Talk:Oversteer

Overstating 'Countersteering'?
Regarding "the critical speed" ...

"Oversteering cars have an associated instability mode, called the critical speed. Above this speed control is reversed, that is, the steering wheel must be turned left in order to turn to the right.(Gillespie :Fundamentals of Vehicle Dynamics, or any basic vehicle dynamics text). Understeering cars do not suffer from this, which is one of the reasons why high speed cars tend to be set up to understeer."

For this excerpt from the article, can anyone clarify whether ALL turns to the right above the critical speed really require that the wheel be turned to the left OF CENTER? (That's the case with motorcycle turns at speed, but a car doesn't lean on a tire edge the same way unless wheels are coming off the pavement.) I suspect the author may have meant that a steering wheel turned excessively to the right will have to be backed off leftward TOWARDS the center but still right of center, in order to regain enough traction to resume oversteering (unless the vehicle is decelerated instead). If I am incorrect, then not knowing your exact critical speed or some way to sense it could be astonishingly dangerous; but if I am correct, then it is more a matter of sensing that during a turn you are losing front traction, and simply backing off on the steering angle somewhat. Which is it?


 * Why not read Gillespie and find out? If you don't believe my summary of the situation is correct there is not much point in me repeating myself.

UPDATE: From further reading, it appears our widely quoted Wikipedia article as it stands is quite incorrect. As the critical speed is approached, the steering angle must be backed off towards the centerline. But, apparently, at the moment the ideal driver is forced to back off the steering completely to the center position, the vehicle becomes unstable and there is NO position of the wheel which will allow controlled steering until speed is reduced. So the article is wrong to say control is possible (just reversed) above critical,


 * No, the article is correct. Read Gillespie.

and it's wrong to allow the reader to think the wheel might need to be turned left of center to cause a vehicle to rotate to the right. (The article may be confusing the direction of the curve in the road with the temporary turn direction of the nose of the car, which can be opposite: an oversteering car below critical speed may be deliberately rotated leftward while negotiating a turn to the right. See opposite lock.)


 * No, the article is correct.

(Part 2: can someone also add a functional citation for this "basic" vehicle dynamics issue? Preferably one which explains the mechanism  involved if it is not as obvious as it now seems.  I've found only bits and pieces on the web.)


 * Gillespie

(Part 3: It would be very helpful to have some idea just how easily one can reach critical speed in a non-racetrack environment. Is the diminished angle of the steering wheel an obvious warning that it is approaching?  How likely is critical speed to interfere with opposite lock steering in practical driving situations?)


 * You can't reach the critial speed in a modern normal road going car. You can, in a race-car, if it is not set up correctly

--

Critical speed removes steering control from even the most skilled driver. Great, but what then? How does the vehicle behave in this unstable state? One site says some drivers can still control the vehicle, with no details given -- but I suspect they're actually just waiting to resume active control until their speed comes down, while taking advantage of the fact that they're sliding faster than other drivers might dare. Or is it that the driver can still "influence" direction (partial predictability) without fully "controlling" it?

--User Parsiferon 03:33, 22 December 2006 (UTC)


 * At the critical speed I suspect that various non linear effects will tend to keep the car in a straight line, briefly. However moving the steering wheel would be a very bad move. Above the critical speed turning the steering wheel to the left will cause a turn to the right. Hey I've got a great idea. Why not read the reference given instead of pontificating?Greglocock 22:51, 22 December 2006 (UTC)


 * You've given a reference which is only available on paper in a seriously expensive work. How about quoting the relevant passage instead of just using the name as a talisman?
 * The statement doesn't make sense to me as it stands. I can see that moving the wheel in one direction produces an opposing change in the attitude of the vehicle, but in those conditions that isn't the same as making the vehicle turn in that direction - it doesn't have that simple an effect on the course followed. -- Ian Dalziel 12:17, 31 December 2006 (UTC)


 * I don't know whether people drive through the critical speed. It may well be that they recognise that the steering is going very light and back off. I agree that the behaviour above critical speed is unlikely to be as simple as control reversal, that is merely the prediction from the standard bicycle model, which is linearised. Try google print for the reference, pages 199 through to 206. p204 in particular for a relevant graph. Greglocock 21:28, 31 December 2006 (UTC)


