Advantages of 25mm tyres over 28mm

[Hot Flatus]

Indeed.  My tires (25mm) have been down to 80psi these  past few weeks.  There is no question that the grip is better, especially on corners.

[Cudzoziemiec]

Could the higher pressure in narrower tyres  enable them to melt a thin frost layer and therefore achieve better grip with the road? I'm not suggesting that this would apply to black ice, BTW.

I don't know. But would you actually want it to? Surely water over ice is the ultimate in slipperiness? I suppose if it could melt a very thin frost layer all the way to the road surface, it might work, but if would have to be almost instantaneous melting to give a grip advantage, I'd have thought.

<Far from physics bod, I'm afraid.

[frankly frankie]

This is a common fallacy. Higher pressure per unit area increases friction per unit area, so reducing pressure doesn't increase grip.

Except, it works that way in practice. Many's the time I've dropped the pressure right down on icy roads to avoid back wheel spin when climbing. Maybe it has more to do with the tyre conforming to the road better and the bike not bouncing as much.

Isn't it partly because most of the grip is generated at the edge of the contact patch?  Which would also explein why tread can be helpful, even when all logic insists it does nothing.