Neutral Steer and Car Stability - Tying everything together

Added: 2026-05-22

[00:00:01]Hello, my name is Danny Nolan and I'm the director of ChassisSim Technologies and welcome to this latest episode of Dan's Vehicle Dynamics Corner. And today, ladies and gentlemen, what I'd like what we're going to do today is to revisit a topic that I covered would have been good lord probably would have been out 10-15 years ago now and I really was speaking about things like neutral steer introducing you all to the stability index etc. etc. Now, since then a lot of water has flown under the under the bridge and what I really want to do in this tutorial is to introduce or to really put into perspective what these two concepts are of neutral steer and the stability index and how they feed into each other. So, without further ado, let's get started. Okay, so we're going to review the concept of neutral steer particularly when it comes to car stability.

[00:01:04]Neutral steer is usually your first point of call because when we talk about understeer and oversteer, I mean in automotive we tend to throw this around like frisbees.

[00:01:13]Then we're going to talk about dynamic and transient stability and I'll talk about the stability index and really where this came from is a good friend of mine number Tom Turner did a really good blog post talking about these very concepts and it really got me to thinking that particularly in automotive, particularly in motorsport in particular, we tend to throw things like understeer, oversteer, does the car take a set? We tend to throw this around like frisbees and when you turn to someone and say, "Okay, well, define what you want from the car." It's just like you get you get a lot of vague words salad and really the goal of today is to really cement these concepts for you.

[00:02:01]Okay, first things first, let's take a look at the free body diagram of the forces acting on the car. That folks is going to tell you everything you're going to need to know about the way the car is going to act. Now, I detail this more in chapter five of my book, but you'll see here that we've got our that we've got our forces acting on the contact patch and you can then see in terms of other forces that get that get applied to the car, whether they be drag and other forces. But, that's a really good outline and from that you can then start to really quantify what you're doing. Now, when we talk about understeer and oversteer, and this is again one of these concepts that us in automotive, we've really made a little bit of a porridge of it. But, again, it's well worth to define what we're talking about here. So, when we've got a car that's understeering, typically the front slip angle is greater than the rear slip angle. When we're oversteering, we've got the rear slip angle is greater than the front slip angle. So, understeer is where you're going like this, oversteer where you go, "Woohoo!"

[00:03:12]Um where you're doing that sort of a situation.

[00:03:16]And that is something that is probably where our first port of call when we understand what we're talking about in terms of car handling and car stability.

[00:03:27]But, here's where things can get a little bit can start to go off the rails a little bit. So, our first port of call is to find neutral steer and static stability. So, what we've got to do is we take a look at the bicycle equations of motion. So, here we're just simplifying it a little bit. It's a little bit of a cheat, but not a massive cheat. In one of my earlier one of my first articles is Race Car Engineering, I did a full error analysis of this and I transplanted that in the chapter five of my book. So, what we've got is alpha front is your steer angle minus your yaw and your side slip terms. Your rear slip angle is pretty much your yaw rate minus your side slip.

[00:04:05]So, when we force them to be equal, we get your your delta neutral is the wheel base times the yaw rate on VX. Now, all of that is in strict SI units. That's in meters, that's in radians per second, that's in meters per second.

[00:04:19]Now, and what you can really do is that you can actually this that neutral steer channel is a fantastic math channel.

[00:04:26]Now, here, to quote the the Joker from The Dark Knight, I'm going to show you a magic trick.

[00:04:31]So, what we can do here is we can use a really handy little approximation. So, AY is equal to VX times R and R is about AY and VX. Now, you'll see here there's an approximation. That's actually for a very very good reason because that isn't totally the full story in terms of what level of accelerate what level of acceleration is, but that approximation of AY is about VX times R, it's a good start point. It's not the complete story, but it's a good start point. So, putting that together, you can now combine a neutral steer channel, which is wheel base times AY and VX squared.

