How To Build A Rally Car

By automotive-mag.com 8 Min Read

In 2006, Subaru Motorsports USA partnered up with a boutique shop deep in the mountains of New England known as Vermont SportsCar (VSC). VSC was tasked with building competitive cars for Rally America; driver Travis Pastrana proceeded to win four straight national championships. Pastrana left in 2011 to pursue NASCAR, and VSC’s new driver, Dave Higgins, went on to win six straight titles. Since the demise of Rally America and the inception of the American Rally Association (ARA) in 2017, a VSC Subaru has won every ARA championship except one. 

If you want to know how to build a winning rally car, these are the people to ask.

Anatomy of a Champion

In 2022, the ARA rulebook was revamped. The goal of the new ruleset was primarily to bring down costs in top-tier entries by lowering car speeds and reducing the maximum amount of aero allowed. Luckily, in 2023, Subaru redesigned the WRX, giving VSC a new chassis to work from. The VSC team took the twin opportunities to their fullest and built a new car that could maximize both the new WRX’s abilities and the rulebook’s potential. The new platform just started competing this year in the 2024 ARA season, and the results are already spectacular: Three entries and three wins.  I spoke with Dylan Edmiston, a rally car engineer at VSC, to dive into how the new car came to be.

The process started in late 2021, as ARA announced rule changes were coming. “The pure design phase was a bit over a year” from start to finish, with the first car entering physical testing in March of 2023. In that design phase, “we start[ed] with the rulebook…  the car we’re running right now that was designed from the ground up around the regulations.” Only the WRX’s shell is retained from the street car. That shell is stripped completely bare, new control arm mounts are installed, the entire chassis is seam-welded, and new mounting points are installed for a roll cage. 

The aesthetics of the current WRX are one of the larger limiting factors, as the car still needs to look like the Subarus you can buy. Beyond that, the VSC team can go crazy within the (still permissive) ARA rules. With such a blank slate, aero—a heavily rule-limited factor—and suspension were the starting points. Aero came primarily via computer-aided design based on previous rally car design knowledge VSC has accrued, and suspension was developed via a partnership with R53, a company that builds shock absorbers for a variety of high-performance vehicles (including the Gordon Murray T.50). The transmission, a straight-cut six-speed sequential, comes from SADEV. 

From there, virtually everything is handled within VSC. Edmiston himself worked on rewiring the entire WRX. The ‘24 ARA Impreza’s “whole wiring harness [is] designed from scratch… and custom made for this car” although he noted the taillights use a modified stock loom. Engine management computing comes via Cosworth. The engine itself, a 2.0-liter turbocharged boxer-four, is built to take fullest advantage of ARA’s open-class ruleset and pushes the maximum 22 psi through its single turbo. 

Test ‘N Tune

After the lengthy design period and initial construction, the car went through a continual test-and-tune process. “The designers were always on standby… we had design changes happening from test to test.” Unlike most race cars, which compete in controlled, replicable conditions, rally is grueling and damage is frequent. Since VSC also handles running the Subaru car on top of building it, they’re also responsible for fixing it, and that means rapidly finding weak points and eliminating them with iterative testing. 

Mechanics start training on the design, and attention is paid to how repairs could be done more quickly. “The brakes need to be extremely serviceable… it’s not unknown for a brake disc to crack [on stage] and we need the ability to swap that out in a matter of minutes.” Anything that can minimize time spent in repairs between stages is key at a rally. The mechanics, at this point, start memorizing important torque specs and common repairs, which ideally won’t change much from the old car to the new one. 

“We start[ed] with the rulebook…  the car we’re running right now that was designed from the ground up around the regulations.”

Subaru Motorsport USA drivers Brandon Semeneuk and Travis Pastrana offer feedback on the car during testing. “We’d ask them, like, hey, how do you feel in the car?” And issues with comfort, pedal placement, or chassis tuning can be tweaked to taste. “We keep comments at all the rallies… so we can always hear about things they don’t like.”

This means the design process still isn’t really finished, despite the new WRX having run (and won) three rallies already this year, explained Edmiston. “We’re still at the point of doing development… So every rally, we’ll send an update to the drivers and co-drivers [about] things we’ve changed on the car.” This process will continue through the rest of the season, with the big gaps between stages getting the most drastic overhauls. 

Subaru WRX ARA Rally Car Build Process

The Secret Sauce: Parts Tracking, Strangely

The biggest development in the new car, however? “It’s our cleanest build of a race car… I’ve worked on here,” said Edmiston. The clean-sheet design and methodical iteration allowed for much better documentation of how, exactly, the new cars were put together. “In the past we’ve had some parts that [were] fabricated by the technicians… we say ‘oh, that’s the only one we’ll need. We’ll never have to build another.’ Five years later… we need a replacement, how do we make that part?” It’s not the sexiest or highest-tech, but documentation ensures that “when we build any future cars, they can come out exactly the same.” 

Currently, VSC has two ready-to-race WRXs and a third development car; their team is just sixty people, which constrains how much the company can do at any one time. The company is always shifting priorities depending on the time of the rally season, how the cars have held up, and what other automotive projects are on the table (VSC also builds rallycross Subarus and has even created some of the late Ken Block’s one-off Gymkhana stunt cars). 

Olympus Rally 2024

The next stage the VSC Subaru will run is the Oregon Trail Rally on the outskirts of Portland the weekend of May 17. Having recently attended a national ARA event, all I can say is that if you really want to understand how much of an engineering marvel a rally car is, go watch it fly through the air yourself. 

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