But you are not seeing shaft horsepower. The dyno is not measuring shaft horsepower.
Correct, hence my statement: "...
the computer takes in to account the separately specified gear ratio to determine what the shaft torque (and thus HP) is"
Like I said, I was on the phone for 31 minutes going ever exactly how this dyno measures power. Torque is not a factor. Acceleration delta and roller mass are the factors used. The engineer made it very, very clear that the dyno does not see, and cannot calculate the torque from those 2 parameters. It only sees force, which is not torque, because it's a linear delivery. I don't recall exactly how the engineer worded it, but "tangent," "linear," "force," "acceleration delta," "horsepower," and "not torque" were all used in one sentence to describe how the dyno measures power. I drew this kindergarten skill level picture while he was describing the way the dyno measures power. Since there is no load applied to the roller, there is no twisting force, thus, no torque.[/quote]
Torque definition:
"Torque, moment, or moment of force is rotational force.[1] Just as a linear force is a push or a pull, a torque can be thought of as a twist to an object.
Torque is defined mathematically as the rate of change of angular momentum of an object. The definition of torque states that one or both of the angular velocity or the moment of inertia of an object are changing. Moment is the general term used for the tendency of one or more applied forces to rotate an object about an axis, but not necessarily to change the angular momentum of the object (the concept which is called torque in physics).[4] For example, a rotational force applied to a shaft causing acceleration, such as a drill bit accelerating from rest, results in a moment called a torque."
No toque applied from tires, no roller movement. That's simply physics. As for "
It only sees force, which is not torque", by it's very definition torque
IS rotational force.
So torque
must be applied to the rollers (which have mass) to change their angular momentum (i.e. make them spin).
Now the DynoJet does not attempt to resist this force in order to attempt to measure it (as other dyno designs do), likely because it's cheaper to not try to do so. But the rollers are absolutely subject to it. It's what moves them. And that torque (force) has already been multiplied by the gear ratio of the driveline.
The dyno is measuring how long it takes for the roller to move from point "a" to point "b", and factoring in the weight of the roller to see how much linear force was applied to it. Torque is not a linear measurement. You would get the same data from running the car on a treadmill.
Give the rollers are moving about a central axis, there is no "linear" motion going on here at all. And as they rollers are stationary, but their rotational speed is increasing, hence that measurement along with their mass can allow you to derive torque and power.
So back to your point that "
No, dynos do not measure torque. If they did, you’d be seeing numbers in the thousands", my point is that they DO experience "numbers in the thousands, regardless if it measures the torque on the drums directly, or it calculates it from mass of the drums and their change in RPM.
That dyno then divides by the final drive ratio (and a few other corrections) to estimate the value at the motor output shaft.
Think about it. The rear motor in a Tesla can easily make 400+ ft/lbs of force. The gear reduction is almost 10:1. Given that the radial distance from axle centerline to the tread of a 21" rim + tire is roughly a foot, the rollers are seeing
~4,000 ft/lbs of torque!
