Formula Student Torsional Stiffness FEA Validation

Every year the Loughborough Formula Student team (LUM) use 1D FEA to test the torsional stiffness of their chassis as a comparison to previous years, other teams and the effect on the handling of the car, yet very little work has been carried out as to the suitability of this method.

Along side the minimal work of comparing experimental and theoretical results there has also not been any inclusion as to the effect of any of the other attachments to the chassis, such as the engine or the carbon fibre body panels.

By doing the tests with the theory with and without the body panels the accuracy of the theory can be found as well as the effect of the body panels

Experimental work

Due to this being a university project the full use of equipment was allowed and as such the torsional stiffness was measured using a 4 poster rig, this rig has been used in the past to put a car on the 4 posts to test the suspension of a simulated road. This can be seen below along with the chassis mounted on the rig.

Fig 1. LFS09 Chassis on the 4 poster rig

Pretty quickly during experimental testing it was found that a number of issues of strength needed addressing such as the bending of bolts and the tearing of suspension tabs. After this has been solved by changing the mounting method and welding up the suspension tabs the testing could be carried out.

By inputting a displacement over a time into the posts, the forces are measured using the pressure in the hydraulic posts. The demanded displacement profile for both the left and the right hand posts can be seen below. Running this profile 3 times allowed for an average of each of the three runs at the 4 different displacement points to be averaged.

Fig 2. Displacement post demand

Theoretical Work

The FEA was set up in Nx 8.5 with 1D PBeam elements with T45/CDS tubing in the appropriate places with the fixed nodes to match that in the experimental testing along side the forces being applied at the equivalent suspension node. With the 2D elements a Tri3 PCOMPG element was used with 1 ply of CYCOM-2040 2x2 twill weave T800 as the material.

In order to get deformation of the 1D and the 2D synchronised a nodal equivalence was used. This does have the unfortunate side effect of producing an infinitely stiff bond but was deemed to be a small factor in the comparison.

A mesh sensitivity study was carried out and very little change in the displacement was seen below 100mm for the space frame only and 20mm for the combined FEA. An example of the sort of results acquired can be seen below (this animation is a early FEA set-up test, not a CFRP body panel and not the final FEA).

Fig 3. Space frame and body panel.

 

Comparison of results

Using simple trigonometry calculations the values have been corrected so that the theory and experimental results can be compared as the experimental tests are measured at the suspension tabs but the FEA values are measured at the centre of the tube elements.

After the corrections had been carried out, another calculation was carried out to find the torsional stiffness value.

	Space frame	Space frame with body panels	Stiffness Increase (%)
Experimental (Nm/deg)	797.45	1079.20	35
FEA (Nm/deg)	720.77	1054.39	48
Error (%)	9.6	2.4

It can be seen from the table above that the value with body panels is within a working experimental error however the bare space frame error is ~10%.

The low error when including the body panels is likely due to the properties of the carbon panels not being known fully due to the age of the chassis, and as such it suggests a much larger contribution in the FEA when compared the experimental results.

Whilst this doesn't show that 1D FEA is perfect, it does show that there is a significant increase regardless of theory or practice when it comes to including carbon body panels on a tubular steel space frame and these should not be omitted in future FEA testing.

The experimental results being stiffer than that of the FEA is likely due to the fact that the tubes are not joined at the nodes on a chassis, but are actually ~12.7mm off node, on some of the elements this constitutes 10% of the length of that member. This shows that if you wish to get a more accurate value for torsional stiffness then a 3D FEA analysis is required, but is not necessary for value comparison with previous years so long as there is a consistent method used.

If you would like any of the data then all you need to do is message me and I will try my best to get it to you.