TIMING CHAIN ADJUSTMENT
Timing chain adjustment on modern tandems of any notoriety is generally via an elliptical front bottom bracket mount whereby rotation of this part within the frame causes the front bottom bracket to change its fore and aft position. The component which holds the captain's bottom bracket is most commonly referred to as the "eccentric".
Eccentrics should be lubricated with grease to prevent corrosion and seizing to the frame. As preventative maintenance, it is advisable to remove the eccentric once per year for cleaning and lubrication which can save much heartache in the future.
The eccentric locking mechanism varies depending upon the frame builder's personal design and philosophy. Santanas use two set screws to secure the eccentric while older Co-Motions and Longbikes use a split clamping type bottom bracket shell on their steel frames and set screws on their aluminum models. Newer Co-Motions use an internal expanding design as does the Bushnell eccentric found on Bushnell built tandems, Precision Triplets, and Paketas. Trek, Cannondale and probably others have used the problematic wedge system, a design which is similar to that utilized on handlebar stems. These have a history of problems since they stick, freeze up, binder bolts break, and threads sometimes strip.
Generally the chain tension fluctuates with crank and chainring rotation due to slight elliptical variances in the manufacturing of the rings and cranks. One can often minimize this effect by rotating the cranks until the chain is in the tightest position followed by loosening all of the bolts on both timing rings. With all chainring bolts loose and with the chain rotated to the "tight spot", tap on the chain with a hammer. This will have the effect of centering the timing sprockets on the crankarms, at least by the amount of play available within the tolerances of the design.
Secure the chainring bolts after this procedure and hopefully your tight spot will have been minimized thus creating a more uniform chain tension upon rotation. If this has no positive effect on evening out the chain adjustement, consider rotating one of the chainrings to a different position. Sometimes this will help.
The generally accepted school of thought is to tighten the chain to the point where one does not feel it bind upon rotation by hand. Here is where my thoughts diverge from the general belief system concerning this topic, at least for teams of strength.
To observe how I came to this conclusion, find a friend who has a tandem that can ride at a 16.5 mph average or greater so that you may actually observe fluctuations in timing chain tension as the bike is ridden. But first check the timing chain adjustment and make sure it turns freely without binding and has the typical .5 or .75 inch deflection in the tight spot via fingertip pressure.
Next, watch the timing chain under real life conditions which require a bit of power such as the aforementioned 16.5 or greater average speeds on the flat, hill climbing and standing out of the saddle. Notice how the timing chain is always slack on the bottom under these conditions? This means that the chain could be adjusted tighter without putting any additional stress on the bottom bracket bearings, the chain or the chainrings.
A timing chain permitted to run too loose while under load is prone to chain derailment which can cause a variety of problems ranging anywhere from frame and chainring damage to loss of paint.
The extreme frame damage pictured was resultant of final drive chain derailment despite the use of a Chain Watcher. There is no practical way to repair an aluminum frame that has sustained damage of this extent.
Timing chain derailment can also cause frame damage, hopefully not this severe!
See our article for more information concerning Timing Chain Derailment prevention.
Why does the timing chain appear to sag or become loose under power? The frame flex associated with the captain's power required to move the tandem down the road under conditions of resistance has the effect of winching the two cranks together resulting in frame flex. The bowing of the frame is not unlike that of a bow and arrow. This thought also applies to stoker positions on triplets and quads which supply power via a timing chain as seen below.
The slack timing chains are literally flopping around!
Imagine the chain slop if we we were in the middle of our power stroke!
Note the banjo string chain tension under coast conditions.
According to most, the chains are adjusted too tight but not so when actually ridden.
So... should ideal adjustment be based upon whether the bike is parked or ridden?
It is my belief that tensioning the chain to the point that chain sag is minimized while riding under power on the flats is likely very close to the ideal tension. The only way this extra tension can add additional stress to the bottom brackets or chain and accelerate wear of the components, is if the team spends a great deal of time riding in low power situations that result in minimal or no frame flex.
I would recommend having another rider observe your timing chain while you ride. If it has a very slight sag most of the time while pedaling under load, you likely have it adjusted close to its ideal point.