The wheel is the most important invention after fire and when these two combine (among many other inventions), you get a car, the most complex commercially available product in the world. On the other hands, axles are what makes your car steer, while the suspension is what keeps it firmly on the road. So, these parts of your car are what makes it move both horizontally and vertically. Let’s take a closer look into these parts of your car to see how they work to keep you safe and moving.
In general terms, an axle is a shaft between the wheels, which can be fixed, or rotated in unison with the wheels. Historically, the main examples of the wheel and axle setup were winch, mill, and even a doorknob. The main mechanical advantage of the wheel and axle system is the ability to exert a large force on the load attached to the axle through the force used on the periphery of the wheel. As an example of this advantage, think of an old winch well, and how much harder would it be to crank the axle itself instead of a large wheel.
In automotive terms, this component has several definitions. Typically it’s either the conventional definition, or it’s referred to the entire system between the two parallel wheels, including axle casing, and even independent suspension. Aside from holding the weight of the vehicle and its occupants, an axle also needs to withstand the stress of acceleration and braking.
General Types of Axles
A straight axle represents the use of a rigid shaft system in vehicles, mostly locomotives, train wagons, rear axles of trucks and some off-road vehicles. It puts the emphasis on rigidity and load carrying, making sure the wheels remain steady under pressure. For additional strength, they can be reinforced in a housing running the entire length of the axle shaft.
A tandem axle setup is usually seen on heavy load trucks and trailers and it’s a system where two or more straight axles are grouped together. A common example of a tandem axle is a truck with a pair of axles in the rear, towing a trailer with three axles towards its end. This allows the truck to burden some of the load on its rear axle, to improve overall load capacity. Tandem axles usually come with double-stacked wheels on either side of the individual axle shaft.
Split-axle is used in most conventional cars. Yours one as well probably, due to the design where each of the wheels has its own separate shaft, and in some designs complete independent suspension. While this lowers the load capacity, it makes for a much smoother and comfortable ride. Both designs can use a differential, which allows the parallel wheels to corner at different speeds. As the car turns left, the left wheel would move slower than the right, and vice-versa. This system improves traction, handling, reduces tire wear and extends their lifespan.
Drive Axle Construction
The drive axle is a split axle design which also transfers the power from the engine to the wheels. To better understand the concept of the drive axle, we need to have a look at the powertrain, or drivetrain, when the engine is excluded.
The engine generates power and sends it to the transmission, which consists of a gearbox and clutch. Engine’s power is converted into rotational force by the gearbox, while the gears optimize the power depending on the speed. The clutch separates the connections between the engine and gearbox, which allows safe gear changes and for the car to remain still while running.
The driveshaft is a rotating shaft that connects the gearbox to the transmission in a vehicle where the drive axle is on the opposite end from the vehicle in relation to the gearbox. If they’re located on the same side, the driveshaft is not needed since the axle and the gearbox are so close to each other, and instead, utilize a transaxle design, where the transmission, axle, and differential are packed into a single unit. Differential, as previously explained, allows the wheels to rotate at different speeds, and is the final component before the drive axle.
Dead Axle Construction
Dead axle or lazy axle is free-rotating, as it is not powered by the drivetrain. Unless it is an all-wheel-drive car, where both the front and rear axles are drive axles, cars usually have one dead axle, opposite of its drive system – rear-wheel-drive cars have a dead front axle, while front-wheel-drive cars have a dead rear axle.
It can be commonly seen on farm equipment, construction machinery, car trailers, caravans, and many other types of vehicles. On trucks and trailers, they’re suitable for load-bearing purposes (in straight axle variant), and in tandem, if a dead axle is located in front of the drive axle, it is called a pusher axle. On motor coaches, they can even be steerable.
What Is a Transaxle?
As I previously explained, the transaxle combines the transmission, axle, and differential into a single unit. The vehicle’s layout generally determines whether the transaxle is used. It can be found on front-engine, front-wheel-drive layout, rear-engine, rear-wheel-drive layout, and mid-engine, rear-wheel-drive layout.
While the front-engine, rear-wheel-drive layout usually positions the transmission in the front, some designs incorporate a rear-mounted transaxle to better distribute weight and are commonly used on sports cars. Since the driveshaft doesn’t have to endure the torque multiplied by the ratio of the first gear, there is less stress applied to it too.
Notable examples of transaxle designs include: Porsche 924, 944 and 968, but Porsche 911 too. Italian cars using a transaxle setup are Alfa Romeo Giulietta, Ferrari 456 and 550, but a good old Corvette C5 has it as well. Audi’s quattro four-wheel drive, alongside VW Group equivalents, utilize longitudinal engine orientation with a transaxle mounted behind the engine, containing the gearbox, center differential, front differential, and final drive unit all in one. Nissan GT-R and Ferrari FF are unique in their design, as they utilize a front-engine, all-wheel-drive layout with the transaxle mounted in the rear.
Lift Axle Design
Lift axle can be found on trucks and trailers and its defining feature is a mechanism that raises and lowers the axle if needed. It is useful for increasing the cargo weight threshold, spreading out the weight, which can allow the vehicle to cross a weight-restricted bridge.
The reasoning behind the lift system lies in the fact that such a vehicle has a very poor turn radius, and puts a lot of stress on the lift axle, which is usually positioned in as the first in the rear tandem.
Other benefits include better traction of other wheels and mitigation of unnecessary tire wear. Modern systems include electronically managed systems, which lowers the axle automatically once a weight threshold has been reached, manually risible when better maneuverability is required.
Semi-Floating Axle Design
While there’s the floating axle, the semi-floating axle design is used in cars, namely lighter all-wheel-drive trucks, as well as SUV and rear-wheel-drive cars. In this design, the axle shaft serves both to support the weight of the vehicle as well as torque transfer from the differential to the wheels.
The main difference compared to the full-floating axle is that the semi-floating axle has one bearing and the method of axle removal. Semi-axle can only be removed after the wheel comes off, and if it were to break, the wheel is likely to fall off.
Off-Road Vehicles and Portal Axles
The portal axles often called portal gear lifts is a system used for off-road vehicles to provide higher ground clearance without the need for oversized wheels. This is achieved by having the axle tube or half-shaft above the central link point of the wheel, with a simple gearbox connected to each wheel to transfer power. Another benefit of this design is the reduced load on the differential and axle crown wheel. It has seen use on the Hummer H1, Mercedes G500 4×4², G63 AMG 6×6, Toyota Mega Cruiser, as well as Land Rover Defender.
Parts of a Wheel
The brakes and the wheels are the outermost parts of the axles that work to drive and stop your car.
The wheel itself is simple in construction, as it consists of a rim constructed in steel or aluminum alloy and a tire. We’ve already covered the topic of rims together, and why you should invest in high quality wheels or keep your car stock, but let’s see what’s more to the wheel.
A wheel also acts as protection and cooling of disc or drum brakes. The brakes are usually attached to strut bars, and if your brakes are discs, they consist of brake rotor, a disc that rotates alongside the wheel and a brake caliper that grips it when the brake pedal is pressed. Brake pads are there to ensure friction and stop the car. Drum brakes are rarely seen today, except on subcompact cars and they work via a rotating drum and the shoe, the frictional part of the brake.