Factors effecting stopping distance:
As far as stopping distance requirements:
It may have changed, but when I took the CDL test, the official view was an
empty truck would take longer to stop.
He was going downhill so you have the sign wrong, it should be f-G for
this case. That's not a big difference and not your major error. Your
major error is thinking that truck tires and truck brakes will come
close to the maximum available surface friction of the pavement. For
estimating purposes a more realistic effective coefficient of friction
would be 0.50. Crunching those numbers gives you around 230 feet.
On Fri, 11 Jul 2014 19:30:48 -0500, Unquestionably Confused
In the real world it would not be immaterial. The empty truck with
it's 120psi tires and stiff springs would most likely "bounce" around
if the wheels locked which would greatly increase the stopping
distance. OTOH, a fully loaded truck might not even be able to come
close to locking all the tires or even doing a good job of "braking"
them so it too could have long stopping distances. But it extremely
unlikely the stopping distances would be the same, in the real world.
An empty truck will take longer to stop than a properly loded truck -
which will stop faster than an overloaded truck.
The load rating of a truck takes into account the tire load rating,
the axle load rating,the spring load rating, the braking capacity, and
the horsepower and gearing of the prime mover. If there is no Or not
enough) weight on the braking wheels, the truck will slide.
If there is enough weight on the braking wheels, the tires will hold.
If the tires hold more than the brakes, the brakes will limit the
If the brakes hold more than the tires, the tires limit the stopping
If the brakes and tires are adequate to stop the load, you stop in
time. If they are not adequate to stop the load, you do not stop in
Required Brake horsepower on a grade is determined by the distance
travelled in feet times the weight being stopped (ft lbs) devided by
the time required to stop it devided by 33000.
1 HP is 33000 ft lbs per minute.
So to stop the descent of a 40,000lb truck on a 100 foot high hill
hill at 60 MPH in 1320 feet (1/4 mile) over stopping the truck on the
level, requires the addition of approxemately 757 HP of braking force.
The same amount of extra power it would require to acclerate the same
load from 0 to 60 up a 100 ft hill over a quarter mile, over just
accellerating it on the level.
Double the weight, double the required horsepower. Double the speed,
double the horsepower. Double the incline, double the horsepower.
Just accellerating, (or stopping) that 40,000 lb truck on the level
requiers 475 HP 0-60 or 60-0 in a quarter mile in 25 secconds.
Increasing the load to 60000 lbs raises the power required to 715HP
Increasing the load to 80000lb requires 950 HP.
Lets stop a 40000 lb truck from 60MPH to a dead stop on a 100 ft high
1/4 mile long hill. The brakes will have to dissipate 1232 HP.
Double the weight of the truck and the brakes are required to
dissipate 2464 HP.Double the incline and the 80000 lb truck requires
Now, not only do we need enough brake to absorb that much Horsepower,
we need enough tire to put that 2464 HP to the ground without breaking
traction with the road surface. The number of tires on braking axles
most definitely comes into play here. (as does the number and capacity
of wheel brakes available) - because as stated before, stopping
distance is limitted by the lesser of brake power and tire traction.
In the case referenced by the OP, the tires ran out before the brakes
as the trailers slid and jacknifed - and if the tired HAD been able to
hold all of the braking force, the brakes most likely would still not
have been adequate to stop the loaded truck in the time/space allowed
from the speed he was going.
Reducing the speed coming into the situation reduces the required
braking power for both the decelleration of the load and the control
of descent of the load much more than the weight by both reducing the
feet per second AND increasing the time/distance available to stop.
Increase the load by 10%, reduce speed by 5% - more or less for the
same stopping distance.
I likely missed a few factors that will skew the numbers slightly -
but theyw are close and show the "trend"
Weight and tires/brakes as well as speed all have a BIG effect on
Ashton Crusher wrote, on Fri, 11 Jul 2014 22:43:24 -0700:
The first article in the OP says that the driver, Ravinderbal Singh, said
he jammed on the brakes, which began to smoke, but did little to slow him down.
He's quoted as saying "It wasn't decreasing speed. It kept going up 'cause it was,
like, too steep for me,".
Does that indicate the tires or the brakes were the limiting factor in his
inability to stop until he had crushed the 10 cars in front of him?
The actual limiting factor was gravity.
