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Thanks Jack for the great information. Do you have books you would recommend reading about suspension setup and handling?Jay,
Changes to the swaybar stiffness at either end of the car make relative changes in the handling. If you double the front swaybar stiffness, the car is going to understeer more, relative to what it did before. It could still be oversteering, but just much less than it was. My comment about the Eibach front swaybar and stock IRS rear swaybar handling balance has a number of other assumptions. Weight distribution of the car, front/rear wheel rate ratio, etc. If any of these are different, the car could oversteer or understeer absolutely with that swaybar combination.
Without understanding slip angles, it really isn't possible to understanding handling or really drive a car fast.
Slip angle is the difference in angle between the direction the tire is pointing and the direction that the tire is rolling on the ground. Anytime the car is cornering at all, there is an angle developed. The harder the cornering force, the greater the angle. The two links below provide a good description with one exception. When the slip angles gets too high, the maximum amount of cornering grip that the tire can develop reaches a peak and then starts decreasing past this peak. The graphs at both show the grip staying constant.
Slip angle - Wikipedia
en.wikipedia.org
It is the driver's job to drive the car such that the slip angles of the tires are at the maximum grip point. If the slip angle is higher than that, there is less grip and car goes slower. It is important to understand that anytime the slip angle is lower than the angle for peak grip, the tire is not sliding in the conventional sense. In other words, it isn't leaving black marks on the ground.
How does the driver feel the slip angle? A couple of different ways. The primary way is through the self aligning torque. See graph below.
Slip angle versus grip is the red curve. The blue curve is the self aligning torque. This the torque required to turn the tire to keep the car on its current heading. This torque develops because the rubber at the contact patch is being deformed as it touches the ground and then rebounds in a different direction the instant it is picked up at the back of the contact patch. Stretching the rubber requires work to be done, thus the torque. The higher the slip angle, the more that the rubber is deformed and the greater the torque, up to a point. If you drive the car to the right of the peak in the blue curve, then the steering starts to get lighter, because more and more of the rubber is actually sliding on the ground, so even though the slip angle goes up, which requires more torque, the total amount of rubber being deformed is dropping at a higher rate, so the total steering torque is decreasing. When you get to the slip angle where maximum grip occurs, more or less the entire tire is sliding and the steering gets much lighter. This change in self aligning torque and the grip changes is how the driver can feel what the front tires are doing. Every driver, with any cornering experience whatsoever knows this through experience. This doesn't mean that they can describe it. It how a good racecar driver can jump into a car they have never driven and get 99% of its speed out of it in a few laps. They don't have to spin the car 10-20 times to find out where the grip limit is. They can feel it, even below the limit.
How does this affect handling balance?
Oversteer is when the slip angle of the rear tires is higher than that of the front tires. Understeer is when the slip angle of the front tires is higher than that of the rear tires. See the image linked below.
The above image doesn't really make sense until you add some more lines to it. See below.
In the above image there are two pairs of lines added. The two dotted lines are drawn at right angles to the direction that the tires are pointing. The intersection of these two lines on the left is at what is called the turn center. This is the point is space around which the car is turning. This part of the drawing is inaccurate for deformable tires. This is because any deformable tire will develop a slip angle whenever it also develops a cornering force.
The two solid lines in the sketch are drawn at right angles to the direction that the tread at the ground is rolling. Thus they are affected by the slip angle. The intersection of them generates another turn center, which is an accurate one. You can see that if the rear slip angle is much higher than the front slip angles, the turn center will be located further forward. If the opposite is true, the turn center will be located more rearward. If one looks at a 4WD car doing donuts, you can see that they oversteer so much so that the turn center is way in front of the car. In some cases, the car is actually pointing at it!
Different types of racecars operate at different slip angles. This is mostly a function of the tire construction. F1 cars operate at low slip angles, so they more or less seem to be going the direction that the front tires are pointed. When one gets overdriven in a corner and the slip angle goes past the optimal angle, you can then see the car get a lot more sideways. NASCAR cars with tall, soft sidewall tires have a much larger slip angle. On a road course, the slip angle in some corners is nearly 10 degrees and is very visible.
This should explain how you can determine the handling balance of the car, even when it is driven at significantly less than than its maximum cornering potential.
