A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles.
Originally called a laminated or carriage spring, and sometimes referred to as a semi-elliptical
spring or cart spring, it is one of the oldest forms of springing, dating back to medieval
times.
A leaf spring takes the form of a slender arc-shaped length of spring steel of rectangular
cross-section.
The center of the arc provides location for the axle, while tie holes are provided at
either end for attaching to the vehicle body.
For very heavy vehicles, a leaf spring can be made from several leaves stacked on top
of each other in several layers, often with progressively shorter leaves.
Leaf springs can serve locating and to some extent damping as well as springing functions.
While the interleaf friction provides a damping action, it is not well controlled and results
in stiction in the motion of the suspension.
For this reason some manufacturers have used mono-leaf springs.
A leaf spring can either be attached directly to the frame at both ends or attached directly
at one end, usually the front, with the other end attached through a shackle, a short swinging
arm.
The shackle takes up the tendency of the leaf spring to elongate when compressed and thus
makes for softer springiness.
Some springs terminated in a concave end, called a spoon end, to carry a swivelling
member.
History There were a variety of leaf springs, usually
employing the word "elliptical".
"Elliptical" or "full elliptical" leaf springs referred to two circular arcs linked at their
tips.
This was joined to the frame at the top center of the upper arc, the bottom center was joined
to the "live" suspension components, such as a solid front axle.
Additional suspension components, such as trailing arms, would usually be needed for
this design, but not for "semi-elliptical" leaf springs as used in the Hotchkiss drive.
That employed the lower arc, hence its name.
"Quarter-elliptic" springs often had the thickest part of the stack of leaves stuck into the
rear end of the side pieces of a short ladder frame, with the free end attached to the differential,
as in the Austin Seven of the 1920s.
As an example of non-elliptic leaf springs, the Ford Model T had multiple leaf springs
over its differential that were curved in the shape of a yoke.
As a substitute for dampers, some manufacturers laid non-metallic sheets in between the metal
leaves, such as wood.
Leaf springs were very common on automobiles, right up to the 1970s in Europe and Japan
and late '70s in America when the move to front-wheel drive, and more sophisticated
suspension designs saw automobile manufacturers use coil springs instead.
Today leaf springs are still used in heavy commercial vehicles such as vans and trucks,
SUVs, and railway carriages.
For heavy vehicles, they have the advantage of spreading the load more widely over the
vehicle's chassis, whereas coil springs transfer it to a single point.
Unlike coil springs, leaf springs also locate the rear axle, eliminating the need for trailing
arms and a Panhard rod, thereby saving cost and weight in a simple live axle rear suspension.
A further advantage of a leaf spring over a helical spring is that the end of the leaf
spring may be guided along a definite path.
A more modern implementation is the parabolic leaf spring.
This design is characterised by fewer leaves whose thickness varies from centre to ends
following a parabolic curve.
In this design, inter-leaf friction is unwanted, and therefore there is only contact between
the springs at the ends and at the centre where the axle is connected.
Spacers prevent contact at other points.
Aside from a weight saving, the main advantage of parabolic springs is their greater flexibility,
which translates into vehicle ride quality that approaches that of coil springs.
There is a trade-off in the form of reduced load carrying capability, however.
The characteristic of parabolic springs is better riding comfort and not as "stiff" as
conventional "multi-leaf springs".
It is widely used on buses for better comfort.
A further development by the British GKN company and by Chevrolet with the Corvette amongst
others, is the move to composite plastic leaf springs.
Typically when used in automobile suspension the leaf both supports an axle and locates/
partially locates the axle.
This can lead to handling issues, as the flexible nature of the spring makes precise control
of the unsprung mass of the axle difficult.
Some suspension designs use a Watts link and radius arms to locate the axle and do not
have this drawback.
Such designs can use softer springs, resulting in better ride.
The various Austin-Healey 3000's and Fiat 128's rear suspension are examples.
Characteristics 1.
The leaf spring acts as a linkage for holding the axle in position and thus separate linkage
are not necessary.
It makes the construction of the suspension simple and strong.
2.
As the positioning of the axle is carried out by the leaf springs so it makes it disadvantageous
to use soft springs i.e. a spring with low spring constant.
3.
Therefore, this type of suspension does not provide good riding comfort.
The inter-leaf friction between the leaf springs affects the riding comfort.
4.
Acceleration and braking torque cause wind-up and vibration.
Also wind-up causes rear-end squat and nose-diving.
5.
The inter-leaf friction damps the spring's motion and reduces rebound, which until shock
absorbers were widely adopted was a great advantage over helical springs.
Manufacturing process Multi-leaf springs are made as follows:
Shearing of flat bar Center hole punching / Drilling
End Heating process forming Eye Forming / Wrapper Forming
Diamond cutting / end trimming / width cutting / end tapering
End punching / end grooving / end bending / end forging / eye grinding
Center hole punching / Drilling / nibbing
Heat Treatment Heating
Camber forming Hardening
Quenching Tempering
Surface preparation Shot peening / stress peening
Primary painting
Eye bush preparation process Eye reaming / eye boring
Bush insertion Bush reaming
Assemble Presetting & load testing
Finish painting Marking & packing
Other uses By blacksmiths
Because leaf springs are made of relatively high quality steel, they are a favorite material
for blacksmiths.
In countries such as Nepal, Bangladesh, the Philippines and Pakistan, where traditional
blacksmiths still produce a large amount of the country's tools, leaf springs from scrapped
cars are frequently used to make knives, kukris, and other tools.
They are also commonly used by amateur and hobbyist blacksmiths in Western countries
and India.
In trampolines Leaf springs have also replaced traditional
coil springs in some trampolines, which improves safety for users and reduces risk of concussion.
The leaf springs are spaced around the frame as 'legs' that branch from the base frame
to suspend the jumping mat, providing flexibility and resilience.
See also Corvette leaf spring
Coil spring Suspension lift
Carriage References