# Six Factors on Properly Sizing a Towing Dynamometer

March 04, 2016 - "Industry Insight"

Discover Six Factors to Consider When Properly Sizing a Towing Dynamometer for your Application

Towing dynamometers come in various sizes and weights for all types of car and truck applications. They are designed to safely and accurately simulate real world loads for the vehicle groups they are designed for. Below are six factors to consider when properly sizing a Towing Dynamometer for your application:

tow 1. Power Absorption

Sizing a towing dynamometer is more than just power absorption, it requires a basic understanding of towing dynamometer physics. In short, the ability of a towing dynamometer to absorb power is two-fold, the capacity of the power absorption unit (PAU), and the tractive force needed to hold that power to the ground.

2. The Vehicle’s Power

Likewise, the test vehicle (or tow vehicle) must have the power to pull the towing dynamometer and enough tractive force to put the power to the ground. In other words, if a high performance vehicle has 500 hp but only 1000 lb. on the rear axle, there will not be enough weight on the rear axle to maintain grip when under full load. Yes, the car will be able to accelerate quickly on its own, but when towing a towing dynamometer under load it will quickly loose traction.

A common mistake is to look for a towing dynamometer that is capable of completely absorbing all of the power the vehicle has to offer, at any speed, indefinitely. This notion is unrealistic and is not the design function of a towing dynamometer. Towing dynamometers are designed to simulate real world conditions such as head winds, hauling loads, gentle to steep grades and the like. Sure, they can also be used for more extreme simulated conditions but only within the mechanical limits of the dynamometer’s capacity and the ability of the vehicle towing it.

3. Sizing Formulas

There are two basic formulas to keep in mind when properly sizing a dyno:
drawbar in lb. x speed in mph / 375 = power hp.
drawbar pull in lb. / .7 = required weight in lb. on the vehicle drive tires and dyno tires.
By following these two equations it is easy to identify the power needed to pull a measured amount of drawbar and the weight needed over the axle of both the dynamometer and the test vehicle to achieve the drawbar pull. It is important to determine the power, the drawbar requirement, and the weight requirement for traction and or ballast needed.

4. Weight of the Towing Dyno

Most towing dynamometers are made as light as possible so that they can accommodate a wide range of vehicle weights. The goal is to have enough weight to fully load the vehicle but not so much weight that you are overloading the test vehicle. In some cases an operator may choose to lighten the weight (remove ballast) as much as possible when doing light-duty draw bar loads in an effort to minimize the draw bar load. The opposite is true by adding weight (increasing ballast) for more demanding power requirements.

5. Ballast

Supplying the right amount of ballast is critical to a successful towing dynamometer test.  The weight on the towing dynamometer axle(s) need to be enough so that the tires don’t “skid” under the desired drawbar pull. Likewise, there needs to be enough weight on the drive axle(s) of the test vehicle for the same reason. Properly placed and applied ballast will greatly improve the test results and consistency of the towing dynamometer operation.

6. Speed

Speed is also a factor that needs to be considered when selecting a towing dynamometer. As mentioned earlier, towing dynamometers are mainly used to simulate real-world conditions. However, they can also be used to simulate other more extreme conditions or for specialized tests such as thermal loading, NOX sensor tests and others.

If high power testing is needed at low speeds (15 mph and lower) it may require the use of taller gears in the towing dyno differential and more PAU capacity. In a low speed test, the PAU is rotating at a much lower speed with far less moving air and therefore does not receive the same amount of cooling as it would at higher speeds resulting in reduced capacity as indicated on the dynamometer performance curves. Therefore, if low speed tests are required, the PAU and gearing needs to be sized correctly in order to achieve the maximum length/time required for the test.

Conversely, at higher or high speeds, the dynamometer has higher power absorption capacity because of the natural aero loading and the advantage of greater cooling. If higher speeds are the norm, then it may be advisable to select an axle ratio that is optimized for the higher power and speed requirement.

Towing dynamometers are made in various sizes and weights for all types of car and truck applications. They are designed to safely and accurately simulate real world loads and some extreme loads (within limits) for the vehicle groups they are designed for. Correctly sizing a dynamometer for the test application is critical so that the objectives can be achieved, safely, accurately and efficiently as possible.