question about tag and weight law

To meet the Federal Bridge laws a tandem axle combination has to have a minimum spacing between axles of 49 inches. Then you are allowed a maximum weight of 17,000 Lbs per axle, for a total weight of 34,000 lbs per tandem. If the weight enforcement officer wants to be picky and you have 20,000 lbs on one axle and 14,000 lbs on the other for a total weight of 34,000 lbs on the tandem combination, he can ticket/fine you for being overweight on the 20,000 lbs axle in that tandem combination.

The only way to have a single axle to be able to carry more than 17,000 lbs is if the next closest axle is further than 102 inches away from it centerline, then you're allowed for each axle to carry 19,000 lbs or 38,000 lbs as a spread axle tandem combination. This can also be spread to a 122 inches centerline to centerline of each axle for a for 20,000 lbs per axle and 40,000 lbs for the tandem. Any single axle in the center of a trailer can carry 20,000 lbs if each of the tandem axles in front or behind it are at least 122 inches away from it. If it is closer than 122 inches than the axle combination will be considered a tri-axle combination and the total weight carrying capacity will be reduced, depending on a math formula you have to use. A tri-axle combination with a 117 inch centerline from the two outside axles is only capable by the US Federal Bridge Law to carry 42,500 lbs on three axles. Now this combination doesn't have to have the weight equally spread between the 3 axles. You can have a standard tandem carry 34,000 lbs while the third axle carries 8,500 lbs. Here is a picture of a typical tri-axle combination trying to take full advantage of the bridge law with a light weight single tire third axle and wide spread. This axle combination is 117 1/2 inches and can carry 42,500 lbs on it. The single tire front axle is rated to carry 12,500 lbs, but as you add weight above the 8,500 lbs to this axle you have to reduce the weight the tandems carry. Any two axles that are closer than 102 inches from centerline to centerline are considered a close tandem and can only carry 17,000 lbs or 34,000 lbs as a tandem, so you either keep them at 49 inches or spread them past the 102 inches depending on your application

 

I edited my post to reflect a mistake I made in a measurement of the tri-axle combination. I said 105 1/2 inches, when it should have been 117 1/2 inches

 

is a big mess, since I hardly ever drive on inter state hwy, here in Florida you can put 44k on any tandem, but no more than 80k on a 51 feet or more wheel base, on my try axle dump trucks I can gross 70k which goes like this steering axle I have 425 65 22.5 tires 19,965lbs, drop axle are 11 22.5 you can put 13,310 lbs and the tandem 44k but not to exceed 70 gross that's Florida.

 

I'm going to assume that the 103 inch measurement is with two axles. Here that would limit us to 38,000 lbs for the tandem axle combination, with 19,000 lbs per axle. But now you have another problem using the 385/65R22.5 tire here in the US. This tire is only rated for 9370 lbs, so two of them on an axle is 18,740 lb per axle or 37,480 lbs per tandem axle combination. At a 103 inch spread the 385 tires will limit the tandem axle combination to 37,480 lbs or 18,740 per axle. Yes, a picky weight enforcement officer will fine you for exceeding the tires weight carrying ability if this example was carrying 37,500 lbs on the tandem.....20 lbs overweight.....I've seen them do it in VA if they stop you with a portable scale. With a stationary scale they wouldn't bother you, but with the portables they've already gone to the trouble to stop you and figure they need to make money to justify their jobs.

If you use the 425/65R22.5 than the weight rating of the tire will exceed the weight carrying ability of the axle restrictions by using the Federal Bridge Law.

If you look at the picture this trailer has the 445/55R22.5 tires on the rear tandem axle combination. These tires are rated for 10,400 lbs or 20,800 per axle with a combined rating of 41,600 lbs on the tandem, but this closed tandem can only carry 34,000 lbs on these two axles by the US Federal Bridge Law. The tires have to be rated to carry more than the axles by law, otherwise the tire becomes the limiting factor if a weight enforcement officer wants to get picky. If the axle and suspension are rated at 15,000 lbs by the suspension manufacturer, then the axle/suspension will be the limiting factor regardless of what tires are mounted or axle spacing the truck/trailer is built with. So everybody builds the truck or trailer to handle more weight per axle than the Bridge Formula will allow, uses tires built to handle more weight than the axles can carry by the axle manufacturer rating, just to be able to cover every possible possibility of weight enforcement in the different states.

 

There is another factor to also consider, but I haven't seen enforced........yet anyway

The 385 tire is narrow. The Federal Bridge Law also goes into depth about tread width and the tires surface contact area with the road. This tire isn't used at all in this country due to the weight limiting factor of this, you may see it on steer axles on construction vehicles, but that is usually the only application it's used in. You will see a few trucks/trailers with the 425 tires, but again this tire is limited by its width/contact patch to it weight carrying ability.

The reason the newer 445/55R22.5 tires have become popular in the US is they are wide enough in tread width, as to not limit the axle weight that can be carried.