Swing axle
Encyclopedia
A swing axle is a simple type of independent suspension
first used in early aircraft (1910 or before), such as the Sopwith and Fokker
, usually with rubber bungee
and no damping.
Some later motor-car rear swing axles have universal joint
s connecting the driveshaft
s to the differential
, which is attached to the chassis
. They do not have universal joints at the wheels: the wheels are always perpendicular to the driveshafts. Swing axle suspensions traditionally used leaf spring
s and shock absorber
s. Volkswagens built before 1967 used torsion bars as their spring.
for two reasons:
These problems were evident on the Mercedes-Benz 190SL
and 300SL
, the early versions of the Porsche 356
, the Triumph Herald
, Vitesse
and Spitfire
, Tatra T603
, Renault Dauphine
, Volkswagen Beetle (until rear suspension modifications, c. 1967) and others.
in his 1965 book Unsafe at Any Speed
highlighted accidents related to 1960-1963 models of the first generation Chevrolet Corvair
, swing-axle design, and identified a Chevrolet engineer that had fought management who eliminated a front anti-roll bar for cost reasons. 1964 models were fitted with a front anti-roll bar as standard equipment, in addition to a rear transverse leaf spring thus improving emergency maneuver stability. Second generation Corvairs (1965–1969) used a true independent rear suspension (IRS) system.
The Hillman Imp
designers learned from the problems with the Corvair, having crashed one at a relatively low speed, and designed their rear-engined car with semi-trailing arm suspension at the rear. However, to attain correct handling balance, they actually used swing axle geometry at the front, with the steering pivots mounted at the outer ends of single swing wishbones. However, these caused too much understeer
and uneven tyre wear, and modifications were made to reduce the positive camber
of the front wheels by lowering the swing axle pivot points. Aftermarket kits were also available to do this, and an inexpensive alternative was to insert a tapered shim to change the inclination of the kingpin carrier relative to the wishbone.
axles in the late 1960s, though live axle
s remained the most common. Most rear suspensions have been replaced by more modern independent suspension
s in recent years, and both swing and deDion types are virtually unused today. One exception is the Czech truck manufacturer Tatra, which uses swing axles and a central 'backbone' tube instead of more common solid axles. This system is claimed to give greater rigidity and better performance on poor quality roads and off road.
suspension system
is not present. Each tire in fact moves with a similar camber change to that of the powered swing axles for the rear wheels listed above. But the pivot point of the axles is located not in the middle of the car but nearly on the other beam of the chassis, so the effect is far less hazardous.
Independent suspension
Independent suspension is a broad term for any automobile suspension system that allows each wheel on the same axle to move vertically independently of each other. This is contrasted with a beam axle, live axle or deDion axle system in which the wheels are linked – movement on one side affects...
first used in early aircraft (1910 or before), such as the Sopwith and Fokker
Fokker
Fokker was a Dutch aircraft manufacturer named after its founder, Anthony Fokker. The company operated under several different names, starting out in 1912 in Schwerin, Germany, moving to the Netherlands in 1919....
, usually with rubber bungee
Bungee cord
A bungee cord , also known as a shock cord, is an elastic cord composed of one or more elastic strands forming a core, usually covered in a woven cotton or polypropylene sheath...
and no damping.
Universal joint
A universal joint, universal coupling, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion...
s connecting the driveshaft
Driveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
s to the differential
Differential (mechanics)
A differential is a device, usually, but not necessarily, employing gears, capable of transmitting torque and rotation through three shafts, almost always used in one of two ways: in one way, it receives one input and provides two outputs—this is found in most automobiles—and in the other way, it...
, which is attached to the chassis
Chassis
A chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the...
. They do not have universal joints at the wheels: the wheels are always perpendicular to the driveshafts. Swing axle suspensions traditionally used leaf spring
Leaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles...
s and shock absorber
Shock absorber
A shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...
s. Volkswagens built before 1967 used torsion bars as their spring.
Comparison
This type of suspension was considered better than the more typical live axleLive axle
A live axle, sometimes called a solid axle, is a type of beam axle suspension system that uses the driveshafts that transmit power to the wheels to connect the wheels laterally so that they move together as a unit....
for two reasons:
- It reduced unsprung weightUnsprung weightIn a ground vehicle with a suspension, the unsprung weight is the mass of the suspension, wheels or tracks , and other components directly connected to them, rather than supported by the suspension...
since the differential is mounted to the chassisChassisA chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the... - It eliminates sympathetic camberCamber anglethumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...
changes on opposite wheels
Shortcomings
However, there are a number of shortcomings to this arrangement:- A great amount of single-wheel camber change is experienced, since the wheel is always perpendicular to the driveshaft
- "Jacking" on suspension unloading (or rebound) causes positive camber changes on both sides, which (In extreme cases) can overturn the car.
