Crutches – Spring Rate Crutches by Bob Bolles There have been quite a few articles written about springs. It is very important to know as much good information about how springs are made, what affects springs, and how things like spring sag can affect your setup. But the big question is, once we have evaluated and rated our springs, how do we know which ones to install in our car for each race track? One thing racers do to tune the handling of their race cars is to continually change spring rates. Of course, you might say, how else am I going to setup my car to suit the current track conditions or find the right setup at a new track? This method of chassis tuning is all trial and error based. It is very time consuming and can be very frustrating taking many weeks or years to find the right setup. Also this method is prone to using the spring rates as a crutch to get the car to handle. What we know now is that for every condition and for every different race track there is a certain combination of springs that will be just what your car wants. Wow, what a concept. We’re going to give the car what IT wants for a change. Instead of forcing incorrect spring rate combinations on our race car, it is much better to figure out what exact combination of springs it needs to produce the fastest and most consistent lap times. We will explain how to find that perfect combination, but first let’s study how spring rates affect the chassis and learn how some combinations can hurt race car performance. Common Crutches related to springs are: 1. A stiff right front spring to control shock travel. We’ll look at why we had to do this in the first place and see if we can do better. 2. A soft right rear spring rate. Usually this can help give us bite, but is mainly connected to increased rear steer that in-turn produces more bite, but in reality it hurts our turning performance. 3. Low rear spring rates – not necessarily bad if other settings are adjusted to compensate for the low rates and decreased resistance to roll. 4. Improper positioning of springs to facilitate race car design. The springs may be mounted inside the chassis to make way for other components, but this may produce a narrow spring base in some cars which hurts the overall turn performance. To begin to understand what we need to do to get away from these crutches, we need a basic understanding of how the combination of spring rates can affect the chassis performance. One new thing we’ve learned over the past few years is that the race car FEELS two separate suspension systems. Each system has a mind of it’s own, so to speak. That means that each wants to do it’s own thing. They each have their own spring rate combination, weight they support, and unique way that they are each put together. The two systems are joined by the chassis, true, but each still retains it’s own characteristics. The most important goal you must have in order to find the best setup for your car is to try to help the two ends of the car work together. If you can do that, the car will be faster, more consistent and handle better for a longer period of time. And, we all agree, those should be our primary goals. The term "working together" means that as the car runs around the turns, each system will want to roll to it’s own specific angle to the track. That is because each end of the car has it’s own axle system, supports it’s own weight, and has it’s own combination of springs. Because of the forces exerted on it, each will want to roll, physically, to a specific angle in relation to the track surface. This is a fairly new way to look at race car setup. It is, in fact, a patented method of analysis. It was developed by trying to understand what each race car wants, instead of forcing our individual setup ideas on it. But what if the two ends aren’t working together? How will I know? There are some basic telltale signs to look for. Without getting too technical, let’s try to understand how this works. There are two conditions of forces which together determine the roll angle a suspension will want to achieve. The first is the lateral force which tries to take us to the outside wall. The other is downforce produced by the track banking angle which tries to pull the car down. The greater the track banking angle, the more downforce that exists. When we combine these two effects, we get a true desired roll angle that is a result of the spring rates we have installed in our car. The most common condition is when the rear suspension wants to out-roll the front. Very soft rear springs, a low panard bar, and/or a rear spring split with the Right Rear spring rate less than the Left Rear spring rate all work to produce a higher rear roll angle. If the front roll angle is less than this rear roll angle, the car will push and be inconsistent. Let’s assume the above is our car. As we roll through the turns, the rear may want to roll to say four degrees and the front will only want to roll to two degrees. In reality, because they are joined by the chassis, the car will roll to an average of the two desired angles which is about three degrees. Neither end will be where it wants to be. Here is what happens: The Front – It wanted to roll to two degrees, but it was forced to roll to three degrees. At two degrees, it would have transferred the right amount of weight from the Left Front tire to the Right Front tire. But when it rolled farther, more weight was transferred and the Right Front tire now has to support more of the front load causing it to work harder. The Left Front tire likes it because it doesn’t have to work as hard. It has less weight on it, less tire temperatures, less traction and as a result, the car doesn’t turn as well. The Rear - In the rear suspension we see the opposite. It wanted to roll to four degrees, but only was allowed to roll to three degrees. The correct amount of weight transfer would have occurred at a four degree roll angle, but at three, less weight transferred. More weight was retained on the Left Rear tire. There are two major problems that come from this very common scenario. With more mid-turn weight on the Right Front and Left Rear tires, the car will experience a higher cross weight percent and will be tight handling. You must then reduce the static cross weight percent to compensate in order to make the car handle neutral in the corners. The other big problem is consistency. Each suspension system will "fight" the other trying to achieve it’s own desired roll angle. This struggle is exactly why some cars are very inconsistent in handling when you run a different groove or the track conditions change. The true picture of this car is that it is racing on less than four tires. The Left Front tire is being used at maybe half of it’s capacity. Less overall traction results in less turn speed. It’s that simple. We only need to look at some of the dirt cars that lift the Left Front tire off the ground to understand this principle. Those guys are racing on exactly three tires. Many automotive engineering experts agree that the ideal situation to produce maximum traction and handling consistency is to have equal weight on the left side tires, front to rear, and equal weight on the right side tires front to rear after the weight has transferred as the car turns. This produces the ideal balance of traction for each wheel. Note that the left side tires will not necessarily have the same weight on them as the right side tires. That is a product of the amount of static left to right side percents of weight. The only way to achieve this ideal weight balance is to have the correct combination of spring rates, roll center locations front and rear, and weight distribution that will all serve to work together. If the correct spring rates have been used in relation to the design of the car, the ideal weight transfer situation can be achieved and all four tires will be working for you. The easiest way we do this is to use a computer program that will calculate the front and rear roll angles in our car for a given combination of springs, etc. The program will ask for the weights of the car, some simple measurements and information about the race track. It then simulates the dynamic condition of the middle of the turn. It will calculate a predicted roll angle at each end of the car. Simply change the roll angles by installing different springs, raising or lowering the panard bar, or adjusting the weight distribution. You can simulate what the car will do before you go to the track. When you get both ends working together, the car will be very close to the ideal setup. The two ends will be balanced for roll angles at all attitudes, turn entry, the middle of the turns, and on exit. You will still need to tune for entry braking and exit acceleration with shock combinations and rear "bite" devises, but the chassis will have what it wants for spring rates in combination. Once you find the ideal setup for your car, the tire temperatures will be more even on each side front to rear. The tires will wear longer and more evenly. And, best of all, the handling balance will be more consistent over a wider range of track conditions. |
RMDCRA
Rocky Mountain Dwarf Car Racing Association
RMDCRA
| Colorado |