Tony Raven said:wafflycat said:
Quoted message said:
The back roads *are* slippy, and some ditches with the obligatory 4x4
off-roader type vehicle in them as driver totally overestimates
his/her skill, thinking, "I'm in 4-wheel drive so I can still drive
fast on icy roads and take the corner really fast! Oops..."
As I was taught it, 4WD may give you more grip for forwards and
backwards, but sideways you've still got just the same four tyres.
Thats a workable first approximation - there is, however, a gain to be
had with 4 wheel drive in cornering if you need to simultaneously apply
acceleration, as you can apply more power before you lose control.
Causing any wheel to lose traction moves it from static to dynamic
friction, and that effect can cause you to lose lateral grip at that
wheel aswell. It is only relevant to the difference between 2wd and 4wd
if you have enough power to break traction in the conditions you are
driving in, of course, but it is a significant factor in why 4wd rally
cars are generally faster than their 2wd couterparts. So, its not giving
you more grip, but allowing you to use more acceleration in a corner
before you run out of the grip you do have. You wouldn't need a rally
car to be able to demonstrate this difference on ice.
As I was taught it, you have a finite amount of grip per tyre, and once
the vector of acceleration/braking forces and cornering forces exceeds
the available static friction in that direction, that tyre is skidding,
and its contribution to overall grip will drop until the external
parameters change to bring it back to the static friction model. By this
time you may require the services of a doctor.
--
JimP
--
"We don't have a plan, so nothing can go wrong" - Spike Milligan
