jump chain devices, and the STS reflects this design experience in this burly device.

Installation is a 5-minute job (assuming you have already replaced your cassette with a cog, and shortened your chain as much as possible). Simply remove the skewer nut and slide the black aluminum mounting bracket onto the dropout. Then loosely bolt the stainless steel arm to the bracket and the derailleur hanger with two 5mm bolts. Replace the skewer nut. Rotate the cranks until the chain is at its tightest. (Very few chainrings and cogs are perfectly round.) Lift up on the arm so that the red pulley pushes the chain upward, removing the slack, and tighten the two 5mm bolts. That's it.

Because the pulley is fixed in place, rather than spring-loaded, the STS feels most like a "true" singlespeed, that is, a bike with horizontal or sliding dropouts. There is no possibility of chain bounce, which keeps things quiet on the trail. There is also increased chain wrap on both the cog and chainring, which feels really solid when mashing the pedals.




There are a few caveats, of course. First, you must only use a narrow (3/16th's) multispeed chain, as wider chains will not ride comfortably in the pulley groove. Having ridden singlespeed mountain bikes for 5+ years, I would recommend this anyhow. Multispeed chains are designed to experience side-loads and drastic changes in tension while shifting. On a singlespeed, they last a long, long time.

Second, the derailleur hanger bolt is made out of sacrificial aluminum. If you crash badly, the bolt will break before your derailleur hanger does. This is a good design, but it means you will strip the bolt if you try to torque the crap out of it during installation.

Third, you need to unbolt the arm to remove the rear wheel.

Finally, like any tensioner (or slack remover) do not use with fixed-gears, ever.

Conclusion:
  • Cheaper ($20) than spring-loaded devices from Surly ($40), Paul ($60), and Rohloff ($80).
  • Simple, bomb-proof design. Feels like a proper singlespeed.
  • Looks really cool on a mountain bike (probably on a road bike too!).
  • Takes a smidge of mechanical ability to install correctly.
  • Needs to be removed and reinstalled each time you take the rear wheel off.

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