Tuesday, October 18, 2011

Auto Antics: Snow driving mythology

There is no magic to driving in the snow.  How can I make such a claim? 
It is generally accepted that driving in control involves maintaining something that we call “traction.”  Any number of rituals are employed to placate the traction gods and to keep them from taking this 'traction-chi-energy-juice' away from us.  One such ritual involves putting the vehicle into a low gear and using the compression of the engine to slow it down while driving downhill. 

To be read aloud:

CURLY DRIBBLEDRIVER:  "HARGGHhmb, kid, this is better than brakes.  The engine is slowing us down, GUHHBbbl.   It’s different.
KID:  “Oh.  How is it different?”
CURLY DRIBBLEDRIVER:  “HMMBGhhaaahh…complicated, kid...engine...traction.  You can’t do it with the brakes, kid.  Don't even try.”

The Calm Canary (KID) has alarm bells that go off at this point and you can’t guess the rest. 

Here’s the rest:
Unfortunately, the driver in the selection above has not given her assertion any conscious thought and thus is unaware that there is, in principle, no difference between slowing down with the brakes of a car or with the engine.  No laws of physics have changed.  The force of static friction is what does the work.  I think we all do hope that in the case of moving vehicles the only surfaces in contact with the road are four rubber tire treads.  This does not change when the engine is used to slow the vehicle instead of the brakes.  There are no new contact points established between the vehicle and the road by the sheer merit, virtue, and honor of using the engine to slow the car, and the surface area of the tires is certainly not extended or improved. 
However, what has just been debunked as a theoretical myth can strangely be a practical boon.  Using the compression of the engine can be very handy because of one mysterious wrench: operator error.  (Again, we see here an example of misconceptions due to inborn errors of physics education.)  The ‘smoothness’ of a gear is what actually and truly makes it useful.  Without it the tires would be dependent on the ‘smoothness’ of the foot of the operator on the brakes, which is not as reliable as a mechanical device.  So, the ability to slow the tires down slowly – that is, not in a herky-jerky fashion – is what makes the compression of the engine so special.  The tire faces in contact with the road do not decelerate so quickly as they might if a human were responsible for their course.  (The heel of modern man strikes hard.)
Of course, there is the problem of shifting.
And a driver can quite easily outpace his or her benefit (from using a low gear to go down a hill) by braking too hard or starting out too swiftly and boldly. 
All well and good, you say, but where’s the great revelation?
Here it is:
We clever monkeys know from coil-spring and woodblock experiments that the force due to static friction for an object on a given surface is always substantially greater than the force due to kinetic (sliding) friction for the same object and surface.  So it seems best to keep from skidding, yes?
That’s the secret/the only object of snow driving, my fellows: don’t slide!

No comments: