Hot Rod mag suspension build up - what's your opinion?
A "G" skid pad test isn't the best nor only way to test handling. A skid pad test is steady statecornering going around in a circle.
Read this written by legendary racer Dick Guldstrand:
http://www.guldstrand.com/gtheory.asp
Some Handling Theory
By now you have probably read enough "straight talk" articles about high-speed handling to be totally confused. Your condition stems from the fact that most such articles are written to sell rather than to inform. This section will outline a few basic concepts, which will help you make an enlightened choice about how to modify your car for superior handling. Some parts will get a bit theoretical, but the knowledge gained will be worth the effort, so bear with us.
Where to start?
Where do we start this discussion of high-speed handling? I can tell you it doesn't start with G-forces, gumball tires, spring rates or any other thing the enthusiast considers central to high performance.
It starts with Joe Average. You've met him. He's the driver of that car acting as a moving chicane on your favorite back road. Joe is also the typical car buyer and the key to General Motor's plans for making big profits. They want to sell a large volume of cars all based on the same basic design. To do this they must appeal to the large group of buyers in the middle of the market. Sport coupes like the Z51 Corvette, Z-28 Camaro, and Trans Am Firebird represent the most that GM is willing to do for the enthusiast. If GM can't sell hundreds of thousands of a particular part over the life of a model run, they are simply not interested. Their marketing plans rely on volume.
We need to look at how Joe drives and what he expects from a car. Then we can compare his style with that of "The Enthusiast Driver". Once we understand these differences, we can look at how they affect the overall design of the car. The best way to examine these differences is by watching our two drivers negotiate a typical turn.
A Typical Turn
Let's assume a 180°, medium-speed corner with a radius of approximately 230 feet. This turn will have a total distance along its circumference of 722 feet. If you are having a hard time visualizing it, think of a 180° freeway on-ramp with a recommended speed of 25 mph. Assume also that Joe and Enthusiastic are driving base-model Coupes, which weigh about 3500 pounds. Cornering in a normal manner, Joe will round the turn at 30 mph. His subjective reaction to the cornering experience will be that the car handles just fine. Enthusiastic will corner at 45 mph. He feels that the car leans too much and is not precise.
Why do Joe and Enthusiastic have such different reactions to driving the same car around the same turn? The simple answers are that Enthusiastic is going faster than Joe or that the base-model coupe is designed for Joe's driving style rather than Enthusiastic's. While these answers are valid, they don't help us decide how to obtain superior handling. We're looking for a more fundamental understanding. To get it, we have to discuss some basic physics and the nature of human response to time-distance relationships.
It's Only Natural
The basic physics we need to examine is the concept of energy. Of particular interest is kinetic energy, the so-called energy of motion, which is present in all moving objects. The amount of kinetic energy in any particular moving object is determined by both the weight and velocity of the object. It is important to realize how weight and velocity influence the amount of energy. This relationship is expressed by the formula:
Energy =
½ Weight x Velocity²
This tells us the amount of kinetic energy increases in direct proportion to added weight and in geometric proportion with added velocity. Thus if the weight doubles the kinetic energy also doubles, but if the velocity doubles the energy will be four times greater. Let's get down to earth by seeing how much energy is involved as our two drivers round the typical turn as above. When Joe Average rounds the corner at 30 mph, he is cornering at about 0.279 g. The amount of energy involved is about 142,785 Newton-meters. Enthusiastic Driver goes around the turn at 45 mph, or about 0.628 g, which is not that slow for a base-model car. The amount of energy involved as Enthusiastic corners is about 321,183 Newton-meters. Notice that while Enthusiastic is going 50 percent faster than Joe (45 vs. 30 mph) the amount of kinetic energy involved has increased by 125 percent (321,183 vs. 142,785 Newton-meters).
You're thinking this is all very interesting but wondering how it relates to improving your car. Well, it means that the amount of energy involved with your car during cornering is the basic physical design criteria used in the construction of all suspension components. Think of the suspension components as devices to resist, store and control energy. The spring is a good example. We always hear people talking about spring rates. What does "spring rate" really mean? When we say that a spring has a rate of 250 pounds per inch, it means the spring can store 250 inch/pounds of energy for each inch of compression.
Why don't we use our example again? If we assume that as Joe goes round the turn his outside springs are compressed one inch; then as Enthusiastic Driver corners, the outside springs will be compressed an additional 1.25 inches. How does this extra compression affect the driver's subjective reaction to handling? At this point we get to the second important concept: the nature of human response to time-distance relationship.
It's Just Human
Back at our typical turn, we can watch more closely as our two drivers negotiate the corner. Recall that Joe Average went around the turn at 30 mph, or to put it another way, at 4 feet per second. Assume that as Joe enters the turn, the car takes 2 seconds to roll over, compress the outside springs and come to a steady-state cornering attitude. Likewise, at the exit of the turn, Joe's car takes about 2 seconds to unroll and get comfortably straight again. Watching Joe go through our 722 foot long turn, we see that it takes him 88 feet to transition into the turn, that he has 546 feet of steady-state cornering and another 88 feet of transition back onto the straight. Joe spends a total of 16.4 seconds in the turn.
Of these, 4 seconds (24%) are spent in difficult transitional cornering maneuvers, and 12.4 seconds (76%) are spent in relatively easy steady state cornering. This is why Joe feels that the car handles fine. At 30 mph, he spends relatively little time doing the difficult tasks of getting on and off the proper line and has a good deal of time in the middle of the turn to make corrections.
Enthusiast Driver experiences quite a different situation. He is going 45 mph or 66 feet per second. Remember that basic physics indicates that there is 125% more energy involved because Enthusiast is cornering 50% faster. Remember also that the greater energy causes the outside springs to compress an additional 1-¼ inches. Assuming a "linear" suspension response time, it will take Enthusiastic 4.5 seconds of transition at each end of the turn. He spends 297 feet of transition during turn entry (66 feet per second x 4.5 seconds), has only 128 feet of steady-state cornering 297 feet of transition at the exit. This is why Enthusiast thinks the car is unresponsive. He is in the turn for a total of 10.9 seconds. Of these, he spends 9 seconds (82%) of the time in difficult transitional cornering and 1.9 seconds (18%) in relatively easy steady state cornering.
The technical name for this phenomenon is yaw response. The yaw response characteristic of a car is the single most important of that group of traits we call "handling". The parameters for determining an ideal yaw response characteristic are derived from study of the human nervous system. The yaw response must be designed to make the driver feel comfortable at the speed he wished to go. It cannot be too slow or too fast. It must be slow enough so the driver can react to steering inputs; yet, it must be fast enough so that corrections can be completed before an off-road excursion occurs. The "base-model coupe" has been designed by GM with an ideal yaw response for Joe Average, who normally corners at about 0.300 g. If a driver wants to corner at some higher speed, then the suspension must be modified to provide an ideal yaw response at the higher speed. In other words, we must keep the yaw time, when expressed as a percentage of total cornering time, constant. To keep Enthusiast Driver comfortable going around our typical turn at 45 mph, we must modify the suspension so about 18% of total cornering time is spent in transitional cornering. This is what keeping yaw time constant means.
The Indicators Game
At this point we should scrutinize the primary indicator of cornering performance â namely cornering force as expressed in g's. At the present time, we all tend to focus on how many g's a car can generate. We normally equate high g-forces with good handling. Does this equation really hold-up? Sometimes it does and sometimes it doesn't. We have all read comparison road tests where the car with the highest cornering force was also the slowest through a slalom course. This happens because g-forces are measured on a skid pad, which tests only steady state cornering. The skid pad tells nothing about yaw response characteristics during transitional cornering. Yet, it is these characteristics which are the most important factor contributing to superior handling.
Whether high cornering forces translate to superior handling or not depends on the honesty of the suspension designer. If he designs a complete system which takes into account both transient and steady-state cornering, then yes, high cornering forces will mean superior handling. However, if he uses tricks just to yield high cornering force numbers and does not do his homework on the remaining suspension components, the no, superior handling will not result. Remember, skid pad results are just indicators; the real test of good handling is how your car performs over your favorite back road, in transition as well as "steady-state".
How can we judge the honesty of the designer's work? A good place to start is by examining how General Motors upgrades a base-model coupe into either a Z-51 Corvette, Z-28 Camaro or Trans Am Firebird.
A Lesson from GM
It is a myth that General Motors cannot design good cars. Their engineers and technical staff are among the best in the world. If you have any doubts just look at GM's impressive competition record. It includes numerous successes doing both officially sponsored corporate projects as well as countless backdoor efforts. You may remember their NASCAR efforts of the 50's, the Grand Sport Corvette, and the racing Camaros of the late 60's. From our point of view, the only problem is that these superior technical capabilities are generally used to produce designs for the average buyer rather than the true enthusiast.
