Chassis and Aerobody Attachment (WIP)

Owned by Jason Chen
Last updated: Apr 01, 2024
3 min read

OBJECTIVE:

Attach the carbon fiber and foam aerobody to the metal chassis using two clamp fasteners (two bolts for one clamp). Assuming that other external loads are insignificant to the vehicle weight, determine the minimum amount of fasteners needed to support the gravitational load. Say the aerobody weighs 100 kg (which includes a safety factor of x2) - the fasteners must support 980 N of gravitational load. Because of how the fasteners are set up in the vehicle, the joint is primarily considered shear, where the load is perpendicular to the bolt axis.

The objective is a two part question:

  1. How much do we need to/can tighten our bolts to produce enough friction for the aerobody to not slide with the chassis?

When we tighten a bolt, it stretches and creates a tensile or “preload” force opposite to the force pulling the fastener apart as the material attempts to return to its natural position. This preloading force will directly correlate to how much normal or “clamping” force is produced. Because it is a shear joint, the clamping force will be proportional to the frictional force that stops the aerobody from sliding down and falling.

Therefore, we must determine how much preloading force will occur for every x distance tightened and the maximum amount for a given bolt, depending on the relevant surfaces. We must also determine the relevant frictional coefficient between the chassis and clamp (we will introduce rubber to increase it).

If we observe the given grade of a bolt (8.8), the first digit represents the ultimate tensile strength in hundreds of MPa and the second digit represents the yield to tensile strength ratio. The preload force should be less than the yield strength to prevent plastic deformation - 70% is the factor that is often used in practice.

To determine how much preloading force is applied, there are three methods:

  1. Torque method: use a torque wrench to measure the amount of applied torque and isolate the preload force in the equation T = F⋅D⋅K.

  2. Turn of nut method: tighten the suit enough to bring the two mating surfaces together and then turn it through a defined angle. The amount of preload force will depend on the thread pitch, bolt length, and the material’s Young modulus.

  3. Bolt elongation method: measure the elongation of the bolt from before the point in which torque is applied and after. It is assumed that preload force will linearly increase as the bolt elongates. F = k⋅ΔL. 'ΔL’ is the change in length of the bolt due to tightening. ‘k' is the bolt's stiffness, which can be calculated using the bolt's material properties and geometry k = (E⋅A​)/L. 'E' is the Young's modulus, 'A' is the cross-sectional area, and 'L' is the effective length of the bolt.

We will continue with the 1st method. This force over the area of the bolt will be compared to 70% of the yield strength value, and the lower value will be taken to prevent bolt damage.

  1. How much load can the clamp survive before fracturing?

After further research, it seems that the clamp has the highest likelihood of fracturing when the entire system is under load.

I’ll need to download a model of the clamp from McMaster-Carr, create fixtures to emulate bolts and physical walls and apply force until I reach its plastic deformation value (maximum stress value = material yield strength). With that, I can determine if the two clamp fasteners setup will survive its given load.

According to my current calculations, each clamp can support a maximum load of around 100 N before occurring plastic deformation (which could be an incorrect result). This value is greater than the expected load a given clamp will receive. See Figure X for fixture and force setup and results.

With these two questions answered, it can be determined how many two-clamp fasteners are needed to support the load of the aerobody. To be clear, since each clamp has two bolts, the amount of load the system can hold considers both bolts totaled.

 

Misc notes:

Could also FEA the entire body to determine the positioning of the fasteners, but it's not that accurate.