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Handling and Suspension Setup
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Track Testing

When we set up our customer's racing cars, our main focus is to develop a set up that we can go to a circuit and test.

This excerpt, says a lot about the value of steady state testing on a circular skid pad.  This is exactly the situation we are modeling in the Weight Transfer Worksheet.  If you have done your WTW numbers, you are in good shape to analyse what is happening to your car at the track test.

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Handling, Skidpad and Track Tests

Skid pad tests can be the most productive and exciting tests for a road racing car, but they are highly demanding on driver precision and consistency. It may take a lot of practise laps before the driver can maintain a nearly constant throttle and steer angle and get lap times to repeat within a few hundreths of a second. But a lot of valuable data can be obtained at low cost and low risk on a skid pad, so it is well worth the effort.

The test setup will vary depending on the data needed, but driving technique is simply to keep the car on a precise circular path, at the highest possible constant speed. To a driver who is used to clipping corner apexes with the inside front tire, it is probably easiest to try to keep the inside front tire on the painted circular stripe. That tire will also be least affected by a change in friction coefficient due to the painted stripe.

Maximum lateral capability can be measured by one of three methods, or all three together, if accuracy demands it. The first is to use an electronic speedometer and record the speed constantly. The disadvantage is that it is hard to take the average speed, and the recorded speed is affected by lateral tire slippage. A more direct method is to use an electronic accelerometer mounted laterally in the car. But again, it is hard to take an average of all the fluctuations, and the reading has to be corrected for roll angle. The easiest, fastest, and probably most accurate method is to take the lap-time for each complete revolution on the circle. This merely requires a split-action stopwatch and someone with a very precise eye and thumb, or an automatic trap switch to start and stop the watch. A timer's accuracy can be improved by having a distinct mark on the circle and noting the instant the car's front tire hits it. It is also a good idea for the timer or someone else to watch the car and make a note whenever it moves noticeably off the marked path. As with any other data-taking, any particularly low figure will have to be backed-up before it is credible. Of course, tires should always be scrubbed in and
warmed up until lap times reach a consistent and low level, and turn direction should be reversed frequently.

The relationship between lateral acceleration, radius, speed, and elapsed time is given by:
(.067) (mph)2 (1.22)(r)
g =
(r)
(t)2
where the radius (r) is to the center of gravity of the race car, or the inside tire path plus one-half the tread width, plus some slip angle displacement.

Steady-state stability can be evaluated at the same time as maximum lateral acceleration. This is the tendency of the race car to maintain a stable non-oversteering condition at top speed on the skidpad, as discussed in Chapter 7. As various factors, such as springs, anti-roll bars, or tires are being changed, the steering angle necessary to balance the car will change. It should be possible to see the increase in lap times as the car becomes unstable and uncontrollable with oversteer.   But it is also possible to measure the oversteer or understeer bv recording the steer angle. At a very low speed say 5 miles per hour, or minimum possible,a given steer angle (Ackermann steering) is necessary to keep the car on the circle. As the speed is increased,the necessary steer angle will either increase (understeer). remain constant (neutral steer), or decrease and require quick and violent corrections (oversteer).

Examples of understeer/oversteer plots are shown in Fig. 45. The vertical scale is the steer angle required to drive the car on a 100 foot radius circle. The horizontal scale is lateral acceleration in g's, which increases as the square of increasing speed. (Speed is shown just for comparison on the lower scale. The speed scale would change with different radius skidpads, while the g scale remains relatively constant.)

In the lower sublimit area of the curve usually up to perhaps 0.3 g the curve is relatively flat, and can be presented in a single number of degrees per g. But up closer to the limit of lateral acceleration is where the interesting things happen. With a normal understeering car, the curve will continue to climb, usually at an increasing rate. In other words, it requires more and more steer angle, until no amount of steering will keep the car on the path. An oversteering car, however, will reach a point where the curve flattens out and makes a sudden break downward. This is where "countersteering" is necessary to stay on the path a very unstable situation. The rare neutral-steer car will have a very flat curve from zero and not break up or down. It is generally accepted that the straightest curve is best, and with enough rise that it never breaks downward under any transient conditions.

It is important to mention that many static vehicle parameters can affect these curves, and so they are sometimes factored out. The Ackerman steer angle is the steering necessary to stay on the radius at just above zero speed or g's, and is a function of wheelbase and steering gear ratio. Therefore it is usually subtracted out, defining zero steer at just above zero speed. Steering gear ratio will also directly affect the slope of the curve, doubling the slope with a doubling of the gear ratio. However, because the change in steering wheel angle is what the driver actually perceives, gear ratio is not normalized out except for engineering analysis of front/rear tire slip angles.

