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4x4 Truck Axle Tech - Axle 101

Posted in How To: Transmission Drivetrain on January 1, 2005 Comment (0)
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Rockcrawling is the most difficult work for axles because in this form of 'wheeling, there is generally more grip and more weight transfer. The transfer is even greater with shorter rigs. This flatfender is at about 30 degrees. At that angle, the rear tires have approximately 35 percent more weight than they do on level ground and the weight on the front tires has been reduced by about the same amount.

Out on the trail you'll see axle extremes-big-tire rigs with spindly little axles, and others with monster-sized axles. The little axles would probably run from those big tires, screaming in terror, if they could. Those massive axles weigh as much as a V-8 engine, are three times stronger than they need to be and cost big bucks. Neither extreme is particularly elegant or efficient. There's got to be a "Three Bears" just-right point. And there is. We're going to show you how to find it.

The axle in your 4x4 is caught in the middle of a tussle between driveline torque (see glossary below for unfamiliar terms) and the grip offered by your tires. As long as the tires lose grip before the torque load exceeds the breaking point of some part in the axle, you're fine. The problem is that tire grip is 50 percent of the trail-performance equation (traction + clearance = trail performance) and it is grip that makes difficult 'wheeling possible. Fortunately, grip is in short supply on most trails. That's why you see rigs with smallish axles surviving.

Grip starts with the ground, or driving surface and is measured as a Coefficient of Friction (CoF). It's really more than just the ground, because some tires will provide better grip on a particular surface than others. Perfect grip is expressed as a 1.0 CoF. Actually, some racecar tires on special pavement can reach 1.5 or higher. Normal pavement and normal tires usually achieve a CoF of between 0.6 and 0.8. Rock may have about the same CoF as pavement. Hard dirt runs from 0.3 to 0.4. Mud can run from 0.1 to 0.3, while ice can run from a fraction above 0 to 0.15.

Vehicle weight plays a big part in this. More weight on a tire equals more grip on a hard surface, but it takes more drivetrain torque (and strength) to move more weight. Plus, heavy rigs are a liability in soft ground, so weight definitely is a double-edged sword. Bottom line, more weight equals more traction on a hard surface (to a point), but it then becomes more possible to exceed the torque capacity of the axle. Keeping the tires aired up, by the way, can act as a fuse. An aired-up tire will slip sooner than an aired-down tire 98 percent of the time. You won't make every obstacle, but you aren't as likely to break axle components. Tire grip is also expressed as traction torque, or the amount of torque the tire/ground combo can support. Traction torque is an equation that factors the weight of the tire, the CoF and the tire radius. Increase or decrease any of these values and you increase or decrease traction.

Axle-torque loads increase with tire diameter. This is because a larger-diameter tire makes more traction torque at the same weight than a smaller one. Going from a 31- to a 37-inch tire increases the torque load on the axle by about 20 percent because the tire can hold 20 percent more torque. That's independent of the increased loads that may come from other factors, such as lockers or really sticky tires. Another element is the rotational inertia, or flywheel effect, from the added rotational mass of a big tire. The larger the tire, the more torque it takes to start it into motion and the more it takes to slow it down. Additionally, four-wheeling tires and wheels are usually pretty heavy. This is mostly a negative issue when tires are spinning because it can multiply momentary or "spiked" torque loads. This added weight also is what makes bigger tires harder to stop when you've got them out on the road.

Most of you know that the traction available to an open differential is essentially limited to the amount of grip one tire can supply. That's the primary reason for lockers and limited-slips, so that when one tire on an axle loses traction, the other can still move the vehicle. That's great for getting through an obstacle but it may be tough on the axle. Normally, a 4x4 shares the total traction torque required to move the vehicle between all four tires. Weight transfer (see Vehicle Dynamics, below) or poor ground surfaces may reduce the traction of tires at one end or one side of the vehicle and increase it at the other. In rockcrawling situations, you may have a weight-transfer situation where one tire has a whole lot of the total weight and is providing 90 percent of the tractive effort. A locker will allow 100 percent of the axle torque to go to that one axleshaft, at least to the limit of grip for that tire. If the grip exceeds what that one axle can stand, the shaft breaks. A rig with an open diff or even a limited-slip will long since have given up, spun the lightly loaded tire and saved the axle. The bottom line is that if you run a true locker, your axle strength needs are greater than if you run an open diff or a limited-slip-but if you do run a locker, your rig's ability to conquer terrain is greater.

