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Nuts & Bolts: Rear Axle Beef

Posted in How To: Tech Qa on June 22, 2018
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Dear 4-WHEEL & OFF-ROAD: Please explain the differences (both pluses and minuses) of the Dana 80, the Dana 70, and the GM 14-bolt 1-ton rearends. For the sake of discussion, assume that all are full-floating rear axles and have the same gear ratio. Which is stronger and can pull or carry the heaviest load?

Dwight B.
Via nuts@4wor.com

You ask some great questions, but unfortunately the answers aren’t so cut-and-dried. As you suggested, we are limiting this discussion to rear axles (there’s such a thing as a Dana 70 front) and full-floating designs. A full-floating axle means that vehicle and cargo weight are carried by the axlehousing, while the axleshafts only transfer torque. Full-floaters have that distinctive, large-diameter center hub that protrudes from the center of the wheel, whereas semifloating axle designs require the axleshafts to both carry vehicle and cargo weight as well as transfer torque. There were semifloating 14-bolt axles that are sometimes confused with their heavy-duty counterparts, but they share little in common with full-floating 14-bolts other than the name.

There are many other differences to take into account when talking about the overall strength of an axle, including pinion shaft diameter and spline count, axle tube diameter and thickness, single or dual wheels, and more. But ring gear diameter and axleshaft thickness are good baseline comparisons when discussing the overall capacity of axle. A Dana 80 has a ring gear diameter of 11.25 inches, the Dana 70 measures 10.50, and the 14-bolt measures 10.25. Why is ring gear diameter important? The larger the ring gear, the greater the contact area or engagement between the ring and the pinion and therefore the greater the torque capacity of the axle. Dana 80 axles can be 35- or 37-spline and 1.50 or 1.58 inches, respectively. Most Dana 70s are 35-spline with a diameter of 1.50 inches, while most 14-bolts have 30-spline, 1.50-inch-diameter shafts. The smaller spline count of the 14-bolt is a little misleading because the splines are a different pitch than the Dana axles and are generally considered to be the equivalent of a 35-spline shaft.

The most important statistic to compare between the axles is the axle weight rating stated by the manufacturer, but this is also where things get really muddy. According to several sources, a Dana 80 can have a Gross Axle Weight Rating (GAWR) of up to 11,000 pounds. A Dana 70 can have a weight rating of up to 10,000 pounds, but 8,000 pounds is much more common. A 14-bolt can have up to an 8,600-pound GAWR. The greater the load-carrying capacity of the axle, the greater the strength.

So what’s with all of this “up to” nonsense? Well, the weight rating of the axle was technically rated by the vehicle manufacturer as opposed to the axle manufacturer. In reality the vehicle manufacturers often de-rated the axle based on the specific vehicle it was being used under.

Several factors are in play here, the biggest of which are gear ratio and tire size/configuration. It’s common to see a lighter weight rating with 3.55 gears and a heavier rating with 4.10 gears on trucks that are otherwise identical. Single and dual tires make a big difference, as do tire specifications. All three axles can be found under trucks that touch the medium-duty truck category and can sometimes be found spec’d with 19.5-inch-diameter wheels, which in turn enable a whole different weight class of tires from 16- and 17-inch sizes. These, plus a whole bunch of truck-related factors (engine, transmission, and much more) dictate the final rating of the specific axle. In the end we can speak generally about the capacity of an axle, and people often default to the maximum rating available for each model. While that’s not entirely wrong, technically the rating of the axle is more dependent on the truck it came from than anything else.

So, back to your question. By the numbers, the load-carrying and therefore torque capacity of the three axles from weakest to strongest is: 14-bolt, Dana 70, and Dana 80. Even so, reality and experience says 14-bolt axles are more than adequate for all but the most extreme off-road uses. Dana 70s are also good but have comparatively limited ratio and locker availability, while Dana 80s are generally overkill and can hurt ground clearance more than the others. But if you’re strictly wanting to know which one is the strongest and can carry the most weight, it’s the Dana 80.

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