Front tire & downtube clearance - extremely dangerous? I recently moved cities and met someone to buy a used &quot;vintage&quot; bike from them. While test riding at a slow speed I gradually applied the brakes. The front brake seemed to jump from light pressure to completely locking the front wheel at once and threw me over the top promptly!
My first thought was that the front wheel was out of true - causing the brake to apply harder as the other side of the wheel came around. However, the wheel was mostly straight.
Eventually I discovered that when braking, the frame and fork were flexing enough to cause the downtube to contact the front wheel. This contact caused the downtube to completely seize the front wheel instantly.
Has anyone else encountered this before? What fix would you even go about to stop the downtube from striking the front tire? I searched online and didn't find similar stories - so perhaps this super dangerous maintenance issues is rather uncommon?

Shogun Selectra - old frame, probably from the 90's
62cm frame
700c wheels
Rubino 28mm tires (seller said previous owner had 32mm tires) I didn't get the exact rim measurements. Tires seamed an average height for modern road tires
wheels mostly true
plenty of clearance for the tires in the fork (next time I'll have to check the downtube too)


edit - photos from comment


The frame paint was indeed worn away at the contact point between the front tyre and downtube - though looked like erosion over a long period of rubbing.
Yes, extremely dangerous, as evidenced by your falling over the handlebars
It's evident from the photograph you linked, that the fork is pushed backwards, bringing the wheel much too close to the frame. The most likely reason for this would be crash damage, given that the fork is facing the right way.
There are two hazards then, one is locking the front wheel instantly under braking, while the other is hidden (or even visible) damage to the fork, risking structural failure. This is likely repairable with a new fork and inspection of the frame, but happily you've passed on purchasing this time. Caveat emptor I suppose