The Effect Of Vehicle Weight On Stopping Distance

I'm putting together a new version of our High Road Training Program and I came across a statement in the CDL manual that I've always questioned. Here is the exact quote from the CDL manual:

The Effect of Vehicle Weight on Stopping Distance. The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb. The brakes, tires, springs and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded. Empty trucks require greater stopping distances because an empty vehicle has less traction.

The extra weight on snow-covered or icy roads can certainly help you maintain a grip on the road surface, potentially allowing a heavier truck to stop faster than a lighter truck.

But what about on dry roads?

I don't know about you guys, but a heavily loaded truck on dry roads did not stop faster than an empty truck. No way.

I've always thought they should revise that statement to specify the difference between dry and slick roads.

What do you guys think?

I find that when I'm heavy the truck stops much better. Light or empty, and the trailer tends to bounce. For me, I'm always happier with heavier loads. Low and heavy, even better. Give me 44,000 pounds of beans any day.

If you're on ice, heavy may be better, but I would be parked. Ice has no traction and if you hit the brakes you will skate instead of stopping.

Snow, can vary greatly from light to heavy. I'll drive through snow if there are not too many others on the road. In snow, however, if you don't slow down you will be the one rolled over in a ditch or jackknifed on the shoulder.

The key to driving in snow, ice or heavy rain is to slow down and increase your following distance.

I still prefer weight in my trailer.

That was taught to us in school and I really didn't remember from 21 yrs before.

So, I have kept track of "emergency" stops and heavier is definitely smoother and quicker. Lighter/empty bounces more, so I think a person lets off on the brakes before applying them again and that adds to the length of a quick/hard stop.

Laura

In my opinion I agree with Brett. I’d much rather approach a stale green light empty than loaded. I’m not understanding how the opposite could be true. You may have tendency to jackknife easier while empty on slick conditions but other than that definitely rather be empty.

Basic science and experience support Brett’s post.

An empty semi on a dry road will come to a complete stop faster than a loaded semi.

I hope Errol chines in and gives us a physics lesson on objects in motion and momentum.

Did someone invoke the Science Teacher's name???

First, I don't know how much research went into the CDL driver manual, but I bet it's more than "Yeah, of course it is!" So even the counter-intuitive (reverse logic) "Empty trucks take longer than loaded trucks to stop" is true.

Experiment one: rub your hands together, like you are thinking "I'm hungry and it's dinner time!! Yum!" Did your hands rub fairly fast (unless you are fascinated with your hands rubbing together)?

Experiment two: Press your hands firmly together then rub them at about the same speed. The 6th grade physics answer is that they got hot faster. True. But.... I bet you needed more force to make them rub just as fast as experiment one.

A heavy load provides more traction between the road and the tire. You know, the tire tread (remember, minimum 2/32 inch!) is pressed deeper into the road surface. More traction makes for better braking. An empty load does not have as much traction as a heavy load. The tires do not "skid", but they do not grip the roadway as much. They do slide because the traction isn't there.

=+=+=+=+=+=+

Now, I can always support this kind of stuff that I write - facts - with references to more authoritative sources. This is not just something I made up in my mind (The hand rubbing yes, but the traction thing, no.) The traction part comes from the The Keating Firm LTD law firm, who specializes in our favorite law topic: Big Truck Accidents. they have developed expert testimony in Big Truck Accidents and are proud to post it in their blog. You can read the details here: How Long Does it Take to Stop a Fully Loaded Semi-Truck?

Now for the empty truck and loaded truck question: The same truck even, with and without 42,000 lbs in back. Heavy: more traction no sliding. Light: less traction more sliding - longer stopping distance.

Thank you Errol.

Errol, from the article you quoted:

As a general rule of thumb, the faster the truck is going, and the heavier it is, the longer it takes for it to come to a complete stop.

I've yet to hear anyone mention the momentum generated by the heavier truck as a factor in stopping distance. You keep talking about traction as if that's the primary factor in getting stopped. When you have 46,000 fewer pounds to stop it makes a huge difference.

I'm not yet convinced that empty trucks take longer to stop than loaded trucks on dry pavement, especially with antilock brakes.

Yes, a heavier truck has more grip on the tires, but there is only so much braking force to stop the momentum of that extra 46,000 pounds the loaded truck has.

It just sounds odd to say, "I can't get this vehicle to stop as quickly because it's so light." That doesn't seem to be how physics works. Ask a race car driver, "If you take more weight out of the car but keep the same brakes and tires, does it stop faster? Or does it take longer to stop when it's lighter?"

The race car driver will wonder why you're asking such an obvious question. "Of course it's going to stop faster if it's lighter. "

I'm afraid the "money quote" from that article was copy/pasted from the CDL manual. I just read that passage recently, so I recognized it.

From the article:

This would lead one to think that loaded trucks take longer to stop than empty ones, but I'm afraid that's not right. The breaks, springs, shock absorbers, and tires on heavy load trucks are specifically designed to work better when the vehicle is loaded.

From the CDL Manual:

The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb. The brakes, tires, springs and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded. Empty trucks require greater stopping distances because an empty vehicle has less traction.

So you can see they took that passage from the CDL manual and tried to massage it to appear "original enough."

Oh well.

That's fine Brett. Often different passages covering the same fact will sound similar. There are other legal blogs I've found that look like they may be copied from each other. I couldn't find direct research but as I pointed out, the CDL manual authors aren't out to write things just because they sound right.

It's very hard to find research on the same topic that will come to different conclusions.

You asked for member opinions & thoughts, but if they disagree with yours that's a problem?

Drops mic.