Thunderbike machines, except single cylinder machines, are based upon production models, sold by manufacturers and their dealers in the UK for street use. Proof of compliance rests with the competitor entering the motorcycle.
Single cylinder machines are exempt from production and street use requirements. Single cylinder 2 stroke GP road race machines are not allowed in this class. All machines must have unaltered VIN numbers.
NOTE: Ducati 749 Series machines are excluded from this class.
The organizers will appoint eligibility officers for the series. Their decision on the interpretation of the rules will be final.
1. Production machines not sold by manufacturers and their dealers for street use in
UK via normal commercial channels may be approved on an individual basis. The organisers will maintain a list of non-standard approved models.
2. All machines must comply with the machine preparation regulations as set out in the 2006ACU Handbook for road race meetings.
Frame & Swinging Arm
Frame, cylinder head(s) and engine cases must be from the same production model
motorcycle. Single cylinder motorcycles may use any frame and engine.
The frame must be as originally supplied by the manufacturer of the approved
model. Strengthening gussets or tubes may be added. Only brackets or tubes not
supporting suspension, engine, or drive line components may be removed. Swinging arms may be modified or replaced and rear shocks may be replaced or relocated.
Seat subframes, where fixed by the manufacturer to the main frame by means of welding or other permanent fixing, may be replaced by bolt on replaceable rear subframes in the interest of economical crash repair.
Single cylinder motorcycles are excluded from this restriction and may use any frame.
Engine
Reducing engine size of machines from stock displacement to meet Thunder class
displacement limits is not allowed.
Liquid cooling is not allowed unless original equipment on the model being used.
Original type of induction system must be retained. If a machine originally was sold
with fuel injection, then it must remain fuel injected. If the machine originally
came with carburetors, then it must use carburetors, however they need not be the
original type or size.
All drain plugs must be safety wired. External oil filter screws and bolts that enter the oil or water cavities must be safety wired.
Engine displacement limits are absolute and are set as follows:
Single cylinder, Unlimited displacement
Twin cylinder, air cooled, Unlimited displacement
Twin cylinder, liquid cooled, 3 or less valves per cylinder, Unlimited displacement
Twin cylinder, liquid cooled, 4 valve per cylinder, up to 750cc
Twin Cylinder Water Cooled Desmo valves, up to 750cc (Ducati 749 not eligible)
Three cylinder, non-fuel injected, up to 1200cc
Four cylinder, air-cooled, up to 1200cc
Four cylinder, liquid cooled, 1990 to 1992 model year, up to 650cc
Four cylinder, liquid cooled (oil or water), pre-1990 model year, up to 860cc
All air-cooled, 2-valve, unlimited displacement.
(Model year refers to year of manufacturer, proof of which lays with the competitor)
Power Restrictions
Thunderbike machines are required to meet a maximum power to weight limit of 1 SAE horsepower for every 4.0lbs in weight (based on a conversion of 1kg equal to 2.204lb), i.e. a maximum of 1hp per 1.814kg
Power to weight ratio will be calculated as the weight of the machine divided by the tested horsepower when inspected. Inspections can occur during post race or post qualifying inspections.
Any machine found to exceed the power to weight ratio on a dyno or weighing device selected by the organizers may be excluded from the results.
Controls
Throttle stops or cable adjustments must be fixed or secured in a manner that will prevent adjustment by the rider or crew without the use of tools prior to dyno testing.
Switches or other operator controlled methods designed to affect horsepower readings during dyno testing are strictly prohibited.
Machines are restricted to the mounting and use of one ignition or engine management module during all competition. Adjustable modules or modules with more than one setting must be mounted in such a way that the settings cannot be altered by the rider or crew while on course or prior to dyno testing.
Motorcycles must be fitted with a functional ignition kill switch or button mounted on the handlebar that is capable of stopping the engine from running.
Race Numbers
Race numbers must be as laid out in the ACU handbook and displayed on the front of the fairing and on both sides of the seat unit. If no fairing is fitted then a number board must be fitted to the front of the machine capable of displaying the regulation size numbers.
Technical Inspections
The dyno operator, scrutineer or person appointed by the race or series organisers to ensure eligibility, may request removal of bodywork before, during or after dyno testing, and may request removal of other components for inspection.
Any machine not capable of post race dyno testing will be deemed to have failed the dyno test.
Machines will be run on the dyno until, in the opinion of the dyno operator, the maximum output is achieved. If, in the opinion of the dyno operator or race official, an unsafe condition exists while performing the dyno testing, the test will be stopped and the Race Director or Chief Scrutineer will make the final decision on the dyno test results.
Machines will be selected at random by race officials to run on the dyno as soon as possible following the end of the race. Failure to report to the dyno when requested will result in disqualification.
