Contact patch size x tyre pressure = rig weight* ( x some constant)
(*probably as good as constant)
This is O-level physics <\cliched old duffer>
Note the absence of tyre width in that equation. That's all I'm saying on this ...
But contact patch size is (to a first approximation) proportional to the tyre width, no?
(actually, to a very first approximation (for any curved tyre that doesn't deform...), the contact patch size is zero, or rather infinitesimally small, ).
Actually, thinking about this a but longer I think my O-level (and beyond
) physics may have let me down.
My intuition was that the threshold where static friction would give way would depend on the pressure exerted on the ground by the tyre --- it does --- but it also depends on the area in such a way that both factors cancel (mattc's equation above boils it down exactly) leaving you with a constant multiplied by the downward force (i.e. the rig weight) ... the RHS of his equation is independent of the contatc patch size, and is (for the correct (material dependent) constant) then equal to the maximum horizontal force before static friction gives way and the tyre starts to slide.
The "constant" in question is certainly dependent on the tyre compound (and the road surface material/conditions etc.).
That said, I still think that something in the above may be too simplistic an approximation.
Echoing MV's comment upthread -- I notice a fair difference between the propensity to lock up on my road bike's 25s compared to the 38s that used to be on this rig. The M+ 28s still seemed more keen to slip than I'd have imagined from the two data points I have (not as extreme as 19mm and 40mm difference, but certainly noticeable).
