Re: Truck stopping distances?

[David Bowman <dbowman@TIGER.GEORGETOWNCOLLEGE.EDU>]

Regarding Scott G.'s conundrum:
> ...
> I worked out that large (80,000 lb) truck has roughly 4 to 5 times the
> braking surface area as a car - discussion with several of my students who
> do repairs gave me the info. The truck needs to transfer 20 to 40 times more
> KE than a car in stopping. Graphing some data from the web yields that truck
> to car stopping distances are 1.8 on average.
>
> This does not seem to add up. It would seem like a truck must have about 10
> times the braking surface of a car to stop in 1.8 times the distance.
>
> Where am I wrong?

Scott, don't forget that an 80000 lb truck probably has a lot more
wheels, and hence a lot more brakes per vehicle than a car does.  The
truck has about 4.5 times as many energy dissipating brake surfaces
as the car does.  If both the truck and car have equally loaded
brakes on all their wheels this means that the truck may only need to
have its brake surfaces designed to be able to withstand about twice
the temperature jump that occurs in a car--and this is even assuming
that the truck brakes are not appropriately proportionally oversized
compared to those on a car.  Also, considering the crudeness of the
scaling argument, a factor of 2 discrepancy is not all that bad.

David Bowman