I have a '91 Trek aluminum road bike which was originally equipped with a 7-speed drivetrain. I upgraded the drivetrain on another bike, so I transferred the old 10-speed drivetrain, to the Trek. The 7-speed hub spacing is 126mm and the 10-speed is 130mm. I spread the stays to accommodate the wheel (not cold setting). My question is it safe to ride given that the frame is aluminum?
With aluminum, dropouts cracking from the fatigue caused by this is a real thing that can happen. It may take a long time, or it may not in the case of a frame with a long life of fatigue already on it. There are a lot of factors like rider strength/weight and how good the dropout alignment was to begin with.
Chunky dropouts can somewhat mitigate the risk, but the problem there is the axle will flex instead of the dropouts, which can ruin bearing life and cause drag.