I assembled a Drive from some old parts, shortened the spine and lopped off part of the fore and aft tabs for lots of drivewell play:
I took this down to the water yesterday and pedaled around. Lo and behold, no clunk! Not satisfied, I broke out the trusty saw and hacked some more -- fully eliminated the tabs. Surely this badly mangled Drive would clunk while pedaling, right?
Not a chance! The ONLY way I could get any rocking motion from this drive while pedaling on the water was to pedal one footed (pulling and pushing) or stop pedaling it and rock it with my hands. There was no combination of pulling and pushing with both legs while pedaling that would induce any rocking.
So unless you're one legged, I will say categorically that Mirage Drives do not rock back and forth while being pedaled. Even accomplished bicyclists who pull back on the pedals will push much harder and prevent the Drive from rocking.
There are a variety of clinks or clanks that could occur -- the crank arms can clack back and forth if loose when pulling back, the fins could slap the hull (some models) if you don't have the Drive adjusted far enough forward, or the sprockets could rap against the drivewell lip (other models) for the same reason. Loose cables can snap the fins back and forth ( this is hard on the Drive).
It's not always easy to figure out which is which, but one thing is for certain -- the Drive, regardless of amount of play, does not pivot fore and aft during operation. I can't find any problem here and I think it's a mistake to confuse the increased rocking motion for a poor fit. Anybody in the market for a custom spine?