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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: email@example.com. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
WHAT EXACTLY ARE SHAKER RIGS FOR?
Over the past few years I have seen and heard about all of the developments with 4-post, 5-post, and 7-post shaker rigs. However, it seems that most articles focus on the technology employed by the latest and greatest rig rather than what type of results the rig is going to yield. My question is this, when teams test on a shaker rig, what exactly are they going to get in return in terms of results (i.e. lateral and longitudinal G forces or something else)? Also, maybe even a better question, what is it they are testing for (i.e. is there a way these rigs measure “grip” in particular) and what type of testing regiment is going to be employed? For instance if you are a team that is on a limited budget and can only afford to put your car on the rig once per year, do you put the car through a generic set of accelerations and simulated corners or do you try and tune for something more specific or maybe even a particular track? And lastly, at the end of the day how do you know what was the better setup (if there isn’t a way to measure “grip” specifically)?
Shaker rigs are an example of street driving improving the breed for racing. They originated in passenger car development, and were later applied to race cars. The original objective was to explore the car's response to excitation at the wheels, in a more controlled and observable situation than could be achieved on a test track. Improving handling wasn't the main goal. Engineers were more concerned with isolating frequency-sensitive effects that would impact durability and noise, vibration, and harshness (NVH). Does a portion of the roof or the floor pan resonate at a certain frequency? Are there brackets or ducts that buzz or might break? Does anything rattle? At what speeds and frequencies do the wheels "dance", and does this agree with calculated natural frequencies?
Early shaker rigs had posts only under the wheels, and the sprung mass was allowed to do whatever it wanted to do in response to excitation at the contact patch. It soon became apparent that it would be more desirable to allow the contact patches to float both laterally and longitudinally, so that the track and wheelbase could vary with suspension movement, without having tire scrub fight the movements. A fifth post attached to the sprung mass allowed the car to be anchored horizontally
while all four contact patches could float. The car still needed either an anti-rotation feature in the fifth post, or some other mechanism, to constrain the car in yaw.
This was good for replicating highway driving, but it was impossible to explore suspension behavior in conditions of sprung mass displacement caused by aerodynamic loads, banked turns, or pitch and roll due to longitudinal and lateral acceleration. To reproduce these displacements, three posts were attached to the sprung mass. Conveniently, three posts also constrain the car in yaw, without any additional devices. With this addition, it is possible to reproduce any combination of suspension displacements that the data acquisition system may have recorded on-track, or that an engineer might imagine.
Importantly, this does not mean that the rig reproduces all the forces acting on the suspension. The posts only move vertically. The rig cannot apply or reproduce any horizontal forces. It also cannot measure any horizontal forces. Therefore, we cannot measure grip at all. We also cannot reproduce loads and frictional influences in the suspension that result from horizontal forces at the contact patches. We do not even know vertical wheel loads with any accuracy, because horizontal forces at the contact patches affect these.
This last phenomenon is particularly significant in stock car rear suspensions, and in any suspension with large geometric "anti" effects. (Stock car rear suspensions, and most beam axle suspensions, have ample geometric anti-roll.) With formula cars, where there is modest anti-roll and anti-pitch, the wheel load values on the 7-post are closer to reality, but still not highly accurate.
I am referring here to individual wheel loads. On the rig, we do get reasonably accurate total loads for all four wheels, and for the left, right, front, and rear wheel pairs. It's the loads for individual wheels and for diagonally opposite pairs that are inaccurate.
Even though we do not read accurate individual wheel loads on the shaker rig, we can get a reasonable comparative evaluation of how much these loads vary dynamically in the excitation conditions that we test. What we are after is minimum load variation. We cannot measure grip itself, but we do know that variation of normal load is bad for grip, and minimizing such variation will improve grip.
This is true for two reasons. First, if the tires unload for any significant length of time, the car is limited by whatever grip the tires have at that load. If the grip limit at that load is exceeded, the car will break traction, and once traction is broken it is hard to regain. A slide, once initiated, tends to persist.
Second, even if we are dealing with load variations over small time spans, so that vehicle inertia masks the intervals of low load and grip, we do not fully recover during the highly loaded periods what we lose in the minimally loaded periods. Why? It's our old friend, load sensitivity of the coefficient of friction. Adding 100 pounds of load doesn't help as much as losing 100 pounds of load hurts.
So we try different combinations of shock valving and springing, and we try to minimize load variation at the wheels.
I was asked recently how you valve shocks based on suspension displacement traces provided by data acquisition. I had to reply that I don't know of any way to do that, and I don't know anybody who claims to be able to do that. However, with the use of a 7-post rig, we do at least have a means to make gains by trial and error.
There is another kind of test rig that does let us look at the effects of horizontal forces at the contact patches. It's called a kinematics and compliance tester, or K&C rig. Until recently, only passenger car manufacturers had these, and nobody was offering testing by the day or hour to racers. There is now a K&C facility available to the motorsports community, just up the road from me in Salisbury, NC. It's called Morse Measurements. The people who run this facility are friends of mine, so I'll give them a plug. They are at www.morsemeasurements.com or 704-638-6515.
What the K&C rig does is grab hold of the frame of the car, and then apply horizontal loads at the tire contact patches. The sprung mass can be kept from moving, and the changes in wheel loads measured that way. Alternatively, the sprung mass motion can be controlled to keep the total normal force at the contact patches constant and we can measure the displacements and load changes that result. Roll and pitch moments are applied to the frame, based on an input sprung mass c.g. height. Individual wheel horizontal forces are applied based on estimates, which in turn are based on tested or estimated tire properties and calculated wheel loads.
We then see from the test results whether our estimated wheel loads approximate those produced in the test, and we may want to adjust our contact patch forces and do another iteration on the rig.
There are other tests on the K&C rig as well, including simply cycling the car slowly in heave through its suspension travel and measuring the camber changes, contact patch scrubs, and wheel loads, with the contact patches allowed to float horizontally so that all horizontal forces at the contact patches are eliminated. This test can be done on either a K&C rig or a 7-post, so it can be used to compare the agreement of the two types of rig.
What the K&C rig does not do is move the chassis fast. The 7-post can load and move the chassis at racing speed, but only up and down. The K&C can load and move the chassis in all directions, but only slowly. At this writing, there is no test rig that moves and loads and measures a car in all directions at once, at realistic on-track speeds. And even the low-speed testing done on the K&C rig uses estimated or assumed grip levels at the contact patches.
We do have a test that does measure grip: the skid pad. It measures average grip over a lap, by stop watch, or over smaller intervals, using accelerometers or other sensors. Of course, we have difficulty observing the car during testing, because the car is in motion rather than on a test rig. And the grip measured is only on a particular surface, on a particular radius.
We thus have three useful methods of testing, all of which provide useful comparative measurements, but none of which allows us to fully replicate track events in a controlled environment. I could even say we have four methods, if we include wind tunnels, or a much larger number if we include component testing devices such as tire testing machines, shock dynos, and engine dynos. But so far we have no device that really puts the car through its paces in a manner that replicates in full what happens on the track, on a stationary platform where we can safely watch and measure it. The best we can do is look through a number of windows, each of which affords a view from a different angle, and use our mind to try to put these glimpses of reality together.