8th Generation Honda Civic Forum - View Single Post - Everything You Wanted To Know About Car Measurements(HP, TQ, Gear Ratios etc.)

ryker · 12-14-2007, 08:31 AM

Gears

Gears are used to change the torque and rotational speed of a part of a system of rotating shafts, or to change the direction of the transmitted motion. An example of the former would be the car's transmission, while an example of the latter would be the rear axle gears.
An ideal (lossless) gear set transmits an equal amount of power to the output shaft as it received from the input shaft. This means that if a gearbox has a 2:1 gear ratio, the output shaft will be rotating half as fast as the input shaft, but will have double the torque. Below is a drawing that shows the effects of a gear set, using an abitrary ratio, GR.

The gearbox is given a certain amount of power (from the engine), in the form of torque and revs (tq x revs = hp). It then puts out an equal amount of power, with the revs and torque adjusted according to the gear ratio.

Formulae used for gears.

Drivetrain Gearing

A car's drivetrain uses multiple sets of gears to control how much of the engine's total power is going to torque, and how much is going to rotating the wheels.

All gasoline piston engines produce too little torque and too many revs to properly turn the wheels. With 27 inch tires (13.5" radius), 6000RPM at the wheels is 450mph! It also takes a lot more than a few hundred pounds of force to even move something as heavy as a passenger car. This is why all cars have drivetrains that divide the revs and multiply the torque. (divide revs portion is what the high rev Honda engines do best it is how they can make a 130ft'lb of torque Si compete with turbochaged 250ftlbs from the GTI.

)

To accomplish this, most cars are fitted with two sets of gearing between the engine and the wheels. The first set is the transmission, which multiplies the torque a certain amount, depending on what gear it is in. Typically, first gear has a ratio near 3:1, while the top gear has a ratio near 0.8:1. After the transmission, there is another set of gears which usually have a ratio of around 2.5:1 through 4.8:1, depending on the vehicle. Below is a diagram of a typical drivetrain found in most cars:

The drivetrain of a car is fitted with a transmission and final drive gearing to adjust the engine's torque and revs to accelerate the car.

The wheel torque and revs vary with the engine torque and revs, and the gear ratios in between.

The reason that cars have transmissions with multiple gears is so that the engine can be kept within its operating rev range (which is why a good flat torque curve is great) while the vehicle accelerates from rest to possibly over 200mph. In first gear, there is plenty of acceleration because of the torque multiplication, but very little speed before the engine hits its redline. In second gear, there is slightly less acceleration, but a slightly higher top speed before hitting the redline. This trend continues through each gear in the transmission.

The combination of speed and acceleration is related to the power available (not HP but power as described above that reaches the ground).

12-14-2007, 08:31 AM	#18 (permalink)
ryker Senior Member Join Date: Sep 2007 Posts: 10,065	Gears Gears are used to change the torque and rotational speed of a part of a system of rotating shafts, or to change the direction of the transmitted motion. An example of the former would be the car's transmission, while an example of the latter would be the rear axle gears. An ideal (lossless) gear set transmits an equal amount of power to the output shaft as it received from the input shaft. This means that if a gearbox has a 2:1 gear ratio, the output shaft will be rotating half as fast as the input shaft, but will have double the torque. Below is a drawing that shows the effects of a gear set, using an abitrary ratio, GR. The gearbox is given a certain amount of power (from the engine), in the form of torque and revs (tq x revs = hp). It then puts out an equal amount of power, with the revs and torque adjusted according to the gear ratio. Formulae used for gears. Drivetrain Gearing A car's drivetrain uses multiple sets of gears to control how much of the engine's total power is going to torque, and how much is going to rotating the wheels. All gasoline piston engines produce too little torque and too many revs to properly turn the wheels. With 27 inch tires (13.5" radius), 6000RPM at the wheels is 450mph! It also takes a lot more than a few hundred pounds of force to even move something as heavy as a passenger car. This is why all cars have drivetrains that divide the revs and multiply the torque. (divide revs portion is what the high rev Honda engines do best it is how they can make a 130ft'lb of torque Si compete with turbochaged 250ftlbs from the GTI. ) To accomplish this, most cars are fitted with two sets of gearing between the engine and the wheels. The first set is the transmission, which multiplies the torque a certain amount, depending on what gear it is in. Typically, first gear has a ratio near 3:1, while the top gear has a ratio near 0.8:1. After the transmission, there is another set of gears which usually have a ratio of around 2.5:1 through 4.8:1, depending on the vehicle. Below is a diagram of a typical drivetrain found in most cars: The drivetrain of a car is fitted with a transmission and final drive gearing to adjust the engine's torque and revs to accelerate the car. The wheel torque and revs vary with the engine torque and revs, and the gear ratios in between. The reason that cars have transmissions with multiple gears is so that the engine can be kept within its operating rev range (which is why a good flat torque curve is great) while the vehicle accelerates from rest to possibly over 200mph. In first gear, there is plenty of acceleration because of the torque multiplication, but very little speed before the engine hits its redline. In second gear, there is slightly less acceleration, but a slightly higher top speed before hitting the redline. This trend continues through each gear in the transmission. The combination of speed and acceleration is related to the power available (not HP but power as described above that reaches the ground). Last edited by ryker; 12-14-2007 at 01:25 PM.