The Case of the Bent Trunk - Solved!

I've had a few guys message me if I can try and prevent their trunks from bending when they install a Voltex, or other, wing. I didn't know this was a thing but apparently mine has the same problem. My APR wing has ever so slightly bent the sides of my trunk.

Therefore I began to look into this problem a little further. Research on the internet speculates that the added downforce bends the trunk, but I am skeptical that the downforce produced by our cars can cause this. I had a friend send me his rep Voltex hardware to get some measurements from. It turns out the baseplates they provide are perfectly flat. This is a problem because the trunk is not flat at all, but has a complex curved surface. When tightening the baseplate on the trunk one of the two will deform to fit the other. The axial strength of aluminum is much lower than a FRP composite so of course the trunk deforms to fit the endplate.

I took some measurements of my trunk and put together a simplified stress model to see how the trunk reacts under downforce. This is not a 100% accurate model, but it serves it's purpose to show the load path and some results to determine paths forward.

I setup my model using 6061 aluminum for the trunk, rubber for the trunk supports, and FRP for the endplates. I constrained it in the y-direction on the rubber stoppers and also constrained the trunk where it mounts to the frame. I added 400lbf of downforce to the endplates since I could not find online how much the Voltex wing generates.

The results can be seen in the pictures below, but the displacement is low as I expected (.006"). The max stress is 12ksi. However the max displacement is right where we are seeing the bending in the above picture, so maybe there is some merit to the study.


My plan is to redesign the baseplates and wing stand mounts to better distribute the stress across the trunk. My goal is to reduce max stress from 12ksi to 4ksi and more importantly provide a profile that fits the trunk.

I was also not a fan on how the baseplate to wing stand brackets are in a single shear condition. This seems like a very weak part of the wing design that I plan on fixing with my new baseplates. I was to change it to a double shear condition and remove the hardware from a direct shear load.


After spending some design time, I think I have the solution to the problem at hand. The goals I initially set to achieve (reducing stress by 1/3 and matching the trunk profile) have both been achieved.

Here is the new stress plot. As you can see the peak stress was reduced from 12.18 ksi to 3.62 ksi. That's just over 70% reduction in stress on the trunk! More importantly, the stress concentration when the bends were seen have been eliminated.


Below is the new displacement plot. Max displacement was reduced from .0062" to .0025", a reduction of over 40%!


Here is a look at the profile of the bottom of the baseplate so that is can sit flat on the trunk. I printed a test piece and it sits perfectly on the bottom of my trunk. Most of the bending is from installation since the original Voltex baseplates were perfectly flat. These will not cause the trunk to conform to it, therefore preventing bending.

Trunk Profile.JPG

To take it one step further, I think proper installation should also be followed. The trunk should be loosened from the frame mounts so that is completely unstressed. Then I would hand-tighten all of the new baseplates bolts, stand bolts, and wing bolts in stages (think a cam install). So handtighten everything, then go in 5 ft-lb stages until you get to the final torque. We want to easy the trunk into it's final shape to prevent any strain.

My next steps are to bring these into production. Since they are in direct sunlight I cannot fab them out of PLA. I am going to do some tests in PETG and ABS and have them sit in the sun for a few days. If they pass the test, then they will be put up for sale.

Here's a view of the final shape: