One of the biggest appeals of the Thunder Roadster class is the low price of admission. There are no donors because these are not production automobiles converted to racing trim. They are designed and built by U.S. Legend Cars International as purpose-built racecars. Prices range from $7,500 to $30,000 for used, race ready NASA spec racecars.

There are two preparation levels in Thunder Roadster: prep level 1 and prep level 2. Be aware that Prep level 1 is gradually being phased out in favor of the faster and more reliable Prep level 2. Prep level 1 is Thunder Roadster TR, which uses Yamaha 1200 or 1250 with stock Mikuni BS36 carburetors and black ignition coils / box as delivered from US Legends Cars. Owners may convert air cooled to water-cooled, but the TR cars must run the 2:93:1 or 3:30:1 rear axle ratios.

Prep level 2 cars, known as GTR cars, use first- and secondgeneration Suzuki Hayabusa engines with the stock Mikuni BS36 carburetors and the spec manifold. Any performance modifications to the engine to alter the factory stock horsepower are strictly prohibited. However, oil pump gears, oil pans and oiling system components may be upgraded to maintain adequate oil pressure. The 3:58:1 rear axle ratio is mandatory.

Varies by preparation level, but both are weighed with driver.

Prep Level 1 1,500 lbs.

Prep Level 2 1,600 lbs.

New cars are available from Toys by Tink for right around $39,000.

Prices on used cars range from $7,500 to $30,000.

The Thunder Roadster GTR rules are fairly well locked down, which means modifications are limited.

• Adjustments to bump steer settings will only be permitted by placing spacers between the steering rack and the heim ends of the tie rods or between the front spindle pickup points and the heim ends of the tie rods.
• Frame repairs may include only front clips forward of the A-arm mounts and rear clips (behind the main rail of the sub frame) may be replaced as long as they are replaced with the exact material that it is replacing, and all pickup points must remain in the stock locations as delivered new from U.S. Legend Cars International.
• The stock front control arm clevis may be replaced with a double sheer control arm clevis as long as it does not change the geometry and pickup points.

Tires, size, brand and prices

Hoosier 26.5/8.0-15 Roadster Edition, $130

Brakes, brands and prices

Carbotech, Hawk, Wilwood, $90 to $225

8.2. Air Filters

Only stock and K&N air filters and Outerwears Pre-Filters coverings as delivered new by US Legend Cars are permitted. Velocity/ram stacks are not permitted. No components that direct airflow to the carburetors or increase airflow are permitted.

8.19.2. Metal Fatigue

It is highly recommended to have the frame checked periodically by an expert for metal fatigue. Cracked or broken frames are strictly prohibited from any NASA sanctioned events.

8.31. Radios are Permitted

Two-way radios will be permitted in competition.

8.37. Shocks

All cars must use spec Bilstein shocks as delivered new by U.S. Legend Cars International for use on a Thunder Roadster. Changing or altering the shock or fluid or pressure inside is illegal. Altering or tampering with the shocks or competing with the same, will result in a penalty from probation to an indefinite suspension. Shocks may be turned upside down. Shock bumpers are permitted (maximum 1/4” tall).

9.5. Exhaust System

The header, gaskets and muffler must remain within the stock dimensions, steel thickness, location and configurations of the original, as delivered by U.S. Legend Cars International. Stainless or standard steel headers allowed. The internal components of the header may not be altered. Header wrapping (tape) is acceptable. Ceramic or baked on coatings are not permitted.

10.10. Oil Catch Cans

Oil catch cans (maximum 1 quart capacity each) can be used. It must be securely fastened and remain within the engine compartment. It may only be plumbed in by hose from the stock crankcase breather opening and/or the oil fill cap. A return line may be run to the stock return fitting on the oil pan.