Straight Axle Front Suspension ( A Brief Insight)

In this brief article, we will discuss the Straight Axle, the advantages and drawbacks of a Straight Axle, and the types of Straight Axle Front Suspension.

What is a Straight Axle Front Suspension?

From the Oxford definition, the word Axle means –

“..a rod or spindle (either fixed or rotating) passing through the center of a wheel or group of wheels…”

In a vehicle, a front solid axle is also called a ‘beam axle’, or a ‘rigid axle’. The Front Straight axle is basically a Straight Hollow Tube structure holding the left and right wheels through Kingpin joints. 

What Vehicles use a Straight Axle Front Suspension?

The application for a Straight front axle is mostly in drag racing cars and “traditional hot rods”. A traditional Hot-Rod is a car from the 1920s – 1940s that was converted into a high performance vehicle that is also capable of Drag Racing.

What are the Different Types of Solid Axles?

A Non-Driven Front axle can also be called a beam axle. A beam axle is mostly found in the front suspension of traditional cars from the 1920s -1940s, and in trucks that are rear wheel drive. There are basically 2 types of Front Solid Beam Axles:

  • Straight Axle
  • Drop Axle

Straight Axle

The Straight Axle Beam, as the name suggests, is an axle beam that has no ‘Drop’ or gooseneck.  The beam would be either made of Hollow Tube or solid “I section”. In a real hot rod, the straight axle is not literally straight. It is curved downwards in an arc. 

Drop Axle

The axle beam that is “Gooseneck” shaped is a Drop Axle. A Drop front axle means a Solid beam structure that would mostly have an ‘I-Shaped’ Cross-section in order to get maximum bending strength. The Stub-Axles are attached at each end using a Kingpin joint which is serviceable.

What are the Advantages of a Straight Axle Beam?

In practical use, a straight axle is not literally ramrod straight. It is purposely shaped as an arc. The purpose of the arc shaped straight-Axle is for adjustability of Camber. For adjusting Camber, the Axle beam is bent using a simple hydraulic arrangement.

Another purpose of the straight-axle is to raise the front end of the vehicle so that more weight is transferred to the rear-end. In a drag race, rear-wheel drive cars need the maximum grip and downforce at the rear end. Therefore, rearwards weight transfer during launch is an important factor.

What is the Suspension used on the Front Straight Axle?

A Solid Axle suspension can also be called a dependent suspension. The earliest form of suspension for the solid front axles was the leaf spring type.

Because the left and right wheels are connected rigidly, the vertical motion of the left and right wheel is always coupled. In other words, when the left wheel went upwards over a bump, it pushed the right wheel to go downwards.

What are the Different Types of Straight Axle Front Suspension?

The different types of Straight Axle Front suspension are:

  • Transverse Leaf-spring with 4-Bar Linkage
  • Transverse Leaf-spring with Hairpin control arms
  • Hairpin control arms with Coil Springs

Transverse Leaf-spring with 4-Bar Linkage

The Transverse Leafspring-type suspension is the most popular among all hot rod front suspension systems. The Axle beam is connected to the chassis using 4 parallel “Leading Arms” or control rods (One upper and one lower control arm on each side) from the chassis to the axle. At the Axle, the 2 control arms on each side are connected to a “Batwing” axle Bracket that has the 2 hinges for the upper and lower control arm. 

This takes care of the longitudinal and braking forces. The leaf spring is transversely mounted with the leaf spring middle clamped to the chassis Front Cross-member and leaf spring eye-ends hinged to the axle. The lateral forces are taken care of using a Panhard rod.

Transverse Leaf-spring with Hairpin control arms

The Hairpin arrangement is very similar to the 4-bar Leading arm arrangement. The only difference is that the 2 parallel leading arms on each side are joined together at the chassis end in a U-Shape resembling a Hairpin. Instead of 2 hinges at the chassis, there is only one hinge. The axle has the same Batwing bolted axle bracket to connect to the Hairpin control arm.

Hairpin Control Arms with Coil Springs

The Coil Spring setup is similar to the transverse leaf hairpin control arm setup. The difference is that the single transverse leaf spring is replaced by 2 inward-tilted Coil Springs that are connected from the axle Batwing bracket to a chassis–rail bracket. The Panhard rod restricts lateral movement of the axle.

Drawbacks of the Straight Axle Front Suspension

  • Due to the higher unsprung mass and the inability of one wheel to follow the unevenness of the road without affecting the opposite wheel on the vehicle, the ride and handling quality in a solid axle suspension is much less as compared to an independent suspension.
  • The Straight Axle was typical of the hot rods built in the 1960s. It is only suitable for building a race car out of a vintage era car. The Kinematics and Compliance adjustability in a solid axle is too limited.
  • The straight axle Camber adjustment is done by bending the axle is a crude and inaccurate method. Modern day race oriented independent suspensions provide a wide range of adjustability.

Conclusion

In this brief article, we have discussed the Straight Axle, the advantages and drawbacks of a Straight Axle, and the types of Straight Axle Front Suspension.

In case of any queries or comments, please feel free to ask.

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