Originally Posted by
pdlpsher
Due to the heavier weight and drivetrain inefficiencies, a tandem experiences a higher variability of speed as compared to a single bike. It has a much higher terminal velocity than a single. Similarly it reaches a lower speed when climbing a hill. So speed changes are more frequent and the rate of change is more drastic than a single. When dealing with a higher speed variability, it's desirable to have a wide-range cassette as opposed to a close-ratio, narrow-range cassette. A wide-range cassette reduces the amount of rear shifts. For example, if your speed slows down from 20mph to 8mph, you may only need four shifts on a wide-range cassette as opposed to five or six shifts on a close-ratio cassette..
I'm going to gently push back on this argument, with the understanding that all teams are different and equipment needs will vary accordingly.
My experience has been that teams often have a narrower "sweet spot" in cadence compared to single riders. Some teams may be well-matched in preferred cadence and have a relatively wide useful cadence range, but most teams will need to compromise their cadence due to differences between the riders.
This narrower range of useful cadence results in a need for closer - not wider - ratios. Di2 is great for this since shifts are flawless and require no effort.
While the low and high speed limits of a tandem are usually wider than a single, the higher momentum of the tandem results in lower - not higher - accellerations and decellerations.
An automotive analogy would be a a large diesel truck with a relatively low-reving engine (normally equipped with 18+ speed transmissions) vs. a high reving sports car with a 5 or 6 speed.
Originally Posted by
pdlpsher
The compromises of a triple is a significant increase in number of front shifts as compared to a double system equipped with a wide-range rear cassette. A front shift requires both the captain and stoker to back off the power, which disrupts the rhythm and hence the speed.
I'm basically in agreement here. Many tandems are operating gruppos significantly out of manufacturer's specifications (smaller chainrings or larger cogs), resulting in decreased shifting performance between the small and middle chainrings. The tooth difference between the small and middle rings can often be quite large.
This results in a need for coordination for these shifts so teams can be hesitant to use the small ring unless absolutely necessary. A wider ratio double-chainring drivetrain sidesteps this issue.
Di2 triples are such unicorns at this point that I have no idea if the limitations of mechanical triples also apply to electronic applications.