None of the constants are imaginary, but some aren't constants. The moment of inertia of the drum is known to a high degree of accuracy and it is a constant. The drag from bearings and wind could be estimated, but I don't know if they include that in their calculations or not. If it is, the bearing drag is probably pretty constant, and the wind drag would depend on the speed of the rotating drum.Mouse wrote:I'm off on a tangent but;
Are you saying that the drum type Dyno measures how quickly the drum is accelerated then multiplies/divides that with some (imaginary?) constants to get HP and Torque numbers?
Either way, at any moment with the information about the speed at which the drum is spinning (which changes during a run) and the moment of inertia of the drum (which is constant), it is easy to calculate the energy stored in the spinning drum. Power is defined as the rate of change of energy (which is also equivalent to the rate at which work is done - the "work" in physics has a specific definition, not work in the general sense), so by measuring how quickly the drum is accelerated, you're measuring how quickly the energy of spinning drum is changing, which is equal to the power being applied to the drum.
If the dyno gets a tach signal from the bike, then it can calculate engine RPM at the same time that it's making its calculations of the drum speed. With the power and engine RPM, engine torque (after driveline losses) can be calculated. If it does not get that tach signal, then the dyno can plot power vs drum RPM or drum MPH equivalent at the perimeter. Some dynos will let you say that this bike turns X RPM at Y MPH in the gear I'm using for this run. That ratio is a constant for a given run, but could vary from bike to bike, or from run to run (if you wanted to compare results in 4th gear vs 5th gear, for example). With that ratio, the dyno can calculate the engine RPM from the drum speed and calculate engine torque from hp. This is what rperacing was referring to when he talked about changing the "RPM - roller speed relationship". This changes the dyno's engine torque calculation, because it changes the dyno's estimation of engine RPM.
This is all for a simple inertial dyno where the only load is the inertia of the drum. There are more sophisticated systems that can vary the load based on a bunch of different parameters by using a brake or eddy current system which can be controlled by the computer in the dyno or by the operator. In that case, there would be some kind of model or input used to add the drag from the additional load to all of the calculations and there may be some measurements of torque applied to the dyno as part of the hp & engine torque applications.
Does that make sense? Should this be split into a different thread?