Car suspension

Suspension is the system of tires, tire air, springs, shock absorbers and linkages that connects a vehicle to its wheels and allows relative motion between the two. Suspension systems must support both road holding/handling and ride quality, which are at odds with each other. The tuning of suspensions involves finding the right compromise. The suspension is crucial for maintaining consistent contact between the road wheel and the road surface, as all forces exerted on the vehicle by the road or ground are transmitted through the tires' contact patches. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.

History

An early form of suspension on ox-drawn carts had the platform swing on iron chains attached to the wheeled frame of the carriage. This system remained the basis for most suspension systems until the turn of the 19th century, although the iron chains were replaced with the use of leather straps called thoroughbraces by the 17th century. No modern automobiles have used the thoroughbrace suspension system.
By approximately 1750, leaf springs began appearing on certain types of carriage, such as the Landau.
By the middle of the 19th century, elliptical springs might additionally start to be used on carriages.

Modern suspension

Automobiles were initially developed as self-propelled versions of horse-drawn vehicles. However, horse-drawn vehicles had been designed for relatively slow speeds, and their suspension was not well suited to the higher speeds permitted by the internal combustion engine.
The first workable spring-suspension required advanced metallurgical knowledge and skill, and only became possible with the advent of industrialisation. Obadiah Elliott registered the first patent for a spring-suspension vehicle; each wheel had two durable steel leaf springs on each side and the body of the carriage was fixed directly to the springs which were attached to the axles. Within a decade, most British horse carriages were equipped with springs; wooden springs in the case of light one-horse vehicles to avoid taxation, and steel springs in larger vehicles. These were often made of low-carbon steel and usually took the form of multiple layer leaf springs.
Leaf springs have been around since the early Egyptians. Ancient military engineers used leaf springs in the form of bows to power their siege engines, with little success at first. The use of leaf springs in catapults was later refined and made to work years later. Springs were not only made of metal; a sturdy tree branch could be used as a spring, such as with a bow. Horse-drawn carriages and Ford Model T used this system, and it is still used today in larger vehicles, mainly mounted in the rear suspension.
Leaf springs were the first modern suspension system, and, along with advances in the construction of roads, heralded the single greatest improvement in road transport until the advent of the automobile. The British steel springs were not well-suited for use on America's rough roads of the time, so the Abbot-Downing Company of Concord, New Hampshire re-introduced leather strap suspension, which gave a swinging motion instead of the jolting up-and-down of spring suspension.
In 1901, Mors of Paris first fitted an automobile with shock absorbers. With the advantage of a damped suspension system on his 'Mors Machine', Henri Fournier won the prestigious Paris-to-Berlin race on 20 June 1901. Fournier's superior time was 11 hours 46 minutes and 10 seconds, while the best competitor was Léonce Girardot in a Panhard with a time of 12 hours, 15 minutes, and 40 seconds.
Coil springs first appeared on a production vehicle in 1906 in the Brush Runabout made by the Brush Motor Company.
Today, coil springs are used in most cars.
In 1920, Leyland Motors used torsion bars in a suspension system.
In 1922, independent front suspension was pioneered on Lancia Lambda, and became more common in mass market cars from 1932. Today, most cars have independent suspension on all four wheels.
The part on which pre-1950 springs were supported is called a dumb iron.
In 2002, a new passive suspension component, the inerter, was invented by Malcolm C. Smith. This has the ability to increase the effective inertia of wheel suspension using a geared flywheel, but without adding significant mass. It was initially employed in Formula One in secrecy, but has since spread to wider motorsport.

Difference between rear suspension and front suspension

For front-wheel drive cars, rear suspension has few constraints, and a variety of beam axles and independent suspensions are used. For rear-wheel drive cars, rear suspension has many constraints, and the development of the superior, but more expensive independent suspension layout has been difficult.

