What is True Rear Wheel Horsepower?
from http://www.factorypro.com/dyno/true1.html
at the end of this page there is a scale to see what bikes typically read on the Factory Eddy Current dyno horsepower scale
What is True Rear Wheel (tm) Horsepower?
A Dynamometer can only measure actual power at the output location. Actual power produced AND delivered by an engine will be highest if measured at the crankshaft, lower at the transmission output shaft and even lower, but more meaningful, still, at the rear wheel.
The power that you use is the power at the rear wheel.
Some Dynamometer companies add to measured rear wheel power readings a factor that is based on ESTIMATED rear wheel power losses (under what power conditions? 125cc? 1200cc? under coasting conditions? with a 3.00x17 bias ply tire? a 190x17 radial tire? New heavy radial tire vs. worn old, light, stock bias ply tire? Who knows?)
In short, there is NO meaningful "average" tire to get a correct rear tire power transmission loss measurement for all bikes - so obviously, unless they actually measure the power lost in the rear tire, under driven load conditions, NO dyno company should BE ADDING incorrect power figures into the measured power. It's simply wrong.
The fact that they add varying amounts of power to the actual, "true" amount of power delivered and measured to the surface of the drive roller creates a situation that makes it an onerous task to compare power figures from different brands of dynamometer systems.
On simple inertial dynamometers, some companies use an average for the inertial value of the engine, transmission, rear wheel, sprocket and chain on every bike - as if a YZ125 has the same rear wheel or internal rotating mass as a 1340cc Harley Davidson. Needless to say, if the software thinks that the YZ125 had a HD rear wheel on it, it would look like the 125 makes more HP at the rear wheel than it does at the crankshaft. It's simply wrong.
It's expensive to measure frictional losses in the engine and drivetrain, requiring the dyno to be able to drive the vehicle with engine off. Add the cost of a 50+hp electric motor, controlled power supply, etc. It's just not likely that $10,000 dyno will be equipped with that equipment.
It is also common for dynamometer companies to add to the power readings by adding transmission and primary gear/chain losses back into the measured power readings. Some companies make a concerted effort try to measure frictional losses and, optionally, add the power to the measured readings. Other companies - some that would surprise you - say that it's not important and give a blanket, single factor for frictional losses in every engine. That includes some $25,000-$35,000 dynes.
Some simply say that there is a meaningful "average" for every motorcycle,(2 stroke, 4 stroke, 1 cylinder/1 transmission, 4 cylinder/1 transmission) and apply it to every bike and that it is not a significant difference.
Blanket estimates of "average" losses and corrections are, quite simply, incorrect. At the upper levels of the industry, (we are talking about $150,000 - $500,000 AC or DC 4 quadrant dynamometers) it is not tolerated - shouldn't be - and needn't be.
True Rear Wheel Horsepower (tm) is Factory Pro's standard of measuring the power that is actually delivered to the rear wheel. It is honest, true, fair and duplicable. It is the ONLY standard that can be duplicated by the entire industry - regardless of the dyno manufacturer.
