Author: occamsracers

If I was to grade the average splitter I see on the average track car, I’d give it a D+. I know that grades are supposed to average out to a C, but the average person sucks at aero. I see better splitters on Miatas than other cars, and those generally fall in the C+ range.

• Grade F: Flimsy splitters or front lips, usually bought on Amazon or eBay, but some reputable aero companies also produce shit. These look like splitters, but that’s where the similarity ends.
• Grade D: A misunderstanding of how splitters work results in splitters that are optimized for the top surface, not the bottom. They are are often too high or too low, and universally have sharp leading edges and are flat. Most have splitters wider than the front fascia, but do nothing with the exposed ends. Some might have a spill board on the outer edge, which is at least something, but that does nothing for the underside.
• Grade C: A splitter gains downforce through suction, which is achieved primarily through proximity to the ground and using diffusers and/or curvature to accelerate the air underneath. A splitter without height adjustment is like a wing without angle adjustment. A splitter without any diffusion is as useless as a flat wing.
• Grade B: With great suction comes great responsibility. Because the air under the splitter is low pressure, everything around it is higher pressure and wants to get in there. You need to extract air using hood vents and fender vents, and kick air upwards and outwards wherever possible.
• Grade A: At this point the splitter is making a lot of downforce, but there are still minor problems that occur around the wheels. Air hitting the rotating tires compresses and squirts out under the splitter, and air intrudes through the wheel spokes as well. Mitigating losses around the wheels and tires is the last frontier.

Grade A

Let me show you what a solid Grade-A splitter looks like. It’s not surprising that it’s on a Miata, because we do aero better. We have to; our cars suck. It’s also not surprising my teammate Alyssa Merrill made this. We bounce ideas back and forth, and while I think I’m pretty clever, she’s got a lot of great ideas of her own.

First, you have to use the right material. Alumalite is an automatic grade C, not because of the material properties, but every single person using Alumalite leaves a sharp leading edge. Despite the fact that Alumalite curves nicely, pretty much all of them are flat, as well.

Alyssa chose to use my favorite splitter material, Meranti BS6566 plywood. It’s very light (32 lbs/cu-ft), stiff, easy to work with, boil tested for delamination, and x-rayed for voids. It’s expensive, but you get what you pay for. She’s doubled the thickness of the lip to about an inch and done a good job of rounding the underside of the leading edge, so that air stays attached along the bottom surface.

The location of the radiator opening makes a difference to how the pressure side of the splitter functions, and has an effect on drag and cooling. I’ve seen a lot of radiator openings just barely off the splitter. When you put a hole there (for radiator or brake ducts), you lose the high pressure air bubble on top of the splitter, and with it, some of the downforce you’re trying to make. You might be thinking that a lower opening is better for cooling, but with the radiator opening down low, the air has to take a hard upward turn inside your radiator duct. Air can’t change directions at more than 12 degrees, and because the duct is much steeper than that, air separates from the surface of the duct causing drag and losing cooling efficiency.

Alyssa made the radiator opening as high as is practical while retaining the center bumper support. The higher opening holds a larger/taller high-pressure air bubble on top of the splitter, making more downforce. In addition, this provides a straighter route to the radiator, increasing cooling efficiency and decreasing drag.

All the air that goes through the radiator has to go someplace, and the worst place for that is under the car. The best place to send that air is upwards, because it creates downforce. Some people will use louvered hood vents, while others will use just a front wicker and a gaping hole behind. Louvers smooth the air over the hood, and result in less downstream losses on the wing. But since most cars are front-limited with respect to downforce, a gaping hole is a better solution for a track-only car. For a street car, louvers look better for sure.

Having a splitter without any height adjustment is akin to a rear wing without any angle adjustment. You see that kind of junk on OEM body kits, and you can do better. With a proper wing, tuning the high-speed handling of your car is as simple as changing wing angle.

It’s like that with splitters as well. Most of the downforce comes via interaction with the ground, so setting the height is critical. I’ve seen a lot of splitters that adjust for height initially, but then once you mount the airdam or fascia, the height is fixed. This is a C grade. At least there was some height adjustment at first, and that’s an acceptable situation if you can later modify it.

People getting a B grade with a fixed height splitter do so because they understand aero maps and have set their splitter height for maximum downforce, and thereafter do all of their adjustments via wing angle. But having both ends adjustable for downforce is obviously even better.

A-grade splitters are adjustable after the fact. Alyssa used turnbuckles to support the splitter, which are sturdy and make height adjustment a simple twist of the wrist. The turnbuckles are behind the airdam, which makes height adjustment more difficult, but it also completely hides them, for less drag. This is some trick shit.

What’s even more trick is that the turnbuckles fasten with pins, and so removing the splitter entirely is super easy. Just yank four pins and the splitter drops off the front.

Another area to address is the width of the splitter, and what to do with the ends. Under most racing/TT rules, splitters can be the width of the car or the front tires. In most cases, this results in some unused material on the outside edge of the splitter.

You get a D for doing nothing with the exposed end. A flat splitter with a flat exposed end is just additional drag and an invitation to catch on something. You get a C for putting a spill board on the outside, because while this can reduce drag, there are so many better uses of that real estate. Higher grades are achieved by putting a vertical dam on the outside edge (front tire spat if you will), which builds local pressure on top of the blade, creating more downforce. Also, air spilling off that will direct air sideways, extracting air beneath the splitter, making more downforce.

Alyssa chose to do something similar, but pulled the bottom edge of the airdam outwards to meet the end of the splitter. This won’t make quite as much local pressure on top, but should be better at sideways extraction and have less drag. Hard to say which is better, both work.

Let me address another huge mistake: flat splitters. Most people understand wings, and and flat splitters are like flat wings; meaningless. You need some curvature in the rear of the splitter to diffuse the air. When air expands behind a restriction, it accelerates the air in front of it, lowering the pressure. This suction is the purpose of a splitter.

Time attack cars often curve the entire rear of splitter blade upwards, essentially making a splitter diffuser the full width of the car. Miatas have historically used the rear subframe as mounting points for the back of the splitter, and the consequence of that is that Miata splitters are flat. To get suction, Miatas need to use splitter diffusers.

Splitter diffusers operate on the same principle as rear diffusers; they expand air in the tunnel. The diffuser itself doesn’t create the downforce, but it helps air expand and creates downforce in front of it.

Alyssa and I are designing splitter diffusers and will make them for sale sometime this summer. Here’s your first sneak peak.

These splitter diffusers have one large tunnel that diffuses air inboard of the wheels. This is an area of loss to begin with, and also an easy place to extract air from. Most splitters diffusers have only the one main tunnel.

But you’ll notice our splitter diffuser has additional area outside the main diffuser, with strakes. The purpose of this area is to mitigate tire squirt. As a tire rolls forward, it compresses the high-velocity air underneath it, squirting air out the sides. Some of this air goes under the splitter, which decreases the suction and downforce. By adding a diffuser in this area, it slows the air down, hitting the tires with less velocity.

The strakes spin a vortex off the trailing edge. Because vortexes take energy to create, the air hitting the tire has less energy, which reduces the amount of tire squirt. The strakes are also angled just-so, to achieve the maximum amount of extraction from under the splitter and send it outboard, protecting the main tunnel of the splitter diffuser.

Recall that most of the air is dumped inside the wheel well. Air here is extracted via vents on top of the fender, and behind the wheel. By extracting air upwards, you create an opposing force downwards. And by sending air sideways, you mitigate losses from air intruding underneath. Fender vents are important to make the most out of splitter diffusers.

A+ splitter

There’s an old saying I think attributed to fighter aircraft: 90% of the cost is the last 10% of performance. The same applies to car aerodynamics. While this splitter is a solid A, to make this into an A+ requires a lot of labor and cost, for little return.

First, I’d cut out the center bumper support completely, it isn’t necessary for chassis rigidity, and I can fashion a custom bumper after ducting the radiator. Removing that center support allows a slightly higher radiator intake, with the benefits I already discussed. I’d put a 100mm radius on the edge of the radiator duct, and reduce the opening size slightly.

I’d also tilt the radiator forward to reduce the frontal area, and duct the radiator exit straight upwards through an extractor vent to create more downforce. You might call this more engine bay management than splitter improvement, but it’s all related to creating more front downforce, and goes hand-in-hand with designing the splitter and ducting.

I’d also make the splitter slightly wider. Alyssa’s car is for HPDEs and is unbound by racing rules, so an extra couple inches on the end isn’t a problem. With that extra space I’d place place a simple piece of angle aluminum. This end-wicker, if you will, holds pressure on top of the blade and locates a higher stagnation point behind. This pressure differential creates more downforce, and it’ll still kick air sideways for extraction.

With a clean sheet, I’d also add curvature to the entire undertray. I’d laminate two pieces of 6mm plywood together, rather than using a single piece of 12mm. This would allow the rear edge to sweep upwards, creating more suction under the middle of the splitter blade.

Alyssa’s car doesn’t have a flat bottom, so it’s not a huge problem to diffuse some air into the center of the car. There will be some additional downstream drag due to local flow separations around the suspension bits, exhaust, transmission etc, but this is just a little bit of extra drag on things that were already creating drag to begin with.

I’d also put some height differential between the center and sides of the splitter. Remember early last year when all the F1 cars were porpoising? That was a cyclic gain and loss of downforce via ground effect, and it’s the natural consequence of trying to make as much downforce as possible, losing that downforce, and watching the car rebound on its springs. Once the car is back at a taller ride height, it again gains more downforce, repeating the cycle.

Many cars mitigate that effect by having a higher center tunnel, which gives two different heights of the splitter. This way if the lower part of the splitter goes under 1.5″ height, you don’t suddenly lose all of the downforce. The higher center portion also feeds the underside aero, which isn’t a factor on Alyssa’s car, which has neither a center tunnel nor a flat bottom.

So on her car I’d make the sides of the splitter slightly higher, rather than the middle. This will have the same effect of balancing ground effect losses, and will also protect the splitter blade, because the sides will ground out before the middle when cornering.

Finally, I might address some of the aerodynamic losses that come through the wheel spokes. By covering the bottom half of the wheel with caketin covers and spinning a vortex outboard, I’d reduce losses and redirect air down and out, with a minor gain in splitter suction.

But like I started with, modifying her splitter to go from A to A+ is a 90% effort for 10% gain situation. Addressing other parts of the car to the 90% threshold is time better spent.

I’ve had a tow hitch on just about every car I’ve owned, even my Mini and Miata. I’ve never towed anything with my Miata, but I have a cargo carrier I use for taking an extra set of tires to the race track.

The tow hitch also makes a convenient jacking point for the rear of the car, and I thought I might one day use it as the base for a quick-release diffuser.

So naturally I ordered a tow hitch for my Veloster. I got the Curt model from eTrailer, and in my stupidity, didn’t realize it was for the non-N versions of the car. FML.

The tow hitch sat on the shelf for a couple months while I pondered whether to return it or modify it. I saw a post on the Veloster N forum, where someone modified the Curt hitch to fit, and so I thought I might try to do that.

Yesterday was Father’s Day, and I had the day to myself to mess around, so I went after it. The first thing I did was remove the faux rear diffuser. I thought this would make access a lot easier, and it did, but it wasn’t the easiest thing to remove. Then I dropped the exhaust off the three rubber connectors, and test fit the hitch.

Fitting the tow hitch is easy by using a floor jack to raise it in position, then attaching the four supplied bolts. There was some body putty on my frame, which I had to scrape off before the ends would go into place.

Bolted up as intended, the hitch sits quite low. When hitches are this low, they can ground out going up a driveway, and the height is generally too low most trailers with tongue jacks, and even some bike racks. More significantly, the hitch is also directly in the way of the muffler.

I had a few different ideas on how to modify the hitch, including making a new one using the existing side plates. But after measuring some more, I reasoned it might actually fit if the hitch went above the exhaust.

I cut 3.5” out of the side mounting plates and tack welded it together, and then test fitted it. So close! The muffler just barely made contact with the hitch.

I cut off the tack welds with an angle grinder and moved the entire hitch portion half an inch rearward. Then I test fit it again and found it fit quite nicely. But I had mis-aligned the plates a couple degrees and the bolt holes were not lined up perfectly.

I could have widened the mounting holes, but instead cut the tack welds off one side, re-angled the end plate, and tacked it together again. Third time is the charm, right? Right. This time it fit like a glove, and so I removed it one last time, welded it all up, and spray painted the grinds and welds.

This sounds like a lot of work, and it was. Cutting and welding the mounting plates is a 20-30 minute job, but the trial and error part took hours. But probably the most time consuming part is removing the diffuser, which mounts with a bunch of annoying plastic tabs.

It would have been pretty easy to cut a square hole so that the trailer hitch protrudes through the bodywork. But I decided not to put the faux diffuser back on. I don’t like fake aero to begin with, and I actually think it looks way better this way. (Please don’t ask me what a cut bumper is worth for drag reduction!)

I’m not sure I’ll tow anything with the VN (the manual says not to), but now I can use my cargo carrier for race tires. I also got a hitch mounted bike rack.

Side note, the Harbor Freight hitch-mounted two-bike rack is a very well thought out design and a bargain at double the price. If you get a short hitch extender, you won’t need to pull the pin and swing the rack down; it’ll clear the hatch lid easily, so you can get to things in the trunk.

For the few dozen of you who subscribe to the blog, let me update you on some of the things that have been happening in my life, and how that’s going to affect this site and future content.

I bought a Veloster N for what I thought were pretty good and mature reasons. I took the car to Watkins Glen for Grid Life, but I didn’t really get a chance to open it up in the monsoon conditions. An NC Miata would have been a better choice (if you don’t get that joke, it’s a boat reference).

Then I drove the VN out to Detroit to see my buddy Chris Gailey, and in preparation for that, I built a bunch of aero parts for testing. No plan survives the enemy, and my Veloster didn’t survive one lap.

I accelerated out of T2 at Waterford and heard a loud noise, which then got louder, kind of like an exhaust gasket getting blown out. I’d recently replaced the straight pipe with OEM, so I thought maybe this was a gasket issue. But then the car lost power, and then died as I entered the pits.

We took off the undertray and found oil; it didn’t look good. A warning came up on the screen, I touched that, and it called Hyundai service. They said they’d pick up the car and take it to the Hyundai dealership, Glassman Hyundai in Southfield.

While waiting for the tow truck driver to arrive (high as a kite), I let Chris’s son have my track day. The good folks at Summer Track Days allowed Griffin to drive in my place, and I let Griffin borrow my helmet. Unbeknownst to me, Griffin let his buddy borrow my helmet so he could ride shotgun. Welp, the passenger got sick, couldn’t get my helmet off in time, and threw up all over the inside of my lid. No good deed goes unpunished.

We rented a Jeep Grand Cherokee, possibly the worst car I’ve ever driven, and drove 500 miles home to the smell of stale cigarettes. People who smoke in rental cars should have their lungs ripped out. Fuck you.

The people at Glassman Hyundai were awesome, especially Ralph, and kept in touch with me. The good news: the motor will be replaced under warranty. The bad news: N motors are on “international backorder status.” I’m pretty much expecting October.

I had some track days coming up, so I dusted off my 1.6 Miata, which recently had HLAs serviced and new valve springs. I spent a bunch of time on new aero for that, including fitting my fastback to it.

Then I took it up to Pineview, and it dropped a valve in the first session.

So now I’ve got three broken cars and four broken engines (my race car’s engines are also currently apart) and I have nothing to drive for the PCA event coming in a few days. So I figure I’ll find a 1.6 engine, throw it in there stock, and at least I can drive something for two days at Watkins Glen.

I found a 1.6 a few hours away for $750, and tried to trace back the ownership. It was apparently Dieter’s engine once upon a time, and I get his assurances that it’s decent. So I buy the engine, take it over the Shade Tree, and have them install it. Except the engine is rusted on the inside and worthless. WTF?

Well, apparently Dieter had two engines, this one isn’t the one he was talking about. This engine used to be Stefan’s, and he pulled it out of a junkyard car and sold it before checking into it. Fucking boat anchor, that’s all it’s good for.

So now I’ve got five broken engines and no car, so I figure I’ll go to the PCA event and just do data coaching. That didn’t really work out great due to some scheduling mishaps (the classroom sessions were double booked with my data sessions), and then just a lack of people wanting to do data. So I didn’t get a chance to use my vMin coaching tools or show the slide deck I worked so hard on.

So I sat in the stands with Josh and watched turn 10 for a bit. In an entire session (20+ minutes), four people hit the rumble strip on exit. I’d say 90% of the cars left a car width or more on the exit of the turn. A good number of cars exited turn 10 in the middle of the track. Did I mention this was the advanced run group? Maybe data coaching isn’t that important.

All of this has kind of reframed what I want to do with my life. I’ve spent 12 years racing and being a prisoner of this hobby. And it isn’t just the racing I got into, there’s the instructing, data coaching, racing rules, Pineview bullshit, etc. And of course all of the aerodynamic crap: theory, building, testing, simulations, and eventually writing it down here.

I’m not sure what comes next, but I’m due a break from the chaos. One thing I’ve decided for sure is I’m no longer owning a racing team or any Miatas. I’m selling or giving away everything Miata related. It’s been a great journey, but that part of the journey has come to an end.

As soon as I’d made that decision, I felt better. My wife simply said “It’s about fucking time!”

I’m still going to write; I have a lot of unfinished drafts and some really good ones coming up. I’m still going to race, but it’ll be as an arrive-and-drive. I’m still going to track my Veloster and do a bunch of aero experiments, but it’s going to be a while before I get back to it. The future is undefined, but looks brighter already.

Just today Ralph from Hyundai sent me a little movie – the new engine is in, the car is running, and it’s ready to be picked up. Hallefuckinglujah.

My brother recently wrote a blog post on the Driving Progression, which is a Dunning-Kruger of performance driving. I thought it would be fun to do the same thing for aerodynamics.

If you don’t know what the Dunning-Kruger effect is, it’s that people with very little knowledge have a lot of confidence. They’ll freely share the benefit of their inexperience with everyone. As they gain knowledge, they start to realize they don’t know anything, and lose confidence, eventually reaching a low point of “I don’t know shit.” After hitting rock bottom, an increase in knowledge steadily rebuilds confidence to the point that they master the topic.

The D-K effect is not about being stupid, it’s about not knowing any better. That’s certainly how I started my journey through aerodynamics, and it’s probably similar to how other people got started.

I’ve identified four general stages of the D-K effect, which I’m calling Fanboi, Learning, Experiments, and Optimization.

• Fanboi – Buying aero parts for looks, drag reduction, or blind trust.
• Learning – The apple of knowledge brings despair.
• Experiments– Tools and experiments to expand knowledge.
• Optimizations – Applying aerodynamic principles to any object with good results.

Fanboi

The first phase of aerodynamics is largely acquiring parts without understanding their specific use.

• Appearance – Many people buy aero parts because they look cool. Most dealer options like a front lip, side skirts, or spoiler are early on the upward slope.
• Drag reduction – People easily grasp the concept of drag reduction, and not knowing any better, it defines the next level of purchases or DIY projects.
• Blind trust – At this point a person has bought a wing or spoiler and some other aero that has good results (I dropped 3 seconds with a wing!), and now they think more aero is better and buy it all, whether or not it’s designed well or would be useful on their car.

You can ask a fanboi why they have this aero part, or how it works, and you’ll get a reply that has nothing to do with personal aero knowledge and everything to do with blind trust in the manufacturer. Businesses have a vested interest in parting you with your money, are they really the people you should trust on this matter?

Some people never get past the Fanboi phase, and good for them, ignorance is bliss. But some of them watch a video or read a book, or god forbid they find this website, and then they fall down a very deep hole into despair.

Learning

The next phase begins when a person does actual research into aerodynamics, and thus begins a slippery slope down Dunning-Kruger’s backside. Often YouTube is their first inkling that things don’t work exactly the way they’d hoped. For example, maybe they saw Kyle Forster’s video on how wings don’t work with convertibles. Or they watch Julian Edgar read yet another bedtime story from his book.

