Well, I won't make any claim to being an expert on trailer axles but here's what I do know from recent personal experience.
Dexter axles are ALL made to order. You specify the beam length, diameter, spindle type, brake wiring, brake mounting flange, stud number, diameter, thread pitch and centers, brake size and type, spring seat centers, any camber, E-Z lube or standard spindles etc. If you call Dexter with a recent axle serial number they will tell you all about it on the spot however Dexter did not have computerized records for each axle until much later than 1984. I suspect that if you were willing to risk your head exploding you could look up the specs of the NSN for the axle beam but remember the specification for an MRE chocolate chip cookie is almost 20 pages long.......
If a trailer manufacturer, parts store or online seller has axles in stock it's because they ordered an agreed upon buyers part number or they specified the entire axle bit by bit. Most stock axles have the spring seats loose so they can be welded on as desired so take the info below on spring seat centers to heart, please.
A 3" axle tube would point to a 3,500 pound or heavier axle beam rating and DOES NOT infer any particular rating without much more information. A 3" axle beam can have any of several wall thicknesses each having a different nominal rating for a particular length and overhang.
And the single biggest variable in the actual working load rating will be the overhang from the spring seats to the hub face. The longer the overhang the lower the actual weight rating regardless of beam diameter or wall thickness. To determine the overhang measure the hub face to hub face length and subtract the spring seat center to center distance. The combined overhang on the left and right sides is the axle overhang.
12x2" brakes will generally be rated for up to a maximum of 7,000 pounds per axle but are also stock for many 5,200 pound axles and can be mounted on 3,500 pound axles by simply ordering the five hole brake mounting flange on the axle in place of the more standard 4 hole on 2-7/8" centers brake mounting flange seen on most 3,500 pound axles.
As an example of how much can change from the nominal axle rating when you have all of the dimensions we recently had to replace the front axle under my cargo trailer. It's a 7,000 pound rated trailer so all I need is a 3,500 pound axle, right? Well, let's see here.....
The hub face to hub face length is 94.65", the axle has a 4" drop, the spring seats are on the bottom of the axle, I need 10x2/4" electric self adjusting brakes and E-Z lube spindles and all I have to do is buy one off the shelf with the loose spring seats and weld the spring seats on. Or maybe not....
In this case the spring seats are on 70" centers leaving an overhang of 24.65". Any overhang more than 17" for this particular length axle de-rates the axle carrying capacity and in my case the 2-3/8" tube would have been de-rated to 2,556 pounds. I don't want to make my 7,000 pound trailer into a 5,200 pound trailer so I need a 3" axle beam. Simple, huh?
But, as they say in the infomercials "Wait!! There's more!!" Now, because if the 4" drop and the way those axle ends twist on the axle beam when combined with the long overhanging ends they recommend a slightly thicker axle beam wall. So in this case the overhang and drop at the spindles require a larger axle diameter and thicker wall and my original choice of a 2-3/8" axle has become a 3" axle with a heavier than normal wall.
Clearly, reading the axle weight rating may not be all you need to know as in my case the D35 2-3/8" axles could have been shipped with loose spring seats and a weight rating tag showing a 3,500 pound rating but when I welded the seats at 70" OC the axle would have become a 2,556 pound axle without anyone reading the label knowing it. Or maybe without me knowing it either.....
In short, I strongly advise that to be very careful when determining the actual safe axle rating. In the case of they M101A2 trailer I suspect (but sure can't prove) that the axle was ordered as a complete assembly with welded spring seats that was ready to bolt to the springs and that any weight rating on the axle is accurate. But I also suspect that the already stated idea that the axle was re-rated from any civilian equivalent due to off road use is an accurate one and none of us have an easy and accurate way to translate that to a standard on-road axle rating.
BTW, the correct 3" axle with the thicker wall was only $40 more than the dangerously under-rated one I would have had if I had just assumed that any 3,500 pound axle would do the job..... If I had added a parking brake feature that would have added a total of about $200 to the installation for the upgraded brakes, cables and handle. I'm kicking myself now for not adding them when I had the chance.
Lance
