Why do semi trucks have to weigh in?

Answer 1

There are two reasons. First, the whole truck can only weigh a certain amount--usually that's 80,000 pounds.

And second, each axle group has a maximum legal weight. If you only have one axle, it can weigh 20,000 pounds. This covers the steering axle, and the axles on double trailers. If you have tandem axles--two axles close together--they can weigh 34,000 pounds.

Let's say I just picked up a load of sugar. Weight of the cargo is 42,000 pounds. I've got my tandems in the middle of their range, and on this trailer there are 21 holes--10 in front of the tandem locking pin, 10 behind. (I better explain this. The wheels on a trailer will slide back and forth along rails. The tandem carriage has two pins on it that fit into holes in the rails, and those pins keep the tandems from sliding if you don't want them to.) I know that when I roll across a scale the whole truck is going to weigh about 78,000 pounds...too heavy to chance it, right? So I head to a truck stop with a scale. The first thing I want to do is to fill my fuel tanks. If I add 100 gallons of fuel to my truck, the weight will go up by 730 pounds, and I don't want to take a chance of going across a DOT scale with a really heavy load if I don't know what the whole truck weighs. Anyway, once I do that I put the truck on the scale. I receive this piece of paper from the weighmaster:

Tractor 1234 Total Weight 78495 Axle 1: 17478 Axle 2: 34088 Axle 3: 26949

You can see the problem at a glance: I have to move 88 pounds of weight from the front tandem to the back one, and you do it by sliding the tandems. On my trailer I can shift 300 pounds of weight by moving the pin one hole--if I move the tandems forward it will shift weight back (because more of the trailer is hanging out behind the axle group) and if I move them back it will shift weight forward. We gotta move 'em forward because I'm already illegal on the tractor's tandem. And y'know, I could get away with just moving them one hole, but I'm really not going to. I'm going to live it up and slide the tandems all the way forward, which will do two things to me...most important from the DOT's standpoint:

Axle 1: 17478 Axle 2: 31088 Axle 3: 29949

(No, the steer axle didn't change. The only way it would have is if I had slid my fifth wheel, but a lot of trucks don't have sliding fifth wheels and a lot of drivers who DO have them don't use them.)

Now I'm really legal, and it feels good to be really legal. But from my standpoint, it's easier to drive the truck with the tandems all the way up; it makes the truck effectively shorter so it's easier to turn.

There's one other reason a tractor-trailer will weigh in. If you drive for a moving company, they charge by distance AND by weight. You go across the scale empty, pick up the customer's stuff and go across again. The difference is the weight of the customer's things.

Edit: There are several things wrong with the above answer. First in the US you aren't allowed 20,000 lbs. on the steering axle, you are only allowed 12,000 lbs.! The setup above is the standard "18-wheeler", A 5-axle setup that can weigh a maximum of 80,000 lbs. You're allowed 12,000 lbs. on the steering axle, 34,000 lbs. on the next two combined (or 17,000 lbs. each), and 34,000 lbs. on the tandems (again 17,000 lbs. each). 34,000 lbs.+34,000 lbs.+12,000 lbs.=80,000 lbs. Now, if you have one axle with dual tires on each side (like a single drive axle, or single trailer axle) that axle is allowed to weigh 20,000 lbs. So a truck with a single drive axle and a single trailer axle is only allowed to weigh 52,000 lbs. 12,000 lbs.+20,000 lbs.+20,000 lbs.=52,000 lbs. And me I like to keep my tandems as far back as possible even though it's a little harder to turn. The reason for that is that it rides so much better. And any driver who loads 42,000 lbs. then fills his truck with fuel before he puts it on the scales has either hauled that load more than a few times, or hasn't been driving very long! If you load 42,000 lbs. the first thing you do is weigh your truck to see how much fuel you can get without going over. And the last thing is that moving your tandem WILL change the weight of your steering axle because it changes how much of the trailers weight is sitting on the tractor!