Axles: We use them and abuse them, every 4x4 has them, and changing parts on them is what we do to turn bigger tires, climb steeper hills, and crawl massive boulders. But what are all those parts in an axle and how do they work together? Axles do the job of taking torque from the engine, transmission, and transfer case and redirecting it to the tires. This month we'll address the multiple components including the housing, ring-and-pinion, and the differential. Next month we'll reveal the secrets of axle-shafts, various bearings and seals, and front axles that have some sort of steering joint, stub shaft, and steering knuckles. If all this sounds like mumbo jumbo to you then hopefully these stories will help clear the fog.
There are two main styles of axles: independent and solid. The tried-and-true off-roader will swear by his solid axle, and considers independent units nothing but junk, better suited for grocery-getters and sissified dork-utes. But with nearly 20 years of independent axles being delivered from the OEM factories they are definitely common and being used on many trails, especially at high speeds.
Independent Suspension Axle An independent suspension axle has the housing solidly mounted to the frame and has half shafts running from the housing to knuckles or end housings at each wheel which can move independent of each other (thus the term independent). Most independent axles are found in the front of '87-and-newer 4x4s and are referred to as independent front suspension, IFS for short.
Solid Axle Under any 4x4 the most basic style is a solid axle (also known as a straight-axle). A solid axle is compromised of a housing that solidly attaches the wheels from side to side such that when one wheel articulates (moves up and down) the other wheel moves in the same or opposite direction. This housing is also attached to the vehicle's frame via some sort of suspension (spring), allowing the entire axlehousing to travel up and down with the tires following suit.
A solid axle often has more unsprung weight (this refers to any weight not supported by the suspension) than an independent suspension axle and less ground clearance. When doing high-speed driving, minimal unsprung weight is beneficial and thus independent suspension is chosen in most motorsports, but currently there is not an affordable independent suspended axle design that can match the strength and reliability of a solid axle for severe off-road use in rocks (though we would like to see one). Since most four-wheelers desire a solid-axle vehicle, we will be concentrating on them for this story, though many of the components are universal to both.
The axlehousing is basically a metal container that protects and holds all the important moving parts inside. The most important job of the housing is to keep the ends of the axle in a straight, rigid line, thus preventing gear or bearing failure or leaks at the seals. There are two major types of housing: the integral carrier version (also known as the Salisbury or Dana style) and the removable carrier version (also known as the Hotchkiss or drop-out style).
Integral Carrier housing An integral carrier housing usually consists of a cast centersection with tubes pressed in that are then plug-welded or riveted through holes in the centersection. In an integral carrier housing the differential must be assembled within the centersection and is supported by two carrier bearings that are clamped to the internal edges of the housing with bearing caps. Common versions of this housing include all the Dana or AMC axles found in Jeeps and other SUVs and trucks as well as the GM 10-, 12-, and 14-bolt. This type of housing requires that the gears and differential be set up within the housing, and as such it requires a heavier cast centersection.
Removable Carrier housing The removable carrier style housing is commonly a stamped- or forged-steel assembly that a cast third member, which holds the gears and differential, is then bolted into. Common versions of this style axle are found in Toyota trucks as well as Suzuki Samurais and older GM 3/4-ton trucks. The most common and well known, however, is the Ford 9-inch. This style axle is known for being lightweight, yet not usually as strong as a comparable integral carrier style unit. However, they can be made very strong with gusset reinforcements or completely fabricated aftermarket housings.
Residing within the housing centersection or third member is the ring-and-pinion, also referred to as the gearset. These are the parts that redirect the rotational force coming longitudinally down the chassis from the engine via the driveshafts and send it laterally outwards to the tires. The pinion is attached to the driveshaft via a U-joint and pinion yoke or flange, and turns at the same speed as the driveshaft while supported by two or three pinion bearings. At the end of the pinion are gear teeth, and they mesh with the teeth on the ring gear. The ring gear turns at the same rotations per minute (rpm) as the tires, and the ratio of the number of gear teeth on the pinion to gear teeth on the ring gear determines your ring-and-pinion ratio. So if the pinion has eight teeth and the ring gear has 41 then the pinion must turn roughly 5.13 times (41 divided by 8) in order for the ring gear to turn just once, thus this gearset has a 5.13:1 ratio.
The torque that the engine makes in one rotation (after being multiplied by the transmission and transfer case) is multiplied by the ratio of the ring-and-pinion. Using 5.13: 1 as the example, the engine torque in one rotation of the driveshaft is applied to turning the tires just 1/5.13 turn or about 20 percent of a revolution. If the gearing were 4.10 it would have to turn 25 percent of the revolution. However, with equal size tires in equal transmission and transfer-case gears, the 5.13 ratio would also require the engine to spin approximately 25 percent faster per single rotation of the ring gear versus the 4.10 ratio gears. Thus when that revolution is put over time you can see why a 5.13 gear would need to turn 5,000 rotations per minute (rpm) to turn the tires at the same speed a 4.10 gear would be turning them at around 4,000 rpm.
