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How to make a carbon fibre racecar splitter

The black art of aero has to be one of the most misunderstood, yet best bang for buck car modifications around. As such i decided to get handy and build a front splitter for the evo. My car lacked any type of front undertray, even the oem undertray was missing so i couldn’t help but think i am losing time due to this and not capitalising on the rear GT wing as a result.

I am hoping this mod alone will be a 1 second improvemernt around EC, i know for a fact that my car lacks front downforce as i always have to modulate the throttle to avoid understeering and be very patient round some turns. My rear wing isn’t providing the necessary dividends without having decent front downforce do it was a no brainer to start with a splitter. I have done a lot of research on this and spent a fair bit of time under the car planning how to mount it.

I have fabricated a bracket out of a metal fence post to use as a brace to mount the splitter from. The bracket is super strong and actually not all that heavy, it weighs about 2kg and it will certainly be able to stand over 100kg and actually probably twice that. This bracket attaches directly to the chassis of the car, the part that is right behind the radiator at the front and extends across. The bracket is held on by the two cross member bolts and high tensile M8 bolt each side, it just happens that there is already a threaded hole in the chassis each side which has made this much easier. The bracket is 100mm tall by 5mm wide (extends 100mm down from the chassis) i got it from Bunnings, i was super lucky that the fence post was the perfect dimensions otherwise i would have had to go to a metal fabricator.

The splitter is going to attach to the bottom of the bracket. As there will obviously be no access to the bracket once the flat splitter is connected, i needed some way to have some sort of fixed nuts on the bracket so i bought an entry level arc welder and had a crack! It isn’t pretty but it is functional. I have since cleaned it up and galvanised it. I have made it so the splitter itself is a pretty easy job to take off and on for transport, takes about 20mins to mount and 5mins to take off.

Following is my documented build process.

Firstly i used 12mm structural ply to make the initial template and this can also serve as a backup should i ever destroy the carbon fibre splitter.

metal bracket:
diy racecar splitter

diy racecar splitter

Welded some nuts on:
diy racecar splitter

diy racecar splitter

bracket mounted to the car:
diy racecar splitter

Mounting the splitter at the rear to the 2 existing holes in the cross member. You cant see it but there is a nut wedged in the aluminium tubing:
diy racecar splitter

Lining up to mark where to drill holes to mount the splitter to the bracket and then i’ll use a jigsaw to cut the splitter to the correct shape:
diy racecar splitter

diy racecar splitter

diy racecar splitter

template now jigsawed into shape:
diy racecar splitter

diy racecar splitter

diy racecar splitter

diy racecar splitter

The splitter extends 11cm out from the lip:
diy racecar splitter

diy racecar splitter

view from the rear showing the attachments:
diy racecar splitter

Next, create a front aluminium sheet to gap the hole between the splitter:
diy racecar splitter

diy racecar splitter

diy racecar splitter

diy racecar splitter

diy racecar splitter

Now the template and bracing are complete it is time to make the real carbon fibre splitter. I decided to make a foam cored sheet from scratch. Bought all materials from www.easycomposites.co.uk whom i found very helpful. I even called the owner via Skype one night and asked a bunch of questions and he was very helpful. Shipping from the UK to Australia was a bit of a killer though. It would be cheaper to get the foam core locally and maybe also the epoxy resin but it’s definitely cheaper buying the Carbon Fibre fabric from o/s.

So with the help from a mate we spent a day laying the carbon fibre fabric and foam core to create the 2m x 1m sheet. It turned out pretty good, a few bubbles but definitely OK for my purposes, after all a race car splitter is going to get dinged up over time anyway. The sheet is 2 layers of 195gsm Carbon fibre fabric each side of a 5mm foam core centre. I wet laid it using epoxy resin.

Preparing a 2400 x 1200 mellamine sheet with the chemical release agent:
diy racecar splitter

This is the first layer of carbon fibre fabric over the ‘show’ layer of epoxy, spent ages with a bristle roller and hairdryer trying to ensure no bubbles on the show surface. This is probably the most critical stage of the exercise:
diy racecar splitter

Laying the foam core:
diy racecar splitter

diy racecar splitter

The sheet is finished and left for 48hrs to dry and cure:
diy racecar splitter

The scariest part of the progress, releasing the sheet from the mellamine:
diy racecar splitter

After jisawing the new splitter from the plywood mould i drilled all the necessary holes and added the support bracing:
diy racecar splitter

The quality is pretty good. there are some imperfections but it’s a race car!
diy racecar splitter

diy racecar splitter

Then i added some side fins to the splitter. It is now ready to fit to the car. I just need to paint the edges black:
diy racecar splitter

I decided to add some extra bracing to hold the splitter on. It was probably not needed but it has really strengthened the splitter and reduced the distance from the main cross brace to the front of the splitter:
diy racecar splitter

Finally i got some adjustable and removeable ARP splitter rods and fitted the splitter to the car:
diy racecar splitter

diy racecar splitter

diy racecar splitter

I then raced the car at the 2011 World Time Attack Challenge (WTAC) – aka Superlap Australia – and the car looked and performed great. I managed a new PB at Eastern Creek Raceway of 1.40.06 on Hankook Z221 R spec tyres. To put this into perspective my previous PB on R specs was at the previous year’s Superlap where i did a 1.41.78. Since then the only mods to the car have been an AP Racing front brake Kit / different brand tyres (Hankook Z221s instead of Advans A050s) and the splitter. So that’s a 1.8 second improvement, a huge amount in motorsport.

Also a month ago i did a 1.40.41 time at Eastern Creek on Michelin slicks at the Evonationals. So basically i’ve managed to go 0.3 seconds quicker using R-specs, than my previous best time which was set on full slicks!

In summary i believe the splitter has resulted in about 0.8 – 1 second lap time improvement so is an excellent bang for buck mod. I can feel the car more planted through the corners and my data has shown an average of 5-6km/hr speed improvement through Turn 1 at Eastern Creek as well as speed improvements on the majority of the other turns as well. Of course the Hankook tyres are partly responsible but with the splitter and Hankooks i am going faster than when i was using Michelin full slicks. So the splitter is certainly doing something.

Here are some shots from WTAC 2011, a full write up is coming soon:

Sonic Interactive Evo 9 - WTAC 2011

Sonic Interactive Evo - WTAC 2011

Sonic Interactive Evo - WTAC 2011

5 Responses to “How to make a carbon fibre racecar splitter”

  1. John Hunt says:

    Nice work Mick! Not only is it functional, it looks cool too 😉

  2. Tinto says:

    Great work – the DIY steps are much appreciated!
    Was the aluminium air dam section making contact with your existing lip?
    How did you manage movement between those surfaces? Some rubber piping or foam padding?
    Maybe I’m just overcomplicating it 🙂

    • Michael says:

      It does make contact with the existing lip but it’s quite tight so it doesn’t move much. It rubs on the lip a bit but as it’s hidden it doesn’t really matter. Putting a foam strip on would be an option but i haven’t bothered as yet.

  3. Marc says:

    Hi… great work.
    How Many Layers did you use above and under the foam core?

    interestingly in the pictures of the finished Lip, I can’t really see the foam core.

    • Michael says:

      Hi Marc, from memory there were 4 layers both sides of the foam core. I painted the edges black after jigsawing which is why you can’t see the foam core.