I suppose that cycle tyres,motor-cycle tyres and indeed car tyres are all reliant upon the type of surface that is actually beneath the water and the relevant tread-depth in relation to the load that is being carried - in the case of a cycle - the weight and bulk of the rider.
True, but testing with a representative surface in controlled (limited) conditions would probably give information that could be usefully extrapolated to all surfaces.
I believe this to be the case because grip comes from two sources: molecular adhesion and hysteresis grip, with the latter dominant in the wet and resulting exclusively from surface roughness.
I crudely described this here. Note that maximum grip occurs around the glass transition temperature of the compound, so a perfect test would vary the ambient (and therefore tyre) temperature to find the optimum temperature for each tyre. In fact, testing winter tyres at room temperature as I casually proposed might be daft. Already a complication rears its head!
Smaller riders will have a much lower center of gravity whilst larger and particularly heavy riders with be TOP HEAVY and therefore much more unstable in any given situation.
I think it’s the other way about: tall riders are more stable. Bicycles must be balanced, unlike cars, and we all know it’s easier to balance a broomstick than a pencil on a fingertip. Riders with a lower centre of mass must make more frequent corrections.
Wouldn't the test have to take into account the fact that the tyre is rotating? A stationary tyre on an angled plate might not give the same results.
Why, though? My guess is it doesn’t matter whether the tyre is spinning or not. Similarly, braking and cornering grip amount to the same thing.
I'd say that the risk of locking a brake and skidding when going straight is largely down to the user and not the tyre.
My Michelin Pro4 Grip tyres are greatly less likely to skid than my Schwalbe One tyres when I apply the rear brake in the wet, so the tyre does matter. Of course, slipping when cornering is usually far more hazardous than the rear wheel locking up (which is usually innocuous and sometimes deliberate).