SAE CO414 Applied Vehicle Dynamics

mazdeuce - Seth said:

In reply to spacecadet :

Almost forgot, there are two drivers/engineers from a major tire manufacturer. We were discussing tires (because fundamentally all vehicle dynamics is about tires) and a bit about contact patch shape and lateral vs. longitudinal grip. They said that the manufacturers can and do change contact patch shape with the carcass construction. That means that they will test two tires the same size with the same rubber compound that will give different responses laterally and longitudinally and often times the OE version of a tire is different than the consumer version in the same size because of this. That information mostly made me to be a billionaire so I could have bespoke tires made that perform just the way I want them to. 

That's cool, you need to meet my buddy Dave at OLOA. He and his dad are running a GT3. Dave works for a major tire manufacturer here in TX as a test driver.

On your multiple carcass comment.

The BFG rival S is a time where we got it to our advantage and see it in real life. The Rival S vs Rival S 1.5 was autocrossers and track guys complaining about how numb the response was from the original Rival S vs the RE71R and so BFG went back on the small sizes and added more sidewall stiffness to make it better.

I can't feel the numbness in the Audi or the focus. But I do love that glorious BFG slip angle.

spacecadet said:

But I do love that glorious BFG slip angle.

You love the lateral force that the BFG creates at relatively high slip angles. 

Tires generate more lateral force (grip) as slip angle increases - up to a point - where the grip saturated and the tire starts to slide. A tire with a lateral force vs slip angle curve that plateaus at peak force will be more forgiving at the limit (or over the limit) than a tire with a sharp peak at peak lateral force. 

(I work with Dave, but not in TX. Small world!)

collinskl1 said:

spacecadet said:

But I do love that glorious BFG slip angle.

You love the lateral force that the BFG creates at relatively high slip angles. 

Tires generate more lateral force (grip) as slip angle increases - up to a point - where the grip saturated and the tire starts to slide. A tire with a lateral force vs slip angle curve that plateaus at peak force will be more forgiving at the limit (or over the limit) than a tire with a sharp peak at peak lateral force. 

(I work with Dave, but not in TX. Small world!)
 

Dave is a cool dude. Also yes, all the tire technical jargon. I love the slip angle the BFG lets me get away with. 

Today was fun. Too much information to type out in the few minutes before I go to sleep. I will tell spacecadet that I think some very smart people explained to me why the TT was freaking out in autocross. Cars with electronic stability control are magic as long as they're happy. 

For now, this is the "road course" side of the facility that we get to play on. 

And off to the right of that is a big black lake that's the same square footage with a polished skid pad at the end. We got to do fun stuff like spin cars on purpose. 

In reply to mazdeuce - Seth :

Yes, the TT freaks out if a wheels gets off the ground because it doesn't know what to do with all the electronics that control the AWD system. 

Looks like you're learning a lot. We'll have a lot to download when you get back. 

Lots more driving today giving a lot to think about in relation to testing. It will certainly help me develop cars in the future. We also got drift rides from the BMW instructors. Anyone who has rallycrossed a RWD car is familiar with a winding course of tail out, but the forces involved in doing it on asphalt are something else. 

I have a lot to write about. The class structure, the teachers, the rest of the students, content and how valuable I think it is. Tons. After three days I think the class was worth it for me. I have an understanding of vehicle dynamics that I didn't three days ago, and more importantly I learned it in an environment filled with curious people and knowledgeable teachers. A good course is more than some guy reading power point slides to you. A good course uses those slides as a step off for further explanation, discussion, and wide ranging side tracks. 

The plan right now is to get on a plane tomorrow morning and go meet my family in New Orleans for the rest of their spring break. That flight goes through Denver however, so we'll see how that works out in the morning. My intention is to sit down and type all of this out when I get home, both in long form and in a condensed easy to read version, but maybe I'll have a lot of quality airport time in the next little bit. 

In reply to mazdeuce - Seth :

Drifting on asphalt is a WHOLE OTHER ball game and knowing what I have learned from you the past 4 years is why I was in total awe of the professional drifters at #Gridlife last year. UNREAL car control and heads up driving. 

I'm now REAAAAAALLY excited to download with you. 7 weeks to South Bend from Today! 

The students. SAE has a "who should take this class" section in their class description, but what's better than who should, is who did. There were two tire engineers, a marketing guy from a Canadian wheel manufacturer, someone who engineers golf carts and another guy who dynamically tests heavy trucks for military contractors. Super nice guy from an auto manufacturer who works mostly with the dual camera system that runs the auto cruise control and emergency braking stuff. There were also a bunch of people from accident reconstruction firms that were learning why cars do what they do in the moments before they do very bad things and how they cars systems affect and attempt to save them from physics. In that group you can include a police officer with a couple decades of experience collecting evidence and helping to decide if a judge and jury need to have a sit down with you about your actions. The odd man out was me, the weird dude who plays with cars. 

