The Effect Of Vehicle Weight On Stopping Distance

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Brett Aquila's Comment
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Driving a tractor without the trailer attached (Bobtail) is very tricky and indeed does require extra care when driving. They bounce around like a hard rubber ball...

So the statement in the manual is highly accurate.

That's true. They also put a limiting valve in tractors to limit the braking force on the drums when you're not attached to a trailer. This is to help prevent the drive tires from locking up. Unfortunately, that also means it takes a lot more pressure on the brake pedal than it normally does to stop. So you ease on the brake and the tractor barely slows down. You press harder and harder until finally the tractor slows down.

When I was bobtailing, I would always remind myself that I would need more pressure on the pedal.

Bobtail:

"Bobtailing" means you are driving a tractor without a trailer attached.

Turtle's Comment
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I feel the manual is correct.

In a normal everyday situation, less pressure on the brake pedal is needed to stop an empty trailer vs a loaded trailer in the same distance. It's easy to see where one could think this equates to less weight equals less stopping distance.

In an emergency situation however, when hard maximum braking is required, I'd put my faith in the loaded trailer stopping me quicker than an empty one, due to the drastically increased traction.

Sure, momentum plays a role. But I just think the effectiveness of the suspension and braking system in a loaded truck will outweigh (pun intended) the force of momentum.

I just feel that, under hard braking, an empty trailer will either lock up the tires or kick in the antilock brakes. Both will increase the stopping distance.

Fortunately I've never had to test this theory while empty or loaded, so it's only gut feeling and opinion on my part.

Brett Aquila's Comment
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Fortunately I've never had to test this theory while empty or loaded, so it's only gut feeling and opinion on my part.

It's interesting that after years of driving, none of us can really say with 100% certainty that we've experienced this for ourselves. We all understand the variables that factor into stopping distance, but how that equates to the minimum stopping distance in the real world is hard to say.

I will say this.........I do not expect that the rule will apply under all circumstances. I suspect there are some circumstances that a fully loaded truck will stop faster and others where the empty truck will stop faster. Slick roads vs dry, uphill vs downhill, a road surface temperature of 120 degrees versus -10 degrees, etc.

Who knows? Someone must.

Keep in mind, also, that rarely do they build anything to perform its best at the maximum recommended usage. If the GVWR is 80,000 pounds, did they really build the entire suspension and braking system to perform at its peak at 80,000 lbs? It's possible, but I do not know.

Interesting conversation. I enjoy these.

GVWR:

Gross Vehicle Weight Rating

GVWR is the maximum operating weight of a vehicle as specified by the manufacturer, minus any trailers.

Turtle's Comment
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I do not expect that the rule will apply under all circumstances. I suspect there are some circumstances that a fully loaded truck will stop faster and others where the empty truck will stop faster. Slick roads vs dry, uphill vs downhill, a road surface temperature of 120 degrees versus -10 degrees, etc.

Agreed.

Hopefully none of us find ourselves on the wrong end of either theory, whatever the scenario.

Bird-One's Comment
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Empty trailers no doubt will skid but i think overall it still stops quicker. If I’m approaching a light empty, guy next to me loaded, I just grapple that he is having a easier time stopping. But than again all I haul is milk. There is definitely a surge in the reefer. Not like a tanker but it’s there. You could probably nitpick this all day.

Another thing I think of is what about If your going down a sharp decline and you have to hard brake? I think I would prefer empty in that scenario.

Reefer:

A refrigerated trailer.

Keith A.'s Comment
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Quality of road surface enters the conversation too: stick an empty trailer on a potholed driveway, then run the same one loaded -- my experience has been that you can keep more consistent pressure applied with the loaded trailer than the empty.

On brand new, dry, asphalt though, I'd take the empty trailer for stopping in a hurry.... I think that by sheer math the heavier you are the longer you take to stop, but after road conditions, driver comfort, driver attentiveness, and load weight there is only a general trend of more weight = longer time to stop.

