Help design torque, power needed

1. Jul 22, 2011

tebrown3

I am designing a transmission for my class but i have a couple of concepts i kinda confused, i have a 550lbs load, and i want to move 35mph=15.64m/s at a constant speed, assuming the drag coefficient to be Cd=.55 and the coefficient of rolling resistance to be Crr=.3 (tires on sand), i found out the two frictional forces going against a rigid body(my off road car) which are the sum of the Rolling resistance force and the force of aerodynamic drag which turned our to be F=815.21 N, i want to like to learn how to find the torque necessary to move it and the power required, do we keep power constant?, and does the power depend on torque or vice versa? Thanks in advance
here are the eqn used

Last edited: Jul 22, 2011
2. Jul 22, 2011

edgepflow

Note that:

Work = Force X Distance

Power = Work / Time = Force X Distance / Time

Velocity = Distance / Time

Thus,

Power = Force X Velocity

You can compute your power from the 815 N force you figured and your 35 mph velocity.

Then the power and torque are related as:

Power = Torque X Angular velocity

And you can find the angular velocity from linear velocity and gear ratios.

3. Jul 22, 2011

tebrown3

thank you that cleared up alot, but how do you calculate the power and torque needed when the car is at rest to 35 mph?

Last edited: Jul 22, 2011
4. Jul 22, 2011

edgepflow

Compute Power = Force X Velocity from your numbers above.

Then look at your engine rpm at 35 mph. Use the transmission gear ratio to find the transmission rpm. Using this power and rpm, figure the transmission torque.

5. Aug 26, 2011

To calculate the rate of acceleration don't we need to know the average torque? That is from rest (to get the load moving) through the engines rpm range to 35 mph. Which would be calculated as a function F(x) torque (or maybe F(x) power)?

Willy

6. Aug 26, 2011