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Shouldn't being massively heavy mean that we keep this speed for longer? Pretty sure this is GCSE-level physics that I've forgotten. Who is stealing our speed?
Weight has nothing really to do with it.
Kinetic energy gets converted into gravitational potential energy. If it were perfectly efficient then:-
(We'll ignore the rotational energy of the wheels...)
1/2 * m * v * v = m * g * h
(g = gravitational constant of 9.8m/s^2)
The mass on each side cancels out and you're left with:-
h = 1/2 * v * v / g
So if you're in a vacuum and no rolling resistance then it doesn't matter if you're on a bike, skateboard, tandem or whatever.
But the KE to GPE transfer isn't perfectly efficient, the tandem will have a higher rolling resistance and higher drag coefficient. That's why you don't go as far up the hill as you do on a solo bike.
Greenbank-
There are other terms in there too that have no mass dependence (e.g. aero drag) so you do end up going faster down the hill. It doesn't explain the sensation of losing speed more quickly up hill than with a solo (probably to do with a non-linear increase in rolling resistance) but I feel it's mostly a synchronisation problem. As @Hovis suggested, the stoker doesn't know exactly what's going on and, assuming you spun out down the hill, you both have a reduced sensation of what's going on with the freewheel. Is the resistance you feel the weight of the bike or your stoker? So you can't pick up the work as early as you would on a solo and end up dying a slow and painful death in your granny gear.
hamrack
Good to know that mine are common complaints. The flip-side, of course, is that on a decent bit of flat, it basically feels like we're floating because it's so easy to keep this "bike the size of a sofa" moving.
But what I don't understand is why, on rolling terrain, we lose speed so quickly when starting a little climb after a long descent? On an individual bike, I'd basically use speed gained on the descent to give me a head-start on the hill; on the tandem this speed seems to evaporate into thin air.
Shouldn't being massively heavy mean that we keep this speed for longer? Pretty sure this is GCSE-level physics that I've forgotten. Who is stealing our speed?