The aspect of vehicle dynamics that causes many broken axle parts is weight transfer. When your vehicle climbs, descends or is tilted on any axis, weight is transferred from the high side to the low side. That unloads the high tires, reducing grip, and loads the low ones, increasing grip. Bear in mind that the distance between the tires, whether that's the wheelbase length or axle width, has an effect on weight transfer. More distance equals less transfer at any given angle, and vice versa.

The pinion from a very tall 2.76:1 ratio (left) has much more tooth contact than the 5.38 gear on the right and will be stronger. If you need the really tall gears, an upgrade to an axle with a larger ring gear will allow the pinion teeth to be larger and more robust, even at the same ratio.

The axle manufacturer rates an axle several ways, including Gross Axle Weight Rating (GAWR, or GAW) and Output Torque (OT). GAW is the amount of weight the axle is rated to carry. OT is divided into two areas-Maximum Continuous Output Torque (COT) and Maximum Short Duration Output Torque (MOT). Both are measured in pound-feet. COT is simply the amount of torque the axle can handle hour after hour, day after day without failure. MOT rates the momentary torque spike the axle can handle, such as from a spinning tire suddenly gaining grip.

Eighty percent of the time, the axleshafts are the weakest link in any axle assembly. Their strength is generally a major part of the OT rating offered by the manufacturer because they are at the end of the torque multiplication. Lower gears produce the most axle load from torque multiplication, and tall gears the least. OT is generally calculated according to worst-case loads, i.e., the lowest ratio offered by the axle manufacturer (generally about 4.56:1). Yes, really low aftermarket ratios may push axle loads outside of the original manufacturer's parameters and you should factor this into your calculations. Bear in mind that as ratios get lower, the pinion-tooth count decreases, tooth engagement is less and tooth size gets smaller. It's difficult to calculate exactly where the "magic number" dividing line is for failure. It's far more likely if you are running a lot of gear reduction (torque multiplication) in front of the pinion from a deep First gear, splitter or low-geared T-case, or even if you have a very torquey engine. This scenario may also overstress the pinion shaft and you can calculate pinion-shaft strength the same way as with axles below. If your existing axle is close to the ragged edge of strength, you plan difficult four-wheeling, have a very heavy rig and need ratios lower than 4.56:1, the safest cure is to go up a ring-gear size so the pinion shaft and teeth will be more robust.

Differentials and carriers have weak links that are difficult to calculate. Remember that all the torque, including from the reduction of the ring-and-pinion gears, will be transferred through the carrier, pinion (spider) gears, pinion shaft and side gears. Some open diffs are notoriously strong, especially four-pinion carriers, and some are notoriously weak. There are some improved open carriers available for certain axles, but most people upgrade by installing aftermarket lockers or limited-slips that include new carriers, which are vastly stronger than the OE units.

Weak carriers may provide an even weaker link when used with plug-in lockers (i.e., Lock Right, E-Z Locker, Aussie Locker). If the carrier is a strong one, the plug-in locker is a great, inexpensive, effective alternative. If it's weak, it will fail even faster. Bottom line, if you have a notoriously weak carrier, opt for a locker or limited-slip that includes a new carrier, or upgrade the open carrier if that alternative exists.

Hubs, bearings and wheel studs also count in the strength equation. It's not hard to figure that big tires will put greater stress on wheel bearings and hubs. Fortunately, these parts are generally safe if your axle assembly is matched in strength according to your tire size. Often overlooked are wheel studs. No five- or six-lug 4x4 that works hard should have studs smaller than 1/2 an inch in diameter. These studs are the last direct torque connection between the axle and the wheel. It's generally easy to upgrade a hub from 7/16 to 1/2 inch. Going to 5/8-inch studs is more difficult. Small bolt patterns, such as the 5-on-4.5-inch used on many Jeeps, Chevy S10s, Rangers and so on are best upgraded to 5-on-5.5-inch or six-lug patterns if tire diameter will be greater than 35 inches.