[00:05:07]And for those of you who are ChassisSim literate, that's wheel base times curvature. That is one of those math channels I kind of backed into by accident and without a shadow of a doubt, is one of the most useful math channels I've ever I've ever come up with.

[00:05:25]So, what we then do is that we plot neutral steer versus actual steer. Now, I'm got a little bit of a bee in my bonnet about the way that we've always plotted steer cuz if you take a look at the way that we've always plotted steer, we always take a look at steer angle at the steering wheel as opposed to the tire.

[00:05:44]Now, personally, I think that's just complete and utter bark raving mad. And the reason it's bark raving mad is that it really stops you from doing a plot like this cuz when I'm looking at data, and particularly when the car first comes in, once I obviously do the basics of checking, you know, engine health, etc., etc., etc., this is the first channel I look at. And so, what I'm looking at is the comparison between actual steer and neutral steer. Now, if you're over the line, you're understeering. If you're under the line, the you're oversteering or the car is on the edge.

[00:06:21]Now, that is always probably my first go-to. And particularly when we come to reviewing car stability, it's your first port of call.

[00:06:33]However, it's not the complete story.

[00:06:38]That channel works brilliantly when you're dealing with someone of the caliber of an Ayrton Senna, a Michael Schumacher, a Max Verstappen, someone like a Jim Clark, a Juan Fangio, someone who has a really innate feel of the contact patch. But, cross-referencing some of my articles and videos that I've made about driver in the loop, invariably, drivers will always throw in a little bit too much steering steering lock because they because most drivers don't aren't completely attuned into what the grip of a tire is doing. So, let's have a look at this plot.

[00:07:18]Now, if we take a look at this plot, we've got our speed, our RPM, and we've got our neutral steer, actual steer, throttle, lateral G, longitudinal G. Now, because of the fact that this is from actual data, I've redacted all the scalings.

[00:07:33]Now, on paper, you'd look at this and you go, "Okay, so we're neutral on turn-in, we've slight understeer."

[00:07:40]In practice, the driver would be screaming blue murder about it being twitchy on turn-in and about mid-corner um oversteer. But again, and the reason that he'd be but the disconnect is because he's just applied a bit too much steering lock. Now, in this case, the driver hasn't applied masses of masses of over steering lock, but he still applied a bit much of a steering lock. So, consequently, when we're looking at the neutral steer channel, it's a great start point, but it's not the complete story.

[00:08:14]So, to fill that in, we need the stability index. Now, I've obviously done this to death on a number of other um uh different um uh videos. But again, what we're doing with the stability index is we're effectively non-dimensionalizing the moment arm between the center of the lateral forces and the center of gravity. Now, this comes from the concept of static margin used for longitudinal aircraft dynamics, hence why I've shown where this comes from.

[00:08:43]So, in order to define this, again, it is the non-dimensionalized moment arm between the center of the lateral forces and the center of gravity divided by the wheelbase. That's it. The MT is just to make sure that it's that MT terms to make sure that it's unitless. So, the way that we do that is that we've got our lateral forces is that we've got our sum of our lateral forces at the front, sum of lateral forces at the rear. This is effectively your um uh tire cornering uh your tire cornering slope, which is your track uh the sum of your traction circle radii times the non-dimensionalized slope of um uh your uh your tire forces versus slip angle, and that's your stability index. Now, I've pretty much done that to death in um my book The Dynamics of the Race Car.

[00:09:35]Also, too, in my forthcoming Race Car Engineering article, I give you a work example of that. So, in some respects, this this is kind of almost a companion um tutorial um to um that article.

[00:09:48]So, why is the stability index so powerful? Let's consider moving the aero balance forward 5% on an F3 car. Now, this is a simulated data trace, so granted, the steering traces aren't going to be as big for a simulated change versus an actual change. But, if we take a look at the differences in steering, it's not much. You might be looking at 0.2, 0.3 of a degree.

