Dennis wrote:
>Well, there's where the model falls on it's face.
Uhh, no. Take a look at the skidpad figures achieved by various
BMW, Mercedes, Jaguar, Lexus, etc. boats in magazine skidpad
tests and compare them with the same figures for much smaller
cars. You might just be shocked to see how well the barges do.
>While it's true that a tire generates more grip with increased
vertical load,
>that relationship is decidedly non-linear - the rate at which
grip increases
>with load diminishes with load, and the rate that rate changes
is very much a
>function of individual tire construction.
See my comments re the optimal load range for a tire. Within that
range, its Cf doesn't vary by nearly as much as you seem to
believe.
>So while it's true that Car B (assuming the same tires on both
cars) must have
>greater grip as a function of its increased vertical load (by
virtue of being
>physically heavier), one cannot assume the linear relationship
to hold.
It does, however, provide a far more accurate picture than the
model you proposed.
>However, it appears then that if the only difference in f is the
difference
>caused by the non-linear increase in grip with increased
vertical load, then the
>only way to determine how much Car B must slow down is to get
the "grip vs
>vertical load" graph from the tire manufacturer - that they
don't give out.
That isn't enough. You'll also have to know CG location pretty
accurately, so as to accurately model the vertical load on each
tire and add up the contributions each one makes to the total
available grip.
>According to all this, if I drop
>500lbs of lead in the car, and get enough extra power out of the
engine to keep
>power/weight constant, then my performance should be unchanged.
No. Nobody's said that. You've been talking about steady-state
cornering only. Adding 500 lbs to a car may have little or no
effect on its steady state cornering capability, depending on
where the weight is added, the car's suspension behavior, and the
design of the tires. There are plenty of other reasons not to
want to add weight to a car, though.
>Maybe it's not steady-state I should be looking at - maybe it's
the
>transitions....
Now you're getting somewhere.
Jay
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