Transient Response 3

When it comes to transient response many of us have some strange ideas, me included. Let me illustrate this with one true statement and one false statement.

Statement 1:  The polar moment of inertia has a significant effect on how fast a car spins. True.

Statement 2: The polar moment of inertia has a significant effect on transient response. False.

Is the falsity I claim for statement 2 surprising to you? It was to me. I mean, whenever people talk about handling they always say something like a mid-engined car handles quicker and is more responsive because of the lower polar moment than a front engined car. Witness a 2011 in-depth handling comparison between two Porsches, the Cayman R and the 911GT3, done by Car and Driver here in which the 911GT3 puts the Cayman R to shame. They mention polar moment, Center of Gravity, yaw rate and even a “new step-steering-input maneuver.” New to them, I guess. They ran a variation on ISO 7401 which had been around since 1988 and researchers and car companies had been doing step-steer testing long before it was codified into a standard. Why they couldn’t have just run ISO 7401 (it’s not difficult) I don’t know.

They talk about how the Cayman R’s 20 percent less polar moment of inertia “theoretically [makes] it easier to begin and end any cornering maneuver.” Unfortunately, they don’t actually know what they’re talking about. In fact, they totally miss the point of the step-steer test in my opinion.

Car and Driver does a step-steer test by turning the steering wheel 90 degrees and then reports the final rates at which the two cars turn in degrees per second. Who cares about that? The point of a step-steer test is to determine response time. The time it takes for the car to reach 90% of the final turning rate is the result of a step-steer test. That’s a measure of transient response, not the turning rate that it finally reaches for a given angle of steering input. What they report is really just a simple measure of steering ratio!

I don’t want to pick on Car and Driver too much. I think the article I’m referencing is not bad, mostly because of the battery of tests they did. They meant well. One test is worth 10 theoretical analyses in my book. I forgive their lack of real car handling engineering knowledge but can’t quite forgive that they don’t go out and find someone (say, any kid who did chassis design in an FSAE program at college) to explain it to them before they write the article and lord theory over us so much. For instance, they conclude that the 0.6″ lower CG height of the GT3 is not nearly as significant as the 20% less polar moment of the Cayman. Really? How do they know? One might ask: if the smaller polar moment is so important, why didn’t the Cayman R do better? Why did the two cars exhibit exactly the same yaw rate of 30 degrees per second if the Cayman has 20% less polar moment? Seems like that was a hint that they weren’t looking in the right place.

Then there’s the fact that the 911 was wearing vastly superior tires than the Cayman which makes the entire set of tests extremely suspect. Couldn’t they have at least put the same tire brand and model on the two cars at the stock widths and eliminated that glaring discrepancy?

OK, Fisher, if you’re so smart why don’t you tell us what really affects transient response, i.e. step-steer results? Since you ask I will, but I’ll need to refer to the following chart.



Figure 1- Yaw Inertia Diagram


Here we have a plan view of a car with the Center of Gravity (CG) shown near the center and a big black arrow showing where the force comes in to turn the car. The polar moment of inertia would be the sum of all the masses of the car times their distances from the CG squared. That’s a nice thing to know and it does directly affect how fast the car will rotate once it is in a spin. The main problem is that when initiating a turn a car does not rotate about the CG, so polar moment is of suspect importance.

When initiating a turn, say with the ISO 7401 step-steer test, which happens every time a car on a race-track enters a corner from a straight, the car actually rotates about a point that is back behind the rear axle. I have it labeled CR (Center of Rotation.) How far behind the rear axle is it? Using data from testing done by the National Highway Transportation Safety Administration (NHTSA) Bobier, et al in a 1998 paper entitled Transient Responses of Alternative Vehicle Configurations found that most passenger vehicles sold in the U.S. were very similar in that for 90% of all vehicles tested this point was between 1.39 and 1.96 times the distance from the CG to the rear axle, which is dimension b in the figure above.

For my purposes I simplify and approximate this into the formula c = 1.5 x b. I’ll call this equation 1.