Of all the people in the room, I'm the only one who wrote a check for this myself. Everyone else had a boss who looked at the curriculum and thought their employees would be better at their jobs next week than last week. That range of experience also meant that we came at ideas differently. I want to know how to keep a car at the hairy limit, the police officer wanted to know why that hairy limit was exceeded into a wreck, and the heavy truck engineer wanted to know what those numbers really meant when they tipped things over in the desert. 

I'll get into what we learned and who taught us in a bit, but I feel quite confident in saying that if you work with anything that has tires, and you're interested or concerned about that vehicle when those tires are at their limits, you can learn things that will make you better understand your job. 

This is a bit of an intentionally crappy photo, I was trying to protect the identities of the innocent, but this is the buffet style breakfast and lunch that we were served every day at the BMW Performance Center in Thermal California. The staff was spectacular and if buildings at tracks were usually this clean and well lit there would be a lot more spouses tagging along to driving events. I've gotten so used to the mid 80's campground vibe at tracks that I'm shocked when I go somewhere nice with fast cars. 

With Palm Springs right there, Joshus Tree just a short drive away, and a bunch of other things I didn't get to do, it's one of the few tracks that's worth visiting for reasons other than the track. Although we stayed on the slow side of the big wall, on the other side is the Thermal Club track and all of their amenities.  If you sign up for the two day BMW school, day two is over there. It's swanky.

In reply to mazdeuce - Seth :

I appreciate your feedback about the course material, the facilities, etc.  This class was unusual in that we normally have more vehicle engineers and less reconstructionists.  It was also a smaller class than normal.  In SC we typically have 27 students, and I believe we had 18 in our first Thermal class back in December.

It is true that I specialize in making cars go slow, and I’m really good at it.  But, at least in the autocross, you were better at it than me.  ;-)

i also feel that I should point out that my fast lap was not exactly a flyer.  27.7, 27.5 dirty, 27.0.   And perhaps this belongs in the minor confession thread, but I was quite relieved to beat you.  Because internet.

In reply to AngryCorvair :

I was thinking about this a lot, and I've come to the conclusion that the only reason that you beat me is because you're better than me. I think I can fix that.  I also want to point out that the only people that beat me in the autocross were the BMW driver, three of the four SAE instructors, and both of the professional tire testers. This is pretty usual for me. I was the fastest person who isn't actually fast. I'm the king of medium. It's my jam. 

This is a good time to talk about the instructors. 

There were four SAE instructors and two BMW coaches that were in charge of teaching us both in the classroom and in the cars. The last thing you want when you show up at a class like this is a teacher who is an engineer who knows equations backwards and forwards but who has never spun a car when lifting through a kink. I want an instructor who has stressed the middle of his seat cushion through experience. 

First we have AngryCorvair. He builds $Challenge cars and a bunch of you have met him in person. I'd never say it to his face, but he's legit. Then we have a guy who has raced Chump cars and crewed for the guy who originally created the course materials for this course. He's the real deal track guy. I had a sit down with the third instructor talking about the 5 series that he's building. It's an early 80's car that is supposed to replicate the prototype M5 that they built 200 of the year prior to releasing the first M5. He's got all the bit and pieces and what not. Cool guy. I didn't have a proper conversation with the fourth instructor, but during the course it came out that he's the guy responsible to building in launch control to the current ZF transmission. All four of these guys have about a billion hours not only engineering cars, but test driving them in all sorts of conditions all over the world. Unless you've won a jacket somewhere or someone pays to you drive, they can probalby out drive you and their driving experience pales to their engineering experience. Speaking of paid to drive, they BMW instructors that helped us out do get paid to drive, not only the class cars, but actual race cars. They're the real deal. You can find TV footage of them driving. It's always humbling to get in the car with pro level drivers. I suck so much. 

The point of this is to assure you that you're getting what you pay for from an instruction point. Whenever I get instruction in the car or in the classroom I want my teachers to be better than me. All of these guys were way way better than me. 

mazdeuce - Seth said:

In reply to AngryCorvair :

I was thinking about this a lot, and I've come to the conclusion that the only reason that you beat me is because you're better than me. 

In that car, on those tires, on that track, on that day, right?

Edit:  FWIW, I got beat by the BMW guy, one of the Pirelli Engineers (who was 2nd only to the BMW guy), and Tom the SAE guy who also Chumps and has been to the Ring.

Lifetime, I’m about 60/40 with Ed, 30/70 with Erik, 5/95 with Tom, and I don’t think I’ve ever beaten James.  I’ve *definitely* never beaten the BMW guy.