- Reduction in cornering forces due to change in camber can lead to oversteer — a dynamically unstableDirectional stabilityDirectional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion. Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation to the oncoming medium when disturbed away from...
condition where a vehicle can lose control and spin — and in extreme cases lift-off oversteerLift-off oversteerLift-off oversteer is a form of oversteer in an automobile that occurs when the vertical load on the tires shifts from the rear to the front quickly due to throttle release while cornering...
.
These problems were evident on the Mercedes-Benz 190SL
Mercedes-Benz 190SL
The Mercedes-Benz 190SL is a two door grand touring convertible with a removable hardtop that was produced by Mercedes-Benz between May 1955 and February 1963...
and 300SL
Mercedes-Benz 300SL
The Mercedes-Benz 300SL was introduced in 1954 as a two-seat, closed sports car with distinctive gull-wing doors. Later it was offered as an open roadster...
, the early versions of the Porsche 356
Porsche 356
The Porsche 356 was the company's first production automobile. It was a lightweight and nimble handling rear-engine rear-wheel-drive 2 door sports car available in hardtop coupe and open configurations. Design innovations continued during the years of manufacture, contributing to its motorsports...
, the Triumph Herald
Triumph Herald
The Triumph Herald was a small two-door car introduced in 1959 by the Standard-Triumph Company of Coventry. Body design was by the Italian stylist Michelotti and the car was offered in saloon, convertible, coupé, van, and estate models....
, Vitesse
Triumph Vitesse
The Triumph Vitesse was a compact six-cylinder car built by Standard-Triumph from 1962 to 1971. The car was styled by Michelotti, and was available in saloon and convertible variants....
and Spitfire
Triumph Spitfire
The Triumph Spitfire is a small English two-seat sports car, introduced at the London Motor Show in 1962. The vehicle was based on a design produced for Standard-Triumph in 1957 by Italian designer Giovanni Michelotti...
, Tatra T603
Tatra T603
The Tatra 603 is a large rear-engined luxury car made by the Czechoslovak company Tatra. It was a continuation of the series of Tatra streamlined sedans started by the Tatra 77...
, Renault Dauphine
Renault Dauphine
Renault Dauphine is a rear-engined economy car manufactured by Renault in one body style — a three-box, four-door sedan — as the successor to the Renault 4CV, with over two million examples marketed worldwide during its production from 1956-1967....
, Volkswagen Beetle (until rear suspension modifications, c. 1967) and others.
Solutions
A number of engineering options could ameliorate swing axle handling:- Anti-roll bar: As a design option, engineers can include a front anti-roll bar which can ameliorate the swing axle car's handling – shifting weight transferWeight transferWeight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects: the change in load borne by different wheels of even perfectly rigid vehicles during acceleration, and the change in center of mass location relative to the wheels because of...
to the front outboard tire, considerably reducing rear slip angleSlip angleIn vehicle dynamics, slip angle or sideslip angle is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing...
s—thereby avoiding potential oversteer.
- Single-pivot point: Mercedes-Benz addressed the handling issues by producing swing axles with a single-pivot point located under the differential, and thus well below the axle. This configuration markedly reduced the tendency to "jack-up" and the later low pivot swing-axle equipped cars were praised in contemporary publications for their handling. The low-pivot swing-axle remained in production with Mercedes-Benz W108Mercedes-Benz W108The Mercedes-Benz W108 and W109 were luxury cars built by Mercedes-Benz from 1965 through 1972. The line was an update of the predecessor W111 and W112 fintail sedans. The cars were successful in West Germany and in export markets that included including North America and Southeast Asia...
280SE and 300SEL until 1972. It was fitted to the 300SEL 6.3, which was during the early 70s the worlds fastest production sedan. AMG-modified 6.3s were also raced with the stock swing axle.
- Tire pressure differential: The Renault DauphineRenault DauphineRenault Dauphine is a rear-engined economy car manufactured by Renault in one body style — a three-box, four-door sedan — as the successor to the Renault 4CV, with over two million examples marketed worldwide during its production from 1956-1967....
