On 30/12/17 09:51, Rob Morley wrote:
On Fri, 29 Dec 2017 13:56:23 -0800 (PST) Simon Jester
wrote:

Road damage is 4th power of axle loading. A 1000kg car causes 100
000 times as much damage as a 10kg primary vehicle. If primary
vehicle users pay 1p per mile, moronists must pay £10 per mile.
Sounds fair to me.

ITYF it's gross axle weight that matters. I'm not sure how the
calculation varies with pressure at the road surface - me on a bike
with 20mm tyres at 140PSI probably exerts a higher pressure than a
small car. Of course it seems the calculation ignores the existence
of single track vehicles anyway, otherwise they'd be considering
wheel loading rather than axle loading. I'm pretty sure that they
should be considering pressure along with weight though -

It's not a matter of considering bicycles and motor vehicles as objects
delicately placed on the road surface and then left alone - tyre
pressures are utterly insignificant compared to the static load bearing
capacity of road materials. Vehicles are moving along, producing cyclic
effects deep under the surface and it is this that really matters.

From observation, I suggest the road surface (I assume we are talking
about the finishing layer of gravel bound by tarmac or asphalt) usually
only seems to fail when it becomes unstuck or the structure underneath
has failed; it also seems to me that surface erosion is mostly due to
tyre scrub.

a LGP vehicle can do less damage to delicate heathland than an
inconsiderately ridden mountain bike, and I imagine that similarly
applies to road structures.

Depends on whether the damage you are considering is just visual damage,
at the surface, or down amongst the roots. On a muddy track, it is usual
for a motor vehicle to produce deeper/wider ruts than a mountain bike.
On a dry, dusty track, it is usual for a motor vehicle to throw up
bigger clouds of dust than a bike (with any kind of tyre).