We can easily determine the value of b for any particular car by finding the wheel-base (WB) and the weight distribution on the internet, which is usually reported by the manufacturer or someone has measured it.

So, b = WB x (1 – %R)

where WB is the wheelbase and %R is the percentage of weight on the rear axle. Call this equation 2.

What I’m after is an estimate of the yaw inertia about the point CR, not the polar moment inertia about the CG. We can get a good, first-order estimate with this equation

Iy = wt x c^2

where Iy is the yaw inertia, wt is the weight of the car and c is the distance from the CG to the CR.

Now I just substitute equation 2 into equation 1 and take that result and substitute it into equation 3 and we have

Iy = wt x (1.5 x (1-%R) x WB)^2

This gives an estimate of yaw inertia in terms of three parameters that we can easily find for any car.

What does this equation tell us? It tell us that the lower the weight of the car the lower the yaw inertia, all else equal. It tells us that the shorter the wheelbase the lower the yaw inertia, all else equal. And it especially tells us that the further the weight is toward the rear, that is, the closer the mass is to the center of rotation, the lower the yaw inertia.

Guess what car has a really short wheelbase and it’s CG way back toward the rear and is only slightly heavier than a Cayman R? A 911GT3, that’s what car.

So, a mid-engined car can be expected to have faster response primarily because it will have more weight to the rear, not primarily because it’s weight is centralized. That’s why a front mid-engined car, like the fifth generation Corvette I have, is never going to have the response of a Porsche Cayman, even though its’ polar moment might not be higher. The mid-engined Lotus Evora has better response than the mid-engined Cayman because it has a weight distribution more reward, like a 911, compared to a Cayman. (The Lotus designers knew what they were doing!) And if you get a car very light, with an extremely short wheelbase and make it mid-engined, well then you have an MR2 Spyder and when you step into it from your Corvette at the autocross you will run over each and every key cone until you get used to how incredibly fast it turns in!

This is what I use in my autocross speed rating system that some people know about. It gives an idea of the potential that a car has for having rapid transient response. Here are some selected results:

selected yaw inertias

Figure 2- Selected Estimated Yaw Inertias


Stay tuned. More to come.



Transient Response 2

In the earlier post I wrote that “in the Street classes a car’s peak lateral-G capability is of zero importance in determining speed through a slalom.” What?

The analysis was done a while back (see J-Rho covering it in part 5 of his series Comparative Vehicle Dynamics) that all the techno-nerdy autocrossers (like me) know about where someone (I won’t mention his name) derived the time around cones in a slalom using basic physics. In his derivation it came down to how much the car must move left and right and the peak grip (lateral-G capability) of the car. Using his formula you can calculate the time lost by leaving more space around the cones, for instance, and you can compare one car to another based on width and peak lateral acceleration capability. The only problem is that it doesn’t work.

Am I saying that the math was wrong? No. What I am saying is that it is a useless, misleading, nonsensical simplification that totally missed and therefore obscured what really matters since it was done over 15 years ago. Let me prove it. With data.

Soon after acquiring a GPS data device I noticed something weird. I will illustrate it with values from recent runs, but it was clear from the very first run I ever took with data.

In my last autocross in the Corvette the course contained both slaloms and sweepers, as usual. In five different sweepers the data shows the car pulling sustained lateral acceleration values of 1.250, 1.083. 1.084, 1.125 and 1.260 Gs. In various slalom sections the car pulled peak levels of 0.676, 0.774 and 0.552 Gs.

In my most recent autocross I co-drove a 2008 Cayman and in various sweepers achieved 1.299, 1.080 and 1.276 Gs. In the slaloms the car pulled 0.720, 0.435, 0.469 and 0.627 Gs.

See the pattern?

For a long time I thought I was just pitiful at driving slaloms. It made me work really hard to improve. I got faster! The pattern didn’t change, however. I’m so stupid!

If a car can’t achieve it’s peak lateral acceleration capability within a slalom then peak lateral acceleration capability is not important and is no predictor of slalom speed. Any mathematical formula that claims to be able to predict the time from cone to cone in a slalom that uses peak lateral acceleration capability is just. plain. wrong.

