|In my comments on WEIGHT TRANSFER, I described an "inertial force." This force, if you'll recall, has 3 important properties: Its line of action is through the center of gravity, it's proportional to the product of the car's weight and its acceleration, and its sense (direction) is opposite to that of the car's acceleration.
So, a 3000 pound car, launching at 1G of acceleration, would have a horizontal inertial force of 3000 pounds, acting through its CG (center of gravity), and pointing to the rear.
This inertial force causes the car to undergo certain changes. The front of the car rises. The loading on the rear tires increases (weight transfer). And, with some cars, a very noticeable event occurs at the front of the car: The LF (left front) tire begins to leave the track surface before the RF.
But, is it necessary to have an inertial force to bring about these changes? Of course not! The car can't distinguish between an inertial force and any other kind of force. As Mr. Ed might say, "A force is a force, of course, of course." (I'm sorry. I couldn't resist that.)
And, with this realization, we have the basis for a "traction dyno." Just as an engine dyno loads the engine to simulate the conditions when the engine is installed in the car, the traction dyno loads the chassis to simulate the conditions during launch.
So, let's proceed to a "for instance." We'll start with a top fuel dragster, which, of course, lacks those components which we normally associate with a suspension. Yet, as we'll see, it responds to the inertial force in much the same manner as a suspended car.
To provide the inertial force, a horizontal chain is attached to the rear of the dragster. It is centrally located (between the rear tires) and is at a height equal to that of the CG. The engine must be prevented from rotating by some means. The rear tires and wheels, however, are free and able, when the chain is tensioned, to provide a torque into the driveshaft. If, with such an arrangement, the chain is tensioned sufficiently, the LF tire will be seen to leave the shop floor...before the RF...in exactly the same manner as it does at the strip during launch.
But, we need to quantify our results, so we'll ease the chain tension, jack up the front wheels, and place wheel scales under the two front tires. (If you value your wheel scales, DO NOT attempt to also place scales under the rear tires as the chain is tensioned. The rear tires must sit directly on the shop floor. By observing the front wheel scale loadings, it's easy enough to calculate what's happening at the rear tires.)
Before beginning a traction dyno "run," wheel scales are used to determine the corner weights. Then, the scales at the rear are removed and the tensioning chain attached. With the LF and RF scale readings recorded, jack the front up, tension the chain slightly, and east the car back onto the front wheel scales. The total of the LF and RF readings will now be less. This difference in totals is the weight transfer. But, even with this first reading, you might notice that more weight has come off the LF than the RF. This trend will continue as you repeat the procedure, each time jacking up the front, shortening the chain, and then easing the car back onto the scales.
It's not necessary to continue the run until the car is hanging on the chain. Actually, only a few hundred pounds of chain tension is necessary. Since the results tend to be very linear, 3 or 4 "points" of data will give you all the information you need.
But, what do we have? Well, if, for each data point, you subtract the difference in weight removed from the RF from that removed from the LF, you'll see that this difference, when plotted against the total weight transfer, falls on a straight line. And, if you're an engineer or just "mathematically inclined," you might want to complete that exercise. But, for the average racer, all that's a waste of time! The important thing to realize is that the front tires are not being unloaded equally and that's BAD!! Why? Well, if the front tires are not being unloaded equally, that means the rear tires are not being loaded equally and that means both performance and safety are being compromised. As any oval track racer will tell you, maximum performance from a tire pair is achieved when they're equally loaded. As for safety, it's pretty obvious that a car has a better chance of staying in its lane when the tires are equally loaded.
And, here's where the traction dyno becomes a very useful tool. Instead of studying video and 60 foot time slips, you can make your suspension adjustments in the shop without even starting the engine.
Returning to the top fuel dragster, what could be done to equalize rear tire loadings during launch? Since there's no suspension, a totally satisfactory "fix" is not available, but we certainly can do something to improve the situation. We know that most of the run (I'm talking now of the "run" at the dragstrip, not the traction dyno "run"-) will be made with very little load on the front tires. So, we'd like to see the front wheel scales with the same readings under those conditions. To achieve this, it would be necessary to build in a certain amount of chassis "droop" at the LF. In other words, when the front is lifted with a centrally located jack, the LF should droop. When the car is lowered, this means that a torque preload is put into the frame. If the proper amount of droop is used, the rear wheel loadings can, indeed, be nearly equal during most of the strip run.
With suspended cars, there are means of achieving equal rear tire loading with ANY value of driveshaft torque. If anybody's interested, I might comment on these at another time.