Monday, August 2, 2010

My Road to Singlespeed

My single speed craziness started with a Bontrager Privateer and a Jamis Dakar. I bought the Privateer in 1998 and was satisfied with the Trek version of Keith’s classic bicycle, which I still own today; see the photo on the left. If I had to do it again I would probably would have gone with a larger frame over the medium I bought, as Bontragers run small. Although I bought the “S” Sport level, I upgraded the hubs, shifters and cranks to XT, resulting in a XT gruppo with Avid brakes. The original shock was a Rock Shox T2; an elastomer version of a Judy XC, that is upgradeable with cartridge kit to the equivalent of the Judy. After attempting such an upgrade, I blew out the cartridges at a rate of 1 a month, so I replaced it with a Marzocchi Super Fly. The T2 shock also had a special Bontrager designed crown with steeper geometry. The Marzocchi upgrade stabilized the ride not only because it’s a stronger shock, the more relaxed geometry with a standard crown really calmed down the steering. This was made possible as the 1998 (and 1999) were the only Bontrager frames made with a 1 1/8” head tube. The picture here is the current condition of my Privateer. With the exception of the King headset, shorter stem (As I said before Bontrager’s run small, so with the smaller stem it can be setup closer to a small frame size; I did this for my wife), seatpost, pedals (I still have the originals) and front rim (I taco’ed the original), the bike is the same build I bought in 1998.

Well after happily riding my Privateer for 3 years, I decided to go with a dual suspension bike and bought a 1996 Jamis Dakar frame from (later bought by Performance) and built it up. The pictured bike has the same frame with a slightly different (but surprisingly similar) build; imagine this bike with a red Marzocchi Atom 80 fork. Well this left my Bontrager just hanging in the garage, so I figured I’d check out this single speed thing.

My first foray was to buy a Surly Singleater, TruVativ Stylo single speed crank and a spacer kit. I started with a 32:16 gear ratio with the Singleator in the push down mode. I then bought a Surly SS rear wheel from Sheldon Brown and ran it with a 16T and 18T freewheels. For some reason this caused the chain to skip at the rear (16T) freewheel. The reason is as follows,

The usual suspect of a skipping chain is a spring loaded tensioner. Most spring tensioners work by pushing the chain down and away from the rear cog. This results in very poor chain wrap. When peddling the majority of the torque from the chain is on the front chainring is on the teeth at 12:00 –0300 and on the rear cog it is the teeth from 6:00- 9:00. What this means is the very area of the rear cog where, chain wrap is necessary, a push down spring tensioner pulls the chain off the cog. This transfers the torque to the tensioner pulley wheel, which in turn pulls the tensioner arm upward. The chain then rides up in the cog teeth in the 9:00-12:00 position. Since it takes a great deal of torque to turn the rear wheel and there is practically no chain warp in the area, the tensioner will fail and the chain will jump over the teeth at the 9:00-12:00 position on the cog, causing it to “skip”. The answer is to first make sure your chain is as short as possible (a half link is a good idea but it will create a weaker link). Next use a spring tensioner with a push up mode or even better a tensioner without a spring so the arm can be locked up; this way tensioner cannot be defeated by overcoming the spring tension. A quick fix is to zip-tie the tensioner arm to the chain stay. This will create a tensioner in a push up mode that does not rely on spring tension. This is a photo I set up to show how such a setup would look.

When this first happened I was in the middle of a ride. I switched to the 18T freewheel (with a 32T chainring) which turned out to be the so-called “magic gear”, or a setup where the chain does not need a tensioner with vertical dropouts. I then figured out that with a half link, the same was true with a 16T freewheel. At some point later I sold the Surly wheel and bought a Spot wheelset that I really didn’t like and I ended up selling it, and returning to a spaced rear wheel. I have only one picture (of poor quality) of this Bontrager as a single speed I took on a Soquel Demonstration Forest ride. This is the bike I rode until I has custom bike made for me by Rocklobster in 2002.

Next: The Custom Bike


kenjihara said...

Dude, that's my red Jamis Dakar!!

kenjihara said...

And I also ride Bontragers! You're like my brother from another mother.

kenjihara said...

...seriously, that photo was taken in the hall outside my apartment.