When these talented engineers redesign the base coupe into a Z-51 Corvette, Z-28 or Trans Am, they treat the whole car as an interrelated system. They make detail changes to a wide range of suspension components. The Z-51 Corvette, Z-28 or Trans Am differ from the base coupe in the hardness of rubber suspension bushings, the rate of both front and rear springs, the diameter of sway bars, the ratio and feel of the steering gear, the width of the wheels, and the size and compound of the tires. It takes the combined effect of all these changes to maintain a balanced car, which (1) exhibits cornering forces in the 0.800 ranges and (2) has good yaw response characteristics. Notice that they change several of the major system components and not just a single item such as a front or rear sway bar. The total system approach produces a car with an integrated, balanced feel derived from all the parts working in harmony.
The other thing to notice is that all of the revised parts are stiffer than the normal part. The springs are stiffer, the sway bars are stiffer and the rubber bushings are harder (which is another way of saying stiffer). As we discussed above, they must be stiffer to deal with the increased energy generated by higher cornering speeds.
No Free Lunch
We need to understand one final point. Every given set of suspension components has a limited working range. As suspension components are modified for higher cornering speeds, the working range becomes narrower. Thus, the base-model coupe rides well at slow speeds, corners satisfactorily up to about 0.450 g and is uncomfortable at 0.650 g (145%). The Z-28 or Trans Am has a firm ride, corners well to 0.720 g and becomes difficult to control at 0.770 g (107%). A good combination street/slalom car has a very firm ride, corners well to 0.875 g and feels "edgy" at 0.900 g (103%). The typical racecar has no ride comfort, corners well at 1.200 g, and leaves the road at 1.210 g (101%). Notice that as cars are tuned to handle well at higher g-forces, low speed ride comfort is sacrificed.
Under current technology, the twin goals of pillow-soft low-speed ride and superior yaw responses at high cornering forces cannot be accommodated in the same car. The truth is that tuning the suspension to work well at higher cornering speeds always trades off some low-speed ride comfort. We must each individually decide how much low-speed ride comfort should be sacrificed for added high-speed cornering capability. As a wise man once said, "There is no free lunch."
So there it is, the real "straight talk" about high-speed handling. You won't remember all the details but you should remember the following three points. First, superior high-speed handling is more than just high cornering power. It must include a balance of both high cornering power and correct yaw response characteristics. Second, a car with superior high-speed handling is produced only by the systematic modification of a wide range of suspension components. Just bolting on a sway bar or some other part won't get the job done. Finally, as cornering speeds get higher, the suspension system must get stiffer in order to handle the increased energy levels.
Author, Dick Guldstrand
A "G" skid pad test isn't the best nor only way to test handling. A skid pad test is steady statecornering going around in a circle.
Read this written by legendary racer Dick Guldstrand:
http://www.guldstrand.com/gtheory.asp
Some Handling Theory
By now you have probably read enough "straight talk" articles about high-speed handling to be totally confused. Your condition stems from the fact that most such articles are written to sell rather than to inform. This section will outline a few basic concepts, which will help you make an enlightened choice about how to modify your car for superior handling. Some parts will get a bit theoretical, but the knowledge gained will be worth the effort, so bear with us.
Where to start?
Where do we start this discussion of high-speed handling? I can tell you it doesn't start with G-forces, gumball tires, spring rates or any other thing the enthusiast considers central to high performance.
It starts with Joe Average. You've met him. He's the driver of that car acting as a moving chicane on your favorite back road. Joe is also the typical car buyer and the key to General Motor's plans for making big profits. They want to sell a large volume of cars all based on the same basic design. To do this they must appeal to the large group of buyers in the middle of the market. Sport coupes like the Z51 Corvette, Z-28 Camaro, and Trans Am Firebird represent the most that GM is willing to do for the enthusiast. If GM can't sell hundreds of thousands of a particular part over the life of a model run, they are simply not interested. Their marketing plans rely on volume.
We need to look at how Joe drives and what he expects from a car. Then we can compare his style with that of "The Enthusiast Driver". Once we understand these differences, we can look at how they affect the overall design of the car. The best way to examine these differences is by watching our two drivers negotiate a typical turn.
A Typical Turn
Let's assume a 180°, medium-speed corner with a radius of approximately 230 feet. This turn will have a total distance along its circumference of 722 feet. If you are having a hard time visualizing it, think of a 180° freeway on-ramp with a recommended speed of 25 mph. Assume also that Joe and Enthusiastic are driving base-model Coupes, which weigh about 3500 pounds. Cornering in a normal manner, Joe will round the turn at 30 mph. His subjective reaction to the cornering experience will be that the car handles just fine. Enthusiastic will corner at 45 mph. He feels that the car leans too much and is not precise.
