Bridge Formula

[John C.]

A couple of weeks ago I was looking at a three axle equipment trailer for a guy. He had gotten stopped by the state patrol for nonsense and when they could not pin a ticket on him they measured the distance between the axles of the trailer and told him the trailer did not conform to the bridge formula for the weight he was carrying. Anyway they wrote him a ticket for just under $400.

I have heard of the bridge formula but never figured I'd have anything to worry about as the manufacturers would include that in their engineering when they built the trailer. I am apparently wrong.

Can anyone out there explain what is going on? What do I have to know about axle spacing when looking at purchasing an equipment trailer?

[Chaz Murray]

A couple of weeks ago I was looking at a three axle equipment trailer for a guy. He had gotten stopped by the state patrol for nonsense and when they could not pin a ticket on him they measured the distance between the axles of the trailer and told him the trailer did not conform to the bridge formula for the weight he was carrying. Anyway they wrote him a ticket for just under $400.

I have heard of the bridge formula but never figured I'd have anything to worry about as the manufacturers would include that in their engineering when they built the trailer. I am apparently wrong.

Can anyone out there explain what is going on? What do I have to know about axle spacing when looking at purchasing an equipment trailer?

It really depends on the state/states you plan on running in. The bridge just refers to axle spacings like in California with a 16 tire trailer and 4'6" axle spacing they only allow 58,000lbs and if you jump up to 5'9" they allow 60,000lbs. All the states are different so you need to get in touch with of the DOT for your state and get a copy of a weight chart for whats allowed with what spacings.

[JDOFMEMI]

I once downloaded the regs on weight limits here in CA, and they were something like 60 pages long. Quite a few years ago, so I don't remember exactly but i do know it will give you a headache reading.
Allowable loading on an axle group can change noticably due to a 6" change in axle spacing.
If your trailer is too short, you get close coupling, and get reduced for that. If your overall distance from front to rear axle is not enough, overall gross weight drops. Thats why most end dump pullers have a looong truck pulling the shorter trailers.
Just when you think you have it all figured out, there will be a change in rules just to keep you jumping.
Remember, each state is different, and then there is the federal bridge law, but not all states use it, and some states only use part of it.

I would reccomend getting on your states hiway department website and finding out what applies where you live.

[Orchard Ex]

MD adopted the federal bridge formula. Here's a link to the truckers handbook: http://www.marylandmva.com/Resources/trucker.pdf if you go to page 45 in the pdf file (or find page 36 in this document) it gives you the formula, the weight tables and the exceptions to the formula. Enjoy :Banghead

[Kgmz]

We have a TrailMax TRD50-T tri-axle tilt-bed with 17.5" tires.

So I'll give you a example.

The trailer has a gross weight rating of 60,800 lbs.
The trailer weighs 12,100 lbs.
The axles have a spacing of 49" between each axle.
The trailer load capacity is 48,700 lbs.

OK, now bridge law.

With a 8'2" axle spread on the trailer, the legal max on the axles is 42,000 lbs.

We have 2 dump trucks, one with a single lift axle and the other with two lift axles. Both have tandems and the single lift truck has a 14,000 front axle with normal tires, the double lift truck has a 20,000 front axle and the monster 425 tires. Both have what would be a legal 20' axle spread, 19'6" on the single lift truck and 20' on the double lift truck.

So by bridge law the single lift truck can carry 55,500 with the axle down, and the double lift truck can carry 60,500 with the axles down. As long as each axle is not overloaded, 34,000 for the tandems, 10,000 for the single drop as long as the spread for the drop and the tandems is over 11' which it is. The double drop has a spread of 11'7" so legally 12' so I can only carry 8,000 on each of the lift axles. Now the drop and tandem spreads can carry 44,000 for the single lift truck and 50,000 for the double lift truck. So that leaves 11,500 for the front axle to carry on the single lift truck and 10,500 for the double lift truck if the rear axles are maxed out.

