Leaf spring - WikiHQ

right|thumb|upright=1.3|A traditional semi-elliptical Hotchkiss leaf spring arrangement. On the left, the spring is connected to the frame through a shackle.

A leaf spring is a simple form of spring commonly used for suspension in wheeled vehicles. Originally called a laminated or carriage spring, and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, it is one of the oldest forms of vehicle suspension. A leaf spring is one or more narrow, arc-shaped, thin plates that are attached to the axle and chassis in a way that allows the leaf spring to flex vertically in response to irregularities in the road surface. Lateral leaf springs are the most commonly used arrangement, running the length of the vehicle and mounted perpendicular to the wheel axle, but numerous examples of transverse leaf springs exist as well.

Leaf springs can serve multiple suspension functions: location, springing, and to some extent damping as well, through interleaf friction. However, this friction is not well controlled, resulting in stiction and irregular suspension motions. For this reason, some manufacturers have used mono-leaf springs.

Operation and basic design

thumb|right|upright=1.5|Generic diagram of a leaf spring pack, without eyes; leaves are fastened together by the centre bolt, midway along the length of the spring, and lateral alignment is enforced by multiple clips

A leaf spring takes the form of a slender arc-shaped length of spring steel of a rectangular cross-section. In the most common configuration, the centre of the arc provides the location for the axle, while loops formed at either end provide for attaching to the vehicle chassis. 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. The longest leaf is also known as the main, master, or No. 1 leaf, with leaves numbered in descending order of length. The eyes at the end of the leaf spring are formed into the master leaf.

The leaf spring acts as a linkage to hold the axle in position and thus separate linkages are not necessary. The result is a suspension that is simple and strong. Inter-leaf friction dampens the spring's motion and reduces rebound, which, until shock absorbers were widely adopted, was a very significant advantage over helical springs. However, because the leaf spring is also serving to hold the axle in position, soft springs—i.e. springs with low spring constant—are not suitable. The consequent stiffness, in addition to inter-leaf friction, makes this type of suspension not particularly comfortable for the riders.

Types

thumb|Three-quarter-elliptic leaf spring on a carriage.

There are a variety of leaf springs, usually employing the word "elliptical". "Elliptical" or "full elliptical" leaf springs, patented in 1804 by the British inventor Obadiah Elliott, referred to two circular arcs linked at their tips. This was joined to the frame at the top centre of the upper arc, the bottom centre 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 (shock absorbers), some manufacturers laid non-metallic sheets in between the metal leaves, such as wood.

Elliot's invention revolutionized carriage design and construction, removing the need for a heavy perch and making transportation over rough roadways faster, easier, and less expensive.

File:Spring 3 (PSF).png|alt=Carriage with elliptic springs|

File:Semi elliptic spring and mounting (Manual of Driving and Maintenance).jpg|alt=Vehicle suspension with semi-elliptic springs|

File:Back axle suspension springs.jpg|alt=Car suspension with three-quarter elliptic springs|

File:Quarter elliptic spring mounting (Manual of Driving and Maintenance).jpg|alt=Vehicle suspension with quarter-elliptic springs|

File:Transverse leaf spring (Manual of Driving and Maintenance).jpg|alt=Front suspension with transverse leaf spring|

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thumb|Tapered or parabolic leaf spring diagram

A more modern implementation is the parabolic leaf spring. This design is characterized by fewer leaves whose thickness varies from centre to ends following a parabolic curve. The intention of this design is to reduce inter-leaf friction, and therefore there is only contact between the leaves at the ends and at the centre, where the axle is connected. Spacers prevent contact at other points. Aside from weight-saving, the main advantage of parabolic springs is their greater flexibility, which translates into improved ride quality, which approaches that of coil springs; the trade-off is reduced load carrying capability. They are widely used on buses for improved comfort.

A further development by the British GKN company and by Chevrolet, with the Corvette, among others, is the move to composite plastic leaf springs. Nevertheless, due to the lack of inter-leaf friction and other internal dampening effects, this type of spring requires more powerful dampers/shock absorbers.

Typically when used in automobile suspension the leaf both supports an axle and locates/partially locates the axle. This can lead to handling issues (such as "axle tramp"), 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 (or a Panhard rod) and radius arms to locate the axle and do not have this drawback. Such designs can use softer springs, resulting in a better ride. Examples include the various rear suspensions of Austin-Healey 3000s and Fiat 128s.

History

thumb|17th-century coach spring in Lisbon Carriage Museum

The earliest known leaf springs began appearing on carriages in France in the mid-17th century in the form of the two-part elbow spring (as the illustrated example from Lisbon), and later migrated to England and Germany, appearing on the carriages of the wealthy in those countries around 1750. Dr. Richard Lovell Edgeworth was awarded three gold medals by the Society of English Arts and Manufacturers in 1768 for demonstrating the superiority of sprung carriages. By 1796, William Felton's A Treatise on Carriages showed that leaf springs were being marketed regularly by the late 18th century carriage industry. and G.R. Henderson (1894). Improved steel rolling processes, process instruments, and spring steel alloys were developed during the latter half of the 19th century as well, making the manufacture of leaf springs more consistent and less expensive. They are also commonly used by amateur and hobbyist blacksmiths.

In trampolines

Leaf springs have also replaced traditional coil springs in some trampolines (known as soft-edge 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.

Clutches

The "diaphragm" common in automotive clutches is a type of leaf spring.

Modernization

Tuning (from English to tune, to adjust, to harmonise, to fine-tune) in the field of vehicles is a term that denotes the modification of a vehicle relative to mass production standards in order to adapt it to personal tastes or specific needs. Suspension tuning includes changing springs, shock absorbers, sway bars, and other related components. Many trucks are supported by leaf spring suspension. Leaf spring suspensions provide exceptional articulation, higher load capacity, and can withstand significant abuse. With the right methods, they can be modified to help the vehicle carry heavier loads, have better articulation, or fit larger oversized tires. Some vehicles may be equipped with front and rear leaf springs or only rear leaf springs with independent front suspension. There are two main ways to upgrade leaf springs. The first is budget-friendly but very effective — adding a leaf to the factory leaf spring pack. This provides increased ground clearance and incremental load capacity. The second option is to replace the factory leaf spring pack entirely with a heavier duty pack. This provides increased ground clearance, significant additional load carrying capacity, and can increase the travel (droop/compression) of the rear end for improved performance. The third option is to replace the factory leaf spring packs with coil springs. Overall, this approach changes every aspect of the rear suspension dynamics, giving you more control over outcomes including height, load capacity, wheel travel, etc.

Some lifting methods are good for the rear but not for the front, such as lift blocks. Lifting the rear using blocks is a common way to achieve the desired height. This is done by installing a block of the desired lift height between the leaf spring and the leaf spring perch and fitting longer U-bolts. This is a poor method for the front, primarily due to safety issues during braking. When braking, the front wheels generate most of the braking force. The block moves this lateral force caused by braking higher above the axle than in its standard form. This can cause the block to shift out of place and a complete loss of control.

A more acceptable way to build up leaf springs is using add-a-leafs. This is done by inserting an additional leaf into the vehicle’s leaf spring pack. Using add-a-leafs will increase height but sometimes leads to a stiff suspension travel due to the added spring stiffness.

For off-road vehicles, the emphasis is on extending suspension travel and fitting larger tires. Larger tires — with bigger wheels or without — increase ground clearance, provide a smoother ride over rough terrain, offer additional cushioning, and reduce ground pressure (which is important on soft surfaces). Often, the rear suspension of off-road vehicles is represented by leaf springs.

References

External links

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