To get these curves, it is usually necessary to use continuous (analog) strip chart recording. Vehicle dynamics engineers ordinarily use recorders which take a minimum of 4 to 8 channels of data simultaneously. Here, just 2 channels would be adequate: steer angle and speed. Another less precise alternative is to increase speed in steps and make a visual note of the steer angle in degrees at each speed.

Transient tests can only be positively made on a skidpad that allows entry and exit from the steady-state circle, or on a race track, of course. However, if there is even a little extra room on the skidpad, it is possible to get a feel for transient response. There should be enough understeer so the car can be accelerated relatively rapidly without losing traction at the rear. In other words, there shouldn't be any drastic throttle oversteer. Conversely, there shouldn't be a sudden change in stability due to a complete throttle-off at the limit. It can also be an educational experience to brake hard from maximum lateral acceleration while trying to maintain the path radius.

When it comes to checking fuel and oil pressures, the skidpad is far safer than a race track, if not quite a perfect simulation. Naturally the driver should always be aware of oil pressure at high lateral accelerations, but the skidpad allows him to watch it more closely over a longer period. If the oil level is ever going to settle to one side of the pan and starve the pickup, it is better for it to happen on a skidpad where there is a greater opportunity to shut the engine down immediately. The is true of fuel pickup systems, especially in trying to determine how completely they can drain the tank without picking up air and leaning-out the engine. The greatest problem with the test is that in many cases the worst possible situation is compounded by the addition of braking or acceleration forces (which you get on a track, but not on the skid pad).

Suspension deflections can be important to know, especially in development of geometry and ride rates. Deflection measurements on the skidpad will merely show the maximum roll angle, how close the components are to bottoming, and how great the jacking effect is. The test setup is about the same for aerodynamic downforce, except that the deflection sensors are mounted as close to the wheels as possible, instead of in the center of the car. Ideally, there would be a continual recording at each wheel, but if only one channel is available rather than four, the test must be carefully repeated four times. The best location to record suspension deflection is probably at the centerline of the spring, although it may also be useful to know the wheel deflection. In that case, a swivel anchor can be mounted to the center of the wheel, and the sensor can be mounted outside and above me wheel as on a fender lip or extended bracket.

For real-life conditions such as bumps, dips, and combinations including acceleration and braking, only a test on an actual race track will suffice.  (If no data logging use shock travel O rings on the shock shaft).

TIRE TESTS

Tire tests are one of the most valuable uses for a skidpad, once the chassis has been fairly well developed. There is no other way to accurately determine the optimum tire compounds, temperatures, camber angles, or pressures.  Results obtained from race track testing would have to be far more significant to eliminate driver inconsistencies. The test setup simply requires a race car and driver that can run all day long at low speeds and high lateral acceleration, with no fatigue, overheating, or variation in performance. It is also important to remember that only a pair of front or rear tires can be tested at once, since the car will be limited by either front or rear cornering capability. The best practice is probably to develop front tire cornering performance first (since the car should be understeering), and as it becomes better, to keep increasing the front anti-roll bar rate as necessary to avoid oversteer. When no more front cornering power is available or the front roll rate is so great that it lifts the inside front tire off the ground, then it is time to work on increasing the rear tires' capabilities. The rear anti-roll bar rate may then be increased to create oversteer, as a rear tire limiting condition to overcome.

Tire test procedure is simply to record average lap time or lateral acceleration for each configuration change. However, it will be necessary to monitor tire temperature constantly, since its effect is great enough to cancel out other test conditions. The first test with any tire should be a comparison of g's versus temperature, to determine the optimum and the drop off on either side of the optimum. Since it is difficult and expensive to record temperatures continuously, it will be necessary to stop the test at intervals and check the temperatures as rapidly as possible with a tire pyrometer. The best technique is to run in two or three lap increments, with one person timing, and another taking temperatures as fast as the car can be stopped from its high lateral condition. It shouldn't be necessary to take over half a lap to stop, and the tire technician should be right there at the stopping point. Within 10 or 20 laps the tire should be at its maximum temperature, or past its peak cornering capability. It can also be valuable to know just how fast the tire cools off, to get an idea of what the true temperature is while the tire is working. (See Chapter 2) This can be estimated by watching the temperature fell in a given location over a matter of seconds and projecting the result. Since the test accuracy is poor, it is a good idea to repeat it after, or as, the tires cool off, and in both directions around the pad. Of course, the outside front or rear tire temperature is of greatest importance.

