Here we will be exploring some mechanical properties of materials and how they relate to karting. Almost all materials in karting are used under conditions involving some type of load, resulting in forces which cause deformation of the material. In most instances these deformations are relatively small and are not readily noticeable. However the ability of a material to resist these forces is of primary interest to all engineers. The properties that determine the ability of a material to behave in particular fashion under applied forces are the mechanical properties of the material. 

Perhaps the most fundamental mechanical property of a material is its strength. In its more general usage, strength refers to the ability of a material to withstand stress without failure. However failure does not always involve fracture of a component – in many cases a component is considered to have failed if it undergoes a certain amount of deformation such as stretching or bending. 

Strength is probably the most important general property in karting. It is an important consideration to every component, whether it is part of the kart itself, or an accessory such as helmets and race shoes. However it is a rather general property and in the case of axles and chassis components, hardness and toughness are probably more relevant to how the components perform. 


Hardness is a property that is often related to a broad range of mechanical and even physical properties – one’s concept of a hard material may be a material that has high strength, is brittle, or is difficult to scratch or cut. The engineering definition of hardness is “a material’s resistance to permanent indentation under static or dynamic forces”.
Hardness is especially relevant to kart rear axles as you can buy different hardness grades of axle off the shelf. The harder the material is, the less likely it is to bend, and the more it will act like a spring. This is only true up to a certain limit though. Once a harder material is stressed beyond it’s yield point, the more likely it is to stop bending and actually break or fracture, or be bent past the point where it can successfully be bent back into it’s original shape without fracturing. Whenever bending of a material takes place, some localized hardening will occur at the point of the bend. So an axle that has been bent out of shape and then straightened will be harder at the point it was bent. This is commonly known as “work hardening” and refers to an increase in hardness as the material is “worked”, as opposed to heat treatment based hardening processes. This could also have an effect on handling by affecting the flexing properties of the axle.
The harder the axle is, because of increased springiness, the more suddenly it will release any stored energy and increase the likelihood of bouncing.

Toughness is a measure of the amount of energy required to cause failure (or fracture) of a specimen. Toughness is measured in Joules and is determined by impact testing. The more Joules of energy the specimen can absorb, the tougher it is. Toughness is drastically decreased as temperature decreases and in particular below zero degrees Celsius. 

Toughness plays a major role in material choice for the kart chassis. A tougher material will always be better because of it’s inherent strength. Toughness is often closely linked with hardness, which also has the benefit of creating more spring in the chassis. Therefore, a new chassis with high toughness is more likely to return to its original shape after being flexed. Another issue with the chassis is the weld toughness. The filler material used in welding is an alloy, which always has a higher toughness and hardness than the material being welded. This makes the actual weld stronger than the parent metal, which is why a fracture never occurs through the weld itself. If any breakage occurs it will always be next to the weld in the parent metal. 


Fatigue strength (or endurance limit) is defined as the maximum stress below which a material may endure an infinite number of stress cycles. Fatigue failures occur with no visible deformation of the material. The major contributor to fatigue failure in materials is the presence of stress raisers – notches or relatively sharp changes of section that cause a localized concentration of stress. 

Fatigue is a good argument why some people feel that an old, used chassis never performs as well as a brand new one. The continuous bending and vibration stress cycles placed on the chassis must cause some degree of metal fatigue. Over a period of time this results in some weakening of the material, possibly leading to reduced strength, toughness and potentially even hardness. The worst-case scenario is a complete failure of the material resulting in cracking and sagging of the chassis. Unfortunately there is no easy way to stop or remedy metal fatigue in a chassis, so inspect very carefully any potential second hand buys for hairline cracks. This is made even more difficult by the paint or powder coating covering the steel. 

On the other hand, an older chassis that has been through a few of these fatigue cycles, and is a bit more prone to flex, may actually perform better under certain conditions or with a particular driving style.
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