I am happy to report that my love affair with the Thunder Roadster has not waned. If anything, it has sparked an engineering journey that I am super excited about. And throughout that process, I have found the documentation around the Thunder Roadster severely lacking. As I continue down this path, I will use my platform to compile, consolidate, and expand the knowledge base I acquire and make it available here.
So, first up, let’s look at the Thunder Roadster’s classing by comparing the Prep 1 to the Prep 2 (GTR Spec) and TRX Spec.
Background
This post will be less of a history lesson and more of a technical discussion. However, a little background will help. US Legend Cars International introduced the Thunder Roadster in the early 2000s, and all Thunder Roadsters are based on the same chassis. Also, Thunder Roadsters are currently not in production.
However, many chassis are still out there, and mechanical parts are readily available thanks to the popularity of the Legends car series. In fact, Thunder Roadster and Legend in early trim used the same drivetrain and suspension (including suspension arms and rear ends).
Furthermore, the only parts unavailable ‘off the shelf’ are the body panels. And that is something I am exploring alternatives to with my custom build.
Prep Level 1 Thunder Roadster Specs
The Prep Level 1 Thunder Roadster is a somewhat open-wheel ‘hybrid design’. What I mean by that is the wheels are exposed on the top side, and a fiberglass side pod runs the length of the body between the front and rear wheels. There is an additional contact bar that protrudes parallel to this side pod. A quick pic will go a long way to illustrate this.
Drivetrain Specs
(Note: The following is the generally accepted current spec for a Prep Level 1 Thunder Roadster to run with the National Auto Sports Association. And unsurprisingly, and something I will speak about more in the future, the AI summary for these classes is absolutely misrepresented garbage.)
In race trim, Prep 1 with driver weighs 1500lbs. Powered by a front-mounted 1200cc to 1250cc Yamaha air-cooled FJ series engine. Output is optimistically 110hp at the wheels. Power is sent to the rear via a two-piece driveshaft that bolts to the Toyota solid axle rear end. Interestingly, all cars use a spooled diff, which effectively locks the rear to a 50-50 power distribution. This rear-end choice, more akin to a drift build, works surprisingly well in the Thunder Roadster. Combined with the bias-ply Hoosier tires that are more than willing to slip at different rates. The result is a very rewarding and engaging racecar.
Power-to-weight and gearing are where the Prep 1 suffers. With only five gears compared to Prep 2’s 6, top speed is limited to 111-112 mph. And the power-to-weight ratio of around 14 to 1 has left the Prep 1 significantly underpowered compared to the Prep 2 builds and the Super Touring 4 class, where I run.
Oil cooling is also of significant importance for the air-cooled Yamaha motors. The Prep Level 1 Thunder Roadster has a reputation for engine fragility. Mainly because it encloses the air-cooled engine and relies solely on oil cooling to keep it alive. I have seen several solutions involving multiple oil coolers and blower fans over the head fins themselves.
Although these can be effective, I prefer the hybrid cooling loop that my Thunder Roadster and many later Gen 1s employed. It is a liquid-cooled head jacket that circulates water around the finned head. Exchanging heat to the air with a traditional radiator. Although this system is a bit heavier, I believe it is the ideal cooling solution when combined with a single oil cooler to extend the motor’s longevity.
Prep Level 2 Thunder Roadster Specs
The Prep Level 2 Thunder Roadster or GTR Spec is where most Thunder Roadsters compete. The first significant change is the bodywork. New fiberglass panels replace the front, rear, and sides, effectively turning the chassis into a closed-wheel racecar. Furthermore, the side impact bar is now located behind the side pod, making even minor contact much more expensive. These cars also run a rear wing.
Drivetrain Specs
This is, perhaps, the most significant difference between Prep Levels 1 and 2. In race trim, Prep Level 2 GTR cars are 1600lbs with driver and 153hp to the wheels. Accomplished by chucking the old Yamaha tech in favor of a Gen 1 or 2 1300cc Hayabusa motor. For simplicity and reliability, the Prep 2 GTR cars keep the Yamaha carburetor setup. This keeps the power in check as well.
But the engine change delivers several benefits at the cost of a slight weight increase. Namely, the Hayabusa dramatically changes the power-to-weight ratio. How does 10.5 to 1 sound? It also includes a 6th gear and a significant increase in torque. The result puts it into a completely different class compared to the Prep 1 cars.
Other changes include a Ford rear end, outboard rear struts, a larger fuel tank (that Hayabusa is thirsty), and beefier brakes. The trade-off for all this extra performance, significantly higher cost.
Also, the Hayabusa is liquid-cooled by design. Making it much more thermally stable than the air-cooled Yamaha.
Thunder Roadster Extreme (TRX)
Take everything you have in Prep Level 2 and add/keep the Hayabusa’s fuel injection. Add a custom intake manifold and raise horsepower to 170hp and beyond. That is Thunder Roadster Extreme, a.k.a. TRX.
Now we are talking a 9.5lb/hp power-to-weight ratio. (9.5 to 1)! Oh yeah, they can also run a windscreen, which isn’t the worst idea.
Classing my Thunder Roadster (Engineering Teaser)
What does all of this mean for me? Mainly, with most drivers in GTR and TRX builds, I am strung out in no man’s land. My Prep Level 1 car performed well in Super Touring 4 in 2025. However, the significant power deficit needs to be addressed before the 2026 Nationals. Unfortunately, there is no good path to increase the Yamaha engine’s power for ST4. And the Prep Level 2 conversion is way more expensive than it should be. Which means I am going to engineer around the problem. This will be fun.