We need to start over and rethink it. I’ll do that in the next installment.

Transient Response

As autocrossers we all know, or think we know, that Transient Response (TR, for short) is important to getting around the course quickly. Because slaloms and other types of offset features. In no other form of motorsport is the ability to quickly change direction quite so critical.

I recently became convinced that TR is actually much more important than we generally think. So important that it’s the number one thing that causes course dependency. Course dependency is when one particular car or type of car is inordinately favored by the course design. Let me give you an example of course dependency.

A few years ago a friend bought one of the first Porsche Macan Turbos in our area. It had been classed in B-Street, which was also the class for my 2000 Corvette FRC. This friend I willingly admit was a faster driver than me. We had co-driven several times. He was always half a second faster. In my own car, in his car, it didn’t matter.

But he made a mistake. He informed me that he had bought autocross tires for the Macan and intended to beat me and my Corvette at the next event. What he didn’t know was that I was the course designer. I told him what a huge mistake he had just made and that I would design a course where there was no way he could win. He scoffed, confident in his and the Macan’s abilities.

I was good to my word and designed a transitiony course. No pinch points, just a lot of transitions. I was 2nd to an S2000. He didn’t even get 3rd. He was 4th in our local B-Street class. AFAIK, those tires have never been used since. It was his wife’s daily driver and no way would she put up with stiff, howling RE71Rs.

The Macan accelerated faster than the Corvette. It probably pulled as much, or very close to as much, lateral-G in a sweeper. It was slightly wider, but like I said there were no pinch points. What it couldn’t do was transition as fast as a Corvette or S2000. The CG is just too high.

Here’s what I’ve come to realize: in the Street classes a car’s peak lateral-G capability is of zero importance in determining speed through a slalom. Not less than what we thought previously, I mean zero.

In a slalom all that counts is TR and car width.

For instance, it doesn’t even matter what tires you are on or how wide they are, except in so far as the tires affect TR.

(More to come.)

Autocross Revolution 2.0

co-drivers in masks

Aspiring Autocross Car-Show Models

Newly returned from Solo Nationals 2019 is a good time to talk about the elephant in the room that we insist on buying drinks for. I’m not talking about electric cars or spotty availability of the hot new tire or clouds of cement dust. But I have to start with a little history.


Once upon a time you had only your memory to tell you what you did wrong or right after taking an autocross run. If the back end came around 90 degrees that would be an obvious mistake. If a section “felt” faster then you must have done it better, right? Maybe not. It took a lot of experience to know for sure.

We walked the course as much as possible and with a certain discipline. What to do while walking, what to notice, where to look… the walking task was as much a part of the sport as the driving because every course was different, one of the key differentiators for autocross. Then came the mental planning part without any practice runs (all runs are officially timed), another key differentiator. What line to take in each corner, how much to brake, how soon to get on the throttle and much more had to be estimated, planned and remembered, not practiced over and over and refined like for time-trials or road racing. At a national event you had all night(!) to think and plan. Then, after a first run, you had only two chances to make changes where your plan was proven wrong by reality and to fix the inevitable mistakes. We’ve all heard the lament, “I can’t believe I coned away 27th!”

We learned various guidelines to help us, like Slow-In and Fast-Out, Maintain Momentum and Cut Distance and we could judge ourselves based on those guidelines (even if they were sometimes mutually inconsistent) and whatever the timer said. It was difficult to tell what was really going on from the timer, right or wrong, because being fast in one section is often negated by being slow somewhere else, especially in the beginning when we were very inconsistent. A few people set up single corners and used the stop watch to figure out which of several methods was faster. That was about as good as it got until Revolution 1.0.


Revolution 1.0: Portable Data & Video Electronics

The advent of GPS data devices allows us to analyze in detail exactly how we’ve driven a course, even during the event. We can compare doing the same corner different ways from run to run, either driving differently on purpose ourselves or compared to others who drove the same course with different choices. Did we gain on entry or exit or lose time everywhere? Where was it worthwhile to give it up to Save Time somewhere else? Gradually we proved out what actually worked and what didn’t. We learned that a lot of the guidelines carried over from road-racing were not applicable to autocross, at least not without some fine-tuning. Road-racing was going through a similar revolution and learning new things.


