Simple rules. Unlimited everything on suspension, engine and drivetrain.
Minimum weight is the "Class Multiplier" x the total width of the four tires.
If you want to add ANY aero beyond stock, you get an add-on to the multiplier.
If you have a drivetrain that is completely bone stock through the exhaust (except the tune) you can get another add-on to the multiplier.
Have Hoosiers or real slicks rather than 200tw or 100tw tires, add another multiplier
If the engine disparity is too much, you can add a multiplier for engines over/under 3 liters.
Minimum weight is rounded up to the next whole number. And weighed as you come off the track with whatever fuel is in the car, the driver and driver's equipment.
For the sort of middle class, you use something like a multiplier of 2.44 with -0.12 modifier for stock drivetrain and 0.12 modifier for non-stock aero
A Miata on 225 tires in this class would have a minimum weight of 2196 lbs.
Have a Miata with stock drivetrain, subtract 0.12 from the class multiplier (225 tire size x 4 tires x 2.32 stock aero, stock drivetrain modifier = 2088 lbs minimum weight)
Have Miata with modified drivetrain and aero, add 0.12 to the multiplier (225 tire size x 4 tires x 2.56 with modifiers = 2304 lbs minimum weight)
Have an S197 Ford Mustang on 305 tires with no aero, but some drivetrain mods (305 tire size x 4 tires x 2.44 stock aero = 2977 lbs minimum weight)
A Porsche Cayman in this class with it's stock 235 front and 265 rear tires, but stock drivetrain and aero ((235x2 + 265x2) tires x 2.32 = 2320 lbs minimum weight
For the fast class you start with a multiplier like 2.10 and have modifiers of -0.25 for stock drivetrain and 0.28 for non-stock aero.
A Miata on 225 tires in this class would have a minimum weight of 1890 lbs.
Have a Miata with stock drivetrain, subtract 0.25 from the class multiplier (225 tire size x 4 tires x 1.85 stock aero, stock drivetrain modifier = 1665 lbs minimum weight)
Have Miata with modified drivetrain and aero, add 0.28 to the multiplier (225 tire size x 4 tires x 2.38 with modifiers = 2142 lbs minimum weight)
Have an S197 Ford Mustang on 315 tires with no aero, but some drivetrain mods (315 tire size x 4 tires x 2.1 stock aero = 2646 lbs minimum weight)
A Porsche Cayman in this class with it's stock 235 front and 265 rear tires, but stock drivetrain and aero ((235x2 + 265x2) tires x 1.85 = 1850 lbs minimum weight
For a slower class, you use something like a multiplier of 3.00 with -0.10 modifier for stock drivetrain and 0.10 modifier for non-stock aero
A Miata on 205 tires in this class would have a minimum weight of 2460 lbs.
Have a Miata with stock drivetrain, subtract 0.10 from the class multiplier (205 tire size x 4 tires x 2.9 stock aero, stock drivetrain modifier = 2378 lbs minimum weight)
Have Miata with modified drivetrain and aero, add 0.10 to the multiplier (205 tire size x 4 tires x 3.1 with modifiers = 2542 lbs minimum weight)
Have an S197 Ford Mustang on 305 tires with no aero, but some drivetrain mods (305 tire size x 4 tires x 3.0 stock aero = 3660 lbs minimum weight)
A Porsche Cayman in this class with it's stock 235 front and 265 rear tires, but stock drivetrain and aero ((235x2 + 265x2) tires x 2.9 = 2900 lbs minimum weight
If you really want to keep the "big" cars with big displacement engines out of the slower classes, you give the class a crippling penalty for engines over 3 liters.
If you want the slower classes to have less aero, you increase the non-stock aero penalty on those classes.
Because the penalties and modifiers are called out separately for each class, you have more ability to shape the cars performance in the class.
What you want to avoid are rules that are specific to an engine layout, suspension equipment, or finely defined modifications because many of these will apply differently based on different chassis and really only work well when you have a tightly defined group of cars that are acceptable.
