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Re: Truck stopping distances?



Beating the dead horse :) , one must be careful in taking the analysis
too far if we're really trying to look at the real world here. When a
truck driver is faced with stopping a truck, especially if it is to be
done quickly, (not a panic stop, in which all beats are off), he doesn't
just apply the brakes and slow down. He also doesn't pump the brakes,
not because they're anti-lock, but because they're air-brakes. If you
pump air-brakes, especially on a downhill run, you're dead. But the
important factor being missed here if we're trying to look at what
*really* happens, the driver will downshift the transmission, forcing
the wheels to turn the motor at a higher rate than an idol, which
expends energy but not through the brakes. Another brake saver is a
noisy device called a "jake brake" which allows a diesel truck driver to
stop the exhaust valves from opening. This uses the forward motion of
the truck (through the tire/road contact, so it's still friction
dependent) to make the engine work against itself, expending energy
without using the brakes. The fact that it cuts down on brake
replacement is it's whole purpose. Jake brakes are very noisy (they
make a popping sound) and it is generally illegal to "pull" one within
city limits.

It is interesting that rapid downshifting of a truck transmission is to
be avoided, but when you have to it takes skill to do it correctly.
Left foot works the clutch, the right foot heel on the brakes, the right
foot toe on the gas to rev the engine between shifts, right hand
changing gears, and you steer with the left hand.

andy graham

Scott Goelzer wrote:

I thought about this power-temperature issue with the brakes before starting
this thread. I did not think about the scaling of brake surface area
seriously until D.Bowman's post. Was too caught up in the mass of the
brakes...

I tried to make some estimates. I still cannot make the numbers work.

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

*****************************
S.Goelzer
Physics Teacher
Coe - Brown Northwood Academy
Northwood NH 03261
603-942-5531 ext43
sgoelzer@coebrownacademy.com
*****************************