Axle shaft and U-Joint Strength

Have you ever wondered how much torque your axle shafts can handle? How about your u-joints? This article will tell you how to calculate the maximum torque your truck is ever going to send to your axles, as well as list maximum torque ratings for many axle shafts and u-joints. The following info will tell you if your components are strong enough to hit the trails, or if you need to beef up your axles a little.

Calculating Torque Loads

Calculating the maximum torque that will ever be sent to your axles is fairly simple to do if you know the correct specs, but you also need to know an approximate amount of torque required to break the tires loose. If it is easier to break the tires loose, that will happen before something breaks. Let’s start with calculating the amount of torque multiplication the drivetrain can generate:

Engine’s maximum net torque
X first gear ratio
X transfer case low-range ratio
X axle ratio
X “real world” ratio of 0.85

The “real world” factor is multiplied in due to the fact that your engine will not be putting out max torque all the time and the transfer of torque is not 100%. For example if you have a Chevy 350 that produces 290 ft. lbs of torque, an automatic TH-700R4 tranny with a 3:1 first gear, a 2.61:1 low ratio and a 4.10:1 axle ratio, that’s 9,309.8 ft. lbs of torque. Multiply that by the “real world” factor and you come up with 7,913.3 ft. lbs of torque being sent to your carrier or locker, axle shafts, and tires. Please keep in mind that without a locker, 100% of the torque can never be sent to one axle shaft. Also, the total weight on a particular axle may be more or less depending on if you are climbing or coming down a hill.

The previous calculation does not take into consideration one very important factor; traction. The amount of torque it will take for your tires to break free is not always easy to calculate, unless you’re on flat pavement. On the trail you must account for weight transfer, terrain angles, etc. Picking a high coefficient of friction will keep you safe, although some of the time the coefficient will be more or less. To make this calculation, you first need to know the weight on each axle. Load up your truck with fuel and your common trail gear and take it to a truck scale. Pull each axle over the scale one at a time to determine the weight on each axle. After you know that the calculation goes like this:

Weight on axle
X coefficient of friction
X tire radius
divided by 12

This formula only applies if you have a true locker. An open diff will share the load between both axles, and a limited slip will partially share the load. A good general coefficient of friction to use is 0.6. The highest would be 1.0 which would correspond to flat pavement, while soft, wet mud will be very low.

Now you know how much torque your engine can produce, as well as an approximation as to how much torque it will take to break the tires loose. If it takes less to break the tires loose than it takes to break a component, you’re in the clear.

Axle Shaft and U-Joint strength

Calculating the strength of an axle shaft or u-joint requires knowledge of the material the component is made of, as well as diameter or thickness. Instead of trying to calculate the strength, this material will be presented in the form of charts with common axle shaft and u-joint types and sizes.

Axle Shaft Strength
*Please note that these yield torque ratings were generated via an engineering formula and should be considered approximations. They are useful to show the difference that size and material have on yield torque.
 

Notes:

1. Old Jeeps, Dana 23 and 41, as well as old Land Rover series rigs.

2. The necked-down section on GM 28 and 30-spline front axle shafts and 30-spline Dana axles. Also a close approximation of the old Jeep Dana 25 and 27.

3. The OE stub axle of Dana 44 and GM 10-bolt front axles.

4. Dana 30 and 35 front and rear 27-spline axles used on Jeeps and Ford Rangers and Bronco II. The 1.20-inch, 28-spline Ford 8.8 on many small Fords is just slightly stronger. Mitsubishi trucks and SUVs are slightly stronger.

5. Jeep AMC 20 rear 29-spline and AMG Hummer IFS/IRS (not CVs). Newer Nissan trucks and SUVs similar.

6. The GM 10-bolt front or rear 28-spline axle.

7. A 28-spline in 4340.

8. An OE type 30-spline axle, to include Dana 44 in many rigs, most Toyota axles, GM 10 and 12-bolt axles, small axle Dana 60 light duty full-floaters.

9. A slight upgrade in material on a 30-spline axle.

10. A 30-spline axle in 4340.

11. The 31-spline Ford 8.8 OE axle. Old Nissan Patrol similar.

12. An OE 33-spline axle similar to those used in the GM semi-float 14-bolt.

13. A 33-spline axle in 4340.

14. An OE 35-spline axle as found in a front or rear Dana 60 or a 30-spline, 1.5-inch, 14-bolt full-float axle, a Ford 10.25 Sterling full-floater, or a Dana 70.

15. As in number 14 above, but in 4340 alloy.

As you can see from the previous chart, material plays an important role in axle shaft strength. For instance a Dana 30 with the big 297x u-joints and Warn 4340 shafts could possibly be stronger, or as strong as a stock Dana 44. Keep in mind that the stock Dana 44 could also be upgraded to 4340 shafts to be even stronger.

Front Axle U-Joint Strength

As with axle shafts it is easiest to present this material in the form of a chart. The chart below is based on destructive tests on a small sampling of universal joints from various manufacturers. Interestingly, most brands tested within 15% of each other regardless of price. These tests do not include the super beefy CTM u-joints or the OX u-joints.
 

When determining how much your u-joints can handle it is also important to consider angles while the tires are turned. The more your tires are turned, the more torque is multiplied. The below chart will give you an idea of how much torque can increase with greater angles.
 

As you can see, when your tires are fully turned, and you are applying a lot of throttle combined with a lot of traction you can snap a u-joint pretty quick.

Driveshaft U-Joint Strength

Another area you may want to evaluate in the driveline is the driveshaft u-joints. The first thing to do is to calculate how much torque is on the drive shaft when the tires break loose. The formula isn’t much different from calculating how much torque is required to break the tires loose from before. It goes like this:

Weight on axle
X coefficient of friction
X rolling radius of tire
divided by 12 times the axle ratio

(ex. for 4.10:1, you would divide by 49.2)

This calculation will yield the amount of torque on the driveshaft. As long as this figure is less than the short term max torque listed in the following chart, you’re in the clear.

Driveshaft Universal Joint Dimensions and Torque Ratings
 

In case you are curious, the Spicer 297 u-joints are similar in strength to the 1310 series listed above, and the Dana 60 332 u-joints are similar to the 1410 series.

Conclusion

While performing these calculations, please keep in mind that nothing is perfect. Just because a lot of tests show that a particular component will hold up to a certain amount of abuse, doesn’t mean that one or two may come out of the factory a lot weaker. Or they could come out stronger for that matter. There is also no way of knowing exactly how much traction you have on a particular spot in the trail. These figures are just for a general approximation and even if your calculations say you shouldn’t break anything, you very well may, especially if you give your truck a lot of abuse regularly.


Write up by: Aaron Powers (ghuru x86)

****All specs and formulas in this article were taken from “4-Wheelers Bible” by Jim Allen. I suggest you purchase this book or at least check it out. While you will probably already know a lot of the information contained in it, there is still plenty of useful stuff. If your local book store doesn’t carry it, it can be purchased here:  4-Wheelers Bible

Last Modified: 7-13-05 JCH