Re: IML: 1966 Imperial checkin & brakes

["Frederick Joslin" <fljoslin@xxxxxxxx>]

I'll give it a shot.

You refer to static and kinetic friction. Static friction is what you get when no sliding is occurring which is what you want to have happen at the tire-road interface. Kinetic friction is the friction that you get when there is sliding occurring which is what you want to have happening at the pad or shoe to rotor or drum interface.

The following information is from this link: http://physics.clarku.edu/courses/110labs/Lab4.pdf

Theory: We encounter friction at almost all times during the day. Friction between our foot and the floor helps us walk. In spite of its importance, friction is still not well understood. However, empirical laws describe the friction between two surfaces. These laws are as follows:

1.       The ratio of the maximum frictional force and the normal force is a constant and equals the coefficient of friction, u, and depends only on the nature of the two surfaces in contact. i.e.:

u>= (Frictional Force) / (Normal Force).

2.       The coefficient of friction is independent of the area of contact.

3.       The coefficient of kinetic friction uk (the object is in motion) is lower than the coefficient of static friction us(the object is stationary.)

If we assume that the tires do not change, any difference in breaking must arise from changes in the pad/shoe and drum/rotor interaction. Assuming the same materials for these and the fact that the friction is independent of the area of contact (see #2 above), it looks to me as if greater pressure is the only variable. Thus if disc brake systems can exert more force between the two surfaces than drum brake systems there will be more friction.

The real trick is to maximize the kinetic friction at the brakes without getting into the static friction mode because that will then lock up the wheels and put the tires into the kinetic friction mode (sliding) which provides less friction that in the static mode for stopping the vehicle.

I personally feel that part of the issue is that disc brakes really are self-adjusting and work optimally all of the time where as drum brakes do not.  

----- Original Message -----
From: "Dick Benjamin"
To: mailing-list@xxxxxxxxxxxxxxxx
Subject: IML: 1966 Imperial checkin & brakes
Date: Wed, 22 Feb 2006 09:41:37 -0800


Interesting ideas, Dan.

But, somebody educate me here, please!

Way back in 1952, when I was wet behind the ears, I went off to a big
building in Cambridge MA, where they fill skulls full of mush with facts and
theories. One of the subjects I had to study was physics. I must have
passed it, because they let me out of the place, and even gave me a piece of
parchment to show I'd been there.

In physics we learned that stopping a wheeled vehicle required only that the
wheels be prevented from turning, and that the wheels grip the road surface
well enough to avoid slipping. All that was required was that the brakes be
able to lock up the wheels, and that the tires have an adequate coefficient
of friction.

The optimum stopping distance requires that the brakes do not quite lock up
the wheels, as the rolling coefficient of friction is higher than the
sliding coefficient of friction. A very skilled driver knows just how hard
to push the pedal to stop in the shortest distance. Most of us, myself
included, only jam down the pedal, and hope that the coefficient of sliding
friction is good enough. (Modern ABS brakes attempt to duplicate the very
skilled driver by moderating the braking force to prevent wheel lockup.)

If the tires won't grip the road well enough, something is wrong with the
tires. Better tires are worth buying, for that reason alone. (I recommend
the Tire Rack stores web site for information on what tires stop better). I
just buy Michelin tires.

Coefficient of friction gives the relationship between the weight of the car
and the deceleration force. In other words, the weight of the car falls out
of the equation, meaning a heavy car will stop just as short as a light car,
ignoring some minor factors.

OK, the minor factors are: In a heavier car, more heat will be generated at
the tire/road interface, and if the contact patch isn't larger, the tires
will overheat quicker on the heavier car. A mitigating fact is that since
almost all passenger cars use the same tire pressure (30 to 35 PSI), the
contact patch also automatically adjusts for the car's weight, thus reducing
or eliminating the heat difference. If this doesn't sound right to you,
visualize what happens when you add weight to a wheel - the tire flattens
out so the contact patch grows - what sets the relationship? Right, the
pressure in the tire. A tire with 30 PSI inside it has a weight supporting
ability given by the contact patch size times 30 PSI. In our cars, where
the weight on the front wheels gets up to around 1600 pounds per wheel
(Guessing here) during a fast stop, the contact patch grows to 53 square
inches. In the little modern car, which let say weighs half as much, the
contact patch will grow to 27 square inches, or half that size, so the heat
per square inch applied to the rubber is exactly the same.

If a car is driven with brakes that cannot lock up the wheels, something is
wrong with it, or the driver is too weak to press the pedal hard enough. If
all the wheels don't lock up at the same pressure on the pedal, something is
wrong with the balance front to back or side to side, but the car will still
stop, although not straight in its lane.


Since all Imperials built since 1951 have power brakes, I doubt anyone has
trouble pushing the pedal hard enough on a car that is working right.

This all brings me to the conclusion, that (except for cars in bad
condition, repeated stop situations, or looooong downgrades where the driver
doesn't downshift, thus causing the drums to overheat and expand, also
lowering the linings coefficient of friction to the drum due to
overheating,) there is no significant difference in stopping ability between
disc brakes and drum brakes.

I have a feeling I'm missing something here - so please take me seriously
and tell me where I'm wrong.

Thanks,

Dick Benjamin
-----Original Message-----
From: mailing-list-owner@xxxxxxxxxxxxxxxx
[mailto:mailing-list-owner@xxxxxxxxxxxxxxxx] On Behalf Of
dansgarage@xxxxxxxxxxxx

Hi All.....I know I know, "to convert or not to convert" has been kicked
around LOTS on this list but I have had a number of close calls, regarding
braking distances, when on the interstates here in southern California. As
anyone who travels these Interstates knows its almost impossible to leave a
'safe' stopping distance beween your (Imperial) vehicle and the ones ahead
of you since either 3 people try and move into that space or you find out
real quick that the other (newer) cars can stop in MUCH shorter distances
than you can! I have heard many discussions about changing to front disc's
and whether or not its actually going to make a big enough difference to try
it.



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Fred Joslin