, Volkswagen Beetle and first generation Chevrolet CorvairChevrolet Corvair-First generation :The 1960 Corvair 500 and 700 series four-door sedans were conceived as economy cars offering few amenities in order to keep the price competitive, with the 500 selling for under $2,000...
(1960–1964) used a tire pressure differential strategy to eliminate oversteer characteristics of their swing axle suspensions — specifically low front and high rear tire pressure — which induced understeerUndersteerUndersteer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...
. Nonetheless, the tire pressure differential strategy offered a significant disadvantage: owners and mechanics could inadvertently but easily re-introduce oversteer characteristics by over-inflating the front tires (e.g., to typical pressures for other cars with other suspension systems).
Safety
Ralph NaderRalph Nader
Ralph Nader is an American political activist, as well as an author, lecturer, and attorney. Areas of particular concern to Nader include consumer protection, humanitarianism, environmentalism, and democratic government....
in his 1965 book Unsafe at Any Speed
Unsafe at Any Speed
Unsafe at Any Speed: The Designed-In Dangers of the American Automobile by Ralph Nader, published in 1965, is a book detailing resistance by car manufacturers to the introduction of safety features, like seat belts, and their general reluctance to spend money on improving safety...
highlighted accidents related to 1960-1963 models of the first generation Chevrolet Corvair
Chevrolet Corvair
-First generation :The 1960 Corvair 500 and 700 series four-door sedans were conceived as economy cars offering few amenities in order to keep the price competitive, with the 500 selling for under $2,000...
, swing-axle design, and identified a Chevrolet engineer that had fought management who eliminated a front anti-roll bar for cost reasons. 1964 models were fitted with a front anti-roll bar as standard equipment, in addition to a rear transverse leaf spring thus improving emergency maneuver stability. Second generation Corvairs (1965–1969) used a true independent rear suspension (IRS) system.
The Hillman Imp
Hillman Imp
The Hillman Imp is a compact, rear-engined saloon car that was manufactured under the Hillman marque by the Rootes Group from 1963 to 1976...
designers learned from the problems with the Corvair, having crashed one at a relatively low speed, and designed their rear-engined car with semi-trailing arm suspension at the rear. However, to attain correct handling balance, they actually used swing axle geometry at the front, with the steering pivots mounted at the outer ends of single swing wishbones. However, these caused too much understeer
Understeer
Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...
and uneven tyre wear, and modifications were made to reduce the positive camber
Camber angle
thumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...
of the front wheels by lowering the swing axle pivot points. Aftermarket kits were also available to do this, and an inexpensive alternative was to insert a tapered shim to change the inclination of the kingpin carrier relative to the wishbone.
Replacement
Swing axles were supplanted by De Dion tubeDe Dion tube
A de Dion tube is an automobile suspension technology. It is a sophisticated form of non-independent suspension and is a considerable improvement over the alternative swing axle and Hotchkiss drive types. A de Dion suspension uses universal joints at both the wheel hubs and differential, and uses a...
axles in the late 1960s, though live axle
Live axle
A live axle, sometimes called a solid axle, is a type of beam axle suspension system that uses the driveshafts that transmit power to the wheels to connect the wheels laterally so that they move together as a unit....
s remained the most common. Most rear suspensions have been replaced by more modern independent suspension
Independent suspension
Independent suspension is a broad term for any automobile suspension system that allows each wheel on the same axle to move vertically independently of each other. This is contrasted with a beam axle, live axle or deDion axle system in which the wheels are linked – movement on one side affects...
s in recent years, and both swing and deDion types are virtually unused today. One exception is the Czech truck manufacturer Tatra, which uses swing axles and a central 'backbone' tube instead of more common solid axles. This system is claimed to give greater rigidity and better performance on poor quality roads and off road.
Variations
Another use of the swing axle concept is Ford's "Twin I-Beam" front suspension for trucks. This has solid axles (so they do not transmit power). Though it is touted as an independent suspension system in that each tire rises and falls without affecting the position of the other, the parallelogram action of the A-armDouble wishbone suspension
In automobiles, a double wishbone suspension is an independent suspension design using two wishbone-shaped arms to locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint at the knuckle. The shock absorber and coil spring mount to the wishbones to control...
suspension system
Suspension (vehicle)
Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...
is not present. Each tire in fact moves with a similar camber change to that of the powered swing axles for the rear wheels listed above. But the pivot point of the axles is located not in the middle of the car but nearly on the other beam of the chassis, so the effect is far less hazardous.