Tomorrow I'll talk about the math, which was light, but suffice to say, this is the first sticker I've made of an equation. Good stuff. 

mazdeuce - Seth said:

Tomorrow I'll talk about the math, which was light, but suffice to say, this is the first sticker I've made of an equation. Good stuff. 

Yeah, I’m gonna need a stack of those.  Please.

In reply to mazdeuce - Seth :

In that equation, you lost me at the triple equals sign thing.

I sit here patiently awaiting any and all pertinent discourse from this event.....in person and on 'TV' I've witnessed drivers(cars)/motorcyclists 'dive bomb' corners 5A-5B at Barber MotorsportsPark with ,for lack of a better explanation, execute a head shake to subtlety  create a lurid slide thru said corners  to facilitate an earlier application of the loud pedal heading down towards 6.....to be able to do that with aplomb would be a highlight in my world.....oh and thank you and Ms. MazD  for sharing this 'once in a lifetime' opportunity with the 'Hive'    

mazdeuce - Seth said:

Tomorrow I'll talk about the math, which was light, but suffice to say, this is the first sticker I've made of an equation. Good stuff. 

Max Acceleration is equal to peak Mu..... Cool....

Also I like where this is going and I'm definitely going to need to bribe you for a couple of them!

Edit: missed triple equal sign on first pass.... 

Max Acceleration can be defined as equal to peak Mu?... Interesting.....

triple equal is either like "exactly equal to" , "defined as" , "has equivalency"... it's a logical equivalence, not an operator

it's kind of like saying maximum acceleration is only achieved at the friction coefficient peak; not <= and not >=, only exactly at

it should be noted, there's no sub-definition... so this applies both laterally and longitudinally

next someone gets to go define ~=  and !=

I was talking to someone about this course and they asked "so will it make you faster?" The answer to that is, maybe. Like all engineering education, CO414 is a course that teaches you to think in a particular way and it's up to you to go forth and apply that to your life. If it was just about being faster it wouldn't really have relevance to crash reconstruction or big trucks or golf carts. Because of that, there isn't really any way for me to sit down and type out what I've learned in any comprehensive way. What I am going to try and do is spell out a couple of the concepts and assure you that reading what I type and listening to/discussing those concepts from very smart experienced people is vastly different. What I'm about to do is the equivalent of asking your six year old about dinosaurs. They can give you a great answer filled with information you didn't know, but they aren't a paleontologist. 

The above equation relating maximum acceleration to the frictional coefficient between the surface and the tire is important both in the simplified form above, and in it's expanded form. In the form above you get your friction circle. You can't go outside it. You can expand it by increasing friction (sticky tires and what not) but you'll always be bound by it. The goal of any driver is to hang out on the edge as much as possible. The goal of any car builder/setup person is to make that possible and to expand the circle. Many of you, most of you, know this. 

One of the most interesting things that I learned about tires had to do with weight and grip. The more weight you put on a tire, the more grip you get. It's a normal force and coefficient of friction equation thing. But........the tire becomes less efficient as you add weight. Check out this chart. As you add weight, your peak coefficient of friction goes down. This is SUPER IMPORTANT because the course was about vehicle dynamics, not vehicle statics. When we  brake or accelerate or turn we're moving weight back and forth and side to side and changing what each tire is doing and what each tire is capable of. This is regardless of whether a particular car has the best double wishbones or the worst struts. Weight moves, things change, dynamics. 

It took me two days of class to realize the significance of a chart like the one above. I'm not quick on the uptake. Let's take a 1200kg car with perfect 50/50 weight distribution so 300kg on each tire while cruising along. Now smash the brakes and the geometry of the car (CG and wheelbase) causes weight to shift forward so that you've now got 400kg on each front tire and 200kg on each rear. Because of algebra, the coefficient of friction for the rear tires goes up, but the fronts go down, and when we sum and average all of our individual tires the total coefficient of friction for the system is less than it was.

Now, I always thought pitch, dive, whatever you want to call it, was part of this and in fact it's really not. Weight transfer is all geometry and the only way to decrease it (if we assume wheelbase is constant) is to lower CG either through lowering the car or taking weight off places above the CG. Pitch is important, but only so far is is makes your suspension do all sorts of wonky things that mess with your contact patch and bump steer and what not, the weight will transfer either way. Dynamics. 

How does this help me be faster? High CG is bad. Sunroofs are really bad. Batteries up high are bad. Glass is so heavy and up high in the car. I should dry sump the motor and drop it two inches. I should absolutely fold the rear seats down for autocross. Lower really is better as long as you can make the suspension work, there are some intersecting curves there. Most of all, most importantly, a max is defined by mu peak, and mu peak is defined by the frictional surface (which is the same for us on a given day as long as we're on line) and the tires. Our best times are going to come from maximizing the performance of the best tires. There are reasons that a particular car will favor one tire over another, but as a car builder/setup person, you're always chasing the highest mu peak. Weight, tire pressure, temperature, camber curves, chasing theoretical mu peak on that surface with that tire.