The rigors of hard trail work and the extra leverage of big tires can stress a housing past the breakage point. Housing strength is a combination of the strength of the iron center housing and the diameter, wall thickness and material of the tubes. Given equal wall thickness, the larger-diameter tube is stronger and given equal diameter, a thicker wall tube is stronger. Cold Drawn or Drawn Over Mandrel (DOM) tube is stronger than the common HREW (Hot Rolled Electric Welded) tube, and seamless tubing is the strongest of all. Center housings are commonly of either grey cast iron or nodular iron, with nodular being stronger by 50 to 100 percent (depending on grade used). Since the late '80s, almost all housings are of nodular iron of one grade or another. Again, it's difficult to calculate precisely what's needed, but we can generalize. Based on observation, lighter rigs like Jeeps that are worked hard need at least 2.75- to 3-inch tubes, 0.250-inch wall, or better, in the rear, and 2.75-inch 0.313 or 0.380-inch wall up front. That's equivalent to many 1/2-ton trucks. Half-tons in the hard-worked category should have 2.75-inch, 0.50 wall or 3-inch 0.313 or 0.380 wall tubes up front (or better), and 3-inch 0.50 wall in back. Many solid-axle 3/4-tons have smallish axles up front, even though they may have 0.50 wall tubes, and should really be upgraded to 1-ton Dana 60s with 3.125 tubes with 0.380 or 0.50-inch-wall tubes.

The difference in strength between the 29-spline (left) and 27-spline Mopar 8.25 axles is on the order of 25 percent. It's a bolt-in swap to boot. This is an easy way to gain strength to compensate for larger tires and/or lower gears.

Shaft strength can be reasonably accurately calculated on paper. Since the axleshaft is the most common first failure, it provides a useful yardstick by which to judge overall strength. There are a lot of ways to look at axleshafts, including diameter, spline count and so on, but the most useful is minimum diameter. The smallest part of the shaft is the weakest part, wherever it is. Most often, but not always, it's at the minor spline diameter (MSD). This is the diameter at the bottom of the splines, with more teeth having a larger MSD and vice versa. You will also see different spline pressure angles, i.e., the angularity of the teeth (45 degrees versus 30 degrees), and these equate to a slight difference in MSD, but it's not a hugely important strength issue. The main thing is that the types don't interchange.

A good acronym to use in evaluating axles is DMD, or Diameter, Material and Design. Diameter (minimum diameter) is the most important. Bigger is better. An increase of 0.100-inch in the diameter of a 1-inch shaft (a 10-percent increase in size) equals a 33-percent improvement in strength. Material is almost as important. There are many steel grades, but only a few used in axles. A nearby sidebar rounds them up. There can be a strength difference of 100 to 150 percent between the high-performance 4340 alloys and the ordinary 1040 carbon steel often used for OEM applications. There are options in between these as well, so there are plenty of choices.

Finally, we come to design. The shape and physical characteristics of the axle can contribute to strength. A "fluted" or "waisted" axle has the main section of the shaft at about the same diameter as the MSD. The transition point at the spline roots, where mainshaft diameter dips into the splines on a common axle, is a high-stress area and where non-fluted axles often break. Fluted axles eliminate these stress risers. Unfortunately, fluted axles are rare.

The smoother the surface of the shaft, the less chance there will be of a stress riser being formed, hence the value of polished or micropolished shafts. Cryogenic treatments, during which the metal is subject to temperatures as low as 300 degrees below Fahrenheit, add slightly to strength and greatly to fatigue resistance.

The way splines are formed can add or detract from strength. At the top of the list are rolled splines, which are rolled into the steel under very high pressure. This forges the steel at the splines and increases strength in that area. Close behind are hobbed splines. In this case, the curved-tooth profile (called an involute spline) is machined, rather than rolled. There are cut splines, which have a flat profile and are not compatible with parts designed for an involute profile. Though nitpicked in many circles, the difference between properly done rolled and hobbed splines is very small.

There is a way to calculate the strength of an axleshaft of a given material and diameter (see the "Useful Formulae" sidebar). It's subject to some variable factors but can get you in the 10-percent ballpark, or better. Use it when you have no other strength yardsticks or if you've upgraded an axle with aftermarket shafts. Remember that a stronger shaft may expose weak links elsewhere in the axle.

1) Weigh the vehicle to get the weight on the front and rear axles individually. You can do this at truck scales, waste-disposal sites, scrapyards, grain elevators and so on. Load your rig as you would for the trail, using the actual gear or simulated weight in the same relative position in the vehicle as it will be on the trail. Drive halfway on, getting the weight on the front tires and halfway off and getting the rear weight. The front and rear weights should equal the total.

2) Calculate traction torque using the axle weight and the radius of the tires you run, or will run. Use a CoF based on your style and type of 'wheeling. We'd recommend a worst case, especially if you rockcrawl in hard-core terrain. Use at least 0.7 and if you are wild and crazy, use 1.0. You can also add weight to account for weight transfer if you need to generate more worst-case scenarios. Add 25 percent more weight to the rear axle to be safe.

