The Bullet and the Foot, Nov 17, 2009

Quote of the Day
Q:	What do you get when you cross a mobster with an international standard?
A:	You get someone who makes you an offer that you can't understand!
Oh how the time goes by...
Why it seems like only last week I was talking about delays getting new pages posted... (foot shuffle, foot shuffle.) I think filling out a time sheet with 114 hours in the past 2 weeks might have had something to do with it.

Well let's not dwell on gaps in postings, lets get back to zen and the art of aggressive driving!

The Zen of Aggressive Driving, Chapter 2: Tempo
In our last session, I described how to pick the best speed for your car. Today, how to pick the best speed for the road.

Every road has a natural speed. This speed changes throughout the day. It varies by the volume of traffic on it, the weather, traffic lights, the presence of bends and bumps, and local conditions like speed trap on the shoulder, or an accident in the left lane.

Roads are as alive as a river. With rapids, and slack, broad expanses, and narrow channels. Deep parts, and shallow parts. They even have tides!

Thus, anyone who selects a speed without taking into consideration the condition of the road is misguided.

But first, why is it important to be aware of natural speed? What is the advantage to coasting at 15mph through a traffic jam vs. ramping up to 60 to come to a screeching halt?

From a pure speed perspective, there is nothing slower than an accident. Your own. Getting into a fender bender is worth at least an hour on the side of the road, phoning out to your insurance and the other guy's insurance, waiting for a cop to make a report, etc., etc. And if you are having a bad day, the police may decide to up the ante on misery if they feel you were driving too fast for conditions, or cite you for failing to control your vehicle. (As happened to the guy who rear-ended me on I-95.)

It doesn't matter that the car in front of you went from 40mph to zero in the blink of an eye.

But beyond the obvious expense mitigation of avoiding accidents, there are genuine savings in fuel economy and wear and tear on your vehicle. Here's a little bit of physics:

Remember from Physics I,
(Newton's Law):
FforceEnergy that comes from the engine, or that is bled off from the brakes
mmassThe amount of matter that makes up you, your car, and the golf clubs in the trunk
aAccelerationA change in your car's speed, either faster or slower

There are no magic fairies. The energy in the system is like a bank account, and you deposit energy in with the gas pedal, and withdraw it using the brake.

Positive "force" comes from the engine, which converts chemical energy stored in you gas tank into mechanical energy that moves your car. It's a slight simplification of the real physics involved, but whenever you are talking about large values of force, you are also talking about large volumes of fuel.

Negative "force" comes from your brakes. They take mechanical energy and convert it to heat. In removing mechanical energy from the system, they slow you down. But that heat wears away your brakes. Large negative values of force involve shorter brake life.

A handy form of this equation:

Kinetic Energy
KKinetic EnergyThe amount of energy in the system that is applied to "motion"
mmassThe amount of matter that makes up you, your car, and the golf clubs in the trunk
vvelocityThe speed at which the car is traveling

So let's say you, your car, and your golf clubs weigh 1800 kilograms. (That's about 4000 pounds.) No matter how fast you accelerate your car, it will take a minimum of X amount of energy to get to that speed, and the same amount of energy (in the form of braking force) to slow you down from that speed to a stand still. To get to 25 miles per hour requires 113 kilojoules. To get to 35 requires 222 kilojoules.

If you were to graph what the formula outputs, you'd see that to double your speed requires four times the energy. . Every kilojoule of energy is a drop of gasoline or a penny towards your next brake job.

So running out of time for today. I hope you are getting the idea that it takes energy to change speed. Tomorrow I'll get into tricks to avoid changing speed in traffic. TTFN

All content copyright 2017, Sean Woods | email: yoda@etoyoc.com | phone: 703-342-2662