# How to Calculate Engine Torque from Wheel Torque?

Not sure if this is the right place to post - so feel free to tell me to go away :)

Im trying to understand how I can use wheel torque curves to estimate the engine torque vs engine rpm relationship.

My understanding is that the following equation relates wheel torque $T_w$ to engine torque $T_e$ where $i_g$ and $i_a$ are the gear and final drive ratio respectively:

$T_w = T_e \cdot i_g \cdot i_a$

Then taking a curve like from http://rototest-research.eu/popup/performancegraphs.php?ChartsID=715 select the [Kw / Nm] option. Can I use the above equation to produce a reasonable engine torque curve estimate?

The graphs from that particular site are presented as follows, "The sum of the torques measured at each wheel hub is presented as total drive wheel torque divided by the total transmission reduction (i.e. the gear ratio times the final drive ratio)"

Furthermore, the chosen gear ratio is always the gear that is closest to 1:1, therefore $i_g$ and $i_a$ are known.

Thanks in advance

## 1 Answer

Yes, your equation is correct and you can calculate the torque at the engine by measuring the torque at the wheels. However the number you get will be less than the true engine torque because of friction in the drive train. The figure you calculate will be the engine torque minus the transmission losses.

Rolling roads estimate the transmission losses by disengaging the clutch and measuring how fast the wheels slow down. This gives an estimate for the power loss in the drive train, and adding this figure to the wheel power gives the engine power. Dividing by the engine speed then gives the engine torque.