What is a Chassis Dynamometer?

Lori Kilchermann

A chassis dynamometer is an instrument used to measure the horsepower and torque a vehicle produces at the drive tires. An engine dynamometer measures horsepower and torque of an engine at the flywheel. While this figure gives a vehicle owner an idea of the power a vehicle is producing, the chassis dynamometer gives an accurate power measurement after all of the parasitic loss has been accounted for. Engine accessories such as transmission, rear-end gears and tires all rob horsepower from an engine. By taking measurements of horsepower on a chassis dynamometer, a real-world and true horsepower reading can be ascertained as power actually going to the tires.

Measuring horsepower on a chassis dynamometer can give an actual horsepower reading.
Measuring horsepower on a chassis dynamometer can give an actual horsepower reading.

Many racing teams have been utilizing a chassis dynamometer for decades. The use of the dynos has led to advancement in wheel design, transmission design and even the style of gears used inside of the transmission and drive axles. Prior to the use of the chassis dynamometer, rotating mass, frictional and parasitic loss meant nothing to most car builders. Once it was learned that the diameter of a drive shaft had an effect on the amount of horsepower it took to rotate it, changes were made.

In auto racing, the old saying of "Win on Sunday and sell on Monday" holds true for the findings of a chassis dynamometer as well. The fuel crisis of the 1970s made it imperative for automobile manufacturers to create vehicles that used less fuel to operate. By using the chassis dynamometer to detail the effects of part design on the amount of horsepower it took to operate them, more efficient vehicles were created. The use of alloys over cast iron as well as the use of smaller diameter parts all require less power to operate.

This type of study also revealed that it took less horsepower to pull a vehicle rather than push it, generating the front-wheel drive evolution in automobile design. The chassis dynamometer proved that a smaller engine could do a better and more efficient job of powering a vehicle than a large engine in a rear-wheel drive application. Even factors such as tire size were evaluated to explore elements that affected power production and usage.

In racing and high-performance automobile tuning, the chassis dynamometer makes it possible to tune an engine while it operates in real-world conditions. By placing the vehicle on the dynamometer and entering certain parameters into the dyno's computer, the actual atmospheric conditions of a race track can be replicated and tuned. This creates a perfectly tuned engine to gain the most power possible in any given situation.

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