[00:10:13]But, if we go down here, which is the stability index, colored is our baseline, black is with the aero balance moved 5%. We can see a very clear change in the movement of stability index. And that, folks, is its resonant power, which is why you've always got to be very, very careful. If you're If you've got, say, active steering, that is, say, looking purely at steering angle as well. You've got to be so careful, cuz that can actually lead you down the garden path.

[00:10:41]Whereas, something like stability index, you see a very, very clear delineation in that change. So, you can go, "Ah, that's the next That's the next step."

[00:10:51]And indeed, my plan of attack when I will look at data is that I'll look at the neutral steer versus actual steer, then I'll look at the simulated data of that change to go, "Okay, well, what did the stability index do?" And that way, both of them feed into each other as a sanity check.

[00:11:09]Now, in terms of some rules of thumb to go by, particularly for rear for rear-wheel drive, so we're basically tooling down the uh front straight, here's [snorts] where you want to be in terms of in terms of the stability index, in terms of trundling down the straight. So, your superstars, typically for your engine power at about 170 kilowatts. Yeah, minus three to minus 4%. Engine power once you get to 400 kilowatts, minus five to minus 7% and your engine power is about minus eight to minus 10%. Your average drivers are about minus six to minus eight percent, minus 10 to minus 12% and your bad drivers are minus 12 minus 16 to minus 20% respectively. Once we're talking mid-corner, some rough rules of thumb, you're typically at about minus 10 to minus 15% mid-corner for your low engine power, about minus 15% for your engine power at 400 kilowatts.

[00:12:10]At about engine power at minus 600 kilowatts, that'll be minus about minus 25% or so. And under brakes, you're typically about plus five to plus 10%.

[00:12:19]Again, these are rules of thumb.

[00:12:22]The ultimate test will be once you start correlating what the driver is saying versus to what the car has done. That's actually a really good rule of thumb about how you correlate what the stability index is doing versus what the driver is saying. And again, these two things all play into each other.

[00:12:44]Okay, one thing that I was probably a little bit remiss in terms of not mentioning properly is what happens when you go past the peak slip angle. Now, when cuz when you go past the slip angle, you're in the post you're in the post slip zone of the tire and so effectively, your traction so effectively, your um your slip curve slope goes to zero. And a lot of people and and when I first introduced this concept, a lot of the automotive guys who wrote car automotive guys who have got who are very wedded to traditional areas of looking at understeer oversteer really got their because they're not over it.

[00:13:23]Now, once you go past the post stall area, the more complete version of that, and I actually have missed that for a reason just to help you understand it, is actually is actually going to be the slip angle curve slope time plus whatever traction forces that you've applied both at the front and the rear. And again, that feeds into each other. So, when we go per postall, this is now the dominant term. This is why when you're looking at dirt light model or you're looking at sprint car or you're looking at drifting, you can see cars you can see cars all of a sudden with the with the rear hanging out, but they're stable. And the reason they're stable is what's happening here is that you've got the FXR term at the rear. Let's just limit our comments to rear-wheel drive.

[00:14:16]You've got this term at the rear FXR that's stabilizing you. However, because you're postall, the diff is now actually destabilizing you. And it's that dance that you play in terms of being able to be able to control the car. And indeed, one of the things on my to-do list is to you actually come up with stability index formulations for the diff, but that will be a subject for another tutorial.

[00:14:44]So, to sum up, the neutral steer concept is a really great starting point when it comes to talking about race car stability. That being said, it's not the complete story. That's when the stability index comes in and complements what you're going to do on the neutral start. And it of the neutral steer channel, I should say. And what that does is it completes the whole picture. And again, you will always reference to what you see in the data versus what you see in simulation.

[00:15:12]And you always cross-correlate that to what the driver is saying. Now, the better the driver, the easier that is to understand. The worse the driver, yeah, it does get a little bit more complicated. But that's kind of the art of what we do. All right. That will do it for this tutorial, and we will catch you in the next episode of Dan's Vehicle Dynamics Corner.