If that has your head spinning, try following this bit: The higher the ratio, the "lower" the gear is said to be, such that a 5.13 gear is considered 'lower" than 4.10 due to the fact that it offers more torque multiplication, similar to how low range offers more torque multiplication than high range in a transfer case.
Reverse Rotation vs. Standard Rotation (also known as High Pinion vs. Low Pinion) If you are confused at this point you may want to skip over to one of our feature truck or trail ride stories for a bit and let the facts sink in a while. If you're coming back for more, we'll now touch on reverse rotation versus standard rotation axles. More common standard rotation ring-and-pinions have a helical-style cut that delivers maximum strength when located in the rear axle of a vehicle. This is because the teeth on the ring gear are cut with a "drive side" designed to take the loads better than the "coast side" or backside of the teeth, but it also results in a lower pinion design with the pinion below the centerline of the axletube. Though this axle ring-and-pinion can be used in the front of a vehicle, it actually makes for a less desired setup because the ring gear is now being turned on the weaker "coast side" of the gear. A reverse rotation gearset is cut in the opposite way, making it stronger when used in the front of the vehicle because it is now running on the drive side of the gear, plus it has the more desirable high-pinion design with the pinion engaging the ring gear above the axle centerline. However, each of these gearsets requires its own specific housing centersection, so you cannot put reverse-rotation gears in a low-pinion housing or vice versa. Also if you turn a low-pinion housing upside down it does not make it a high-pinion housing. In fact it will make the tires turn backwards. To sum it up, a reverse-rotation (high-pinion) axle is stronger in the front of a 4x4 than in the rear, a standard rotation (low-pinion) axle is stronger in the rear of a vehicle, and turning a low-pinion axle upside down does not make it a high-pinion.
Within the centersection the ring gear is bolted to some sort of differential. The differential can be open, limited slip, selectable or automatic locking, or spool style. When your 4x4 is making a turn on hard pavement, the inner and outer tires need to turn at different speeds, yet when you are off-road, you usually want the tires to be locked together so that both can help pull the vehicle through an obstacle.
Open Differentials Most axles come equipped with an open differential from the factory which delivers power to whichever tire has the least resistance by using two or four smaller pinion style "spider" gears that mesh with two side gears through which the axleshafts attach. With an open differential, if one tire is in the mud and the other is on solid ground, the tire in the mud will spin where the tire on the solid ground could pull the truck forward if it was getting power.
Limited Slip A limited-slip type differential is similar to an open differential in that it allows different rotational speeds of the tires in a corner, but when going straight it has clutches that engage, and the friction helps keep both tires turning and receiving equal torque even when one has less traction. However, if one tire has no traction and the torque input coming down the driveshaft and the traction of the tires combined is more than the friction the clutches can handle they will break loose and send power to the tire with no traction and you will go no where. In addition, limited slips often require a special additive to gear oil, and the clutches will wear out over time.
Lockers A locking differential is designed to ensure both wheels receive equal amounts of torque no matter how much traction they have, so that even if one tire is in the air the opposite one will still turn and pull the vehicle forward. There are two main styles of lockers-automatic and selectable. Automatic lockers use gears that engage when torque is applied yet still allow some difference in tire speed during cornering. Selectable lockers offer the driver the ability to lock the differential together via a cable, vacuum, airline, or electrical switch. The benefit of a selectable locker is that it acts as an open differential on the street, and then is completely lockable whenever you need it on the trail. The downfalls of this style are the possibility of damage to the activation device or plumbing, and the fact that you will need to remember to turn on the locker before you attempt an obstacle (almost every selectable-locker owner has forgotten to flip the switch on the trail once when he needed it).
Spool The most basic style of differential is a spool, though it really shouldn't even be called a differential because it never lets the wheels turn at different rates. A spool is basically a piece of steel that the ring gear bolts to and the axleshafts are splined into, but that does not have any internal gears. Spools are nice because they have very few parts to break, however they transmit an enormous amount of stress to the axleshafts when driven on the streets, and since they do not allow differences in speed during turning the tire will scrub and chirp around corners, which is harsh on tires. Plus a front axle equipped with a spool is difficult to steer, as the tires want to plow forward. Many low-budget wheelers weld the spider gears of an open differential into a solid unit, basically forming a spool that is commonly known as a Lincoln locker, referring to the welders made by Lincoln. Though we would happily put a spool in the rear of an off-road-only rig, another thing to consider is that when sidehilling in slippery mud, snow, or loose gravel terrain, having a spool or even a locked selectable locker can cause the vehicle to slide sideways, and if that sidehill is above a tall cliff, you may want to take it slowly or tie off your rig to some solid object as you move along.