how does a rolling road dyno work out power at the fly wheel?

[otispunkmeyer]

do they just rev it up to max then dip the clutch and let it coast or something?

 

[dug777]

i would assume what you suggest...

 

[drnickriviera]

I always though they just used a set percentage for drivetrain loss.

 

[imported_Baloo]

It doesn't, it measure power at the wheel.

 

[LS20]

plug in the gearing

 

[iamaelephant]

Originally posted by: drnickriviera
I always though they just used a set percentage for drivetrain loss.

Bingo. Chassis dynos always measure torque in 4th gear (1:1 ratio in a manual tranny) and divide the figure by the diff ratio. Then for power at the flywheel they just make an educated guess based on the type of drivetrain.

 

[otispunkmeyer]

Originally posted by: iamaelephant

Originally posted by: drnickriviera
I always though they just used a set percentage for drivetrain loss.

Bingo. Chassis dynos always measure torque in 4th gear (1:1 ratio in a manual tranny) and divide the figure by the diff ratio. Then for power at the flywheel they just make an educated guess based on the type of drivetrain.

well theres a guy with his own rolling road it seems, and on his site he explains how his machine can calc HP at the fly wheel by doing as i described.

either way its never gonna beat a proper engine dyno rig