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Dynamometers

Just wondering... How is torque determined using a dynamometer?

I mean, when they report "Torque at wheels", what gear is used... Because it seems that meausuring torque at the wheels would vary with the gear... in fact it would change in direct proportion to the ratio of one gear to the next (e.g. measuring wheel torque in 1st would be twice that as measured in

2nd if the gear ratios were 3.4 and 1.7, respectively). Do dyno results take into account gearing (including final drive) to 'back calculate' the torque produced at the crank? Sorry if I'm rambling... am I making sense???

Thanks!

Reply to
93 Fox
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If you know the power transmitted from the wheels to the dynamometer and the RPMs of the wheels you can calculate the torque at the wheels by a simple formula:

Torque = (Power / RPM) * ( 30 / pi)

where Torque is in Nm (Newton meters) at the wheel, power is in W (Watts) and RPM is the RPM of the wheel, not the engine. (None of them arcane Imperial units). Size of the wheels does not factor in.

Knowing the torque at the wheel you can calculate back to torque at the crank using gear ratios and final ratios as you suggested (ignoring for the moment power losses through the tranny etc., it will get you close)

93 Fox wrote:
Reply to
Randolph

I know how to convert torque to power. In a dyno, though, Torque is what is measured, not power... then power is calculated.

My question is, how do the Torque measurements vary when using different gears. Will the dyno readings change (as I mentioned) from gear 1 to 2 to 3 etc.?

Reply to
93 Fox

Yes, the actual torque on the wheels varies with gear ratio. Ideally you use the highest gear for a chassis dyno test so that you maximize the load on the engine and to minimize the rotational inertia effects of the drivetrain as the dyno test is done. At least with a rolling drum type dyno, where you are measuring the rate of acceleration of the heavy drum to determine the instantanous torque applied.

Reply to
Roger Brown

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