Q:

What role does roller diameter play in the design of a dynamometer, and how do I decide which roller size is best for my application?

  A:

In general, the faster and more powerful the application, the larger the roller diameter you’ll require.  Notwithstanding, it is important to note that roller diameter and accuracy are mutually exclusive.  Roller diameter does have an effect on a number of factors - including top-end speed, Equivalent Inertial Mass, and tire deflection.  Top-end speed and equivalent inertial mass are parameters that should mirror characteristics of the vehicle to be tested; if you plan on testing vehicles to 200-mph, you’ll need to eliminate roller diameters that cannot achieve this rate of speed. 

Equivalent Inertial Mass is a function of the mass and inertia of all the rotating elements of the dynamometer system.  A good rule of thumb: compare a dynamometer’s Equivalent Inertial Mass to the weight of the lightest vehicle you plan to test.  If the Equivalent Inertial Mass exceeds the weight of the lightest vehicle, then you may need to evaluate the pros and cons of the dynamometer in question (i.e. Is the extra speed and power worth “losing” the low-end applications).  If the dynamometer is physically large and “inertia-heavy” its probably designed for muscle cars or heavy-duty trucks - try finding a smaller dynamometer if you application is less aggressive.

Tire deflection describes the interaction between the circumference of the tire and the circumference of the dynamometer roller.  As the diameter of the roller approaches infinity, tire deflection more closely resembles deflection on a level surface.  For this reason, most laboratory grade dynamometers utilize rollers having a diameter of 48-inches or more – depending on the application in question.  However, a direct correlation between tire losses and roller diameter is difficult to define in the absence of misapplication.  In fact, testing conducted on rollers ranging from 8.575-inches to 50-inches has failed to provide any data that would lead us to believe that roller diameter has any discernable effect on accuracy or tire losses when tests are performed in accordance with the product’s specifications.  

A hint: try to first define your needs in terms of torque and speed – then consult a Mustang Dynamometer Sales Engineer to help you define a suitable product.  Moreover, do not use the dynamometer in any way that conflicts with proper procedures as outlined in a manufacturer’s manual.