 * here's the equation of interest. steer angle=57.3*wheelbase/radius of turn +K*latacc eqn (6-16) . K is negative for an oversteering car, hence at sufficiently large latacc the steer angle is negative. I suspect a more detailed analysis will show that the steer angle is really a complex number. I'll re-edit the section to reflect that uncertainity. Greglocock 22:16, 31 December 2006 (UTC)

It seems like the whole critical speed section is a bit wrong/irrelevant, and may have been written by someone who didn't fully understand the situation. The article seems to imply that the understeering car becomes an oversteering car with added steering angle, which is possible, but also irrelevant. Critical speed as I understand it (going off Race Car Vehicle Dynamics by Milliken and Milliken) is an effect of changing road speed, not steering angle. It occurs because at some speed, the oversteering moment generated by having more front lateral than rear lateral at a given vehicle slip angle overcomes the 'yaw damping' effect, which diminishes with speed. As such the yaw rate of the vehicle starts increasing even with the steering angle at zero. Also, you don't 'turn left to go right', you turn right to yaw the vehicle to its desired heading (thus establishing a rear slip angle), then turn left to establish the front slip angle. You have to steer somewhere between the centerline of the car, and the actual course of the car, such that you have less front slip angle than rear, but you are still generating the required lateral force. Nereth (talk) 06:59, 16 December 2007 (UTC)


 * Yes unfortunately somebody clueless has rewritten it. I suggest you rewrite it, or find the old version. &#39;&#39;GregLocock&#39;&#39; (talk) 07:21, 16 December 2007 (UTC)

I'm not particularly experienced with wikipedia (note the lack of discussion page, etc), thus I'm not touching it. I would probably get ripped apart for not having enough references. Maybe I'll look for those wiki pages that explain how to use it -.- Nereth (talk) 07:32, 16 December 2007 (UTC)


 * I found the old one. Is that better? Hey, get in there and edit, we need people with "seriously expensive books" as someone described it. Welcome to wiki Greg Locock (talk) 07:34, 16 December 2007 (UTC)


 * Or as someone described it, "How about quoting the relevant passage?". You still haven't done that - even page numbers would be an improvement. A position of "I've read a book therefore what I say goes" is hardly a citation! -- Ian Dalziel (talk) 11:23, 16 December 2007 (UTC)


 * Does my agreeing with him count as any amount of consensus? I have verified it from a different source (Milliken). As to quoting the relevant pages, I don't see how "I have read pages x to y therefore what I say goes" is any better than "I've read a book therefore what I say goes". That seems to just be the nature of quoting off a book no one is willing to buy. Nereth (talk) 15:23, 16 December 2007 (UTC)


 * Just to be clear, I don't disagree with the passage as it now stands - and the immediately previous version was garbage, I don't know when that crept in. But what would be wrong with quoting the relevant text as a reference? -- Ian Dalziel (talk) 15:48, 16 December 2007 (UTC)


 * Sorry, I wasn't clear in my reply up there, what I should have said was, a) giving page numbers is no better than giving just a book, and b) at least in the case of Milliken, the relevant section you would have to quote would be at least 2 pages long, and also useless (mainly due to a lot of equations that draw on previous equations) without the context of the rest of the chapter.  —Preceding unsigned comment added by Nereth (talk • contribs) 01:13, 17 December 2007 (UTC)


 * Well if you bothered to read my answer to you above, you'll see I gave the pages. I gave the equation. I listed where the graph is. Short of driving round to your house and dropping off a photocopy there isn't a whole lot else I can do. Greg Locock (talk) 17:04, 16 December 2007 (UTC)


 * That would be good - any afternoon this week should work. I suspect it might be a long drive, though?
 * I was thinking more of getting helpful references into the article, really.
 * My problem with the original paragraph was with the suggestion that once you pass the critical point, turning the steering left would cause the car to move right. In my experience, once the tail goes, you're either going to make it round the bend or visit the outside barrier - nothing you can do to the wheel is going to bring you back to the inside of the curve. Since you've got the book and I haven't, can you identity a flaw in that? -- Ian Dalziel (talk) 20:54, 16 December 2007 (UTC)
 * My problem with the original paragraph was with the suggestion that once you pass the critical point, turning the steering left would cause the car to move right. In my experience, once the tail goes, you're either going to make it round the bend or visit the outside barrier - nothing you can do to the wheel is going to bring you back to the inside of the curve. Since you've got the book and I haven't, can you identity a flaw in that? -- Ian Dalziel (talk) 20:54, 16 December 2007 (UTC)


