Do trucks really need more space to stop?

Question

It is generally accepted that a heavy truck can't stop in the same distance that a car can stop. But from the perspective of basic physics, I don't see why this should be true.

If the truck's mass is 30 times that of a car, then its tires are pressed against the road surface with 30 times as much force.
And that force times the coefficient of friction gives the maximum amount of stopping force that can be applied, which is also 30 times as great as the maximum stopping force of the car.
And by applying 30 times as much stopping force to a mass 30 times as great, the truck's maximum deceleration should be exactly equal to the car's deceleration.
So why do trucks (supposedly) need more space to stop?
Is it that they can't achieve this theoretical maximum rate of deceleration? (They do, after all, have more moving parts?)
Or is it that they are harder to control at maximum deceleration, so drivers are reluctant to do a maximum stop?
Or is it just an old wives' tale to make us cautious?

squeezie and procrastination have added useful information to the discussion.
squeezie makes the point that braking power is not just a matter of coefficient of friction between tire and road, multiplied by the weight on the tire. It is also a matter of what the vehicle's brakes can do.
Now for the first stop, it should just be a matter of whether the brake is powerful enough to make the tire skid (or come very close to skidding). But after the brake linings heat up, it's also a matter of whether they can dissipate that heat and prepare for the next stop. (And I notice that the table which procrastination references shows stopping distances with cold brakes and with hot brakes (the latter being longer).
Thanks for your input.

Thanks to Freight Train for the link to Ohio Public Safety.
Just so there's no confusion, this question is not about whether we're being too careful around trucks (statistics say we're not being careful enough), or about whether truckdrivers are not doing their job (to the contrary, they're highly skilled and, in almost all cases, very considerate.
I just wanted to understand the physics of why it should take them a greater distance to stop, and was hoping that an automotive engineer or a physicist could write some formulas that would explain it.
I think that some of the responders did not understand the reasoning in the question. But I think it will be clear enough to the engineer/physicist who has the definitive answer. Meantime, thanks to all for your answers, and special thanks to those mentioned above.

Answer 1

The other post is correct that a truck with greater mass than a car moving at the same speed as a car has more kinetic energy. It is also correct that a truck can possible generate greater friction over it's 18 wheels than a car with it's four wheel.

However, I didn't see anyone else mention braking power, at least not clearly. A car with four wheel disc brakes and power-assist is able to apply more braking power to the wheels. Also, the four wheels with their relative positions provide better braking control, especially with ABS.

A truck by comparison, uses much larger brakes, but they are not 30 times (just using than number because it has been discussed) larger than a car's. That would not be practical due to size and the amount of hydraulic force required. Even more important is that a truck, with it's longer length and articulated trailer, is not as stable during braking. ABS is being used to improve this, but it can still be a hairy ride under full braking power.

As an aside note, trucks generally use drum brakes. These provide better economics that they last for more miles drive. However, they do not provide the same amount of braking power as an equivalent disc brake would. Older cars had drum brakes on all the wheels and did not stop as good as a car today. Some cars had disc brakes in the front and drum in the back for improved braking. Many modern cars use disc brakes all the way around.

And finally, up until the 1970s heavy duty trucks were not required to have brakes on the front axle. It was believed (and often happened) that brakes on the front axle would cause the truck to slide out of control. In reality these were far less likely to stop as quickly as a truck equipped with them.

As both an engineer and someone with truck driving experience (no not a pickup either) a truck does require more distance to stop. Learning to drive a truck will radically change your driving habits. It also makes you learn to trust no one else on the road. Loaded the truck is heavier and puts more load on the brakes when decellerating. Unloaded the truck is lighter and handles different; you can easily loose control if you don't learn to drive each way and brake accordingly.

Oh, by the way, hot brakes provide far less braking power than cool brakes. That's why you don't want to ride the brake when going downhill, whether in a car or a tractor trailer. It can even boil the brake fluid which is VERY bad.