Minimum weight is the "Class Multiplier" x the total width of the four tires.
If you want to add ANY aero beyond stock, you get an add-on to the multiplier.
If you have a drivetrain that is completely bone stock through the exhaust (except the tune) you can get another add-on to the multiplier.
Have Hoosiers or real slicks rather than 200tw or 100tw tires, add another multiplier
If the engine disparity is too much, you can add a multiplier for engines over/under 3 liters.
Minimum weight is rounded up to the next whole number. And weighed as you come off the track with whatever fuel is in the car, the driver and driver's equipment.
For the sort of middle class, you use something like a multiplier of 2.44 with -0.12 modifier for stock drivetrain and 0.12 modifier for non-stock aero
A Miata on 225 tires in this class would have a minimum weight of 2196 lbs.
Have a Miata with stock drivetrain, subtract 0.12 from the class multiplier (225 tire size x 4 tires x 2.32 stock aero, stock drivetrain modifier = 2088 lbs minimum weight)
Have Miata with modified drivetrain and aero, add 0.12 to the multiplier (225 tire size x 4 tires x 2.56 with modifiers = 2304 lbs minimum weight)
Have an S197 Ford Mustang on 305 tires with no aero, but some drivetrain mods (305 tire size x 4 tires x 2.44 stock aero = 2977 lbs minimum weight)
A Porsche Cayman in this class with it's stock 235 front and 265 rear tires, but stock drivetrain and aero ((235x2 + 265x2) tires x 2.32 = 2320 lbs minimum weight
For the fast class you start with a multiplier like 2.10 and have modifiers of -0.25 for stock drivetrain and 0.28 for non-stock aero.
A Miata on 225 tires in this class would have a minimum weight of 1890 lbs.
Have a Miata with stock drivetrain, subtract 0.25 from the class multiplier (225 tire size x 4 tires x 1.85 stock aero, stock drivetrain modifier = 1665 lbs minimum weight)
Have Miata with modified drivetrain and aero, add 0.28 to the multiplier (225 tire size x 4 tires x 2.38 with modifiers = 2142 lbs minimum weight)
Have an S197 Ford Mustang on 315 tires with no aero, but some drivetrain mods (315 tire size x 4 tires x 2.1 stock aero = 2646 lbs minimum weight)
A Porsche Cayman in this class with it's stock 235 front and 265 rear tires, but stock drivetrain and aero ((235x2 + 265x2) tires x 1.85 = 1850 lbs minimum weight
For a slower class, you use something like a multiplier of 3.00 with -0.10 modifier for stock drivetrain and 0.10 modifier for non-stock aero
A Miata on 205 tires in this class would have a minimum weight of 2460 lbs.
Have a Miata with stock drivetrain, subtract 0.10 from the class multiplier (205 tire size x 4 tires x 2.9 stock aero, stock drivetrain modifier = 2378 lbs minimum weight)
Have Miata with modified drivetrain and aero, add 0.10 to the multiplier (205 tire size x 4 tires x 3.1 with modifiers = 2542 lbs minimum weight)
Have an S197 Ford Mustang on 305 tires with no aero, but some drivetrain mods (305 tire size x 4 tires x 3.0 stock aero = 3660 lbs minimum weight)
A Porsche Cayman in this class with it's stock 235 front and 265 rear tires, but stock drivetrain and aero ((235x2 + 265x2) tires x 2.9 = 2900 lbs minimum weight
If you really want to keep the "big" cars with big displacement engines out of the slower classes, you give the class a crippling penalty for engines over 3 liters.
If you want the slower classes to have less aero, you increase the non-stock aero penalty on those classes.
Because the penalties and modifiers are called out separately for each class, you have more ability to shape the cars performance in the class.
What you want to avoid are rules that are specific to an engine layout, suspension equipment, or finely defined modifications because many of these will apply differently based on different chassis and really only work well when you have a tightly defined group of cars that are acceptable.
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