History

's Model T used a torque tube to restrain this force, for his differential was attached to the chassis by a lateral leaf spring and two narrow rods. The torque tube surrounded the true driveshaft and exerted the force to its ball joint at the extreme rear of the transmission, which was attached to the engine. A similar method like this was used in the late 1930s by Buick and by Hudson's bathtub car in 1948, which used helical springs that could not take fore-and-aft thrust.
The Hotchkiss drive, invented by Albert Hotchkiss, was the most popular rear suspension system used in American cars from the 1930s to the 1970s. The system uses longitudinal leaf springs attached both forward and behind the differential of the live axle. These springs transmit torque to the frame. Although scorned by many European car makers of the time, it was accepted by American car makers, because it was inexpensive to manufacture. Also, the dynamic defects of this design were suppressed by the enormous weight of U.S. passenger vehicles before the implementation of the Corporate Average Fuel Economy standard.
Another Frenchman invented the De Dion tube, which is sometimes called "semi-independent". Like true independent rear suspension, this employs two universal joints, or their equivalent from the centre of the differential to each wheel. But the wheels cannot entirely rise and fall independently of each other; they are tied by a yoke that goes around the differential, below and behind it. This method has had little use in the United States. Its use around 1900 was probably due to the poor quality of tires, which wore out quickly. By removing a good deal of unsprung weight, as independent rear suspensions do, it made them last longer.
Rear-wheel drive vehicles today frequently use a fairly complex fully-independent, multi-link suspension to locate the rear wheels securely, while providing decent ride quality.

Spring, wheel, and roll rates

Spring rate

The spring rate is a component in setting the vehicle's ride height or its location in the suspension stroke. When a spring is compressed or stretched, the force it exerts, is proportional to its change in length. The spring rate or spring constant of a spring is the change in the force it exerts, divided by the change in deflection of the spring. Vehicles that carry heavy loads, will often have heavier springs to compensate for the additional weight that would otherwise collapse a vehicle to the bottom of its travel. Heavier springs are also used in performance applications, where the loading conditions experienced are more significant.
Springs that are too hard or too soft cause the suspension to become ineffective – mostly because they fail to properly isolate the vehicle from the road. Vehicles that commonly experience suspension loads heavier than normal, have heavy or hard springs, with a spring rate close to the upper limit for that vehicle's weight. This allows the vehicle to perform properly under a heavy load, when control is limited by the inertia of the load. Riding in an empty truck meant for carrying loads can be uncomfortable for passengers, because of its high spring rate relative to the weight of the vehicle. A race car could also be described as having heavy springs, and would also be uncomfortably bumpy. However, even though we say they both have heavy springs, the actual spring rates for a racecar and a truck are very different. A luxury car, taxi, or passenger bus would be described as having soft springs, for the comfort of their passengers or driver. Vehicles with worn-out or damaged springs ride lower to the ground, which reduces the overall amount of compression available to the suspension, and increases the amount of body lean. Performance vehicles can sometimes have spring rate requirements other than vehicle weight and load.

Wheel rate

Wheel rate is the effective spring rate when measured at the wheel, as opposed to simply measuring the spring rate alone.
Wheel rate is usually equal to or considerably less than the spring rate. Commonly, springs are mounted on control arms, swing arms or some other pivoting suspension member. Consider the example above, where the spring rate was calculated to be 500 lbs/inch, if one were to move the wheel , the spring more than likely compresses a smaller amount. If the spring moved, the lever arm ratio would be 0.75:1. The wheel rate is calculated by taking the square of the ratio times the spring rate, thus obtaining 281.25 lbs/inch. The ratio is squared because it has two effects on the wheel rate: it applies to both the force and the distance traveled.
Wheel rate on independent suspension is fairly straightforward. However, special consideration must be taken with some non-independent suspension designs. Take the case of the straight axle. When viewed from the front or rear, the wheel rate can be measured by the means above. Yet, because the wheels are not independent, when viewed from the side under acceleration or braking, the pivot point is at infinity and the spring is directly inline with the wheel contact patch. The result is often, that the effective wheel rate under cornering is different from what it is under acceleration and braking. This variation in wheel rate may be minimised by locating the spring as close to the wheel as possible.
Wheel rates are usually summed and compared with the sprung mass of a vehicle to create a "ride rate" and the corresponding suspension natural frequency in ride. This can be useful in creating a metric for suspension stiffness and travel requirements for a vehicle.