IRONING OUT THE BUMPS
Understanding how a suspension system works should help diagnose any annoying problems and in our latest beginners guide we take a brief look at several very different classic set ups.
The primary function a car’s suspension system has to cope with is to protect the occupants from road shocks and to ensure the vehicle remains stable and under control when cornering. Solid cart-style front axles mounted on semi- elliptic springs are mainly the preserve of pre-war cars and eventually this crude design gave way to an independent set up consisting of individually sprung front wheels.
One simple form of independent front suspension found on quite a few post-war cars, such as the Austin A30 saloon is the double wishbone arrangement. This set up comprises of wishbone shaped links hinged at the broad end to the car’s body, while the narrow end of the link is attached to a kingpin (the assembly the front wheels swivel on) and a stub axle.
A coil spring sandwiched between the top and bottom links provides the bounce and quite a few sets ups, especially early BMC products, employed a lever arm shock absorber as the top link. However, on cars fitted with a proper upper wishbone a telescopic shock absorber is usually located inside the coil spring to dampen the bounce.
Some front suspension layouts utilise torsion bars for the spring medium. A torsion bar is a length of specially heat-treated steel that twists as the wheel moves up and down. As the bar rotates, it will immediately try to straighten itself out and it’s this movement that provides the bounce. Torsion bars usually have splines at each ends were they attach to a suspension component and the bodywork and feature on such classics as the Morris Minor and VW Beetle.
STRUTS & ANTI- ROLL BARS
Another popular type of front suspension is the MacPherson strut and these are found on a lot of classic Fords, as well as a sprinkling of period Rootes and Vauxhall products. Developed in the US by Chevrolet engineer Earl S. MacPherson in the mid-‘Forties, the first mass produced UK-built car to utilise these space saving components was the 1951-introduced MkI Ford Consul.
A MacPherson set-up consists of a hollow strut containing an internal damper and an external coil spring. The top of the strut is mounted to the inner wing by a flexible swivel and the base is fixed to the axle stub and a lower transverse link assembly by a heavy- duty ball joint. Because a MacPherson strut set up is compact and mechanically simple, this form of front suspension helps the front wheels follow any road irregularities without affecting the suspension’s camber angle (the tilt of the wheels).
Other types of independent front suspensions include the swinging half axle design, where the road wheel is attached to a long ‘axle’ to form a rigid assembly that pivots close to the centre of the car. However, wheel travel on this type of set-up will affect the camber angle, hence it not being used very often.
On most forms of front suspension, an anti-roll bar connects the lower transverse links to the bodywork. As this stout bar ‘winds up’ when the car corners, it transfers weight to the opposite spring and helps minimises body roll. Anti-roll bars can also be fitted at the rear on cars fitted with all-round independent suspension.
The Rover P6’s front suspension was unique in that it featured upper links mounted lengthwise along the car instead of across it. The bonus of this very compact system is that it transfered all the loadings directly into the car’s bulkhead area.
A trailing link front suspension set up comprises of a pair of link bars carrying the front stub axle and some cars, like the classic Mini, ADO16 1100/1300 and large Citroëns do away with conventional springs altogether and use pressurised fluid and/or rubber cones to cushion the ride.
REAR SUSPENSION
The most common type of classic rear suspension is the Hotchkiss drive. This set up comprises of a solid live rear axle mounted on a pair of semi- elliptic leaf springs kept in check by either lever arm or telescopic dampers. One disadvantage of this type of set up is that the leaf springs can often distort under rapid acceleration and heavy braking.
To counter these unwanted effects, a pair of radius arms or an A-frame can be mounted between the rear axle and bodywork to keep everything in check. Another way to prevent a solid rear axle stepping out line is by using an arrangement called a Panhard rod or a more complex Watts linkage.
One of the most popular independently sprung rear suspension arrangements is the double wishbone design. A semi-trailing arm set up comprises of wishbone shaped pivots mounted at an oblique angle to the centre line of the car at one end. Trailing arm set ups are basically similar but the suspension pivots are mounted to the bodywork at right angles to the car’s centre line.
Both these arrangement utilise a combination of coil springs and dampers to provide and control the bounce. Some cars like the Triumph Herald have a very simple swinging half axle layout comprising of a single leaf spring mounted across the car and shackled at each end to the hub assemblies. Yet another variation is the De Dion axle, which features a tubular beam containing a sliding joint mounted across the car and is connected at each end to the hub assemblies.
Lightweight sports cars are often fitted with a pair of small, non-turning rear suspension legs known as Chapman struts. There are obviously variations on all these designs but whichever suspension system is fitted to your classic; everything should be inspected regularly to ensure all the components continue to perform at their peak.