Why do Joe and Enthusiastic have such different reactions to driving the same car around the same turn? The simple answers are that Enthusiastic is going faster than Joe or that the base-model coupe is designed for Joe's driving style rather than Enthusiastic's. While these answers are valid, they don't help us decide how to obtain superior handling. We're looking for a more fundamental understanding. To get it, we have to discuss some basic physics and the nature of human response to time-distance relationships.
It's Only Natural
The basic physics we need to examine is the concept of energy. Of particular interest is kinetic energy, the so-called energy of motion, which is present in all moving objects. The amount of kinetic energy in any particular moving object is determined by both the weight and velocity of the object. It is important to realize how weight and velocity influence the amount of energy. This relationship is expressed by the formula:
Energy =
½ Weight x Velocity²
This tells us the amount of kinetic energy increases in direct proportion to added weight and in geometric proportion with added velocity. Thus if the weight doubles the kinetic energy also doubles, but if the velocity doubles the energy will be four times greater. Let's get down to earth by seeing how much energy is involved as our two drivers round the typical turn as above. When Joe Average rounds the corner at 30 mph, he is cornering at about 0.279 g. The amount of energy involved is about 142,785 Newton-meters. Enthusiastic Driver goes around the turn at 45 mph, or about 0.628 g, which is not that slow for a base-model car. The amount of energy involved as Enthusiastic corners is about 321,183 Newton-meters. Notice that while Enthusiastic is going 50 percent faster than Joe (45 vs. 30 mph) the amount of kinetic energy involved has increased by 125 percent (321,183 vs. 142,785 Newton-meters).
You're thinking this is all very interesting but wondering how it relates to improving your car. Well, it means that the amount of energy involved with your car during cornering is the basic physical design criteria used in the construction of all suspension components. Think of the suspension components as devices to resist, store and control energy. The spring is a good example. We always hear people talking about spring rates. What does "spring rate" really mean? When we say that a spring has a rate of 250 pounds per inch, it means the spring can store 250 inch/pounds of energy for each inch of compression.
Why don't we use our example again? If we assume that as Joe goes round the turn his outside springs are compressed one inch; then as Enthusiastic Driver corners, the outside springs will be compressed an additional 1.25 inches. How does this extra compression affect the driver's subjective reaction to handling? At this point we get to the second important concept: the nature of human response to time-distance relationship.
It's Just Human
Back at our typical turn, we can watch more closely as our two drivers negotiate the corner. Recall that Joe Average went around the turn at 30 mph, or to put it another way, at 4 feet per second. Assume that as Joe enters the turn, the car takes 2 seconds to roll over, compress the outside springs and come to a steady-state cornering attitude. Likewise, at the exit of the turn, Joe's car takes about 2 seconds to unroll and get comfortably straight again. Watching Joe go through our 722 foot long turn, we see that it takes him 88 feet to transition into the turn, that he has 546 feet of steady-state cornering and another 88 feet of transition back onto the straight. Joe spends a total of 16.4 seconds in the turn.
Of these, 4 seconds (24%) are spent in difficult transitional cornering maneuvers, and 12.4 seconds (76%) are spent in relatively easy steady state cornering. This is why Joe feels that the car handles fine. At 30 mph, he spends relatively little time doing the difficult tasks of getting on and off the proper line and has a good deal of time in the middle of the turn to make corrections.
Enthusiast Driver experiences quite a different situation. He is going 45 mph or 66 feet per second. Remember that basic physics indicates that there is 125% more energy involved because Enthusiast is cornering 50% faster. Remember also that the greater energy causes the outside springs to compress an additional 1-¼ inches. Assuming a "linear" suspension response time, it will take Enthusiastic 4.5 seconds of transition at each end of the turn. He spends 297 feet of transition during turn entry (66 feet per second x 4.5 seconds), has only 128 feet of steady-state cornering 297 feet of transition at the exit. This is why Enthusiast thinks the car is unresponsive. He is in the turn for a total of 10.9 seconds. Of these, he spends 9 seconds (82%) of the time in difficult transitional cornering and 1.9 seconds (18%) in relatively easy steady state cornering.
The technical name for this phenomenon is yaw response. The yaw response characteristic of a car is the single most important of that group of traits we call "handling". The parameters for determining an ideal yaw response characteristic are derived from study of the human nervous system. The yaw response must be designed to make the driver feel comfortable at the speed he wished to go. It cannot be too slow or too fast. It must be slow enough so the driver can react to steering inputs; yet, it must be fast enough so that corrections can be completed before an off-road excursion occurs. The "base-model coupe" has been designed by GM with an ideal yaw response for Joe Average, who normally corners at about 0.300 g. If a driver wants to corner at some higher speed, then the suspension must be modified to provide an ideal yaw response at the higher speed. In other words, we must keep the yaw time, when expressed as a percentage of total cornering time, constant. To keep Enthusiast Driver comfortable going around our typical turn at 45 mph, we must modify the suspension so about 18% of total cornering time is spent in transitional cornering. This is what keeping yaw time constant means.