The smoking brakes means that he had been using the brakes coming down
the grade, they started to heat up (which is what they do, convert the
kinetic energy of rotary motion into heat energy through friction) Want
to demonstrate this, take DRY hands and rub them together rapidly, what
happens? They heat up.
Now in this case the brakes were already heated from the earlier usage,
When he "jammed" on the brakes the heated shoe surfaces started to
outgas. This is common and actually normal to an extent. Basically the
binder material that glues the friction material together gets heated
and small amounts of gas/smoke are released. After the first few NORMAL
stops heat the brakes up the process slows because the heat levels never
rise much more.
In a panic stop the brakes create far above normal friction, the heat
rises VERY rapidly and the shoes try to absorb the heat and try to
release the heat into the air. The drums also get VERY hot and they try
to dump the heat into the air as well.
While this is happening the gases from the brake shoes actually act like
lubricant, that destroys the friction coefficient. The remaining
friction keeps heating things up in a vicious cycle.
What the CHP will do is go over the drivers records. Then the trucks
records, including ALL the paperwork. They will look at the weight
ratings of the components as well.
Those ratings will be a key item. Every vehicle on the road has brakes
engineered to stop a given weight. Basically they take the GVWR and
calculate the amount of braking force that will be able to stop that
weight in a given distance at a given speed while still maintaining
control of the vehicle. Then they add in a safety margin to cover some
misuse/abuse and unforeseen circumstances.
Most big trucks have brakes far larger than they actually need, BUT it
is also possible that they don't.
On Fri, 11 Jul 2014 18:53:13 -0700, Evan Platt wrote:
I believe I had asked if the skirting was required by law.
As I had not seen skirting like this on trucks while I was an active
I may have. But then also said something else to clarify and you never
If you have a CCW in state A, you can carry that firearm all you want.
Cross the line into state B, who does not issue CCW, and you could go to
You can carry a firearm in a ttuck as long as it is properly locked and
stored and not within arm's reach.
But if you work for a company and the company owns the truck, when they
find that firearm, they can and will fire you if they do not allow
You're the one who is a frickin laughing joke.
Claiming to be an expert on any topic.
On Fri, 11 Jul 2014 21:54:09 -0500, deadrat wrote:
Actually that was Jeremy Clarkson on "Top Gear".
Two different things.
When on the moon, Neil Armstrong dropped a bowling ball and a feather at
the same time. Both landed at the same time.
That works in a vacuum.
And the real experiment was done using a ramp.
With various sizes and weights of balls.
I think you're confusing theory and practice.
Brakes can overheat, slip, or otherwise fail under enough stress.
I once saw a freight train derail. It was amazing how far the
fairly slow moving train moved while off the tracks and wheels
sliding along the rock train bed. Momentum is a serious force.
I recall from freshman physics that the mass normal to the friction surface
and cancels out the kinetic energy. Of course, they professor also said the
coefficient of friction couldn't exceed 1.0. A new explanation was needed
when the fuelie dragsters started posting E.T.'s and terminal speeds that
On Saturday, July 12, 2014 10:40:10 AM UTC-4, rbowman wrote:
I think the issue here is that the physics are based on certain
assumptions and we don't know how well that conforms to what happens
with a truck in the real world. The friction force is what's key.
We know that the energy of the truck goes up proportional to the mass.
The physics of the friction says that it too goes up proportional to
the mass, but there must be some set of assumptions along with that.
The question is how closely the truck tires fit those assumptions.
What works for friction of a piece of sandpaper going across concrete
with a 5lb or 10lb weight could be different than what happens with
a hot truck tire.
Generally a Prius that somehow overlooks a 65' long, 13' high 78,000 vehicle
with a bright red paintjob and pulls out in front of you. There's also the
BMW Z3 piloted by a Master of the Universe that really thinks he's going to
win a game of chicken with a Volvo White.
The driver trainer at the company I worked for explained that the company
policy forbade firearms in the trucks but common sense suggested a 9mm close
at hand was a good idea. A couple of times I had to go to Canada and left my
firearm with the terminal manager. Nothing was said.
Subject: trailer side skirting ???
Date: Sun, 3 Oct 2010 14:58:03 -0700
"What's up with trailers having plastic or canvas skirting between the
wheels? I've been seeing more and more of them with it."
Maybe because you don't pay attention?
No, others clarified.
ROTFLOL.. How's that dome home coming?
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