It is important to know your sway bar rates in order to relate that to the setup you desire. There are problems associated with looking up the rate in a table or other publications because of several factors that affect the working rate of the bar. We learned of a method that takes into account all of the variables to provide the true working rate of any sway bar.
This all started when, in my spare time, I was helping a friend who owns and manages a Grand Am DP team. The sway bars on those cars can be quite complicated to calculate the rate for because of some unknown factors such as the exact hardness of the metal used, the effectiveness of the blades on the ends of the bar, motion ratios that are non-linear (change with motion), and more.
I did some research and came across a method described by James Hakewill in a July 4, 2006, publication on his website. He is a driver who races formula cars. I convinced the team to go through the process of measuring the rates of all of the combinations of sway bars. We modified the method to suit our car and were able to identify several problems with the current bars we had been using in combination with the blades.
I'd made a significant discovery here, and I immediately related this experience to stock cars. We have the very same situation with not really knowing our true bar working rate. So, I took the method over to a local race shop and proceeded to rate their sway bars on a typical Late Model Asphalt Touring car. What follows is a detailed account of how I did that and, of less importance, the results.
What Affects The Rate of The Sway Bar? There are several things that affect the rate of a sway bar, or more accurately said, the spring rate at the end of the arms related to how that affects the overall wheel rate. The sway bar resists chassis roll and with some of the more popular setups, minimizing and/or eliminating chassis roll is a desired effect helped along by using large-diameter sway bars. Here is a list of what can affect the rate:
1. The length of the portion of the bar that will twist with chassis roll affects its rate.
2. The outer diameter of the bar affects its rate. The larger, the stiffer it will be.
3. Hollow bars are less stiff than solid bars and the wall thickness affects the rate.
4. The arm length affects the rate, the longer the arm, the softer the bar rate.
5. The material the arm is made of ultimately affects the rate if it istoo soft and bends.
6 The material the bar is made of, or the hardness of the steel, affects the rate. There is a modulus of elasticity for each type of steel that depends on the mixture of metals and the hardening process used in the manufacturing process.
7. The installation ratio affects how the bar rate is translated to the wheel rate.
On the DP car, being a prototype means that the systems are necessarily made very compact
The blades on the DP car can be turned to provide varying amounts of resistance to bending
These are the seven variables that affect the amount of work the bar does and there could be more in some applications. What this means for us is that trying to calculate the rate of the bar and how much work it does is very subjective at best.
The method we used here necessarily takes into account all of the variables and any we did not mention. It is fairly simple to do and is very accurate. Here is how it works.
Overall Principle of the Method What we are going to do is cause the sway bar assembly to support the weight of one corner of the car. In our Grand Am car, we had sway bars in the front and rear, so we had to go through this process for both ends of the car. On a stock car, we usually only have one bar up front. We should run through this method for each bar we will possibly use.
We chose to use the Right Front corner to rate the bar. Sitting at ride height, the RF wheel supports X amount of sprung weight of the car. If we can cause the sway bar to act as the spring holding up that corner, then there will be a deflection of the wheel and we can measure the amount of deflection and convert that to a spring rate in pounds per inch, much like the normal spring rate at the wheel.
In the stock car world, sway bars are getting larger. The use of stiffer bar rates means w
Make sure your attachment link at the right lower control arm is perpendicular to both the
We installed solid links in all four corners of the car in place of the shocks. Make sure
Once we find the wheel rate of the bar, we convert that rate back to the end of the arm using the sway bar installed motion ratio and we will then kow the true bar rate at the end of the arm. This rate could be close to the calculated and/or published rate for that particular bar, or it could be much different. In any case, this method cuts through all of the unknowns and gives us the true numbers.
Getting Started We start off by doing the following steps to prepare to take the measurements we will need to determine the sway bar wheel rate.
1. Install the desired bar and arms and leave loose to the lower control arms. Make sure that the link attaching the bar to the right lower control arm is perpendicular to both the sway bar arm and the lower control arm. This is very important and can affect the motion of the bar.
2. Remove the front springs and support bot of the front corners of the car with solid links in place of the shocks. Make sure that the material used for the links is strong enough to support the weight of the car. We usually use tubing of0.75 to 1.0 inch in diameter with nuts welded onto the ends and Heims attached with opposite threading so we can easily adjust the length without removing them.
3. Support the rear of the car at one point, usually using a piece of angle iron placed on top of a stack of wood or other medium and placed at the center of the frame preferably behind the rear-end and halfway between the rear wheels. You will need to support the rearend using either solid links like at the front or by placing ride blocks between the axle tubes and the frame if it is a frame-under car. You can remove the wheels to provide more room for chassis roll.
4. Place the front wheels onto scales and record the RF weight.
Before taking any measurements or weighing the car, lock the steering shaft with the wheel
You will need to support the rear of the car at one point in the middle between the two re
Before getting ready to support the car on the sway bar, snug the left arm up against the
5. Support the frame at the RF on a jack and remove the link. Record the weight of the wheel assembly, or unsprung weight, of the RF wheel. We only want the sprung weight number when calculating the amount of deflection related to the load the bar will support. Move the assembly up and back onto the scales several times to take the friction out of the pivots.
6. Subtract the RF unsprung weight from the total RF weight to find the sprung weight that the bar will support. Record that number.
