Project Gruppe R: Aerodynamics For Rally

Rally Aerodynamics_edited-1We have decided to best classify our 2013 VW Golf R (Project Gruppe R) as a ‘recce car’ — with no intention to race this hot hatch, we are not left beholden to any sort of homologated rule set. We are free to let our creative and technical whims guide the project to the end destination we feel it deserves, and our gearhead’s Ouija board has been telling us that this car deserves a makeover in the form of some functional aerodynamic aids.

This divergence into the performance and aesthetic side of the project is a non sequitur if you actually examine the Federation Internationale de l’Automobile (FiA) rules for recce cars. The FiA mandates that recce cars are “totally unmodified standard cars as offered for sale to the general public,” per section 25.1.1 of the WRC Sporting Regulations — however, the rule set continues to define allowable modification to suspension, protection, and safety.

But, because we are not European, and we declare artistic license is in effect, we are going to forget about that. The end goal here is performance akin to that of a WRC car, without the fuss of homologation and the ability to enjoy this driving experience daily.

Computational Fluid Dynamic (CFD) Predictions

We are certainly not going to just slap on some fiberglass and declare our efforts valid in the crucible of the wind tunnel. Instead, we aim to go about our modifications systematically, addressing known quirks of the hatchback design — while citing authorities on the subject to inform our decisions.

Screen Shot 2016-02-26 at 2.54.52 PM copyWind tunnel time is expensive, obscenely expensive — as powerful computers have become more readily affordable, many engineers and hobbyists are turning to computational fluid dynamics (CFD) to model their cars. CFD uses a ‘mesh’ of digital cells that the user can manipulate to visualize the flow over the surface of the vehicle. These computations are not end-all-be-all answers, but rather reliable springboards for development. The ability to model multiple variations and test them on the computer makes trial and error a lot less expensive. Screen Shot 2016-02-26 at 2.55.06 PM copy

For some scholarly insight, we consulted with Eduardo Cañada, holder of a MSc degree in Motorsport Engineering from Oxford Brooks University, and Mathieu Horsky, a race engineer formerly of M-Sport and also a MSc holder from Oxford Brooks — both of whom have done considerable research on the topic of WRC aerodynamic aides. In Cañada and Horsky’s CFD analyses, an analogous platform, the Subaru WRX hatchback, is used. Thus, our applications of their findings are approximate but supported in theory. Borrowing graphics and verbiage from Cañada’s dissertation Aerodynamic analysis and optimization of the rear wing of a WRC carand Horsky’s Dynarace 2.0 blog simulations, we will attempt to depict the effects of our aerodynamic additions.

Baseline Aerodynamic Characteristics

Screen Shot 2016-02-26 at 2.56.31 PM copyLet us first establish that the boxy profile and cliff-like rear hatch of a car like the VW Golf have about as much aerodynamic merit as a piece of Bauhaus furniture. The airflow over the vehicle mimics that of most normal sedans until it reaches the rear hatch, which drops away steeply. Here begins much of the car’s aerodynamic downfall. Over the years, well-funded motorsports programs like those found in WRC and other series have developed systems to improve the VW’s aerodynamic performance.

Screen Shot 2016-02-26 at 3.00.31 PM copyOn loose surfaces, the abrupt downwash of air off the back may contribute to the rear end going light. The boundary layer of air separates and leaves a low-pressure void (or wake), behind the car, much like the draft behind a semi-truck. This low-pressure force translates into drag, and while a spoiler exacerbates this effect, it lends favorable downforce to level an airborne car (preventing a nose-in landing). A spoiler for rally application is also designed to help stabilize the platform at unusual attitudes and slip angles — flying over crests and sliding around hairpins require different demands than pure downforce. Screen Shot 2016-02-26 at 3.03.46 PM copy

Proposed Modifications 

We are taking a three-pronged approach to the aerodynamic and aesthetic package to adorn the Golf. Our goals are to mimic the VW Motorsports-tested, and third-party-evaluated body additions, both in form and function. The major parts of our unique package include: a clone kit of VW Poland’s Castrol Cup race series Golf wide body fenders (both front and rear), a dual-plane rally spoiler, and a front air dam.


The raw parts, fresh out of the cargo plane. There’s lots of work to do before these will be worthy of Gruppe R.

Allocating the parts for this build has been, and continues to be, a global search. For the body work, we found ourselves brokering freight passage across the Pacific from Hong Kong, and reluctantly conducting a slightly sketchy wire transfer to Warsaw for our spoiler. In the end, both parties came through with the parts.

