Adjusting the Timing Chain

Know Your Eccentric

Split Shell | Set Screw | Internal Wedge

Keeping your tandem in proper mechanical condition is an important part of enjoying the ride. It's not uncommon for a new team to become frustrated with their 'bike' only to find out later that a simple adjustment was all that was needed to quiet things down.

Only two tools are needed for adjusting your timing chain: an Allen tool and a bottom bracket pin spanner.

Unless you have a good selection of bicycle tools at your disposal, a local shop, with lots of tandem experience, is necessary in helping you maintain your tandem's health. Even so, because of the complexity of a tandem and the added wear on components, certain tools and skills should be acquired so that you may perform some routine maintenance on your own. This may help keep costly trips to the shop to a minimum.

A routine inspection should include the timing chain tension. The timing chain connects the pilot and stoker cranks, keeping them in sync (even if they're out of phase, they're still in sync). Over time, the tension of the timing chain will lessen due to the stretching and/or wear of the chain links and pins, and the wear of the chain rings. The drive chain is subjected to the same wear, but because it passes through the rear derailleur, its tension is always maintained. It's recommended that a new tandem or new timing chain be adjusted at 200 miles, then again every 500 miles after that.

No Tom not there. Check the tension of your timing chain at the center of the top run. After rotating your cranks to find the point of highest tension ('high spot') there should only be 1/2 inch of vertical chain deflection.

First you'll need to inspect the tension. Because chain rings and crank fingers are never perfectly round, the inspection and adjustment of the timing chain should be performed at the location of maximum tension or 'high spot'. Rotate the cranks until the chain is at its tightest. As you rotate, press down in the center of the top run of the timing chain and note the amount of vertical deflection allowed. A properly adjusted timing chain should have roughly 1/2 inch of deflection at the 'high spot'. Never adjust the chain tighter than this. Some slack is necessary to accommodate frame flex.

Once it is determined that an adjustment is required, you will need to loosen the eccentric at the front bottom bracket. The clamps holding the eccentric in the bottom bracket shell will be either set screws in the shell, pinch bolts on a split shell, or internal expander bolts. In any case, one or two Allen-head screws are used.

Loosen the eccentric with an Allen tool. Once loose, the eccentric can be rotated with a standard bottom bracket pin spanner.

When loosened, the eccentric can be rotated using a standard bottom bracket pin tool. Rotate the eccentric while checking tension as before. Be certain that the timing chain is still at the 'high spot'. Now that you've adjusted the timing chain back to its ideal 1/2 inch deflection, tighten the eccentric bolts to hold it in place. It may have slipped while being tightened, so reinspect the tension before you call it quits. It's a good practice when adjusting the timing chain, to rotate the eccentric through the lower half of the shell. This allows you to sit just a little lower in the saddle and thus shift your center of gravity. Unless you hammer through the corners and need that little extra ground clearance, keep the eccentric adjusted down.

It's also a good idea to keep a pin tool with you when you ride. A small section of chain and a chain tool is a must. But unless you stokers want to push all of the weight home with your own two legs, you better be prepared to repair the timing chain on the road. Besides, working on your bike puts you more in touch with its every detail, and helps you become more aware of its performance. That way you may learn to locate trouble before it becomes a catastrophe.

TM Online | Tech Tips

Copyright Petzold Publishing, 1994, 1995