Testing Carbon Fibre before I commit – Part 2

So, here is the raw results file:

raw data

I’ve kept it there in the spirit of honesty, but the following distilled data into the graph is what really matters:

new graph


So the numbers that matter are those on top of the line – each of those is the number of KG force required to deflect the part by a given distance. You can correlate the colour to the number to the data in the top chart (click through for details)

Finally, there is the density/deflection ratio, which would show the ideal performing part if absolute strength wasn’t the most desired outcome.



There is a trade-off between core and layers, which is what would be expected. What actually surprised me the most was the difference a core makes.

The top line has two variables set – 4 layers of CF (rather than 2), and a 10mm core, rather than the yellow line, which is 4 layers and a 6mm core. The third highest line is the darker blue line, which is 2 layers and a 10mm core.

So, it’s layers over core but again, there’s a trade-off. The red line is 4 layers over a 3mm core, and you can see it yielded really quickly at 3mm with a very low amount of force (well, 98kg of force). Without doing any statistical analysis, I am observing that each 3mm of core seems to give me an extra 100kg of resistive force before yielding. However, I don’t know how far that scales.

Finally, you can see that there are a selection of flat lines near the bottom, and they are parts made without core. The seem to bend a lot and not yield. For my purposes though, they’re not suitable. Parts 6 and 7 (four and two layers), deflected up to 7mm without yielding, but weren’t much use to me.

As the thicker parts started to yield, we could actually hear them crack (quietly). I’m assuming that’s the fibres snapping. As such, the moment it starts to happen the part is compromised.

Testing Carbon fibre before I commit – Part 1


Some carbon fibre flat pieces were made and tested to see how far they bend for a given load. A load cell was used to measure the force.


So, before I go mad and put several hundred pounds of material into my CF tub, I thought it best that I do some clever investigation into just how does a composite structure bend, yield and break under various loads. So, I made the pieces, tested the pieces, and did some graphs, and came to some conclusions.


8 trial pieces were made of various thicknesses – different cores and layups. Each piece had a few things in common though:

samples 1

  • single layer of 300 gsm aramid for intrusion protection (if it were on the car)
  • this was balanced with a 300 gsm piece on carbon on the opposite layer to the core
  • one layer of 600 gsm either side of the core, and one layer of 200 gsm e-glass either side of the core for a little tolerance and flexibility.


samples 2I could then vary the thicknesses of the cores and extra layers of 600 gsm carbon I wanted to use. I opted for the following core thicknesses:

  • 10mm closed cell foam (CCF)
  • 3mm CCF
  • 6mm (2x 3 layers of 3mm CCF)
  • none


In order to test the bending load, I went to see my good friend Simon at Cornering Force. There are only so many people who like it when you call them at 8pm and say “Hey – do you want to stress test and break some carbon fibre?” After marking up each sample, we then put each on a support, resting on the bed of a mill, and pressed down on it with a load-cell. The load-cell had a 25x50mm contact face. The mill allows us to make vertical adjustments in 0.1mm increments easily, and the load-cell is accurate to the gramme.


The pieces were laid out on one sheet, and infused as one piece, and then cut to size afterwards. The vacuum feed was left on post infusion and for the curing cycle. The resin was degassed. This was to ensure the maximum amount of spare resin was removed until the gel stage. After gelling and the initial cure, they were post-cured at 60C for 12 hours, as per the manufactures instructions. This gave me the maximum strength possible for the sample for the least weight. I decided not to vary curing methods (e.g. length of cure) because a 12 hour, 60C cure is what I can achieve for a part as large as this.

The plan was to work out how much a piece would deflect, how it would yield and fail, and just what were the most significant factors in this – is it core thickness, is it amount of cloth? Did I turn around three times widdershins?

This was also a good opportunity to try a couple of techniques given to me by Vic at Scorpion CDL. I am always astounded at the wealth of technique he has and the quality of work done there.

Lessons Learned

I also had a slight infusion issue, which was the temperature of the resin I infused with. I had it at about 10C which was great for it not going off, but made it too viscous, and meant de-gassing it was a pain. It also meant the infusion was quite slow for such a small part. Further checking says 25C to 30C is a better temperature for this. I should have also wrapped more mesh around the input spiral which would have also alleviated this problem, but that was fixing the wrong problem – get the temperature right – let the resin flow.

Analysis and Conclusions

Coming in the next post.

Fury Front Anti-Roll Bar

I have my front ARB in place and I must say, I am exceedingly happy with it. I didn’t make it myself but comissioned Cornering Force to make it for me. It’s part of a matched pair (one for the rear as well) which was bespoke manufactured for my layout, corner weights and aspirations for the car. I can’t recommend these guys enough. Simon is a leader in the field and has his suspension on cars on pole in the BTCC.

So, to the pictures.

This is a view of the completed, powder coated rocker arm, showing the drop link going down to the blade, set in ‘fully priapic’ mode. There are a couply of other positive observations here – you can see how neatly it’s tucked under the rocker, as well as getting a nice view of the locking pin. The pin is spring loaded, and you just pull it out, turn the blade and it clicks home.

My car is only going to weight 650KG with me in it, so we only need one blade. Both sides are coupled of course so one blade does the work for both sides. The blade itself goes through 10 different metallurgical processes according to Simon so it will last the distance. It’s the single most expensive component in the arrangement.

Here’s a good view going into the adjustor, showing the nylon bearing block as well. As you may have noticed, I’ve decided to make all parts that bolt on to the chassis blue, and the chassis will be yellow again. There are small stops welded onto the ARB to secure it to the nylon bearings.


Here’s the final view, on the fixed link side. Nice and simple. What should also be evident (and is also clear in the top picture) is the line of sight between the chassis force node (top rail where the vertical meets it) and the far corner node where the suspension mounts. If you look at this and the top picture, you will see where the next set of cross-bracing is due to go in. I will make this cross bracing demountable otherwise it won’t be possible to get the right angle of pitch when extracting the engine.

with old hoops comes great liability

So, Simon (Cornering Force) and I have been mulling over my roll-over hoops, and there are concerns about the design. The main one being, if I did end up on my lid going the wrong way then I have a couple of problems:
1) they aren’t high enough to give 5cm clearance for my helmet
2) if i’m going backwards, i stand a chance of them ripping off. This is bad. Brief, but bad.

So, we’re going to bend up some roll-cage tubing to go in behind the seats all the way down to the floor, securing onto the mcpherson strut turrets, cross braced and then with a demountable forward bar to the passenger footwell so for track days I’ll have extra bracing to stop stuff ripping off and squishing my head.

Corner Weights

As part of the work necessary to design and fabricate my ant-roll bars (which I think will be a thing of beauty in twisted metal form), Simon and Mike from Cornering Force came around to take the corner weights. They had to do this on my pair of scissor lifts which shows they are game for anything.

Following is the breakdown of the corner weights, including the driver. It is a wet weighting, and the weights are reasonably well placed with a little bias to the right. This isn’t going to be very easy to change, but most UK circuits have a predominately right-handed bias, so I’ll cling to that as a benefit.

It’s going to be very interesting to see how the weights change once I’ve finished the build.