Here’s an example of how different autocross and road-racing can be. I’ve heard others tell a similar story. In my second year of autocross (before I had data or video) I asked a very successful former road-racer who owned a race shop at the time to co-drive with me. I had maybe 20 events under my belt. It took only a few seconds into the first run for me to be blown away by his car-control skills in a car he’d only ever driven on a test drive after he fixed it for me! He was immediately on the edge of adhesion, using the tires and controlling the car in a way I’d never experienced. That was eight years ago and I still remember my amazement.

But, I killed him on time.

When it was over neither of us had a clear idea as to how I was consistently faster over the day’s runs. He got faster run to run but I got faster even faster. I was too new to know why and he’d only done a couple of autocrosses in the distant past. With data it would have been obvious. Maybe something about not having track edges to set limits. (There’s another key differentiator in there somewhere.)

The portable high-resolution video cameras not only allow us to see our driving after the event but also allow us to video the course as we walk it before the event. Then we can speed it up and see the course at speed. This helps a lot with knowing the course before you ever drive it and determining where to look at each point along the way, a key aspect of being fast. When you can do this a day or even two before you run, like at Nationals, it’s even more helpful.

Early on I took still pictures with a digital camera as I walked my proposed driving line the afternoon before the national event. Then I would flip through the pictures many times before the first run. Later I used a smart-phone camera. Now I video with my Go-Pro as I walk. When viewing the movie I can stop it to get stills at any point, paying attention to the background as well as the immediate course. I think this helps you to unconsciously recognize your position, similar to the strong home-court advantage in basketball.


Revolution 1.5: Pre-Viewing with Virtual Reality Video

Now we have consumer-level devices that create a 3-D picture, really a 3-D visual model, of the entire course which you can later view along the course path wearing a Virtual Reality headset. You can turn your head left and right just as you would when driving to look ahead. If you run the last two days at Nationals, for instance, you can attach the camera to a car that runs the first two days. This gives the ability to visually tour the course at whatever speed desired many times before ever driving it. The old days of everyone walking the course, imagining in their head what it will be like at speed and planning how to drive it are long gone. I haven’t personally used the 3-D VR technology but others are. We’re no longer in Kansas, Toto.

But, wait. You ain’t heard nuthin’ yet.


Revolution 2.0: Pre-Driving with Virtual Reality Simulators


If I go ever back to Nationals I’ll take a complete driving simulator. That means a gaming PC, wheel, pedals, shifter, chair and VR headset. I will have already created a car who physics matches as close as possible to my actual autocross car. I will GPS-mark each key cone on the course and create the course in VR, maybe even combining it with the scene model from a VR camera. I will have developed a way to measure the surface characteristics of various sites (and tested it at previous events) and put that into the physics engine also. I can even change it to a wet surface (dump a bottle of water on the surface and remeasure) and use my VR rain tires. Then I’ll drive the course many times in VR before I ever run it for real. I will analyze what techniques and lines are fast and what are not. These capabilities are available now, if not all in one package, and will only get better and more accurate with time. Soon it will all be very cheap. Incredibly, this is all perfectly legal.

Autocross is about to become more like time-trials where you can practice the road course however many times you can afford to visit the track before the event, limited only by how much you can afford in tires, other consumables and time, or, you can VR drive it if it’s been mapped! Much of the previous mental key differentiator is no longer necessary, except at local events where the course is created the morning of and there’s little time to collect the data and drive the simulator. Does anyone doubt that soon it will take only a few minutes to collect the data, download it and drive the simulator (in the back of your pickup?) a few times in the paddock? Can you imagine what this will do to the sport at the local level the first time someone shows up with one of these?