Irish WRC fixture and driver of the Abu Dhabi Citroen, Kris Meeke, explains the ins and outs of why aero plays such an important part in the dynamics of a rally car. The three elements he defines are the areas we intend to address:

Adding Some Flare

The first endeavor was to allocate our Castrol Cup body work. This series is based out of Poland and cars are constructed by VW factory-backed mechanics with third party accessories. While the genre of the Castrol Cup is road racing, the tenants of their modifications carry over to our application.

VW Motorsports developed angular bodywork for both the WRC Polo and Castrol Cup Golfs. In regards to the former, “The exposed angular fender directs the airflow sideways past the body. The first version of the Polo R WRC had a rounded fender. However, extensive tests in the Volkswagen wind tunnel showed that an angular fender worked better,” VW’s Rally The World website explains.

Additionally, the increased surface area and angular projections of the front fenders presents a surface on which to build high-pressure air, and by extension combat front-end lift.

At the time of purchase, the only available option was a reproduction designed to be fitted on a four-door version of the Mk6. This meant that the rear fender flares were two separate pieces to accommodate spill onto the rear doors. Not afraid of a little glass dust, we tore into the fenders.


With the gelcoat removed and a foam spacer glued in place, we could consider more permanent bonding.

With the rear fender flares spring clamped onto the sides of the car, we could fit the teardrop insert. In order to accommodate the panel gaps, there was a void between the parts that would need to be filled. With some modeling foam and a heat gun, we cut and formed a spacer to locate the door insert and align the edges. Once everything was held in place, and the gray gelcoat was removed and we could consider permanent joining solutions.

While flares are by no means structural components, they will see ample vibration and shock in the rigors of rough driving. We elected to use a fiberglass chop-matt impregnated body filler to join the fender pieces. After packing the green structural filler into the small void, we were able to wet-layup two layers of fiberglass cloth and polyester resin.

We packed the void with fiberglass-reinforced filler, then laid-up two plies of cloth, stippling resin to wet-out the fiberglass.

After allowing the resin to cure, we were able to sand down the rough edges. The remaining trough allowed us to fill a shallow valley with conventional plastic filler and final sand the profile of the fender.

Looking ahead to the front fenders, we still had more bodywork to do. The widened front wings terminate in the stock locations, which means the fender-to-bumper panel gap leaves a considerable step. The kit included two comma-shaped continuations to finish the arch of the fender as it spills onto the bumper. Because this is a streetcar, we wanted to retain all of the indicators and found that some custom profiling would be necessary.

IMG_4185We transferred a template outline of the marker light onto the fiberglass comma and broke out the die grinder. About an hour later, we had a cleanly clearanced part, and our beveled profile would ensure visibility of the marker from oblique rear angles. Finishing the parts involved more of the green fiberglass-reinforced filler, and a little conventional filler to smooth the surface.

Test-fitting the front fenders was a must before we could drop off anything for paint. Painstakingly, we removed the factory front fenders and installed the new widened units in place to inspect panel gaps and mounting hole alignment. After opening a few bolt holes and playing with panel gaps, we were satisfied and ready to move on.

Shades Of Gray

IMG_5518With the parts in presentable shape, we rang up our buddy Ramon Gonzalez at ACM Auto Body And Paint in Escondido, California. Understanding that color match painting can be a dicey proposition, especially with a metallic color like our gray, we needed someone we trusted and had a positive track record of painting numerous other projects in this vein.

Gonzalez ran his hands over the parts and assured us he would tune them up with filler where it was necessary — fiberglass parts are inherently wavy, so we decided to leave the rest to his qualified team. With the car parked in direct sunlight we broke out the OEM VW color code and paint chips, and after concluding on the best fit, we got to mixing.

We blended the pearls, colors and bases of the water-born DuPont Cromax paint at the scale. We sprayed out the initial recipe on a test strip and applied it to the car — the resulting hue was dark initially, and took a reddish tint. With the addition of green and white pearls, we were able to dial back the maroon and lighten the color to a near-exact match. The OEM paint varies in color a surprising amount from panel to panel; and depending on the lighting, it is either green or red.

“This is the only paint that’s wet-on-wet application, so there’s no flash time in between coats,” explained Jose Marchena of DuPont.” Color match is tricky — OEMs paint in many different locations, and that’s why you get so many variations. You might get this one that was painted in Mexico, and then there’s some that are painted in the US; that’s why you’ll get the same codes but have different shades. Typically, every color that you do is blended, so you go into the next panel and blend into it. You always go opposite on the color wheel to try to kill one of the colors off — we didn’t want to completely overdo it by getting rid of red because the factory paint still has some red in it,” he continued.

To color-match the spoiler, we left one of our satin black BRAID wheels with Gonzalez and Marchena. The replication of a satin finish takes special treatment.IMG_5487

“It’s all done here with them at the scale, and it comes down to what sheen level you want and how much flattener you put in the clear coat. That’s a tricky process too, because you can start seeing stripes and stuff if it’s not applied correctly,” Marchena concluded.

Plot Spoiler

For our application we wanted to capture the aesthetics as well as function displayed in rally cars. We have decided the addition of a functional rear aerodynamic device is a prerequisite to do that wholeheartedly.

Screen Shot 2016-02-26 at 3.00.44 PM copy

Added drag is a known side-effect, resulting from the vortex generated under the spoiler. Top speed means nothing if your platform is too unstable to reach it.

When we find ourselves blasting over a blind gravel crest or hunkering into a road course corner, our aerodynamic aid will perform its duty. According to the WRC ruleset, “The rear aerodynamic device must be entirely contained within the frontal projection of the car without its rearview mirrors,” says FiA 5.7.4. This means that no additional frontal area will present itself to the onslaught of the wind, minimizing efficiency losses while improving stability.

“A spoiler interrupts the natural airflow and manipulates it to the advantage of the car. However, with a WRC car, the aim of aerodynamics is not higher speeds on bends, but stability, stability, and more stability,” explains VW Motorsports’ Rally The World Tech Talk.Screen Shot 2016-02-26 at 4.09.44 PM copy

“For small yaw angles, airflow remains attached on the vehicle’s sides. However, when increasing yaw angle, airflow separates from the leeward side, creating a much larger separation bubble when combined with the separation bubble at the back,” Cañada notes. “This is a very important analysis for a rally car, and its aerodynamics should be optimized to minimize the loss of downforce caused by high yaw angle situations as much as possible.”

Why A Dual Plane Spoiler?

“As we can see, cases two and three clearly show a substantial flow separation on the trailing edge of the lower part of the wing. However, when the air dam is introduced, the flow remains completely attached on the surface of the wing,” illustrates Cañada.

The source for our spoiler is a small Polish rally fabrication shop that conducts work for many teams, and supplies parts to well known rally car manufacturer Dytko. The unit is intended for use on a VW Polo, which, while not available to the US market, is a favored platform in other countries. We will not be using the conventional mounting method, but retrofitting this part to the slightly wider Golf.

Further additions to our spoiler include a ‘Gurney’ flap, or ‘wicker,’ which adds additional downforce to the surface and biases the center of pressure. “The Gurney flap really helps to create the stagnation point over the profile, which helps to generate downforce,” Horsky points out, “When the simple wing is added to the vehicle, we can see that downforce is greatly improved. When the Gurney flap is added, efficiency falls as expected, while downforce is increased and the balance is biased towards the rear of the car,” Cañada continues.

Left, the Gurney Flap stagnation point as identified by Horsky. Right, vertical fin modeling.

By adding vertical surfaces behind the center of pressure, the car tends to recover by itself when disturbed by a gust of wind or a back-end slide. -Mathieu Horsky

Secondly, vertical planes were added to aid in slip angle recovery. These surfaces bridge the dual planes of the spoiler and provide little frontal area until exposed to an angle of yaw, then act as a trim tab or rudder to promote stability. “By adding vertical surfaces behind the center of pressure, the car tends to recover by itself when disturbed by a gust of wind or a back-end slide for example,” explained Horsky.

One of the challenges we faced in transforming the aerodynamic characteristics of Gruppe R was mounting the spoiler. As we mentioned, the device was made for the VW Polo, and requires holes for mounting hardware through the rear window. Not about to bore holes through tempered automotive glass, we set out to devise a more fabricated solution.

The first step was to locate the spoiler in its final mounting position. For this, we screwed together a simple support frame to assist us. With the spoiler in place, we marked the extremes and made card-stock templates complete with bend lines. The goal is to capture the edges of the glass, preventing a premature delamination of the large dual-plane addition.

With these templates, we took to the shop where, with a sheet of .063-inch 4130 chromoly, we duplicated the brackets. We took into account the curvature of the glass and thickness of the edge we would ultimately clamp. After a few plasma cuts, some heat — thanks to our faithful rose-bud — and a few persuasive pangs with a hammer ove we had very presentable brackets without the need for a finger brake or CNC mill.

