How do I adjust my timing chain?

Most modern tandems have a tandem-specific crank-set consisting of 4 arms, with the left side arms connected by a chain. This chain is called the timing chain, because it keeps both riders pedaling in unison. When functioning properly, the timing chain is tight enough to run smoothly with no binding, yet not so loose that chain derailment occurs. One of the first tandem maintenance points that needs attention from new tandem owners is the adjustment of the timing chain. On Co-Motion tandems we have our own easy-to-adjust system. The front crank-set bearing is housed in an aluminum shell with an off-center bore. The assembly is referred to as the “eccentric”. Our aluminum and steel frames are different in the way the eccentric is fixed into place, but otherwise the procedure is the same. On our steel tandem models, the eccentric is held in place by an internal binding system. Begin your timing chain adjustment by loosening the two allen bolts in each side of the eccentric with a 4mm allen/hex key. Insert the allen key into one of the bolts, then turn the crank as though pedaling until the crank arm or crank spider bears against the allen wrench. Applying pedaling force will now rotate the eccentric. The eccentric can be rotated in either direction with the effect on the chain tension immediately obvious. You may also use a pin spanner to engage both allen bolts on one side of the eccentric and rotate the eccentric that way. The eccentric used in our steel tandems prior to 2005, and our current aluminum tandems is shown at the right. Loosen the pinch bolts that bind the eccentric in place and rotate it by inserting a thin screwdriver or allen wrench into the hole on the left (non-drive) side of the eccentric. How tight do you want the chain? While moving the eccentric, keep an eye on the timing chain and refrain from applying force to the pedal when it seems that you have eliminated chain slack. Do not re-tighten the allen bolts yet. Check chain tension uniformity by revolving the cranks a few times. Notice that the chain will be tighter in some positions. Try to find the tightest position and leave it there. You want to Steel BB ensure that even in its tightest position, the chain does not bind. With your finger pressing on the bottom of the chain about midway between the bottom brackets, you should be able to push about 1/2”. Reposition the eccentric until you’re satisfied that the chain can move freely and is sufficiently tight. Re-tighten the binder bolts gradually, 1/2 turn at a time until secure. Older Aluminum Tandems: Older Co-Motion Robusta, Roadster and Al Capp models have 4 set screws securing the eccentric in place. Loosen the set screws about 1 turn each with a 4mm allen/hex key and otherwise follow the above procedure.

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