The Bridge Law Explained

The Bridge Law Explained


DOT is not Exciting

You might not realize this but DOT is not that exciting of a topic.

I have never met anyone who has ever said, DOT is the FUNNEST thing I have ever done!

The truth is that I KNOW that DOT is not that exciting. But I have been called to make it more exciting!

Quite the calling, right? 

Ok, that’s not my only calling. But it is one of them.  To take dry, boring, lame topics that have high importance and help make them more exciting to learn about.

If you have been following me for any amount of time you probably also know that I have a HUGE passion for business, my family, and God.

But today is all about DOT!

Yes, I am one crazy dude that actually likes talking, teaching, and learning about DOT.

And that right there is why you need our team in your corner. Simply because we are all crazy and have decided to become hopelessly devoted to the topics that we need to know about but maybe don’t want to take the time to become an expert in.

Recently I was heading to Denver, CO for a week of work and of course a little bit of fun too.

On the way up my phone rang, so I answered it.

The voice on the other end said, Hey Dan, how’s it going?

Great, What’s up? You know me directly and to the point. Lol

We are thinking about buying this truck and we really wanted to make sure that it would work for what we need it to. It is a straight truck, and we need to be able to haul 30K payload. What do you need to be able to tell me if this rig will work for us? We have been getting mixed answers from everyone we talk to.

Yeah, not a problem, you called the right guy!  All I need is the spread measurements between the axles. Take a tape measure and measure from the center of the front axle to the middle of the rear axle. Then measure the distance between both axles.

I also need the tear weight, or in other words the empty weight. I’d prefer to have the truck full of fuel, but if I know how much fuel it has when it is weighed, I can get close.

Let me explain why I needed all this information to come up with what a truck can actually haul.

There is this little thing called the bridge law.

This law tells you how much a truck or truck and trailer can legally haul.

I know you’re probably thinking, Dan I thought that you could haul 80,000 pounds with any truck and trailer set up. And I thought that you could haul 52,000 pounds on any straight truck.

What do you mean there is an equation that I need to work through in order to see if I can actually hold that much weight? I almost failed math; how do you expect me to do this?

Let me tell you, I almost failed my freshman math class. I scraped by with a D-.  That might have been because I was more focused on the girls and not on the math (Jenna & I met in that class, so I blame her, lol.)

The good news is that I have taken the time to break down the bridge law equation here so that you can see exactly how to do it for any Commercial Vehicle.

So, let’s take a second and work through the bridge law equation for a straight truck so you can see how to do it and see it broken down step by step.

The things you need to know about the equation are:

W = Total Weight

L = Total Spacing Between Axle Group / Total Length Between All Axles

N = Number Of Axles

What you need to know about the truck:

L (Total Length Between All Axles) = 27.5 feet

N (Number of Axles) = 3

Ok with that info let’s break down the equation. I’ll do my best to simplify it.

(L*N) 27.5×3 = 82.5

(N-1) 3-1 = 2

(LN / N-1) 82.5 / 2 = 41.25

(12N) 12 * 3 = 36

[(LN / N-1) + 12N] 41.25 + 36 = 77.25

[(LN / N-1) + 12N + 36] 77.25 + 36 = 113.25

500 * [(LN / N-1) + 12N + 36]  500 * 113.25 = 56,625


The total max gross weight could be 56,625 lbs.  Max legal limit without oversize overweight permits is 52,000 lbs. on interstate roads, in Colorado on highways you can weigh a total of 54,000 lbs.

So, for this truck, you would be good to go with the legal max weight limits.

 With a tear weight of 27,200 lbs, the max load you could haul would be, 29,425 lbs on interstates, and 31,425 lbs in Colorado. (Colorado has different weight regulations for secondary [non-interstate] roads

Let’s work through the same equation but this time let’s shorten the overall spread (distance between all the axles)

New Truck Information:

L (Total Length Between All Axles) = 20 feet

N (Number Of Axles) = 3

Now let’s work the equation out.

(L*N) 20×3 = 60

(N-1) 3-1 = 2

(LN / N-1) 60 / 2 = 30

(12N) 12 * 3 = 36

[(LN / N-1) + 12N] 30 + 36 = 66

[(LN / N-1) + 12N + 36] 66 + 36 = 102

500 * [(LN / N-1) + 12N + 36]  500 * 102 = 51,000

This truck would be limited to a max weight of 51,000 lbs. total max weight. That’s lower than the federal and state limits.

I know that this process is not extremely simple. And for that, I would like to apologize.

But here’s the thing. Know that you have this information, and you can easily walk through the equation, and you can see how much weight your vehicles can haul.

Is it less than the legal limits?

I sure hope this helps.

Have a GREAT day and don’t forget that we are offering a membership to Eclipse DOT so that you can get the EXPERTS on your side!

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