My girlfriend and I just picked up our first tandem on Craigslist, after trying out several rides on various tandems. The bike is a beautiful, very early fillet-brazed Santana: I believe around 1982, give or take a couple years (it's got the marathon style frame, which Santana supposedly phased out around then). It's in fantastic shape -- looks like it's never had more than a few hundred miles on it, almost no wear on any parts. Everything was perfectly greased and immaculate (owner was a bike shop owner).
We just did our first "real" ride on it, after our first test ride of a few miles, and it was going fine for the first seven or eight miles. Then I noticed -- and my stoker noticed, with some dismay -- that the handling had suddenly gotten really weird. I felt like the bike would lurch right on right-hand pedal strokes (cranks are in phase). And it really took all my strength and focus to keep the bike on a straight line, especially after any steering adjustment. We checked pretty much everything we could find, and did notice that the rear wheel had slipped sideways a bit in the dropouts, so we straightened that out. It helped a little, but the bike was still very hard to ride. When I tried getting on and riding solo, it was much worse -- as though the stoker's weight had been damping the wobble in the rear. We got back on and limped slowly home, but it was honestly a bit of a struggle to keep the bike out of the ditch. My arms were exhausted from trying to track a line -- as though I were fighting the bike the whole ride home.
When we got home we took the (threaded) fork off and examined everything for cracks. The headset didn't appear notched/indexed, but I cleaned up all the bearings and races, anyway. The rear wheel stayed in place for the whole ride home, so I don't think that was the issue.
Any thoughts? I'm kind of mystified. It felt like a different bike than we started out on.
OP; Sounds like a really loose headset. Given that and the rear wheel issue; it returns to an old axiom; Whenever buying a used bike, always go completely through it to check its condition and then tune it up fully BEFORE riding it the first time (hire a good LBS to do it if you lack the experience and tools). Apologies for being terse, but the buy and fly away method it a good way to get two folks really injured and also damage a good tandem in the process.
/K