Tight, loose, understeer, oversteer, other malarkey

On Matt's start up blog, he has a few comments on under/over-steer and what he's been looking at in DAQ to sort it out. Talks more about analysis than how balance is actually defined. For as much as "we" (engineers, drivers, fans) talk about balance... and if a car is tight or loose, understeer or oversteer... I think it's pretty damn difficult to define, and a lot of the "textbook" definitions are very incomplete.

Part of it, as Matt mentions, is limit trim - which axle's force capacity saturates first, and is the result a car that "plows" (sideslip angle and yaw rate saturate reach and asymptote) or "spins" (yaw rate increases and sideslip angle takes off to some undesired value.. on the order of 180 degrees!). Even then, you could break that up into limit trim of pure cornering... brake-in-turn... on-throttle... high- and low-speed, etc. Furthermore, racecars are not at the grip limit all the way around a racetrack. Ultimately as a setup engineer (or driver) your goal is to minimize time spent over the entire lap. What about the large radius corners and bends where you're not grip limited? Esses? Corners you barely have to brake for? The part of corner exit where the throttle is at 100%?

I'd say most drivers would attest you do not need to get to 100% of the lateral limit to feel something about balance. What's the sensation, and what's good or bad?

Is it yaw (sideslip) angle? I think a lot of people, myself included, immediately associate oversteer with huge body angles and drifting (such as with the Apikol car pictured to the right). If for some given speed and corner radius tire set A requires front slip angle = 2° and rear slip angle = 2°... but tire set B requires front slip angle = 3° and rear slip angle = 3° ... the difference front to rear in both cases is 0. The steering angle in both cases will be the same. There will be a 1° difference in sideslip angle between them, but is that something we notice? Besides, think of the times you've done some power-on oversteer on fresh snow. You can get the car to big body angles, but is the car loose? With "loose" I think of a car that's very hard to control, twitchy, and easily gets away from you. I can drift around on a snowy parking lot for a long with nice easy-to-control, predictable motion. Sideslip angle gradient by itself... insufficient.

A textbook (RCVD) definition of understeer might include difference in front and rear slip angles, the rate they build up, and deviation of actual steering angle from the Ackermann steering angle (not to be confused with Ackermann steering geometry). That might work if you have the same tire on all 4 corners of the car. What if I have really stiff, low-grip tires on the rear... and really soft, high-grip tires on the front? Initially it will take a lot more slip on the fronts than the rears ("textbook understeer") but eventually the car will spin out. Slip angle difference by itself... insufficient.

Claude, from what I recall, isn't as interested in angles as much as reserve yaw moment capacity of the car (really a measure of if the front or rear axle has more grip left - more "headroom"). While that does give more insight to limit trim, I can think of examples of different cars that have identical reserve yaw moment capacity but drive completely differently in terms of stability and response because of tire curve differences. Reserve yaw moment capacity by itself... insufficient.

There's one thing that I think is pretty indicative of both limit and sub-limit balance, at least in a steady state case. Not going to give it away, but it's related to curvature rather than slope.

In any event, all of these are basically related to constant speed, neutral throttle, quasi-steady state cornering. Not to mention combined slip or transient behavior which adds significantly more complexity. Just more evidence that this shit isn't as straight forward as it might seem when you start to wrap your head around it... and there can be a lot that's really based on opinion and personal experience.