With some massaging over the horn of an anvil, we were able to achieve the profile of the glass, allowing the 1/4-inch lip to grab the edges of the glass without applying a pressure point at each end. The spoiler has some natural spring built into it, and as the mid-plane is tightened into place, the outer arch is drawn inwards. We drilled holes in the brackets to take advantage of this effect. This would allow the spoiler to be easily be fitted over the glass and then clamped down with the addition of the mid-plane.

We drilled the holes to match the threaded inserts already inside the wing, and could finally mock up the spoiler. Confident with the security of the piece at both static and dynamic loads, we considered cleaning up the appearance of the mounting brackets. With some leftover fiberglass-reinforced structural filler, we fared-in the gap between the bracket and the spoiler’s outer corner. After a little more primer and some sanding, a few coats of satin black made for a near seamless bridge between car and aerodynamic aid.

Final Installation

The front wings of this aero package bolt right up to the stock fender mounting locations with a little massaging. Getting the OEM front fenders off, however, was a challenge. VW dips their unibodies in a waxy coat to prevent rust intrusion on salty and icy roads. This, combined with the industrial adhesive (it looks like a bead of silicone, but isn’t) meant that the front fenders were effectively glued and bolted to the car.

IMG_2598With the front wings bolted finger-tight to the car, we could do a little fine-tuning of panel gaps. The hood line is easily adjusted with a single bolt on either side to raise or lower the profile to meet the fender, and the side-to-side can be shifted with the hinges. The last step was to re-secure the front bumper.IMG_2706

With the front fenders now securely bolted in place, the next logical parts to address were the ‘comma’ pieces that completed the front arches.


We cut the 3M VHB tape into strips and applied it to the surfaces we cleaned with denatured alcohol.

With the easy part done we had to get creative, the remaining parts of the aero package did not come with any provisions for mounting, and the car is not forthcoming with options. In this day and age of high-tech adhesives, we turned to 3M for some Very High Bond (VHB) double-sided foam tape (model 5952). The 5952 VHB tape is expressly for secure mounting of panels in automotive, architectural and other industrial applications. It’s easy to underestimate the bonding power of this product until you see it in action; I personally have witnessed parts stuck together requiring an arbor press to separate.IMG_2794

We chose the 5952 line of VHB because of the mechanical properties it had. The 1.1 mm thick foam core means it will conform to imperfections in the body work; UV resistance means the sun won’t take a toll; and a wide operating temperature window means parking in a searing parking lot will not cause the bodywork to melt off.


We used painter’s tape to help us line up the fenders before sticking them in place.

The rear fenders were taped up just like the commas, but with much more VHB. With painter’s tape registration guides in place, we carefully pressed the rear arches into place.

We took particular care to align the body parting lines, taillight reliefs and gas cap. The rear fender flares went onto the car with more ease than expected. Don’t be intimidated with this sort of you-only-get-one-chance install. Just take your time and make sure everything is lined up before pressing the fender into the panel.


Because the spoiler was intended for a Polo (which has a much flatter rear window) some of the angles were a little off. With the brackets structurally complete we blended the transition with a little more fiberglass-reinforced filler.

The final part of this aero package was the spoiler. To prevent scratching and slipping on the edges of the glass, we sandwiched some small suede strips – a little motorsports trick used to insulate high-risk parts like fuel cells from metal-to-metal contact, and eventual failure. On the other side of the bracket, clearances are tight. We didn’t want to mar the paint, so we added a protective strip of clear tape.

We used a rubber mallet to tap the brackets onto the window and then clamped everything down by tightening the bolts that secure the mid-plane wing in the spoiler. This pulled the wing tight across the glass. Since the window tapers, the wing will not be able to slide down, and some VHB helps keep it from sliding up.IMG_2938


IMG_3356Finally, the VW Motorsports-developed aero package we envisioned for the car was installed. The results were just what we hoped for: aggressive, wide, and functional. Of course, there are some panel gaps that are less than factory tolerance, but this is a driver.

We have one more thing to address with the aerodynamics – a front air dam. Look for this simple DIY mod in a quick follow up. For now, enjoy the final beauty shots.

Photo gallery


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About the author

Trevor Anderson

Trevor Anderson comes from an eclectic background of technical and creative disciplines. His first racing love can be found in the deserts of Baja California. In 2012 he won the SCORE Baja 1000 driving solo from Ensenada to La Paz in an aircooled VW. Trevor is engaged with hands-on skill sets such as fabrication and engine building, but also the theoretical discussion of design and technology. Trevor has a private pilot's license and is pursuing an MFA in fine art - specifically researching the aesthetics of machines, high performance materials and their social importance to enthusiast culture.
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