From then on, all tests with that particular tire should be run at that optimum temperature or at least corrected for any drop off. This data will also come in handy at the race track, to determine whether a tire compound is too hard or too soft for the work input under a given ambient temperature.

When tire compound and temperature can be held constant, then optimum tire pressure and camber angles can be determined.  Proper camber angle will show up in skid-pad lap times, tire wear profiles, or temperature differences across the tread, but the last method is quickest for tire development work. The pyrometer must be used rapidly to get three readings (inside edge, center, outside edge) before the natural heat conduction in the tread evens out the temperatures. It isn't reasonable to expect them to be exactly equal, however, since the car will be at maximum lateral acceleration camber angle only for very short periods.

When everything else has been developed to the optimum on a skidpad, it can be a good place to teach the driver what extreme variations in handling feel like. If the car is ever going to lose a shock absorber, or break a front anti-roll bar and oversteer, or have a tire go soft, or otherwise become unmanageable, the skidpad is the safest place to learn the feel and the corrections required. Just knowing the feel of ordinary changes from oversteer to neutral steer to understeer is an invaluable aid in later analysis of a race car on a race track.

TRACK TESTING

An actual race course is the last place where any serious or accurate development work can be done. Only after the car has been otherwise ideally set up will race track laps be meaningful, and then primarily with respect to the driver's performance. For vehicle evaluations, it will still be necessary to break the track down into braking, cornering, and acceleration segments, as opposed to over-all laptimes. The timing isn't as difficult (with an electronic split-action watch) as it is to find a spot where the car can be seen at many locations around the track.  At Riverside, for example, it is possible to see a car most of the way around the track from the roof of the timing tower. A sample of segment times is shown in Fig. 46.

This is also the best way to find out how a competitor's car really compares as opposed to average laptimes. If the other driver is sandbagging, it will probably show up in a particular segment. But if the other car is quicker, it is helpful to know exactly where, to know where there are some capabilities to be gained. In the example shown, car B is apparently better in acceleration, which means that car A is probably adequate in handling and braking but should have more power or lower air drag. Some people have also used radar guns or electronic gunsight tracking devices to record comparative speeds around an arc. There are other more sophisticated spying devices to analyze competitor's cars, which are more accurate and more complete.  But, needless to say, they are much more expensive and complex and confidential.

Probably the best race driver teaching' device known to man is a continual recording of speed and horizontal accelerations around a race course. It won't say much about the car unless there are other recordings of the same car in another configuration, or other recordings of other cars, to compare with. But such recordings will tell a great deal about the driver's ability to take advantage of the car's capabilities. The best test setup is to have a two-channel recorder with speed and acceleration inputs. The speed can come from a front wheel pickup, preferably the outside wheel, to avoid lift or lockup problems in cornering and braking. The g sensor can be either a single lateral accelerometer or a combination of two arranged at right angles. An electronic circuit can be designed to calculate the net horizontal acceleration in any direction and produce a signal proportional to the percentage of traction used versus traction available. A more thorough explanation is given in Chapter 12.

Other tests that can be performed on a race track were previously explained under straightaway or skidpad testing. However, in general they tend to be tests of the track configuration rather than of the vehicle. Unusual suspension deflections are mostly dependent on surface condition or grade changes or bankings. Different tire or brake temperatures are a function of track coefficient and distribution of time spent on cornering, acceleration, and braking. Vehicle transient response characteristics will change with respect to the types of corners on a given race track. If there is enough time, a race car can be set up to the optimum for each particular track's predominant characteristics, but it is likely to take at least a few days of track rental and exclusive running. It helps a great deal to have a lot of experience at a particular track and at a wide variety of tracks. But for those with no experience or for a new track, a careful analysis of speed and g recordings can work almost as well.