DOUGLAS P.'s Comment
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I've never had an opportunity to do a maximum effort stop in a semi. I do a hard stop on average once every 30,000 to 50,000 miles. I don't tailgate, so I haven't needed to "lock em up". I've had an older empty trailer skip around a bit on a steep off ramp. My guess is a moderately loaded trailer with 25,000 - 30,000 lbs would stop the best. It also depends on road grade and conditions. I doubt I could stop as fast on a 6% downhill at 80,000 as I can at 60,000 lbs.

Unfortunately we don't really know how fast we can stop until we need to. It's not part of training. If you work for a major carrier you will have to explain every hard stop that you've made. If you lock em up you will likely have some cargo damage.

Errol V.'s Comment
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We can talk all we want about this stopping thing. Keep in mind, the difference in loaded & unloaded stopping distance means all other things being equal, whether it's icy roads, hills, curves or potholes. I have just as much internet resources as anyone else, no special access to academic or legal libraries, so it seems any original research reports on this is not available.

SO.....

I have available a Freightliner Cascadia, an empty trailer and in West Memphis along I-55 several miles of straight 2-lane road with almost no traffic. This road used to be used for teaching shifting back in the day, and that doesn't happen so much any more (sniff). I haven't done it yet, but this weekend I will go to that road and mark out some measure points, and we can do the actual experiment ourselves.

I need a loaded trailer, around 30,000 - 45,000 in the back. Yes, it's against your company policy for a non-company tractor to haul their trailer. But this is in the interest of science and all that stuff. Also if your tractor is stopped, you are on Off Duty break, right?? I will need you and your trailer for a few hours. My plan includes that you can stop in West Memphis or at either truck stop on that road for a break. I'll come get you and your loaded trailer, drive to the "proving grounds", do the loaded stop test a few times to average out a result. I will either before or after borrowing your trailer, run my empty on the same test.

Give me at least one day's notice and a rough idea of the time. If I have class, great - we have some helpers! I want to video this, and post it for all to see on YT. (I will not video your company name on the trailer for your sake.)

Any takers/contributors? If so, email me at tt.errolv@spamgourmet.com.

OWI:

Operating While Intoxicated

Chief Brody's Comment
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This has always bothered me, because, in my limited experience, an empty trailer stops quicker on dry roads than a heavy loaded trailer.

What I found with regard to the pure physics, is that a loaded truck stops in the same distance as an empty truck. The reason is that both the formula to stop a moving truck at a given weight and the formula for the coefficient of friction use the mass. So, if you increase the mass, it takes more force to stop, but at the same time you have increased the friction coefficient by the same quantity. So, it's basically a wash:

Heavy vs. Empty Trucking stopping distance.

But the basic physics, and the article above, assumes that the braking systems are able to leverage that increased friction coefficient. While the CDL manual states that a truck's brakes are designed to stop the vehicle when heavy, that's simply an anecdotal statement. No scientific support for that statement. As we all know, the trucks are also designed with compression brakes for downhill grades to "save" the brakes. Thus, the brake systems are not robust enough to withstand the forces of all expected driving conditions. Consequently, I don't think that the brakes are robust enough to leverage the increase coefficient of friction of a heavy loaded truck.

Also, as Brett points out, the CDL manuals says two different things. First, trucks takes longer to stop because they are heavier than cars. Second, loaded trucks stop better than empty trucks. Both cannot be true.

While I understand that on slick roads heavy trucks have better traction, that is because the coefficient of friction on snow or ice is very low. Adding weight to the truck increases the coefficient of friction so the truck can stop better. However, at some point, increasing the coefficient of friction exceeds the capacity of the brakes to leverage that increased coefficient of friction.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

DOT:

Department Of Transportation

A department of the federal executive branch responsible for the national highways and for railroad and airline safety. It also manages Amtrak, the national railroad system, and the Coast Guard.

State and Federal DOT Officers are responsible for commercial vehicle enforcement. "The truck police" you could call them.

Turtle's Comment
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