The Indicators Game
At this point we should scrutinize the primary indicator of cornering performance â namely cornering force as expressed in g's. At the present time, we all tend to focus on how many g's a car can generate. We normally equate high g-forces with good handling. Does this equation really hold-up? Sometimes it does and sometimes it doesn't. We have all read comparison road tests where the car with the highest cornering force was also the slowest through a slalom course. This happens because g-forces are measured on a skid pad, which tests only steady state cornering. The skid pad tells nothing about yaw response characteristics during transitional cornering. Yet, it is these characteristics which are the most important factor contributing to superior handling.
Whether high cornering forces translate to superior handling or not depends on the honesty of the suspension designer. If he designs a complete system which takes into account both transient and steady-state cornering, then yes, high cornering forces will mean superior handling. However, if he uses tricks just to yield high cornering force numbers and does not do his homework on the remaining suspension components, the no, superior handling will not result. Remember, skid pad results are just indicators; the real test of good handling is how your car performs over your favorite back road, in transition as well as "steady-state".
How can we judge the honesty of the designer's work? A good place to start is by examining how General Motors upgrades a base-model coupe into either a Z-51 Corvette, Z-28 Camaro or Trans Am Firebird.
A Lesson from GM
It is a myth that General Motors cannot design good cars. Their engineers and technical staff are among the best in the world. If you have any doubts just look at GM's impressive competition record. It includes numerous successes doing both officially sponsored corporate projects as well as countless backdoor efforts. You may remember their NASCAR efforts of the 50's, the Grand Sport Corvette, and the racing Camaros of the late 60's. From our point of view, the only problem is that these superior technical capabilities are generally used to produce designs for the average buyer rather than the true enthusiast.
When these talented engineers redesign the base coupe into a Z-51 Corvette, Z-28 or Trans Am, they treat the whole car as an interrelated system. They make detail changes to a wide range of suspension components. The Z-51 Corvette, Z-28 or Trans Am differ from the base coupe in the hardness of rubber suspension bushings, the rate of both front and rear springs, the diameter of sway bars, the ratio and feel of the steering gear, the width of the wheels, and the size and compound of the tires. It takes the combined effect of all these changes to maintain a balanced car, which (1) exhibits cornering forces in the 0.800 ranges and (2) has good yaw response characteristics. Notice that they change several of the major system components and not just a single item such as a front or rear sway bar. The total system approach produces a car with an integrated, balanced feel derived from all the parts working in harmony.
The other thing to notice is that all of the revised parts are stiffer than the normal part. The springs are stiffer, the sway bars are stiffer and the rubber bushings are harder (which is another way of saying stiffer). As we discussed above, they must be stiffer to deal with the increased energy generated by higher cornering speeds.
No Free Lunch
We need to understand one final point. Every given set of suspension components has a limited working range. As suspension components are modified for higher cornering speeds, the working range becomes narrower. Thus, the base-model coupe rides well at slow speeds, corners satisfactorily up to about 0.450 g and is uncomfortable at 0.650 g (145%). The Z-28 or Trans Am has a firm ride, corners well to 0.720 g and becomes difficult to control at 0.770 g (107%). A good combination street/slalom car has a very firm ride, corners well to 0.875 g and feels "edgy" at 0.900 g (103%). The typical racecar has no ride comfort, corners well at 1.200 g, and leaves the road at 1.210 g (101%). Notice that as cars are tuned to handle well at higher g-forces, low speed ride comfort is sacrificed.
Under current technology, the twin goals of pillow-soft low-speed ride and superior yaw responses at high cornering forces cannot be accommodated in the same car. The truth is that tuning the suspension to work well at higher cornering speeds always trades off some low-speed ride comfort. We must each individually decide how much low-speed ride comfort should be sacrificed for added high-speed cornering capability. As a wise man once said, "There is no free lunch."
So there it is, the real "straight talk" about high-speed handling. You won't remember all the details but you should remember the following three points. First, superior high-speed handling is more than just high cornering power. It must include a balance of both high cornering power and correct yaw response characteristics. Second, a car with superior high-speed handling is produced only by the systematic modification of a wide range of suspension components. Just bolting on a sway bar or some other part won't get the job done. Finally, as cornering speeds get higher, the suspension system must get stiffer in order to handle the increased energy levels.
Author, Dick Guldstrand
