Constrained testing in traction

Constrained testing has been used, for many years, to conveniently obtain cornering performance data. (See Chapter 8 of Race Car Vehicle Dynamics). The concept of replacing an inertial force with a static force is even older, of course. But, to my knowledge, constrained testing has not been done to evaluate rear tire loading of a live axle car during acceleration. The following is the basis for a presentation which I'll be making, Lord willing, at the Motorsports Conference later this year. Your comments will help me prepare for those I will receive at that time.

So, the concept is very simple: Replace the inertial force generated during acceleration with a static force. This reduces to simply pulling, in a negative X direction, with a chain or cable. But, of course, the force must be resisted. In other words, the engine must be restrained from rotation. This can be accomplished by any one of a number of means. A driveshaft torque is then generated and that brings us to the reason for the procedure. The reaction to the driveshaft torque which acts on the pinion gear is absorbed by the chassis through the engine and transmission mounts. It is then distributed, front-to-rear, in proportion to the relative roll stiffness. And, at this point, some of you are probably already aware of the possibilities.

Suppose a chain is attached, in the XZ plane and at the CG height, to a car and then further suppose that the chain is extended horizontally out the rear, where it is attached to a "come-along" and then to a stout post. With a couple of transmission gears engaged to prevent engine rotation, the chain tension is then increased. The loading is then the same as when the car is accelerating. (Well, not exactly, for there's the matter of unsprung mass, but there's a "work-around" for that.)

Next, suppose that the aforementioned car's front wheels are supported by wheel scales. It is then possible to measure the left-to-right front tire load differential necessary to overcome the driveshaft torque reaction at any given chain tension. With measurements at 3 or 4 levels of tension, nonlinearities can be determined.

Okay, what can we get out of all this? With a little calculation, we can determine the load removed from the front wheels (what the drag racers call "weight transfer") and, with a bit more effort, we can calculate the driveshaft torque. Since we can measure the torque absorbed by the front suspension, we can then calculate the roll stiffness distribution. That's probably of more interest to most of you than the primary reason for my development of this procedure.

My primary interest was to validate my efforts to achieve equal tire loading with an asymmetrical suspension. With the proper asymmetric setup, the front wheel loads will remain equal (assuming they were equal originally) as the chain is tensioned. And that means, of course, that the rear tires are also equally loaded. I intend to have data, at the Conference, to indicate the effectiveness of such a setup. Non-dynamic means of load equalization -- such as preloads -- could also be studied.

Incidentally, you don't need to have a sturdy post available. A device could be fabricated, having an appearance similar to that of an engine hoist, which would react on the underside of the axle housing and on the shop floor ahead of the axle. Compression links could react against "levers" replacing the rear wheels and extending downward while a hydraulic chain tensioner would replace the come-along and sturdy post. This would allow leveling of the car with any height of front wheel scales.

Braking reactions could also be measured by reversing the chain force.

So, what are your comments?