 * Well, I wouldn't particularly trust the predictions of a linearised model once the system goes unstable. I'm actually running it as a test case in ADAMS, but I've got a few problems to sort out to get it to work. The problem there is again that the tire model is not really designed for abuse cases, so I'll use tiny steering inputs and see what it does. Anecdotally Schumacher is the only F1 driver who sets his car up on the unstable side of neutral. Greg Locock (talk) 23:59, 16 December 2007 (UTC)


 * I haven't simulated it (nor will I attempt to), but I have explained what I understand of the effect in an earlier comment. You do not turn left to go right. Think of it as a high speed drift. Turn right into the turn, then countersteer, but not so much that you get out of the drift... just enough that you do not continue to yaw into the turn. This is not stated in Milliken, but I think it is a very reasonably extrapolation, once you understand the effect. I don't know why someone said you turn left to go right, probably a misunderstanding. Off course, Greg Lococks model may blow my 'theory' right out of the water. I'll wait for that before I put forward any more misguided theories :) Oh, and sorry for not signing that last comment... I'm new to this.Nereth (talk) 01:23, 17 December 2007 (UTC)


 * Greg's orginal statement was "Above this speed control is reversed, that is, the steering wheel must be turned left in order to turn to the right." - that is confusing because it sounds as though it refers to the car's path, rather than its attitude. No problem with the current wording. -- Ian Dalziel (talk) 13:43, 17 December 2007 (UTC)


 * [] That's a 5 degree steering input at 2 Hz, roughly, for an understeering and an oversteering car. I gave up on ADAMS, this is Carsim. Basically what happens is that when you reach the vicinity of the critical speed any steering input sends the car into a spin. If you drive through it and then touch the steering wheel the same thing happens. Greg Locock (talk) 07:08, 17 December 2007 (UTC)


 * It's difficult to make out which direction the steering was turned when the car spun out, because the inputs are so close together. A new simulation with just a single 5 degree right turn at the steering wheel at 150 kph should bring this discussion to a conclusion... Mr Locock, is there any reason you thought the controls were reversed, or did you just assume the steering angle would continue to reduce and then go negative? I think the steering angle for a given yaw response approaches zero, but never reaches it (just as yaw damping follows the reciprocal of speed). Nereth (talk) 14:04, 17 December 2007 (UTC)


 * The equation in one of the previous posts says the yaw gain goes negative. Do I think that'll happen in real life? Well, it certainly happens in aircraft, so it is not unheard of. Cars are more complicated tho. Yes, I think a single half wave pulse at high speed would be a good test. The wiggle experiment is more to do with the onset of instability, rather than control reversal. Greg Locock (talk) 21:04, 17 December 2007 (UTC)


 * Hmm, didn't see that equation before. It is for a steady state turn though. I think (unless I'm grossly misguided) it's easy to imagine what is going on in the formula. Steering angle goes negative only because the vehicle slip angle will be very positive. However, at the start of the turn, vehicle slip angle is still very zero. To get that equation to apply, I think you will still have to turn into the turn and get through a transient phase. Nereth (talk) 02:44, 18 December 2007 (UTC)


 * No, it is an equation that primarily talks about straight line behaviour - one of the many ways we confuse people when we talk about understeer and oversteer. I'll mod the article again!Greg Locock (talk) 03:12, 18 December 2007 (UTC)


 * For a straight line, radius equals infinity, latacc equals zero and your equation gives a steer angle of zero. What is to be learned? Can you explain that further? Nereth (talk) 03:39, 18 December 2007 (UTC)
 * Oh, I think what you mean is lateral acceleration without a turn, eg on a sloped road. In that case, the same thing applies: assuming lateral acceleration towards the left, the vehicle slip angle has to go very positive for the rear to set the rear slip angle. Then, steering angle would equal [required front slip angle] - [vehicle slip angle]. For an oversteer car where the vehicle slip angle will be more than the necessary front slip angle, that will be negative. I don't think the slip angle itself will ever be negative. Again, that formula assumes a transient phase where the rear slides out, has already passed. Nereth (talk) 04:18, 18 December 2007 (UTC)


