What you're taught in college vs. what you're going to work on

I have to admit, I enjoy perusing the interwebs. Sometimes you come across some really great gems of information (case in point). Other times, shit is just disappointing. Can't even make this stuff up, here is a recent post from FSAE.com

i am new member of fsae. i wud like to know what be the optimum values of
1.toe angle (rear and front for a rear wheel drive car)
2.caster angle
3.camber angle and shud all the wheels have same camber??
4.kpi
5.caster trail
6.scrub radius
plz reply soon
its urgent

There was also a thread on F1 Technical asking about benefits of a pull-rod suspension in comparison to a push-rod setup as well... not nearly as bad but generally oversimplified. Point of both of these is that there generally aren't all-encompassing answers to problems, even if problems in undergrad engineering tend to lead you to one precise solution, arrived at by some fixed process. I think most people grasp that things will be more difficult in industry, but perhaps the true scope of which isn't put in the right magnitude.

Allow me to illustrate!

Classroom Engineering Problem

"Find the minimum of this function."

Not too difficult! Illustrates a concept (minimization / optimization) in a pretty straight-forward manner. The problem is well-defined, and even if you're not 100% sure of the best way to solve it, you can fake it. Alternatively, you can just copy the solution off someone, or consult The All-Knowing Oracle.

FSAE-Level Engineering Problem

"Find the minimum of the function with respect to x- and y-. No worries, you have a few months to work on it."

Shit's starting to get real. Not quite as simple and straightforward if you're used to looking things in a purely one dimension manner of y = f(x). You'll get a different answer looking at it along one slice of x as another. Furthermore, if you ask different people how to go about analyzing it, you might start getting different answers. Lay a few line plots on top of each other at discrete input values? Plot a surface?

Harder to "brute force" these problems too. Takes some time to run through thousands, or tens of thousands of combinations of toe, camber, spring rate, ride height, etc. Overall it's manageable, and ultimately you're competing to be less of an idiot and have your shit held together by fewer zip ties than the competitor in the paddock stall next to you - who is asking for 1" chro-moly tube and a welder, after the first round of tech inspection.

Real World / Pro Motorsport Engineering

"Here's a rough idea of what you're looking at. Try and figure out what the problem is, because it's not really even defined. The number of variables you have to worry about is somewhere between 1 and infinity - depending on who you talk to - and the people who claim they have all the answers are usually full of shit. You have 5 minutes to find the global minimum, and at some point between 2 and 4 minutes in, the problem is going to change. Also, you will lose partial credit for every minute you take to solve the problem, even if you get it entirely correct at the end of your allotted time. Try to score the most points."

Ouch. The problems really are that challenging, open-ended, and undefined. There are generally dozens of theories and opinions on how to solve the damn thing, and the people who are probably the best source of an answer are the ones you're in competition with.

It's really challenging to prepare for this sort of work. Can attack the problem a number of ways and get different answers every time, depending on where you start and what you feel is important. Then there's always the feeling that's similar to what inevitably goes through your head during at least one exam you've taken...

Oh hell... I don't know any of this shit! Any confidence I had just flew out the window. Fuck. I wonder if anyone else is as clueless, let me take a look around... OK good we have some other blank stares... who are these people who are already furiously writing answers?! Oh well, guess I'll write something down and hope for the best. So long as everyone else fails out I will look brilliant by comparison!

The point of all this is what I feel is important to teach in engineering. Equations for principle stresses? Guess they're good to know, but you can look those up in a book at some point in the future. More so, how to approach problem solving when you don't know the answers, the process, or even the problem. That's what engineers and scientists get paid to do - pull answers out of chaos, and/or data that's typically shitty, insufficient, and can be interpreted in many ways. Some institutions, and seemingly even areas of the world, emphasize this more than others.

It's another reason why FSAE teaches you really good life skills - beyond just cooking with a blowtorch or heat treating oven (done them both!).

And it's official...

After almost 4 years of being a race tire data engineer at Goodyear, I am hanging up my hat. Next stop - Penske Racing.

Time for a change?

Some decisions in life are easy... like how to determine relative amounts of ride and roll stiffness on a race car to minimize sprung mass force transmissibility while maintaining high levels of yaw response and stability. Others decisions are much more difficult - for example whether to make chicken parmigiana for dinner, or sausage and peppers. Also, whether I want to stick around this part of the country and this place of employment... or take an offer at a very well-established race team in North Carolina.

Decisions...

New way to set target roll stiffness

How the hell are you supposed to come up with a starting point for designed roll stiffness of a racecar? There are ballpark figures given in RCVD, which I'll admit we had used before (in FSAE) and even a couple years ago (wow! Here, and here) when I did some real preliminary estimates. Ballpark values are nice for reference and a sanity check, but you should know by now - that's not how we do legit engineering.

Let me tell you - this is how we do it:

(Not really, this song is awful)

What do we (relatively young, relatively inexperienced engineers) generally associate with changing roll stiffness? I'll admit - turn in responsiveness (overall stiffness) and balance (front to rear balance). I'm not ashamed to own up to it! It works - it's the right association - but not entirely for the right or really complete reason.