This is actually really good for someone like me, an older and somewhat technically savvy person who has trouble walking National courses an unlimited number of times. I think I stand to gain. I can figure out the technical aspects. I can afford the time and at least some level of a VR driving simulator. Being older it’s become harder for me to pick out the key cones at speed. VR simulation driving may help a lot with that limitation and somewhat even the playing field between myself and the younger competitors.

Except I don’t want it. I probably won’t do it. The sport will have lost a lot of it’s interest for me.

Get it done in three, as the saying once went. Now it’s get it done in 100 VR runs in a simulator and then, oh yeah, drive it three times in RR (Real Reality) to prove the accuracy of your simulation.

I wrote a letter to the Solo Events Board on this subject this time last year. Thank you for your input. I see no indication that the technology tsunamis Revolutions 1.0, 1.5 and 2.0 are being given any thought. The elephant is about to step on our foot and it will be painful.

Can anything be done? It’s true that technology, left unabated, changes everything forever. There is no more potent force for change on the face of the Earth. So I think a lot of people are shrugging it off, thinking nothing can be done. Nothing can stand in the way of technology, right? Wrong!

I was a technologist by trade. It’s been proven over and over that Sport can preserve it’s essence in the face of technology. Even a highly dysfunctional sport like Formula 1 was smart enough to outlaw anti-lock brake systems, for example, in order to keep some driver skill relevant. Formula 1 outlaws all sorts of other technology as well. They do it to protect their sport and their revenues.


Automobile technology is changing autocross, of course, what with paddle- shifted automatic and dual-clutch transmissions, launch control, sophisticated traction and differential control, and the (smaller) elephant in the room everyone recognizes: electric cars. The SCCA is working hard to keep the sport relevant. Most seem to think we are headed for split classes: electric vs. internal combustion. This is where everyone is looking. We’re gazing at the wrong elephant.

I think we can and must preserve the mental aspect key differentiator of the sport. We do it with the ban hammer.

I don’t see why we can’t outlaw the problem at the root. The root is acquiring the course data. Don’t publish the course designs until after the event. No GPS or video electronics allowed on course walks. Use of any location-determining or picture taking electronics on course walks, including cell-phones, should be outlawed.

We don’t allow cells phones or other personal electronics in many other places, such as while driving or in construction zones or in classified work areas. Why can’t we ban their use during autocross course walks?

Any off-course use of technology (like LIDAR) to spot the cones and develop a virtual course map should also be banned as that would be an easy evasion of the course walk electronics ban. Finally, any attempt to use a racing simulator to predrive the course or a set of pictures or video to view the course, no matter the source, should be clearly outlawed as well.

Please, let’s have none of this I know, we’ll publish the course maps just before the event so it can’t(?) happen again but we don’t have the guts to clearly say that what happened last year was wrong ‘cause we’re PeeCeed, weak-kneed, chickenshits without any real principles. Stand up for our sport.

Only a clear ban on VR pre-driving will keep the mental aspect of the sport as challenging as it was when it started. Otherwise this key differentiator is certain to disappear.

Our new rallying cry: Protect our key differentiators! (Or something equally catchy.)

Could people cheat? Of course they could. People can cheat at anything. (I just read The Art of the Con. Wow!) But most won’t. Those that do cheat and get caught should be handed severe penalities.


One last thing: the SCCA should publish accurate electronic maps of copywrited as-run Nationals courses after the event is over. These can be obtained with GPS but more accurate methods are available. Why not negotiate a tie-in with one of the big VR driving simulator software houses? Isn’t this a no-brainer? We should be encouraging people to virtually drive previous Nationals courses on their racing simulators just like they VR-drive race-tracks around the world. Imagine a generation of kids that grow up doing VR autocross racing who can’t wait to get into the sport for real!

ed sitting with umbrella

Coolant-Spill Delay

Teaching Points For Novice Autocrossers


Dixie National Tour 2012

Tomorrow I’m heading to Bowling Green to instruct at the Tennessee Region SCCA autocross school in the National Corvette Museum parking lot. I was thinking about what I would tell my students and decided I’d better write it down.

