Front tire & downtube clearance - extremely dangerous?

Question

I recently moved cities and met someone to buy a used "vintage" 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.

Swifty · Accepted Answer

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

Tude Productions · Answer

Sure the hub and skewer on the front wheel is not really messed up? The only other place there would be that much flex is the forks. In which case if the bike is riding like that now, I would pass on it.
Based on the pics you added the fork looks like it is the right way in, but bent in the headset. At the very least you would need a new fork and at the worst the whole frame is trashed. I would pass on buying it if you plan to ride it.

whatsisname · Answer

This should not happen with a normally setup bike. Possible mis-maintenance I'd speculate could have been at play:

Bent or busted fork or frame
Replaced fork with wrong size replacement.
Badly adjusted headset, allowing the steerer to move around while braking
Fork installed backwards (I've seen it happen)
Too large of tires. You said seller claimed to have used a larger size, but they could have been lying.

I used to have an old 80s racing Bianchi that had about 1/4" clearance between the tire and the downtube. Never once had the two contact, even under emergency stopping conditions.
Safe to say, don't buy it.

Criggie · Answer

Based on the photo - the fork is utterly dead and shouldn't be ridden.
The frame may have stresses/subtle bends about 2-3" / 50-75mm aft of the headtube on the top and down tubes, and should be checked closely.  Being a steel frame helps, but I'd absolutely retire the fork and heatset+bearings.  The front wheel (rim/spokes/hub) needs a good check as well.
Seller is either an idiot, disingenuous, or simply too unaware to know what's wrong with their bike.  I recommend you find another bike from another seller.

Jeff · Answer

Cropped picture of bike in question. No question the steerer tube and blades are not in the intended alignment.  Fork ends up deflected back toward frame.   I'm not sure a headset problem could create that look without some form of structural damage. I'd be checking out the bike's head tube pretty thorough as I prepped for a new headset install should the frame pass muster.

Loss of detail closer in, but the fork crown-lower headtube interface looks suspect.  In addition, the 28mm tires are probably the uppermost limit wide and could therefore exceed the expected overall diameter, adding to the problem and severity of the results.

Front tire & downtube clearance - extremely dangerous?

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