I’ve got my bike on my trainer (a Cycleops Fluid2), and when I’m pedalling hard, the bike tends to “wobble” (the trainer is secure, just not the bike). I’ve got the bike clamped in tightly, and everything seems by the book, but I’m concerned that the wobble/flex may weaken the rear triangle (particularly the slots where the rear axle attaches to the frame, of which I don’t know the technical name).
I’m not a small rider (200 pounds), so I wonder if that has anything to do with it.
the bike is going to flex no matter what it’s made from. An infinitely stiff frame wouldn’t be very comfortable. If your seeing the ‘flex’ by looking down at the crank when your pedalling, then most of what your seeing is probably your head moving not your bike. just make sure your quick release is tight and you should be fine.
If your frame didn’t flex to absorb torque loads, it would be subjected to substantial torque that could lead to frame failure. Since almost all frame materials are selected for varying degrees of flex, this is not a problem except for one frame material that I can think of. That material would be aluminum. Since aluminum does not really have a flex property, subjecting aluminum to forces that cause it to bend and flex is actually contributing to its failure. That’s one of the reasons people tout the “stiffness” of aluminum frames. They don’t bend or flex to absorb peddling torque. Many aluminum frame builders/manufacturers do not warranty their aluminum frames for life either. This is one of the reasons. An aluminum frame clamped in a trainer is subjected to enormous torque loads because the training stand "freezes " the rear dropouts and prevents the frame from “moving” naturally under the rider as it does on the road.
So, if your frame is not made of aluminum, you should be just fine. If it is made of aluminum, beware! You could be contributing to your frame’s premature failure.