Time Trials Production Car Classification Rules
These rules are intended to be used for classification of production-based automobiles prepared for road-course time trials competition. The rules recognize that in addition to driver skill, car modification and preparation play an integral part in an ultimate competitive lap time. These rules are not intended to factor out or limit the car or mechanic in order to compare only driver skill, but instead attempt to group cars with similar performance potential
into classes for competition purposes.
Performance potential is grouped based on discrete features of the vehicle being classed, including weight and engine configuration. The performance potential index generated by these rules is not
intended to quantify the performance of a particular car--two cars with equal indexes will not likely have identical performance--but is rather intended simply to group cars of similar power-to-weight potential in order to generate a competitive environment for a wide variety of vehicles.
The purpose of the Time Trials Production rules are to provide a formulaic method for classifying cars for road-course time trials competition. The formula is intended to use discrete features of the car, along with the actual weight of the car, to group cars together that have similar performance potential. The use of a generalized formula for classification over the usual list of cars and allowed modifications is intended to drastically reduce the effort needed to maintain the rules, as well as simplify the process to classify a car for competition.
It is understood that these classifications are based on limited data and experience, and will therefore be inappropriate in some cases. In such cases, the event stewards are given the authority to make changes with regards to an individual's car classification in the interest of balanced competition. Any changes to an individual's car classification should be documented by the event officials with the reasons for the change and sent to the rule authors for review and possible inclusion in future revisions.
Commonly available production automobiles, intended for street use, modified and prepared for time trials competition (must maintain original frame or unibody), and on DOT approved tires (street or R-compound) are eligible for classification under these rules.
For the purpose of eligibility determination, and to ensure the goal that competition remain between production
automobiles, the following modifications violate the "original frame or unibody" requirement and disqualify a vehicle for Time Trials Production competition unless explicitly excused by the event stewards:
At the stewards' discretion, non-production, limited-production, cars with modified frames/unibodies, and purpose built race cars may be classified under the Time Trials Production rules, however it is suggested that these cars should be classified in TT-X to account for their likely advantages over true production based cars.
Cars are classified via Performance Potential Index (PPI)
. The PPI
is a weight/performance potential ratio that is used as an index into a table of classes. Weight is adjusted to compensate for the performance advantage of light cars over heavy cars, as well as for common modifications. Displacement is adjusted to compensate for features of the engine that improve the power potential of the engine.
ratio is calculated with the following formula:
[race weight] + [weight adjustment] + [weight penalty]
[PPI] = —————————————————————————————————————————————————————————
[actual displacement] * [displacement factor]
The race weight
is the actual weight of the vehicle in race trim, including the driver, rounded up to the nearest pound. Weight should include only the amount of fuel left in the vehicle upon completion of timed laps, however it is at the event stewards' discretion to weigh vehicles prior to competition. In this case, vehicle weight may include fuel as ballast. In most cases, the actual weight of the car is unimportant as long as the vehicle is known to be at or above the minimum weight of the car's class.
Cars will have their race weight
signed-off in their log book by a tech inspector, in order to prevent having to reweigh the vehicle at every event. It is the driver's responsibility to maintain the weight of the vehicle at or above the signed-off weight, or have the vehicle reweighed and signed-off if the weight is changed. It may be advantageous to record the race weight
lower than the actual weight as long as it is at or above the class minimum in order to prevent reweighing the vehicle.
If scales aren't available at an event for classification use, the factory specified curb weight of the vehicle plus 100 lbs may be used for classification, or an alternative weighing of the vehicle (such as the results of a certified public scale) may be accepted at the event stewards' discretion. For unmodified cars that should be close to their factory weight, event stewards may elect to use the curb weight plus 100 lbs even if scales are available at an event in order to streamline the classification process, though it is recommended that cars close to their class weight boundary be weighed.
The purpose of the weight adjustment
is to account for the fact that lighter cars will corner and brake better than heavier cars even when both cars have identical power to weight ratios.
The weight adjustment
is the difference between the race weight
and the baseline weight
multiplied by the weight factor
, and rounded to the next largest pound. The weight adjustment
will be negative for cars lighter than the baseline weight
(a penalty) and positive for cars heavier than the baseline weight
[weight adjustment] = ([race weight] - [baseline weight]) * [weight factor]
Common modifications that improve performance but are not reflected in the power/weight ratio of the classification formula have been assigned a penalty weight. A vehicle's weight penalty
is the sum of all penalties from the table below that apply to the vehicle.
The actual displacement
of the vehicle is the displacement of the vehicle's engine as raced, rounded up to the nearest 0.1 liter. Vehicles with modified or swapped engines must use the displacement of the engine in the car, rather than the displacement of the original engine.
The displacement factor
scales the actual displacement
of the vehicle based on the feature's of the vehicle's engine. The values assigned to modifiers are not intended to specifically quantify the performance gain from that feature, rather they are based on the idea that cars with certain features tend to have similar performance potential, and each modifier is weighted to draw similar cars towards an appropriate class, rather than to explicitly quantify the performance of a car.
The displacement factor
is the sum of all the modifiers from the table below that apply to the vehicle.
Use the calculated PPI
as an index into the classification table below to determine the vehicle's classification. The lower the PPI
, the more potential performance the vehicle has, the higher the class
it competes in. Each class is available for both Track Trials (TT) and Club Trials (CT) levels based on the level of preparation per the SCCA Time Trials program rules
and the Reno SCCA Time Trials Supplemental Regulations