Now add the trailer to the trucks. The total legal axle spread on the single lift truck with the trailer is 48', and is 49' on the double lift truck. So by bridge law these combo's can carry 88,000 on the single lift truck and 94,000 on the double lift truck if all axles are down. Now normally towing and trailer and carrying equipment the axles are up because the truck box is empty. So the legal bridge is 83,000 for the single lift truck and 83,500 for the double lift truck.

The single lift truck weighs 23,000 and the double lift weighs 27,000, the trailer weighs 12,100. So that leaves 47,900 that the single lift could carry with axles up, and 44,400 that the double lift can carry with the axles up.

The trailer has a capacity of 48,700 and you can see how the bridge law won't allow the trailer max. Also by bridge you would have to make sure that there is 6,700 lbs. on the tongue if you had a max load on the trailer because of the tri-axle bridge weight of 42,000.

Also our TrailMax is long with a 5' fixed deck ahead of the tilt. There a lot of tri-axle tilts that do not have this fixed deck so they are shorter which affects the overall axle length and the lessens the capacity.

Another thing is I see a lot of people that put the load on the trailer too far back, so then they are not transfering enough weight to the truck and then the trailer axles are overloaded by bridge law.

Remember your overall axle spread, the spreads of each axle group like a tandem axle with the lift axles up and down, 34,000 on the tandems and depending on the spread whatever the lift axle adds. And remember what each axle and axle group can carry.

If you are stopped they will usually check each and every axle and axle group, and each of the lift axles alone to make sure that there is not over 10,000 on the lift axle.

[Bob Horrell]

Well done Kgmz. I recently had to go through the same computations here in California for my dump truck and equipment trailer. Not many people go through the regulation details like you did and many have no clue regarding axle spacing and load limits. I carry all the regulation rules I printed out from the internet along with me for those instances where you run into an inspector who is not up to speed. In California, the rules for dump trucks and some other vehicles differs somewhat from regular tractors. It can get confusing when there is an overlap in the regulations, which isn't that uncommon.

[AtlasRob]

Jeez :eek: Kgmz 1st rule would seem to be when you get pulled put all the axles down and hope your safe :D

Seriously, you boys have a nightmare never mind those that cross state boundries :notworthy

[John C.]

Kgmz,
Thanks for the explaination. I see where our subject trailer has the problem. It is as much in the truck they are using as in the trailer.

I didn't know that the bridge formula encompassed the entire vehicle as well as each individual unit. The constable had made a comment about the tongue on the trailer needing to be eight inches shorter to haul the load that was being carried. Now I think I understand.

Having seen this first hand now I wonder what happens with the trailers with the tongues and reaches that can be extended. Logging trucks and dump truck pup trailers come to mind. Do they get stretched out to comply with the bridge formula? I know logging truck trailer reaches get lengthened to haul longer logs mostly but I see four axle pups get stretched out all the time after they are loaded.

Thanks for the info.

[Kgmz]

The only way a shorter tongue would help the trailer to comply, is to get more weight forward on the truck. And if this is the case then the trailer is poorly designed. What I would do if it is possible with the type of trailer, is to move the axles back.

And yes, dump trucks with the extendable tongue pup trailers get extended when loaded to comply with bridge law.

Our total wheelbase on the trucks and trailers is over 68 ft, long to carry a total weight of 105,500 lbs.

The single lift axle Peterbilt pulls a 4 axle pup, and the double lift axle Kenworth pulls a 3 axle pup. For a total of 8 axles for both. If the Kenworth is hooked up to the 4 axle pup we don't gain any additional weight rating because of the extra axle, as 105,500 is the max under bridge law for normal loads. You can gain more weight with more axles and more weight per axle with a special overweight permit, but these permits are normally only used for non-reduceable loads. But with that extra 9th axle I could carry the max of 105,500 and be 10 ft. shorter in wheelbase.