3) Compare traction torque to MOT or to the calculated axleshaft strength. Traction torque should be lower than MOT or axle strength. If not, it's upgrade time. If it's within 10 percent, you are probably better off upgrading in some way. In front-axle applications, compare traction torque to U-joint strength (see sidebar). Judge this against the terrain in which you generally run. If you don't 'wheel hard, you can get by at or near the limits, but remember those limitations. Moderating tire diameter gives you a lot of flexibility in retaining drivetrain strength without a lot of axle mods.

Use the Traction Torque formula in the "Useful Formulae" sidebar to calculate traction torque at your axle weight at both the existing tire radius and the radius you project. Subtract the lesser from the larger and the difference is the added torque load your axle will see when wearing the bigger tire. Remember, radius is half diameter. Here are some common tire swaps pre-calculated for you.

Tire Diameter (in.) Percent Increase inAxle Torque
28-31 10.6
28-33 17.8
28-35 25.0
28-37 32.1
31-33 6.5
31-35 12.9
31-37 19.4
31-38 22.6
35-38 8.5
35-40 14.3
35-44 25.7

Axle Torque- Aka output torque. The amount of torque delivered to the axleshaft; engine torque multiplied by the transmission gear ratio, transfer-case gear ratio and ring-and-pinion ratio.

Driveline Torque- The amount of torque delivered to the axle at the pinion, which is engine torque multiplied by the transmission gear ratio and transfer-case ratio.

Grip- Weight on the tire plus the coefficient of friction of the ground surface and the added effects of the tread design equals grip. The ground surface is infinitely variable. Weight is variable according to how much stuff you load for a particular trip and according to weight transfer from acceleration or uneven terrain.

Spline Diameter- The axle diameter over the splines. The commonly touted axle-size number, but the least accurate in terms of strength.

Tensile Strength- The point just before metal breaks.

Torque Multiplication- Gearing multiplies torque (but not horsepower). If the engine makes 250 lb-ft, then a 4:1 tranny First gear multiplies it four times to 1,000 lb-ft. That 1,000 lb-ft enters the transfer case and when in low range, it's multiplied by the low-range ratio, and again by the axle ratio.

Traction- Engine torque turned into vehicle motion. Essentially, it's tire grip plus torque.

Yield Strength- The point at which the material permanently deforms. This is the most important steel property, because when it reaches this point, it's lost most of its strength, even if it hasn't broken yet.

(Typical specs shown, actual may vary)

SAE Classification Yield Strength (psi) Tensile Strength (psi) Note
1040 106,000 120,000 1
1050 146,000 162,000 2
1541H 164,000 181,000 3
4340 210,000 228,000 4
300M - 270,000 5

1. Induction-hardened carbon steel, the industry standard for most of the OEM.

2. A higher grade of carbon steel sometimes used by the OEM and the better manufacturers of aftermarket OEM replacements. About 38 percent stronger than most 1040 grades.

3. A high silicon, manganese steel alloy. Approximately 55 percent stronger than 1040.

4. Chrome-moly steel alloy. Excellent strength and resistance to fatigue. Almost 100 percent stronger than 1040

5. Sometimes known as 4340M, it's an aircraft grade of 4340. It has outstanding fatigue resistance. At least 150 percent stronger than 1040, but very expensive and hard to get.

(In lb-ft, from published axle manufacturers' sources)

(lb-ft)* (lb-ft)**
AAM/GM 7.25 IFS - 2,333
AAM/GM 8.25 IFS - 3,581
AAM/GM 9.25 IFS - 4,643
AAM 9.25 Front - 4,663
AAM/GM 7.63 SF Rear - 3,165
AAM/GM 8.5/8.6 SF Rear - 3,829
AAM/GM 9.5 SF Rear - 4,993
AAM/GM 10.50 FF Rear - 6,242+
AAM/GM 11.50 FF Rear - 8,321+
Dana 28 TTB 575 2,350
Dana 30 Front (all) 640 2,350
Dana 35 TTB & IFS 920 3,700
Dana 44 Beam & TTB 1,100 3,460
Dana 50 Beam & TTB 1,200 5,000
Dana 60 Front 1,500 5,550
Dana 70 Front 2,000 8,000
Dana 35 Rear 870 3,480
Dana 44 Rear 1,100 3,460
Dana 60, 61-SF Rear 1,500 5,500
Dana 60, 61HD-FF Rear 1,500 6,000
Dana 70, 70U Rear 2,000 8,000
Dana 70 HD Rear 2,000 8,800
Dana 80 rear 2,500 10,000
Ford 8.8 IFS/IRS 1,250 4,600
Ford 7.5 Rear 870 3,230
Ford 8.8 (28spline) Rear 1,250 4,600
Ford 8.8 (31spline) Rear 1,360 5,100
Ford 9.75 SF Rear 1,600 6,100
Ford 10.25 FF Rear 2,000 8,300
Ford 10.50 FF Rear 2,900 10,660

