Re: Rear wheel BHP
Posted: Fri Jun 26, 2009 6:10 pm
Rohan makes a good point that the rolling road introduces additional losses.
The 15% figure is based on a car on a level road.
For a rolling road in 2nd/3rd gear, you have 2 gear mesh points for at least 4-5% additional loss, additional bearing losses, and an indeterminate and variable additional loss at the tire to roller interface. For proper testing and repeatability, the rolling road should have measured tie down force, so the minimum force neccessary is applied, and can be repeated.
For a given car, a certain horsepower will produce a certain acceleration, no matter what the engine rpm. The drivetrain gearing converts the engine torque/rpm to the rear wheel rpm. This means that high torque at low rpm produces the same acceleration force at the tire interface as lower torque at higher rpm, assuming the product of torque and rpm is the same. There are secondary effects that modify this general statement.
David
1968 36/7988
The 15% figure is based on a car on a level road.
For a rolling road in 2nd/3rd gear, you have 2 gear mesh points for at least 4-5% additional loss, additional bearing losses, and an indeterminate and variable additional loss at the tire to roller interface. For proper testing and repeatability, the rolling road should have measured tie down force, so the minimum force neccessary is applied, and can be repeated.
For a given car, a certain horsepower will produce a certain acceleration, no matter what the engine rpm. The drivetrain gearing converts the engine torque/rpm to the rear wheel rpm. This means that high torque at low rpm produces the same acceleration force at the tire interface as lower torque at higher rpm, assuming the product of torque and rpm is the same. There are secondary effects that modify this general statement.
David
1968 36/7988