Originally Posted by JayB
Over the years I have heard lots of references to frame geometry which cite "angles" such as 73 degrees (apparently that's what Co-Motion design to). Also references to millimeters of rake eg, 55 for Santana and 45 for a Co-Motion with Woundup carbon forks. But . . . what do these figures mean? Can anyone explain in layman's terms how this affects handling and stability of tandems? Perhaps I'm not the only one who would be grateful for an explanation.
Here's something I wrote in Aug '04 during a thread discussing the differences between a Santana Sovereign and a Co-Motion Speedster. I think it's perhaps closer to what you're looking for...
Originally Posted by ottodog
Co-Motion Speedster/Roadster - Total road bike feel. Pretty unstable @ lower speeds, but handles like it's on rails @ higher speeds.
It's probably an overstatement to say the Co-Motion geometry is "unstable" at slower speeds in much the same way it is to say that the Burley, Santana, or Trek tandems are "unstable" at high speeds.
Not to put words into other folks mouths, but having read how various folks have attempted to put riding impressions into words over the years -- myself included -- I think a more accurate way to characterize it is....
A Co-Motion demands more attention and steering control by the captain at speeds under 10 mph and is very sensitive to weight shifts or movement by the stoker. This is sometimes disconcerting to new teams during initial test rides or even experienced tandem teams who have ridden tandems with less steering trail when they first ride a Co-Motion. However, it's also something that almost any but the largest teams or teams with stokers who move around a lot will assimilate into their normal riding experience in short order (like a couple of rides). The other, shorter trail models (Santana, Burley, Trek, Bilenky, and most others) have a more refined feeling at slow speeds and are less sensitive to weight shifts or minor movements by the stoker. The Bilenky is perhaps the most refined in this regard as it has very short steering trail. At the opposite end of the performance spectrum, the Co-Motion comes into it's own as it is very responsive to leaning inputs for directional control and does not exhibit any oversteer throughout the speed range. The tandems with less steering trail will begin to exhibit understeer at higher speeds and do not inspire aggressive cornering as much as the Co-Motion. In the mid-range, the Co-Motion is a livelier ride than any other the others, whereas the Santana and Bilenky are perhaps the most refined. In short, comparing tandems is like comparing fine wines (or coffees and cigars, noting that is what Co-Motion names its tandems after). Some are more sweet, some have more body, and some are just plain awful. Everyone's tastes vary, so you try to find a good match.
If you can find it, my column from Issue #16 (August/September) of Recumbent & Tandem Rider Magazine also addressed tandem steering geometry without getting too tied up in the numbers. However, as others have noted, unless you are having a custom tandem built or changing the fork, a given tandem's steering and frame geometry are factored into the tube selection (material, diameter, thickness, internal or external butting, and shaping), and even the wheel and tire selection to create the steering, handling, and comfort that the designer or marketing department believes are what its customers are looking for and/or what they believe exemplifies how their products should perform. The latter is why test riding is so important, both when shopping for your first tandem and more importantly after you've been riding for a year or so and start considering the things that you'd like to change about your next tandem.