Of course, in the absence of any recording instrumentation at all, stopwatch times, in various track segments will probably indicate the worth of any vehicle changes. Handling or tire improvements should show up in the low-speed cornering segment times, aerodynamic downforce should show up in high-speed cornering segments, engine power should show up in the straightaways and so on. The overall improvement will probably show up in total lap times, but less significantly.  This leads to the question of the relative value of various vehicle improvements. Racers tend to concentrate on making improvements in the areas they understand or enjoy the most rather than those areas where the potential gains may be greatest. It isn't that hard to determine just what these relative values are, however. A computer can be and has been used to put a numerical value on various race car improvements, but any racer can find the numbers for his own car on any particular track by the handicap method. If it isn't easy to improve a car's performance, it is all too easy to diminish it.  All that's necessary is to know the amount by which the effect is reduced and the increase in lap time which results. The effect will be linear enough over a reasonable range to project from a decrease in performance to an increase in performance due to a positive change.

The most obvious example is in determining the effect of reduced weight on lap-times. All that is necessary is to plot lap times versus fuel consumption in pounds. Say that a 2000-pound race car consumes 200 pounds of fuel during a race, while its average lap time decreases from 90.0 seconds to 88.2 seconds. Then, assuming that all other factors remained equal during the race, a person could project that a 10 percent reduction in vehicle weight would produce a 2 percent reduction in lap times at that track. (This is true for race cars with centrally mounted fuel tanks. For extremely rearward tank locations, weight distribution changes are an uncontrollable variable.)

Other factors may be as easy to degrade as weight: power reduction with throttle-stops, air drag increase with a flat plate, tire braking capability reduction with lower temperatures. However, it may be somewhat difficult to quantify the exact value of the change in the factor. But once the relative positive effect of various changes can be estimated, the proper concentration of efforts can be allocated. Of course, even the most scientific approach must be adapted to fit the capriciousness of racing regulations and the availability of time and dollars.

Most racers feel that durability tests are also beyond their limits in available resources, so they tend to use experience, intuition, and luck instead. The only way to really know whether a car can be raced hard for 24 hours or 500 miles is to race it hard for that time or distance. If it is done in a test session and nothing breaks, then the car can be totally rebuilt as new for the real race and still fail due to some random faulty new component. Even if experience is the best judge of durability, that sort of experience can be bought. A brand new design may be faster but it definitely doesn't have a history of reliability. The best insurance is in knowing that a particular design or component has been around for a long time without unusual failures. At the very least, it is a good idea to know a car or component's history, and its average life expectancy before inspection, rebuilding, or replacing is necessary. On the other hand, if durability testing is feasible, the biggest mistake is to try and make the car survive under those conditions. Instead, the idea is to try and break the car under reasonably severe simulated usage, rather than pussyfooting it around. However, it is wise to test on a track where a failure doesn't have consequences as critical as they could be in a race.


 

 

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About this Book

This excerpt is from "Race Car Engineering & Mechanics" by Paul Valkenburg.  Paul worked for Chevy R&D during the golden years of vehicle dynamics development, and still writes today for Race Car Engineering.  You should buy this book.

 

 

 

 

 

 

 

 

 

 

These tests are common for road car development but not usually done by racing people. 

Ackerman steer angle referred to here is correct, but not to be confused with Ackerman steering we talk about, referring to the angle of the steering arms at the straight ahead position. 

Nowadays, this data is easy with get with the DL1 data logger.  Super accurate speed trace and circuit map with the GPS, and high quality G sensors.

 

 

 

 

 

Data recording has advanced a lot since this was written.  In professional racing the shock position sensors are monitored whenever the car is on the track. 

Building the suspension with sufficient suspension travel to accommodate all modes of movement of the chassis is a vital part of the workshop set up.  It is a compromise based on the input numbers in the Weight Transfer Worksheet. 

 

Without the benefit of a skidpad, we have to tire test as part of a circuit test day.  We take tyre temps and pressures throughout the day.  You can see differing requirements for pressure and camber, as the tyres get up to temperature and the car  goes faster.

 

 

 

 

 

 

 

 

Important concept.  Readings are only meaningful if you take them a number of times during the test.

 

 

 

 

 

 

DL1  data logger is brilliant for split times.  Done on the GPS, no track beacon needed.  Compare any laps recorded any time, any car, all on exactly the same split timing sectors.

Contact us to buy a DL1

Many agree with this.  The DL1 is a good driver coaching device.

 

 

 

 

 

 

 

 

Interesting idea. From an experimental view point, sounds promising. 



 

This is only a maybe at best.  Sometimes will not be true.

The F1 teams know this down to the T.  Cromley talks about it on the telecast.

 

 

 

 

Since Paul wrote this, would have to be a hallmark of modern professional racing.