I ride a fixed gear bike in the city, and sometimes perform "hockey stops" as a braking method. This involves locking the rear wheel, and throwing the rear end of the bike sideways so that the bike is more or less perpendicular to the momentum vector. Hence the name "hockey stop."
Anyway, when I do this, I notice a lot of flex in the rear end. I can't tell while doing it if its from the stays, the wheel, or maybe the tire. The frame is a Soma Rush, 631 tubes. The wheel is a suzue promax laced 28h 2x, built by me. Tight spokes. The tires change, but are always around 100-140 psi.
Any hits on where the flex might be? Is it an issue of a wheel that is not laterally stiff, or would it be more likely that this is coming from the stays?