7. While the link is out and the load is off the RF wheel, we will measure the shock motion ratio. Measure the shock length (the distance from the center of each of the mounting bolts) for the shock with load on it.
8. Place a flat block of wood orother material under the RF tire.Remeasure the shock length. Divide the amount the "shock" moved by the height of the block placed under the tire. Example: Ifthe block were 2.0 inches thick and the shock moved 1.0 inch, then our motion ratio would be 0.500. We will use this number later on.
9. Remove the block, put the link back in place, remove the jack and let the front wheels rest on the scales. Snug up the sway bar arms against the left lower control arm being careful not to pre-load the bar. Record the RF ride height from the frame to the floor. We will need to maintain this height throughout the test.
10. Record the weight on the RF scale pad to make sure it is the same as in step No. 4. Jack up the RF corner carefully and remove the link. Let the RF corner back down onto the scale so that the sway bar is supporting that corner's weight. The frame will be lower and we need to get the ride height back. Measure the RF ride height now and subtract that number from the static ride height we measured in step No. 9.
11. Shim under the RF tire the same amount that the RF frame dropped so that we will be back to the original ride height and no load transfer from chassis roll will have taken place. The scale weight should be the same as in steps No. 4 and No. 10.
12. Measure the new, compressed shock length. Subtract the new shock length from the original static shock length and divide that number by the shock motion ratio we found in step No. 8 to find how much the wheel moved (wheel movement) and record that number.
13. Divide the sprung weight of the RF corner by the amount the wheel moved to find the Sway Bar Wheel Rate. Example: If our RF sprung weight were 600 pounds and our wheel moved 1.0 inch, then the sway bar wheel rate would be 600 pounds/inch.
Finding The Bar Rate We now have the wheel rate of the bar. This is not the last step in the process. We need to know the bar rate at the end of the sway bar arm. Here is how we do that.
1. Measure the length of the lower control arm. This is the right angle distance between the center of the lower ball joint to a line drawn between the two chassis points, or pivots, for the lower arm. This is known as the rotational radius.
2. Measure the distance, parallel to the rotational radius, from the center of the lower ball joint to the center of the point that attaches the sway bar arm to the right lower control arm. If it is a Heim joint, measure to the center of that.
3. Subtract step No. 2 from No. 1 and divide by No. 1. This is the sway bar motion ratio. Example: The lower control arm rotational radius is 19.0 inches and the BJ to sway bar mount distance is 3.0 inches, so the sway bar motion ratio is (19.0 - 3.0)/19.0 = 0.84211.
4. To find the sway bar rate at the end of the arm, we divide the sway bar wheel rate by the sway bar motion ratio squared (step No. 3 times No. 3). Example: If we have a sway bar wheel rate of 600 pounds/inch and a sway bar motion ratio of 0.84211 (this number squared is 0.70914), we calculate the sway bar rate at the end of the arm to be (600/0.70914) = 846 pounds/inch.
What This Tells us Once we find the bar rate, we can relate that to our setup. In our case, we found that the arms were bending excessively affecting our bar rate. A 1.75-inch diameter thick wall bar that should have rated around 1,900 pounds/inch or so was only rating just over 900 pounds/inch because the arms were bending so much. The larger the bar we installed, the less the rate went up because the arms were bending more by taking more load. The bars were doing less work.
The arms became a spring with a much lower rate than the bar. When we combine two springs in series, that affects the overall rate and in this case with the much lower rate of the arm, we recorded a lower bar rate. This could be happening to your car. In our case, we need to change to a stiffer arm in order to get the full benefit of the larger sway bar.
Accuracy Versus Results In any routine, we can only expect the results to be as accurate as the effort we put into making sure all of the steps are done correctly and that no other influence affects the results.
The maintenance of the ride heights is important because when the chassis rolls, the center of gravity moves and there is load transfer taking place. This skews the results of our test and gives us false numbers.
Make sure you zero your scales before taking any readings and lock the steering shaft so the wheels don't turn. This could affect the results. Also take your measurements a couple of times to make sure you read the tape correctly. Take your time in all phases of the test and think out the process to maintain accuracy.
When you take your measurements, take your time. Get down at right angles to the tape and
We read 530 on the RF scale pad with all of the unsprung and sprung weight on it, includin
We read 530 on the RF scale pad with all of the unsprung and sprung weight on it, includin
Conclusion What you find for rates in your sway bars might be what you expected or not. This is why we go through this type of exercise. We need to know every influence that affects our setups. In the case of the Grand Am car, we found that the rear sway bar was very soft with the components that were used, and so we stiffened it and the result was a much more balanced car.
You can go to our website and under the heading, "Multimedia" you will find calculators that include a sway/torsion bar calculator. Using that, you can quickly find what your bar should rate and compare those numbers to your test results. The coefficient of elasticity, or the stiffness of the steel factor, used in the calculator is the average for hardened steel. Your bar stiffness factor may vary from that either higher or lower. NASCAR-style three-piece bars tend to be stiffer and stock-type one-piece bars tend to be softer.
In any case, you now know how hard your sway bar is working and you can make some decisions about how large a bar to use or if you need to re-evaluate the arms you are using. Information is the key to being able to properly plan out your overall setup.
Mark prepares our car for testing the true rate of the sway bars at both the wheel and at the end of the arms. This method is both easy to do and very effective and accurate. The results might just surprise you.