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Putting the Power Down: Wheels, Tires, Suspension & Steering


Text by Dmitriy Orlov

The last article defined the drivetrain and explained how engine rotation turns into axle and wheel rotation. Transmissions, differentials and axles work together to most efficiently use the engine’s power to move the weight of a car.

The suspension system of a vehicle connects its drivetrain to the body or frame, allowing the wheels to conform to the road, either to make the ride more comfortable by isolating the harshness of the road, or to ensure the optimal grip and traction needed in racing applications. The steering system allows the driver to define the direction of the car and is part of the suspension system. The wheels and tires provide contact with the ground and can be chosen based on comfort, style and performance.

Wheels

Without the wheel there would be no car – it supports the vehicle’s weight and minimizes the resistance and effort in moving that weight.

Modern wheels have come a long way from their wooden predecessors and their function and design have changed drastically; they are lighter, stronger and contain styling elements. Wheels come in thousands shapes and sizes, and depending on the specific application, can be designed for form, function or both.



Performance wheels are typically wider to allow for the use of wider tires, creating a larger contact patch to increase traction. Various spoke designs are implemented to reduce weight and rotating mass - a lighter wheel enables a car to get rolling that much quicker. Ample brake cooling is another consideration when choosing a spoke design. Premium and performance wheels are made usually made of aluminum alloy, magnesium or even carbon fiber. The cheaper wheels found on economy cars are steel.

With the popularity of automotive customization, wheels have become one of the primary elements that allow enthusiasts to set their cars apart from others. Various colors and finishes are applied to wheels for a different look, and in many cases, looks take precedent over function when selecting wheels. This means wheels can be larger and wider for an optimal fitment, as defined by popular trends, such as having the outside edge of the wheel flush with the fender and the wheel tucked into the wheel arch to best fill it out.



Tires

The tire is a whole other science in itself. Selecting tires and then balancing them with the right wheels is essential in controlling the comfort level and performance capabilities of a car. The vehicle may be an engineering marvel, but without tires to support it, it is useless.

Depending on the required handling and traction characteristics, different compounds and rubbers are engineered for specific tire applications. The tread pattern, or the design of the tire, also defines its purpose. Racing and performance tires have stickier rubber and minimal tread patterns for high grip and maximum contact with the road. Winter tires utilize a softer rubber compound better suited for cold conditions and their deep, knobby tread pattern provides the best possible grip in deep snow. All-season tires are somewhere in the middle and attempt to find a compromise by balancing dry, wet and snow performance with various tread patterns and compounds. Other factors are incorporated into tires, such as sidewall reinforcement, which either helps with cornering performance or allows tires to be run flat without deflating from lack of air pressure.



Tire size is determined by the wheels and the specific application; let’s consider a tire size of 255/20R18. The first digit is the width or "footprint” of the tire in millimeters. The second number is the aspect ratio of the tire, meaning the ratio of the width of the tire to its height. In this case, it would be 20 percent of the width. The last number is the wheel size in inches, which would make this example fit 18-inch wheels. Letters can also be included in tire size nomenclature to designate type of weather, compound and speed rating (how fast it can rotate).

Suspension

The suspension, like the drivetrain, is a critical vehicle system. It connects the car to the road, sets the comfort level, dictates the handling and performance characteristics, and in some cases, augments a vehicle’s styling. The suspension is the link between the chassis and the wheels and supports the entire car. Springs and shocks, connected to control arms and various linkages, absorb bumps and conform to imperfections and variations in the road.

Weight transfer and body roll are controlled by the suspension, which distributes the weight evenly across all four wheels, or wherever is necessary for the best traction. Under acceleration, weight is transferred to the rear of the car, while braking shifts weight to the front end; when cornering, the weight will move to the left or right. By upgrading and fine tuning the suspension, a vehicle can be made to handle better under different driving conditions.



Suspensions designed with comfort in mind utilize larger, softer springs paired with more forgiving shock absorbers, giving the car a floating, "boat-like” feel. Modern, high-end suspensions can be computer-controlled and are adjustable, allowing drivers to switch between comfort and performance settings with the push of a button.

The springs hold the weight of chassis, either compressing or expanding to accommodate bumps and dips in the road’s surface to ensure the wheel is always planted. Shocks, struts and dampers smooth out the response of the spring - without them, the car would be bouncy. A sway bar is a flexible steel rod that connects the left and right side suspension components and is designed to reduce body roll through cornering. Suspension arms are a network of linkages that connect the wheel hubs to the chassis. The geometry of these linkages determines how the wheel travels under compression and rebound, as well as how the tire contacts the ground under various conditions. An alignment service adjusts these links to fine-tune the suspension to the specific style of driving the vehicle is used for.

Headed in the Right Direction

The steering system in a car controls its direction of movement, turning the wheels in the required direction. Most commonly the front wheels are steered, although some vehicles featured rear-wheel steering, either opposing the front wheels to reduce the steering radius, or in the same direction to facilitate lane changes on highways.



The rack and pinion system is the most common steering system found on cars. The left and right wheels are connected by a linkage and a rod with gear teeth. The rod is moved from side to side by a gear, connected to the steering wheel. Turning the steering wheel rotates this gear - the teeth on the gear move the rod one way or the other - and turn the wheels.

Older cars used a mechanical steering system which made it very hard to turn the wheels at low speeds. Modern cars have power steering where a hydraulic pump is used to assist with turning the wheels. Furthermore, electronic power steering is implemented on high end cars to change the rate of steering at various speeds - quicker steering for low speed response and slower for high speed.



The alignment of the steering system is crucial for both performance and safety. Poor alignment can increase the wear rate in tires and change the way the car responds when cornering.

Wrap-Up

It will be interesting to see what new technologies will become available in the world of automotive suspensions. Electronically-controlled, variable suspensions already exist and are changing how vehicles handle and how they are modified. Some mass-produced cars already have fully-adjustable suspensions, where the driver can change the stiffness, ride height and response. This allows one vehicle to adjust to a variety of driving conditions, from the smooth asphalt of a racetrack to the potholes of public roads. In the future, maybe cars will simply adjust themselves, reconfiguring and realigning as necessary.

Forward motion is generated by the engine and drivetrain, made possible with the wheels and tires and supported by the suspension system; however, it also needs to be controlled. If something speeds up, it will inevitably need to slow down. In the next article, we will talk about automotive braking systems and how they contribute to the driving experience.


Other articles in the Building Blocks Series - The Basics:



Power into Propulsion: How the Transmission & Driveline work



RPM To MPH: How Car Engines Work

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