Preach!

Just one (minor but important) correction:

C0414.  Not CO414.  C zero four one four.

OK, maybe one more.  See that “sliding out of control” point outside the friction circle to the bottom left?  Yeah, while that representation is helpful to visualize / conceptualize the idea that the operator has requested more total force than the tire can deliver, that point doesn’t actually exist outside the circle.  What actually happens is the tire gives everything it can to force-balance the longitudinal request, at the expense of losing some lateral output, and the actual operating point would be where that horizontal braking line crosses the circle, at a much smaller left turn value.

As an instructor, it makes me all tingly to see a student grasp the topic, understand it’s application, and effectively communicate it to others.  I know you already had the effective communication part nailed, as well as (at least) some practical knowledge of contact patch management, prior to attending.  So we, the SAE and BMW instructors, don’t claim to have made you better at those things.  But like we say in the intro, we are here to help you build the bridge between the practical experience and theoretical knowledge.  I will go ahead and claim some responsibility for helping build that bridge, or at least painting some stripes on it.  ;-)

I don't have a track car, and math/theory is not my strong suit, but.....  This is fascinating stuff.   I can see how this can be carried over to every day driving, albeit to a lesser degree.  Please carry on.

edit: Knowledge is power.

In reply to AngryCorvair :

I can't stress the difference between knowing something and having someone competent teach it to you. I did know this stuff, but that doesn't mean I knew it in a way that let me apply it to what I was doing. Now I do. At least some of it. 

This is a good time to point out that if this course was just three days in the classroom, it would have been a smidge lame. Not because the information was bad, but because we would have just had to trust what the instructors told us. More than once I crossed my arms and wrinkled my face in an "I don't know......" expression. But then we went outside and got in someone else's cars on tires we didn't buy. As anyone who has ever taken a materials engineering course will tell you, learning about elastic and plastic deformation is interesting, but clamping a piece of steel into a test rig and breaking it is the real magic. 

When you arrive at the BMW Performance Center your eyes are immediately drawn to M cars. 

And more M cars. 

And then a group of 340s. Those are ours.  

No matter, driving cars that belong to someone else is always fun. In relation to braking at the friction circle and the fact that all of this is dynamic, we did a suuuuuuper cool exercise. They pulled the ABS fuse because ABS is for weenies and simply works too well for us to test a lot of things. We went out to the skid pad which has a diamond polished surface, very low mu, super fun. We brought the car up to speed, on the hairy edge of grip, and then did one of two things. We either stabbed the brakes, or we gently but firmly applied them. Because BMW builds such a sublime almost neutral car, we had two different responses. 

The car is close to neutral so the front and rear axle were both near saturation from a lateral grip standpoint. Adding longitudinal force always takes away your ability to have lateral force. Stabbing the brake saturated the front grip before the weight could transfer, locked the fronts, and we sailed off in understeer hell. Applying the brakes firmly but gently allowed the weight to transfer, loaded the front, unloaded the rear, and we saturated rear lateral grip and spun the car. Brake bias plays into this too, probably heavily enough that AngryCorvair is going to step in and correct me about the dynamics. The end point though is that how you apply the brakes, and the rate of weight transfer has dramatic effects on the outcome.

This was reinforced by both of the BMW instructors over the course of the class. One of them commented that he sees more problems with ESC interference and spinning when it's turned off because of brake release than brake application. And second, after the drift session we talked to the other instructor about why he initiated corners with a flick. He said that he could also do it with trail braking but he found that trail braking had a lot of variables with the tires and speeds and surfaces and it wasn't as reliable as a flick. This whole exercise answered some of the whys I had as to "why do I sometimes suck at trail braking" and gives me some things to try to make it more consistent. I always try to get my autocross students to listen to the car and this is something I'd been feeling but had no context with which to apply it. Now I do. That makes me happy. 

In reply to Dirtydog :

There is certainly some real life carry over. If you're driving a car without ABS or stability control, you need to be aware of weight shift and how that affects the quality of your inputs. Sometimes it's ok to stab the brakes, sometimes not so much. If you do have ABS and ESC then your car is trying to do some very specific things based on what it's guessing your inputs are. In that case, panic inputs are probably fine, but again, knowing what they system is trying to do will help you maximize it's effectiveness. Either later today or tomorrow I'm planning to write just a bit about the how and why of intervention without going into too much detail. Every time I start to write one of these things I'm afraid I might be treading on the intellectual property of the course, but by the time all the words are on the page I realize I'm just scratching the surface.