Carbon fiber boot board

TL;DR replacing the wooden part of the boot floor with carbon fiber

A little-known fact about the Caterham is that it contains wood. I would bet that only people who have built their car themselves know about it.

It's discreetly hidden under the carpet in the boot, visible only from beneath the car.

The panel is a 6mm, 5-ply untreated plywood board measuring 19.5 x 76.5 cm and weighing 634 grams.   Just FYI, the part on top of the fuel tank is a 12mm honeycomb aluminium panel.

The plastic plug is an access hatch to fill the diff with oil. This is specific to the 160-165-170-SS600 Suzuki-based models. The hole has a diameter of 64mm.

Despite 23,000 km of use—21,000 of those with me, through all kinds of weather over the past seven months—it's still in excellent condition. The only problem is that the steel screws have become rusty, which is a real shame. 

Let's fix that with stainless steel screws. I've bought a pack of 4.2mm x 13mm ones, with some washers to match. A pair of those was promptly fitted to the filler neck cover as the two black screws did show some hint of corrosion as well.

But why not replace the board with some shiny carbon fiber ? 

Removal only takes a few minutes, requiring just four Phillips screws to be removed once the carpet is carefully peeled back. I didn't touch the honeycomb area, only the part where the carpet covers the plywood.

A cardboard template was traced from the plywood and cut to size, also copying the location of the screw holes.

I’ve ordered two sheets of 200x400mm twill carbon fiber, 1mm thick. This will save about 335 grams. After having cut one panel to match the desired total length, the two parts were joined with quick-set epoxy glue. The seam is doubled with the 35mm piece that was trimmed earlier. After application of masking tape, all the parts were sanded and cleaned before gluing.

I won't bother with a circular opening, it's easy enough to remove the complete panel to access the diff. Only the two slots were copied from the original part. I did plan on not using the Dremel to cut those, but I had to preserve some sanity. I tried with the thin blade, and I would have had to use it without the saw frame.

The four screw holes were punched with the spring-loaded center punch, 3 mm pilot holes were drilled, then finally a 5 mm drill bit was used at high speed to improve the chance of a clean result.

I had strong hope on being able to insert the full 200mm width without trimming. I was mistaken, and ended sawing the nearly 800mm length with the larger metal saw. I was so fed up that I didn't even care to take any measurement of trace any line to follow, I just winged it and removed about 4mm free handed.

The new panel was dropped in and screwed down with the 4.2mm x 13mm stainless steel screws and washers. I did it with the full roof installed on the car, through the rear window opening.

As expected, 1 mm thick carbon fiber is perfectly adequate. There is very minimal flexion when pressure is applied on it. ChatGPT did estimate that it would be similar to the 6mm plywood strength. A 12kg central point load would be no issue with a 1:2 safety margin in the worst bumps (up to 2g). See down below if you want to read the full rambling around that concern.

 Below : excessive rambling about the rigidity of the 1mm carbon fiber panel

The choice of just 1mm in thickness raised some concern about the rigidity and deformation under load. As the carbon fiber sheets can only be curved around a single axis, being supported all around the four edges should prevent significant deformation. It would rather break catastrophically with a bang. My tests with a 200x400mm panel supported only from the long edges did show minimal flexion with a 15kg load placed in the middle. That's more than adequate, especially with the panel supported by all four sides and screwed down along a long side.

The three people I've shared this idea with think that 1 mm is way too thin and that it will brake or bend badly.  My thought is that your everyday car is made of 0.6mm steel. Of course it's stamped to give it its rigidity. But I cannot see a carbon fiber panel that covers a 15cm opening being a big risk.
I can always rib it with a pair of aluminium "L" or "U" profiles if it's too bad.

ChatGPT thinks it's going to be fine as it did compute that the panel would resist a point load of 500 N right in the center. But it would be good to keep a safety factor of 2 or so, and limit the load to 250 N. That's a 25kg static load.  As it told me that very bad bumps on the road could reach 2 g, we should be fine with 12.5 kg sitting right in the center in the worst conditions.
I asked it the same calculation for the 6mm plywood, and it resulted to 540 N static point load.