Some people might stumble onto my Occam’s Racer website, and if you’re a regular reader, you know I do a lot of debunking and peeking behind the curtain. Other notable stops on the way to the pit of despair include Competition Car Aerodynamics, by MacBeath and the more challenging Race Car Aerodynamics, by Katz.

From there you might dig deeper into to the grandfather of texts, Hucho’s Aerodynamics of Road Vehicles, or hit the lowest point, SAE papers and peer-reviewed university studies. At this point aerodynamics, or should I say, fluid dynamics, is so full of numbers, equations, and nerd stuff that you feel you know shit-all of nothing.

Experiments

Further knowledge comes through experimentation. Just as you were once a fool to trust aerodynamics manufacturers, you’d be a fool to trust anything you read or see on YouTube. So this next phase is where you get hands-on, experimenting to see the result.

Personally I jumped right into Airfoil Tools, OptimumLap, and other free resources, then DIY’d my own projects. A more sensible approach, and one I took later, is to enroll in Kyle Forster’s (JKF Aero) online course on aerodynamics.

Most of us aren’t going to have the means to get into CFD, and so Kyle does that for you, performing the experiments you’d do, and doing a nose-to-tail breakdown of different cars in CFD. If you are serious about aero, there’s no better use of your time or money.

It’s worth noting that Kyle uses OptimumLap in his course, since it’s a great way to get instant validation on whether something works or doesn’t, and how it plays out on different race tracks. I was already using the program for years before taking Kyle’s course, and it being FREE, everyone who is serious about aero or validating any other modifications to their car should use it.

The final stop in the Experiments stage is oddly where I started: real-world testing. That could mean going to a wind tunnel, or in my case, hiring a professional to make Watkins Glen into our own personal wind tunnel. And with that hard data, one comes to certain realizations about aero, and it turns out that it’s not that difficult after all.

Optimizations

Once you reach this level of understanding, you realize it’s not about the parts, but by applying certain principles. You are either optimizing the car for a particular rule set, or in the absence or racing rules, to handling characteristics, efficiency, a particular race track, or other factors.

The principles go something like this:

• Attached flow – You want to keep air attached along surfaces. A thicker and turbulent boundary layer is better than detached. Streamlined objects have rounded leading edges and break cleanly at the trailing edge.
• Change the direction of air – If you aren’t changing the direction of airflow, you aren’t doing dick. Most often you want to send air upwards, creating an opposing force: downforce. You don’t want to change direction too much or air detaches, causing drag and destroying downforce.
• Maximize suction – Pressure is good, suction is better. Maximize suction and minimize losses to the underside of splitters, underbody, and wings.
• Manage tires – Tires are draggy, disruptive to airflow, and create a jet of air as they roll forward on the ground. Manage tire drag and squirt to mitigate losses.
• Manipulate vortices – Vortices are usually thought of in terms of loss and wasted energy, but you can manipulate vortices to create air fences, delay flow separation, reduce energy on the face of the tire, etc. At the bleeding edge of performance, like F1, it seems like most of the effort is spent managing and manipulating vortices.

Conclusion

There is nothing wrong with being on the early, upward slope of the Dunning-Kruger graph. Choosing aero parts because you like the way they look, or because you support a particular manufacturer, or because they work and you don’t care why, is a happy place to be. This covers the 90% usecase.

In reflection, I can’t think of a compelling reason to get into early aerodynamic texts or plunge into SAE papers, they can be too much. Instead, leave that shit to me, I’ll translate it for the masses and make it fun to read (I won’t say “dumb it down” because y’all are smart or you wouldn’t be here).

But if you really want to get into aerodynamics because you’re developing something for the last 10% of performance, or because you need a competitive edge, then I’d start with the JKF aero course. You’ll definitely be making your own aero parts, because the aftermarket doesn’t support the kinds of things you’ll be doing, so plan on investing in tools and materials.

This is a worthy journey, but all-consuming, and requires sacrifices. Have fun with it.