Answer 2

I am a truck driver and have been for 17 years. no it is not that we are reluctant to do a maximum application of our brakes it is that even though we have 18+ wheels on the ground the weight of our load has to do with how fast we can stop.
Because of the mass of the combined weight of load and truck It takes a fully loaded truck, traveling at 55 more than 400 feet to come to a complete stop. the average passenger car, traveling at 55 mph, 130-140 feet to stop; that is why we leave such a huge gap between us and the vehicle infront of us it all has to do with momentum and not the coefficient of friction. yes the friction plays a part but it is the momentum that is the key factor here.

for the most part Commercial drivers are just as concerned if not more than the average motorist about safety and on the average we put more miles behind the wheel in two days than most people do in a month. we as drivers are not out to get you we are here to safely share the road with you. But in todays society our driving skills are put to the test when the drivers of cars get in a hurry and cut to close in front of us. Speed up when we are trying to change lanes or other things that endanger the safety of all those around without thinking. If you see our turn signals slow down and let the truck merge then when safe to do so pass safely you wont save any more time cutting the truck off you will only jepordize your safety
by working together on the road we can make our highways and byways a safer place to travel.

P.S. if you see a truck following to close or being opperated in an unsafe manner get the company name truck and or trailer number color and best discription that you can and call 911 and the company to file a complaint I as a driver call on my brothers and sisters because I have NO Tolorance for unsafe truck drivers.

http://www.nozone.org/noZone/noZone.asp

http://www.publicsafety.ohio.gov/odps_publications/HSY%207777%20Trucking%20Safety%207-05.pdf

Answer 3

Good analysis, but your mistake is confusing the force holding the truck down on the ground (gravity), with the force needed to decelerate it. If a truck weighs 30 times more than a car, then it's kinetic energy is 30 times that of the car at the same speed. The brakes need to convert that to heat to stop it. But, the linings cannot be easily made 30 times the area of car linings, so there is less brake surface per unit mass available to stop it. Trucks also are not built with disk brakes, which can be made much more efficient than cylindrical ones and which can disapate heat better.

Answer 4

The trucks are pressed against the road harder than cars, but both cars and trucks (most of them anyway), still only have 4 wheels for traction. As both vehicles move, the momentum of the car is less than that of the truck. This causes the car to slow down easier than the truck, because the truck will have roughly 30 times the momentum than the car to battle against. So when the truck tries to slow down, with only a little more traction than the car had, it requires more space to allow for the traction of the tires to decrease the momentum.

Answer 5

It's the application of '30 times as much stopping force' that isn't achieved. Your speculations lead to your conclusions...but I don't see anything to cause me to believe that any of them are 'true'.

If your 30x mass is correct, a car is in contact with the road in 4 spots...a tractor trailer rig is in contact in 18 places...or about 4 times as many...so the 'force' on each tire from the mass of the vehicle isn't 30x...but more like 8x.

The application of stopping power is applied to the wheels of the vehicle to prevent rotation, not to the road surface. It is approximately the same level of force as in an auto...except that most cars today have 4 wheel braking systems applying breaking on two axles. The truck only has 5 axles, not 60 or even 16, so the application of stopping power is about 2.5 times.

Using the same logic you have, I contend that you get 2.5x stopping power across 8x points...giving you 20x stoppage against 30x math...so a truck should take at least half again as much space to stop....

note...there's no fact to back up my counter argument...just a logic chase...a dangerous way to do physics...

Answer 6

Think of the time it takes a truck to stop as being exponential. The more mass, the more it takes to stop. What doesn't help with the stopping time is with trains, each car has brakes, but because cars bouncing back and forth, it makes it extra hard for the train to stop.
The link below will hopefully help you.

Answer 7

I can't argue with your reasoning directly, but the following link shows some measured braking distances for cars versus large trucks (with truck distances approximately 25% longer).

Answer 8

It does have to do with inertia and the coefficient of friction: Remember, that friction increases with inertia but at a slower rate.

Run it through: assume the same coef of friction and double the mass, then see what happens.

~It's been a while (7 years) but I think I still got it :-)

Answer 9

Stopping is all about Kinetic Energy (KE).

KE = (mass)(velocity)^2

Thus, when you increase the mass, you increase the KE. Increased kinetic energy results in an increased stopping distance.

For example, if you double the mass, you double the KE, and therefore, you double the stopping distance.

Answer 10

momentum equation is all that comes to mind

p = mv
the more mass, the more momentum you're carrying.

of course i'm barely there when it comes to physics knowledge, im one of those people who likes it until they start to explain the electric motor, then i just zone out.

but momentum no? haha i should have started the sentence with "theoretically" that's impressive!