A "+" indicates that the manufacturer offers higher-rated versions.
*Continuous Output Torque
** Maximum Output Torque

(Ultimate strength [UTS] from destructive tests)

Spicer # UTS (lb-ft) Spicer (lb-ft)
or Brand Yield
260X (D30) 2,950 NA
297X (D44) 4,600 1,600
760X (D44) 5,400 NA
627X (D50) 4,890 2,260
332X (D60) 6,200 3,300

Aftermarket super-strength U-joints like the CTM, the OX or the Yukon are at least two to three times stronger than OEM styles, but no exact test info is available as yet.

These are generalized and should be considered approximate. Actual yield may be higher or lower than shown here, depending upon actual steel formula and heat treatment. The selection of diameters covers many of the common dimensions found out there. Most OE axles are of 1040, some are of 1050. Many aftermarket axles are of 1541 or 4340.

Size Material Yield Note
1.00-inch 1040 carbon steel 1,731 1
1.00-inch 1050 carbon steel 2,385
1.00-inch 1541 carbon steel 2,679
1.00-inch 4340 alloy 3,430
1.10-inch 1040 carbon steel 2,313 2
1.10-inch 1050 carbon steel 3,185
1.10-inch 1541carbon steel 3,578
1.10-inch 4340 alloy 4,582
1.11-inch 1040 carbon steel 2,371 3
1.11-inch 1050 carbon steel 3,266
1.11-inch 1541 carbon steel 3,669
1.11-inch 4340 alloy 4,698
1.125-inch 1040 carbon steel 2,569 4
1.125-inch 1050 carbon steel 3,399
1.125-inch 1541 carbon steel 3,818
1.125-inch 4340 alloy 4,889
1.18-inch 1040 carbon steel 2,835 5
1.18-inch 1050 carbon steel 3,939
1.18-inch 1541 carbon steel 4,424
1.18-inch 4340 alloy 5,665
1.25-inch 1040 carbon steel 3,392 6
1.25-inch 1050 carbon steel 4,672
1.25-inch 1541 carbon steel 3,818
1.25-inch 4340 alloy 6,720
1.29inch 1040 carbon steel 3,725 7
1.29-inch 1050 carbon steel 5,131
1.29-inch 1541 carbon steel 5,763
1.29-inch 4340 alloy 7,380
1.34-inch 1040 carbon steel 4,174 8
1.34-inch 1050 carbon steel 5,738
1.34-inch 1541 carbon steel 6,446
1.34-inch 4340 alloy 8,254
1.36-inch 1040 carbon steel 4,325 9
1.36-inch 1050 carbon steel 6,012
1.36-inch 1541 carbon steel 6,753
1.36-inch 4340 alloy 8,647
1.38-inch 1040 carbon steel 4,557 J
1.38-inch 1050 carbon steel 6,277
1.38-inch 1541 carbon steel 7,051
1.38-inch 4340 alloy 9,029
1.42-inch 1040 carbon steel 4,964 K
1.42-inch 1050 carbon steel 6,837
1.42-inch 1541 carbon steel 7,680
1.42-inch 4340 alloy 9,834

1. A typical 10-spline Dana minor spline diameter, as well as old Land Rovers.
2. The necked-down section on GM 28- and 30-spline front axleshafts as well as 30-spline front Dana axleshafts. Also the minor diameter of OE stub axle of Dana 44 and 10-bolt front axles, 19-spline.
3. Dana 30 27-spline minor spline diameter.
4. Dana 44 19-spline minor spline diameter.
5. Dana 30-spline minor diameter, as used in D44, D60 full-float and D60 "Wimpy-60" semi-float. Also Ford 28-spline minor diameter, as used in 7.5-inch, 8.8-inch and 9-inch.
6. Minor spline diameter of Dana 30-spline outer axle on Dana 60 and Dana 50.
7. Ford 31-spline as on 9-inch and 8.8-inch units.
8. Minor spline diameter for GM H-052/072 and 10.25 "14-bolt" 30-spline.
9. Ford 10.25/10.5 full and semi-float axles, 35-spline.
10. Dana 60, 70 and 80 35-spline minor spline diameter.
11. GM 9.25, 9.5, 33-spline minor spline diameter.


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