Q:

What is a Cradle Roll Chassis Dynamometer?

  A:

The Cradle Roll Chassis Dynamometer utilizes two series of rolls situated in parallel, so as to cradle the wheels under test.  Typically, a cradle roll chassis dynamometer consists of either two “split” roll shafts  – resulting in a total of four rolls, or two solid roll shafts – resulting in only two elongated rolls.  Roll diameters measuring between 6.5-inches and 20-inches are common.

The smaller rollers limit the top-end speed rating of most cradle roll dynamometers, which is typically attributed to bearing limitations.  Currently, the fastest available cradle-roll chassis dynamometer boasts a 150-mph speed capacity.  While this figure is not adequate for extreme applications, it more than meets the needs of streetcar performance professionals –not to mention the high performance diesel and the countless applications and market niches that fall under the scope of the repair and diagnostic industries.  For “apples-to-apples” comparison purposes, large cradle-roll chassis dynamometers are capable of measuring more than 1,000-hp at 150-mph, without the inherent installation and space issues typical of large roll systems.

The cradle roll chassis dynamometer was developed in the early twentieth-century and has been in use ever since in countless automotive OEM laboratories, repair shops, high performance shops, and more recently in centralized and de-centralized emissions testing programs.  The cradle roll chassis dynamometer, also referred to as a “rolling-road” in Europe, is typified by an active loading device attached directly to the roll shaft, allowing for variations in the load applied to vehicles under test.  These active loading devices are typically referred to as Power Absorption Units (PAU).  Several different types of loading mechanisms have been employed since 1938, when the first hydrokinetic chassis dynamometer was developed.  Since then, the chassis dynamometer has undergone a major face-lift – with the vast majority of new systems using electric-current driven loading devices (i.e. eddy current, alternating current, and direct current).