I had a friend who ovalized three headtubes on road bikes... what did all three bikes have in common... alloy shimano 600 headset... he was a big guy, so smaller, lighter riders need not worry, however, I'd say they are far from strong.
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1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear