When looking at the responses to my last blog post “Passive Overdriving” I saw a repetitive theme to many of the comments. People either confused or calling me out on an apparent contradiction. If it is so bad to drive mild understeer why do the same people to profess that set their cars up with mild understeer as the vehicles mechanical balance?
First we must understand that mechanical balance and dynamic balance are different things. Mechanical balance is the car “setup”, the springs the dampers, bars, roll centers, alignment all adjusted so when the car is in steady state cornering (for example on a skid pad) going a constant speed as we gently increase the speed when the car slides (this is a slide not a skid since it is intentional) it is the front that slides first. It is considered mild understeer if the rear tires are close to sliding when the fronts let go and gross or excessive understeer if the rears are nowhere near their limit when the front let go.
Dynamic balance is what the car does in response to braking and acceleration forces that are introduced while the car is cornering. Tires are load sensitive so when you are slowing and cornering or accelerating and cornering you are shifting grip forwards and backward proportionally to the end of the car you are “dynamically” giving the load. As you can see dynamic balance is set by the driver while the car is in motion. On the road done well it is a chauffeur able to get you from A to B serenely without spilling your drink on the race track it is a car that can effectively use all of the grip available (all four tires) for more of the corner making it simply faster.
Let’s pause for a moment and explain why it’s faster. If the car is set up neutral on the skidpad when we bump the speed it will slide all four tires at the same time and it will be doing so at a slightly higher speed than the mild understeer setup. Yes, mid corner it is faster but race tracks are not skidpads, they may and often do have sections in the middle of long corners where steady state cornering does exist (no brake and just enough throttle to maintain speed waiting for the corner exit) but the issue is you have to get the car there…dynamically and the neutral car is unstable therefore slower everywhere else which (and this is the key) more than offsets their mid corner speed advantage making it slower over all.
In simple terms the mechanically set up neutral car wants to oversteer/spin on entry and can’t put power down as well on exit so even though it is faster in that one steady state section of the tracks one long corner it loses more time one entry and exit everywhere else than it gains in the middle of that one long corner.
There is another issue with the neutral car, I mentioned it wants to spin and won’t put power down as well so it is much trickier too drive, it’s too “loose” and near it’s limit it will require a lot of cognitive bandwidth to simply keep it on the road. It will also put a lot of heat into its rear tires and overheat them relative to the front tires. You hear people sometimes say (or boast) about their “neutral” qualifying setup but actually it’s technically a closer to neutral mild understeer setup…but still mild understeer. Sometimes in qualifying it is hard to get heat in the tires quickly enough to take advantage of the tires tiny peak grip window so you can end up with strange setups specifically for a single lap flyer that you could never complete a full race distance with.
So, back to the initial point you want mild understeer in setup but you want too dynamically balance the car to not understeer when you are driving it. We do this with the brakes on the way into the corner (trail braking) and the throttle on the way out all relative to steering and how close we are to the cornering limit. We want the car to always be dynamically close to neutral so we are using all of the grip for as much of the corner as possible but the car is not trying to spin on its own we have to put it there due to its own natural set up balance is benign mild understeer.
Airplanes, rockets, ships, swords just about anything you can wield or control follows these same principles of inherent stability. On an aircraft center of pressure is always slightly behind center of gravity, same on a rocket (or an aero dependent race car) and a sword needs balance so that it not only responds when you swing it but more importantly stops or slow and turns precisely when you need it to. You don’t want anything to be inherently (set up to be) unstable. It simply makes that thing dangerous to use so you can’t effectively use all of its potential safely, repeatedly or for extended periods of time. We need inherent stability in the object so it retains benign predictability, that is an object we can hope to control at the limits of our own and hopefully its capabilities.
Speaking of hope, I hope this make things a little clearer and if you like this sort of thing check out my web site (theoptimumdrive.com) and of course the book I wrote that this is all built and based on Optimum Drive (available on Amazon). I also hope I managed to answer what is a very confusing statement: “Set you car up to mild understeer but then never drive it in mild understeer” I hope you now know why.