The frame or roll cage is the base support for all the other components of the vehicle. It undergoes different load and stresses, depending on road conditions and amount of loads. Choosing the right material for the frame or roll cage is beneficial from the safety point of view as well as vehicle performance. now let see what are the requirement of so called best frame material for racing vehicles?
What make a material suitable for racing vehicle frame?
In order to be a good material for racing vehicles the material need to:
- have good strength
- be light in weight
- be stiff enough
- have durability
We can note that, not only the material need to be light in weight (the more you are light more you can go fast and achieve fuel economy also), but also satisfy the need for strength and stiffness. Situation which is a bit challenging with metals.
Viewing the above requirement one could say, carbon fiber is the suitable material. Well, we all know that apart from formula 1 teams and very professional racing teams everyone cannot afford the material. Let concentrate on steel and alloys; aluminum and alloys for the time being.
Now the question is how do we achieve light weight and strength?
There are several ways and each depends on the team design goals and budget available.
When I say design goals I mean the type of performance we need base on the racing terrain (off-road or simple track).Suppose for off road racing the demand is very high for strength and stiffness, so material selection need to be done accordingly.
Base on which criteria material should be choose?
Suitable materials specifications
According to above specifications let now see best material for racing vehicle from weight, strength and stiffness point of view.
Let say length of total tube needed to build our frame or roll cage is L=5m , outer radius of tube Ro=0.0127m thickness of tube t=4mm=0.004m
Then weight of frame or roll cage will be:
Frame weight=ρ×L×π× (Ro^2-Ri^2)
Where: ρ=density of material (densities are converted into kg/m3)
Weight of frame with AISI 4130 =7850×5×π×0.004× (2×0.0127-0.004)
Weight of frame with AISI1018=7870×5×π×0.004× (2×0.0127-0.004)
Weight of frame with aluminum=2700×5×π×0.004× (2×0.0127-0.004)
From above calculations we can see that almost all the steel materials in this table have the same weight.
Bending strenght and stiffness
To find the bending strength and stiffness of the material following formulas can be use:
Bending strength =Sy× I/C
Bending stiffness =E×I
Where: Sy=yield strength of the material
E=modulus of elasticity of the material
I=second moment of area for the structural cross section of the material
I= π/2 × (Ro^4-Ri^4)
C=distance from neutral axis to extreme fiber, generally=Ro
Ro and Ri are respectively the outer and inner radius of the tube.
For material with less yield strength and ultimate strength we can play with Ro and Ri in order to increase bending strength and stiffness.
Care should be taken with material thickness, if it increase the weight also increase.
competition such as Baja SAE the rule book explain very well the material selection criteria, the teams just need to proof their choices with calculations using the bending strength and stiffness formulas.
Now it’s clear that there is not big difference in term of weight the selection criteria should be based on strength to weight ratio, budget available, carbon content of the material.
For applications such as go kart, SAE formula we can use AISI 1018 or AISI 1020 they can satisfy the requirement at low cost, but for off-road application it’s always a good decision to use AISI AISI 4130 or materials with same characteristics even if the budget is causing problem, the solution can be to compromise other part of the design to achieve the good material requirement.
Aluminum may have better strength to weight ratio, good stiffness and corrosion resistance, but the challenging sides are the cost and the good welding skills needed in order to weld it properly.
Geometry considerations for increasing strength and stiffness
Now, you have an idea of the material you can use. Let talk about an important factor which can make the difference: the frame or roll cage geometry. Here it means the arrangement of the frame members.
Consider following two diagrams, which one you think can help to have better strength and stiffness?
It’s the one with diagonal members. The diagonal members allow better distribution of the loads and stresses thus increasing the factor of safety of the design.
I hope you enjoy this article and it can help you select the best frame material for racing . If you like pleases comment and share, and if you will like us to cover any related topic suggest, we will do our best to satisfy your queries.
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