Now, some of this I was told when I started, but a lot of it I wasn’t. And I was told some stuff that turned out to be, shall we say, incomplete or possibly even misleading but certainly “problematic” which is a curious and pretentious word I learned when I was a Philosophy major a little while back. I’ve learned a lot from taking an EVO school or two and I really learned a lot from our Twickenham Automobile Club advanced autocross school that we put on each year with a curriculum developed by Steve Brolliar. (It’s coming up in a few weeks. Registration opens on 9/1 at

So, below is what I wish I’d been told when I was a novice autocrosser, based on what I’ve learned over the last roughly 10 years.

Ed Fisher’s Teaching Points For Novice Autocrossers- 2019

Walking The Course

  1. Plan your driving line and initially drive to your plan but don’t get too wedded to it. Things may not be as they appear and conditions will change each run.
  2. Plan the entry and exit for each corner. A cone is usually (but not always) the apex location. The apex is the point where you will finish slowing and start accelerating. To plan the exit, stand at the apex cone and determine the correct angle (at your estimated minimum speed) to get the fastest possible exit that sets you up correctly for the next feature. Then figure out how to get to that apex location and angle as quickly as possible, i.e. where to initiate braking, how much to brake and where to initiate the turn. That’s your plan for corner entry.
  3. Autocross often contain a series of connected corners. The entire course may, in fact, consist of one long set of connected corners. (This differs from track driving that features mostly stand-alone corners.) Plan a compromised line and set of entries and exits to optimize a connected section in order to find the fastest average speed through it. Fastest average speed wins every time.
  4. As you walk think hard about where to be looking at each point, far enough ahead to see where you want to go. This may mean looking through the side window. 90% of autocross is just “get where you want to go as fast as possible.” Give yourself that command and give your mind and body the space to do it (by looking ahead) and you’ll do it mostly right and will get better with experience.

Driving The Course

  1. Drive in the moment and keep the car working as hard as possible. Forget about being smooth. You can do that when you’re old(er).
  2. Keep visually scanning far ahead from the time you leave your grid spot until you return. Use peripheral vision to aim at the key cones. Six inches from a key cone is four inches too far away.
  3. Constantly test for the lateral limit when turning. There’s grip on the other side of slip. Get comfortable over there. Only the car can tell you, when one end or the other begins to slide, that you’re driving at the limit. Every corner is different from the others. Each corner changes from one run to the next. Only by constantly testing the limit can you stay at the limit. (Don’t do this on the street!)
  4. Adjust your braking points earlier as each run gets faster or later if you’ve been too conservative in your planning.
  5. Minimize the distance travelled, but for short corners, i.e. < ~120 degrees, go wide on entry to get the best angle. This decreases time to the apex and will increase the minimum speed and shorten the time through any roughly symmetrical corner. (Road-racers call this “using all of the available track width.”)
  6. In long corners, i.e. > ~120 degrees, stay tight and limit how wide you go on entry. The shortest time around a circular path for all cars is on the minimum radius. Find the apex (the location and angle where you can throttle up and start opening the wheel) like any other corner.
  7. Really long corners, more than 180 degrees, may technically be double-apexes. There’s a special way to do those (see The Perfect Corner 2) but if you stay tight and keep the car working near the limit you’ll be close to optimum. Determine the exit apex like any other corner.
  8. The lower your car’s power the bigger the cornering arcs you must drive to keep your speed up. This is called maintaining momentum to maximize your average speed. It’s worth the extra distance travelled when you learn to optimize the balance between speed and distance. High power cars should run a smaller corner radius when given the choice, but the difference is not huge within Street classes.
  9. Learn to trail-brake. Now. This is the safe place to practice it. Initially brake hard and fast (ok to activate ABS) then release the brakes slower and smoother as you turn in. Perfectly done, braking ends at the apex.
  10. Mentally review each completed run and find two or three mistakes to eliminate or places to improve. The third run needs to be close to perfect if you have national event aspirations. Consistency will not happen overnight. In the beginning it’s mostly about eliminating major mistakes and minimizing minor ones.