Fitting a huge wing on the back of the car certainly helps rear stability, but there needs to be something at the front of the car to help keep the car aero balance. The simplest way to do this is with a splitter. I ran one on the Golf for many years and tried various materials. Yes, a fancy carbon fibre one would be great, but they are also very expensive. There are many articles and videos online about how splitters work, how diffusers in them improve effectiveness and end fence design for increased efficiency. For now, I’m starting with something simple but that gives me future avenues of development to pursue should I have the desire and more importantly the time!
Walk around a paddock at a race meet and you’ll see a large number of cars running splitters that look to be made of plywood, but look closer and you’ll see it often has a textured surface and has more layers compared to typical marine plywood. They are using what’s called phenolic plywood; one of the most common is branded ‘buffalo board’. It’s very strong and extremely water-resistant and doesn’t delaminate when wet.
I can just fit a full sheet into the boot of the X5.
You don’t have to be particularly observant to spot this is pretty thick. I ran 15mm board on the Golf and had issues with it bending at high speed. A splitter snapping at 160mph or above is certainly not something I want.
I don’t want to fit external guide wires to support the leading edge of the splitter as I had to in the Golf and the mounting points to the chassis on the R8 are set back quite a bit, meaning the leverage on the front of the splitter will be high. To address this, I used 18mm board. Overkill, too heavy? Yes, definitely, and in the future I may look at bracing it internallyand going thinner, but for now, this will do, and I’ll accept this is 7 or 8kg heavier than it needs to be.
First job is to get the car in the air, position the sheet where it’s going to sit, and set the distance from the bumper to the leading edge.
I use a spacer from the bumper to match the contour as you can see with the line I’ve started drawing.
Somewhere around the sides I need to blend back to bumper width and test this with a piece of cardboard. This also means I can flip it and trace the same shape on the other side. There is no particular shape to the splitter, but if you get the proportions wrong it can look pretty bad. Even a small change in profile looks ‘wrong’.
Once I was happy with the profile it was cut with a jigsaw and sanded with a block to smooth out any cutting imperfections.
I don’t have access to CFD or a wind tunnel so yes, I’m sure the splitter could be made more effective and efficient, but this is the best I can do with the tools available. 
The OEM undertray was removed and used to cut the rear edge to ensure the tyres didn’t foul during turning. I also used a router to add a radius to the leading edge. I see so many people leaving them square or with only a tiny curve on them — yes, it takes a little bit of effort, but not only does it look better, it’s also more efficient.
I wanted to add some splitter diffusers to match the OEM ones. In the future I plan to make a version with larger diffusers, but that will mean cutting the OEM undertray, so I’m holding off on that for now.
To make the ‘diffusers’ , and I use the term loosely, they are literally sloped parts of the splitter with no specific design or shape, just a smooth transition from flat to curved. I used a narrow router and cut relief slots in the top side. A straight cut down either side and then it could be curved.
Side pieces to hold it are from some aluminium sheet screwed into the ply.
The mounting bolts are recessed.
Some large washers were then pressed in the middle to add a recess in the centre for the bolts.
The countersunk bolts will then sit flush once secured.
For some reason I don’t have photos of the mounts, which is irritating. The front aluminium crash bar has 2 removable parts circled in orange. I built up a square of aluminium with the TIG as shown in green and then drilled the middle and tapped it M8. I use 150mm long M8 countersunk bolts that I apply a small amount of medium-strength Loctite, which stops them loosening.
For the sides, I made some aluminium brackets that bolt onto the radiator mounts that are also threaded M8. For the rear of the splitter, M6 rivnuts into the crossmember are absolutely fine. Next time the bumper is off I’ll get the photos and edit this post.
The ply is then sprayed. I use etch primer first, then satin black which help tidy up the leading edge.
Bolted up so it’s just pressing against the bumper.
It looks good, but I’m not happy with the height. The lower the splitter, the more effective it is; this looks too high, so I reckon I’ll be tweaking it.
This feeling of it not being efficient enough was borne out at Spa. With a working splitter, the front end loads up at high speed instead of going light through fast corners like Blanchimont. It was feeling vague even with the wing angle reduced, so I knew I needed to focus more on the height and effectiveness of the splitter. It was definitely better than when I ran no Aero at all, but the reduction in front end lift wasn’t as much as I want.
So onto V2 of the splitter several months later.
I used some leftover board to create a stacked airdam; I can then unscrew a layer at a time to adjust the height of the splitter.
3 layers
Or 2.
With the airdam sprayed, unless you know it’s layers of ply, you’d never know.
After one trackday the head of the leading bolts was wearing away. Even with them being recessed, it wasn’t enough to protect them in places like the Karussel. After 2 days the wear was more significant, but I didn’t get a photo.
Time for a rethink of the mounts. I made some ‘top hats’ from 3mm plate and thin-wall 25mm pipe.
The bolt will sit inside the sleeve and be protected from touching the tarmac.
The M8 caphead is recessed.
I also sank some stainless wear bars ahead of the mounts to protect them.
These were recessed and screwed into some threaded inserts in the timber to secure them.
I changed my mind and the rear 2 holes were secured with long coarse-thread bolts. I ground the head flat after installation. (10 trackdays later they are still solidly attached.)
The wear bars are showing signs of touching, but the actual recessed mount is completely untouched.
To make splitter installation easier, I secured the top hats in place with a recessed M6 bolt.
This screws into a T-nut on top of the splitter. It means when I remove the splitter, these top-hats stay in place and I’m not trying to juggle everything when I fit the splitter.
The whole splitter is held on with 4 x M8 caphead bolts and 3 x M6 countersunk along the trailing edge. I can remove it by jacking up the front of the car and bolting it up from underneath. The eagle eyed will spot I made some Aluminium diffusers, they are currently undergoing tests with different sizes, more details will follow in due course.
If you compare the height to V1, the difference is obvious.
V2
Does the V2 feel better? It certainly does! Blanchimont was much more confidence inspiring, the front end had far less lift than V1 and the nose felt more stable at high speed, giving me a lot of confidence when on track in high speed sections. At the Ring, it’s too low and grounds out badly, especially in the Karussel. I need to remove one of the layers of airdam for Ring trips, but that’s easy enough to do. For everywhere else, it hardly ever touches, only at places like the mountain at Cadwell will the loaded leading edge skim the track which is fine, that’s what the wearbars are for anyway…
The downside? The leading edge has taken a battering this year and it needs repainting, but you can see the ply has held up really well and even after grounding out on kerbs it hasn’t delaminated. I’d feel quite upset if that was a shiny Carbon splitter.
Obviously I can’t leave well alone, and V3 will be along in 2026. 
