Axles and CV-Shafts

What are axles and where are they located?

Axles are what connects your wheels to the rest of your car or truck. The axles for the drive wheels send power to those wheels from the transmission. Axles for non-drive wheels simply allow those wheels to spin freely. Since the weight of the vehicle rests, ultimately, on the wheels, the axles have an important role to play in weight bearing. For this reason, some heavy duty trucks (e.g. dump trucks and semis) may have three or more axles. Passenger cars, though, will have two axles—one in the front and one in the rear. Generally, axles will be split into left and right axle shafts that are connected at a differential at the vehicle’s center line. Depending on whether the vehicle is front-wheel drive, rear-wheel drive, or four-wheel drive, and what the suspension layout is, an axle is either a constant velocity axle assembly known as a CV axle, or half-shaft, or a solid axle, which is sometimes called a beam axle. A solid axle that transmits power to the wheels is known as a live axle. Let’s go over the two different types of axles in a bit more detail.

Solid Axles

Solid axles laterally connect the left and right wheel. They act as one unit but are usually made up of two shafts that connect at a differential. Still, in this system, the two wheels move together as one unit. If a bump or dip causes one wheel to move, the other moves in response. This can lead to one wheel leaving the road surface. 

The advantage of solid axles is their solidity. They are durable and easy to manufacture. For this reason they are frequently used on off-road applications. Jeeps have historically used solid axles for both the front and rear wheels. Solid axles’ sturdiness gives them a good load-bearing capacity, so they’re popular for trucks. 

A solid axle that transmits power to the wheels is known as a live axle. By comparison, one that simply allows the wheels to spin freely is known as a dead axle. Some front wheel drive models use dead axles in the rear. One advantage of a solid live axle is that its sturdiness allows it to take a lot of power. This is why, even as many other models have switched to independent suspension, the Ford Mustang has stubbornly held on to the live axle. 

Solid axles may be semi-floating or full-floating designs, with semi-floating being more typical. In a semi-floating axle, the axle ends with the wheel flange where the wheel studs match. The weight of the vehicle, its passengers and its cargo, bear down on the wheel and the shaft. In a full floating axle, the axle shaft ends at a locking hub. The axle housing attaches at the hub via a spindle. Here, the weight rests on the axle housing and not the axle itself. Full floating axles are usually used on heavy duty and commercial trucks, because that system is better for heavier vehicles (since the axle does not take the vehicle weight in addition to torque). Some Land Rovers also use full floating axles. Most cars and light-duty trucks use semi-floating axles.  

CV Axles

Although they aren’t as durable as solid axles, CV axles have the advantage of allowing the wheels to travel independently of one another. They are commonly used for the front wheels since they improve steering and handling. CV axles are an important part of independent suspensions. Each half-shaft is able to move up and down on its own in response to bumps in the road and the response of the suspension. This leads to improved handling compared to if the two wheels were rigidly linked. It keeps from having situations where one wheel is lifted off the ground, which is extra important in front-wheel drive cars, when your drive wheels and your steering wheels are the same. 

On front-wheel drive cars, CV axle shafts transmit power from the transmission and differential to the wheels. Carrying all that rotational power exposes drive axles to high torque stresses. Older axles were connected by universal joints or U-joints that did not rotate with a constant velocity, which exposed the axle shafts to more stress. For this reason, constant velocity joints (CV joints) were invented. A CV axle assembly has two CV joints, one at either end. These allow the axles to move as is necessary for the front steering and suspension and transfer power through the half shaft at a constant velocity. Although designs vary, the movement of the axle in the joint is typically accomplished through a complex system of ball bearings. In recent years, many cars have been made with independent rear suspensions. These use CV axles in the rear as well.

How do I know if my axle needs to be replaced?

An axle is designed to be pretty sturdy, but the powerful forces that act on it can eventually take their toll. Axles can become damaged or worn out, which can affect your vehicle's steering and handling. Potential problems differ on the basis of the construction of the axle shaft. 

CV axle assemblies are more likely to develop problems then their sturdier solid axle cousins. CV axles use constant velocity joints which are covered with rubber accordion like pieces called boots. The most common problem with CV axle shafts is for these boots to become worn, which leads to damage to the CV joint. The boot holds in grease that keeps the CV joint functioning properly. If the boot, which, after all, is only made out of rubber, becomes worn or punctured, it can begin to leak grease, leading to the joint losing lubrication. It can also allow in dirt and debris, which can cause further damage. 

This type of damage can be identified by a clicking noise when you are cornering or even vibration or shuddering. You may also hear a humming or growling sound coming from the wheel area as you accelerate. You can also, of course, visually inspect the boot for damage. 

Solid axles, even though they are sturdier, can still face certain problems. Aggressive driving and bad road conditions could cause a solid axle to become bent. This can lead to wobbling, which turns into a vibration as speed increases. You may also see a visible deflection of the angle of the tires.

Can I replace an axle myself?

First we should note that if your problem is with the CV joint or the boot, it is possible to replace this part alone. That being said, as labor intensive as that process is, you may be better off replacing the entire CV axle assembly. Replacing either type of axle can be accomplished with some effort and patience. You’ll have to put the end of the car where you’re replacing the axle up on jack stands, remove the axle nut in the center of the wheel, remove any steering or suspension parts in your way, and remove the old shaft. Then install the new one and replace whatever other parts you have disconnected. Although in principle this may be simple, be aware that this will take a good amount of time and physical exertion.