If you recall my earlier thoughts on top-level engineering, in my mind there's only a one step difference between a rigid body model with instantaneous load transfer, and an elastic sprung mass model with delayed load transfer. In effect, as you put stiffer and stiffer springs (and eventually hard links) in your suspension, you're just turning your car into a rigid vehicle - like a go-kart. As such, that rigid model is the upper bound on cornering or turn-in responsiveness (fine, maybe barring some roll-steer shit and what have you). That is not to say that it's instant, because there is still only a finite amount of yaw acceleration and there's definitely yaw inertia, but it is an upper bound.

As we add a suspension and start softening the car up, it's obviously going to add a degree of laziness to the rate of load transfer. In addition to having to wait for the car to start yawing and building lateral acceleration, we also then have to wait for the sprung mass to roll, engage the springs, and transfer load across each axle. Less roll stiffness (or more roll inertia) makes the car lazier and lazier to sharp steering inputs. In one of this SAE papers, Chuck Hallum seems to mention that he thinks (thought - RIP) a conventional tire model doesn't necessarily show this, but I disagree given the rudimentary sim outputs below. I've picked a certain parameter which indicates responsiveness, and have removed the actual numbers - do your own work.

As we reach "stupidly stiff" spring rates, we get close to bumping that response limit which ultimately is a function of yaw inertia and tire properties. Want to raise it? Get new tires, or more downforce. It is absolutely eye opening when you do back-to-back tire testing (FSAE kids take note). The math behind it isn't too difficult and is both in RCVD among the concepts of stability and control derivatives. Conceptually it's really not too difficult to grasp. You can keep adding spring to car and eventually there's a limit of what it does (likewise with chassis stiffness). The trick is finding that limit.

How does this tie back into design? By doing some up-front engineering I can determine what that upper response bound is, and how quickly I approach it. From there I can say I want a responsiveness level of 'X' by a variety of ways - even if I say I want the car to be within 10% of the rigid body response. Once that target level is established, the required roll stiffness falls out, and from there the appropriate levels of spring and/or bar. QED. Science: It works, bitches.

SAE Papers - Hit or miss

Fairly on-topic.

Perhaps you're like me - in that your formal training in tire & vehicle dynamics pretty much amounts to nil. To a degree I'm almost happy that's the way things turned out. Forces you to think for yourself in a field immersed in hand-waving, subjective experiences in a narrow scope, and bullshit.

Anyway, the Society of Automotive Engineers website is a nice resource for being able to dig through their massive collection of member-submitted technical papers. Authors range from college students to industry professionals, and topics can be on aerodynamics, vehicle handling, drive train, brakes, design - you name it. Good to peruse when looking for extra bits of knowledge, experience, or information. In fact, I'm even listed as an author / contributor for an upcoming paper. Go me!

Unfortunately the content of the papers is a bit "hit or miss." Can be good, can be crap (don't worry, the paper I'm on isn't crap). Really have to exercise extreme caution in what you take out of some of these papers, given that seemingly a lot of them are based on opinion and theory without a tremendous amount of hard, factual, objective data to back it up. As such, one of my rules of thumb is to avoid papers written by FSAE students. Still worth checking out. If you're an engineering student, I'd say there's a good chance that you can get copies of the papers for free at your engineering / math library. If you're an employee of a large automotive OEM or supplier, chances are they are easily available as well.

Some examples:

• 901734 - Four Parameter Evaluation Method of Lateral Transient Response. I would seduce this paper with sushi and martinis if I could (and if I wasn't already engaged to MATLAB - see below). For a 20+ year old paper, I think it's gold. Domo arigato, Tetsushi sama.
  
• 2002-01-3302 - Dynamic Traction Characteristics of Tires. I have a lot of beef with some of the content of this paper, and I feel that it is generally poorly written without much of an "objective engineering" tone.
  
  In particular, I don't agree with Chuck's assessment of heat generation in tires, and that "improved turn-in" response by adding (front) roll stiffness is contradictory to "classic" (Pacejka) tire force curves. Seems to be ignoring the effect of chassis roll and time-delayed lateral load transfer on yaw response and how quickly the system settles. This is something I'll probably cover with my own thoughts on a later date. Suffice to say, my opinion is that adding roll stiffness (front or overall) moves you toward approximating a rigid body and tire-limited yaw response.
  
• 1999-01-0046 - Roll Centres and Jacking Forces in Independent Suspensions - A First Principles Explanation and a Designer's Toolkit. Pretty good, found this today. Well-written and I think describes lateral load transfer and roll centers well. Also seems to agree with my thoughts here, particularly that jacking forces only arise in a geometrically symmetric suspension when lateral tire forces are asymmetric. Pretty good feeling, makes me feel less like an idiot alone in the world!

There are several dozen other papers of varying degrees of value that I have squirreled away. If you're willing to do some data mining and look for buried gems, go to the website and start looking.

Off topic - how are you people being notified that I post?

Any time I put up a good entry or two I get a pretty big spike in traffic - as one would expect. However, of all the blogs I "follow," I don't get any email notification or anything if there's new content. Just have to check it at random intervals or if I see something new on Pulse. Is there some trick setting I'm missing to be notified?

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.