From: rgolen@UMASSD.EDU
Subject: Learn to properly use your brakes (LOOOONG)
Reply-To: rgolen@UMASSD.EDU
Organization: UMASS DARTMOUTH, NO. DARTMOUTH, MA.
Date: Sun, 10 Jan 1993 01:42:32 GMT

I don't mean to flame anyone out there, but after reading some of
the comments regarding downshifting to supplement braking, I have
to toss in some comments. As I said in my first posting on this,
downshifting as an aid to braking was done at a time when brakes
were not all that efficient. The motor was used to supplement the
brakes. This is really not necessary today, given disk brakes,
vented rotors and the different brake compounds available.

Brakes apply negative acceleration. If you were able to measure
the force applied by your brakes, then you would find that they
apply a force equal to 5 to 6 times the horsepower of the car
they were designed for. Any car designer, or builder worth
his/her salt knows that before putting in a high performance
motor, you first must build the suspension (shocks, springs,
brakes, and tires) to handle the power. The last ingredient to be
added is engine power. For those of you who have built up your
motors without doing anything about your brakes, suspension,
etc., its obvious you are in over your heads.

A couple of comments have cropped up regarding the value of one's
life vs. the cost of engine/transmission/clutch/brake components,
and that the authors of these comments want the extra "safety
margin" of downshifting to augment their brakes. To those
individuals I say learn to use your brakes properly. Where do I
get off on saying this? Ok, I've built rally cars, autocross cars
and a couple of road race cars. I also compete in autocross and
rally, and have spent some time at various "driving " schools, so
I have a fair idea of performance driving.

No I'm not a Mario Andretti, or a Nigel Mansell. But I know how
to drive a car to its limits. When a car is under braking, the
tires are transmitting adhesion to the ground. As long as the
tires are rolling, no matter how slow, you are decelerating under
control. In fact there is a point of maximum braking efficiency
called threshold braking where the tires actually
squeal a bit, but the wheels are not locked up. If you have a
feel for your car, you can reach this point, and actually get
feedback through your brake pedal to tell you whether or not your
wheels are in fact locking up. When you slow down, you are
applying steady increasing pressure. The faster you wish to slow
down, the more pressure you apply, until you reach the threshold
limit, once you have passed that limit, you lock up the tires,
you loose adhesion, and you skid.

Downshifting under the right circumstances can also cause a skid.
By quickly downshifting/dumping the clutch, or downshifting to
too low a gear, you can cause the wheels to break adhesion, and
the car skids. 

So why do racing driver's downshift? When you are coming to the
end of a long straight, and approaching a curve, you need to
enter that curve at a speed which allows the car to enter/exit
that curve at the maximum possible controllable speed. Brakes are
applied in the straight, with ever increasing pressure. Once the
car has slowed down enough then the car is downshifted. The
downshifting is not used to slow the car down, rather it is used
to bring the power/revs/etc. up to a point where the car can use
its engine to accelerate out of a turn. At this point the brakes
are feathered off and the car begins to accelerate by letting out
the clutch and giving it the gas. You actually accelerate through
the turn and out. 

Why accelerate out? Because brakes work best when the tires are
faced in the direction the vehicle is traveling. This concept has
to do with something called the friction circle which basically
says that a tire can use 100 % of its grip either accelerating or
decelerating in a straight line, or use it in a turn left or
right. If you are using 100% of your braking energy and trying to
turn at the same time, the car will go straight. But if you
feather off of the brakes to say 80% of the tire's grip, you can
apply that additional 20% of the grip to turn the car. A tire can
only give you 100% of its grip, no more. You pass 100% grip, you
immediately go to 0 grip, and no control.

On to the argument that there is no harm to the engine/drivetrain
by downshifting to augment braking. If you believe this, I have
some waterfront property in Florida I want to sell you. The
engine/drivetrain is designed to transmit energy from the motor
thru the transmission and out to the wheels. When you downshift,
you are transmitting power in the reverse..from the wheels to the
transmission to the engine. You are applying forces in a
direction not generally intended by the designers of the car. 

Furthermore as you downshift, the engine is not under load, and
this is harmful to the engine. How you ask? Try this experiment.
Park your car, put it in neutral, then run the engine at, say
5,000 or 6,000 RPM. Hold the pedal down and stay at that speed.
Guess how long your engine will last. This demonstration, albeit
a bit extreme, is similar to what happens to your engine when you
downshift. The engine is not under load, yet is turning at a
rapid rate.

Another stress area for the engine is the thrust bearing. This is
the last crank bearing up against the flywheel. Excessive burning
out and downshifting cause the flywheel to pound up against this
bearing. On an aircooled engine this can cause crank & bearing
failure as well as possibly roaching the crankcase. On a
watercooled engine, you roach the crank and the bearing.

I'm sorry for being so long winded, and if I have offended
anyone, I apologize, for it was not my intent. But if you feel
your brakes are not up to par there are two major reasons:
1. They system itself is not up to snuff, and therefore needs
some work;

or 

2. You, the driver, need to learn to properly, and efficiently
learn to use your brakes.
Newsgroups: rec.autos.vw