When designing a homemade sports car, you need to know the method for calculating the maximum speed in order to correctly select the gear ratios of the main gear and gearbox. In addition, knowing the mathematical calculation methods, you can choose the most optimal tire sizes.
Instructions
Step 1
To determine the gear ratio of a pair of gears in the main gear as easily and quickly as possible, look at the instructions for the car on which this gear is used. From a mathematical point of view, in order to determine the gear ratio of two gears, the number of teeth on both is counted and the teeth of the driven gear are divided by the teeth of the driving gear. For example, if the driven gear has 128 teeth, and the drive gear is 26, then the gear ratio is calculated as 128/26 = 3.54.
Step 2
To calculate the composite gear ratio of the transmission, first calculate the gear ratios of the main gear and the included speed in the gearbox in the above way. Then multiply these two values by one another. For example, if the final gear ratio is 3.54 and the top gear ratio is 0.88, then the composite transmission ratio for the gear used (in this case, the first) will be 3.54 * 0.88 = 3.15
Step 3
If you need to calculate the theoretical vehicle speed at a specific engine speed, multiply the speed by the tire outer diameter in centimeters. Divide the result by the compound transmission ratio multiplied by 530, 616. For example, for 245 / 40R18 tires, the outer diameter will be 65, 32 cm. When the engine speed is 5000 rpm, the design speed will be 5000 * 65, 32 / (3, 115 * 530, 616) = 197, 6 km / h. The real speed of the car at these rpm will be lower, since the formula does not take into account the forces of resistance to motion acting on the car.
Step 4
Use the speed formula to calculate the theoretical maximum speed of the vehicle, knowing the maximum RPM the engine is capable of under load. Also, use this formula to calculate the compound transmission ratio required to achieve the design maximum speed. To do this, multiply the maximum engine speed under load by the outer diameter of the tires in centimeters and divide first by the target speed, and then by 530, 616.
Step 5
To calculate the outer dimensions of the tires for the reference vehicle, multiply the target top speed for that vehicle by the compound transmission ratio and 530, 616, and then divide by the maximum engine speed under load.