 * It looks correct to me, but then, I've been neck deep in bicycle model for the last week, so I don't have the mindset to think about it in terms of reality. Anyway, thanks for the welcome. I've seen guides to editing wikipedia before, but never bothered reading them. I'll try to find them back. Nereth (talk) 07:51, 16 December 2007 (UTC)

Contradictory OR Ambiguous
The following statements from the text seem contradictory unless the second relates only to "high-speed" cars mass-produced for the general public, whereas the first clearly relates to professional racing cars. This seems to be the resolution of the matter (unless the 2nd statement is just wrong), but I don't want to edit the article text unless that's what was intended. Would a qualifed "equivalent original author" please step in here?

First statement -- The intention is to make fast cars oversteer (seems appropriate in a racing situation): "Nevertheless, the required front/rear balance to make the cars fast through corners is obtained by setting up the aerodynamics and balancing the suspension. The car's tendency toward oversteer is generally increased by softening the front suspension or stiffening the rear suspension. Camber angles, ride height, and tire pressures can also be used to tune the balance of the car."

Second statement (2 sections down, "Critical Speed") -- Here the intention is to make fast cars understeer?: "Oversteering cars have an associated instability mode, called the critical speed. Above this speed control is reversed, that is, the steering wheel must be turned left in order to turn to the right.(Gillespie :Fundamentals of Vehicle Dynamics, or any basic vehicle dynamics text). Understeering cars do not suffer from this, which is one of the reasons why high speed cars tend to be set up to understeer.

--User Parsiferon 03:33, 22 December 2006 (UTC)


 * The first statement is pretty much wrong, in general. Most racing cars are set up for linear range understeer (but not because of critical speed). However, even an oversteer car may not be able to reach its critical speed due to lack of power. Statement 2 is a reasonable summary of the phenomenon. I'll alter teh first staement Greglocock 22:59, 22 December 2006 (UTC)

Major Edit
This article had rather poor grammar and was a bit inaccurate on some points. I figure that a championship trophy qualifies me to help this article out a bit as an 'expert' (of sorts). So I did a lot of reworking on it. Overall, I mostly just improved the writing without changing what it was trying to say since it was more or less accurate. But there were a number of points in the original article which were presented as absolutes when they are actually not. I did expand on a couple of points as well. Hope this helps. 209.128.67.234 05:17, 29 April 2006 (UTC)

i have made some changes to the article to try and make it more understandable, i will also be having a big grammar cleanup Pratj 16:45, 7 September 2006 (UTC)

i reverted it because i saw that episode with the smart fortwo, and that was dangerous understeer, and i mean dangerous. i will be happy for you to tone it down a bit though Pratj 18:53, 30 November 2006 (UTC)

I doubt that the Prius really has dangerous snap oversteer, it is heavy, and does not have much engine braking. However, I haven't driven one. The Smart is a different kettle of fish, it has a high cg, I could believe it might do some interesting things. Either way you really shouldn't be quoting Top Gear as a technical reference, the games those muppets play are for entertainment, not analysis. How do you know clarkson didn't dab the brakes on the prius, for example?

Greglocock 01:16, 1 December 2006 (UTC)

top gear, muppets. i resent that. anyhow, the prius wasn't too bad really, in fact it was quite near a normal car so lets forget that. its the smart, which showed the most dangerous understeer i have ever seen, it literally just wouldn't turn. even with the stig driving it just would not turn, at all Pratj 16:04, 1 December 2006 (UTC)

Why do you resent 'muppets'? You aren't involved with TG are you? Or the Muppets? Anyway, What you say about the Smart is very interesting. It sounds as though it has a lot of high g understeer, probably to prevent roll-over in the elk test. Did it have throttle off oversteer as well? Greglocock 01:03, 2 December 2006 (UTC)

well, they were testing handling at about 30 - 40 mph going into the hammerhead corner, and essentially, it turned slightly on the way in, but as he tried to go the other way, it just did not turn, dangerous if u needed to take evasive action. as for throttle off oversteer, im not sure. heres a video, im not sure how u make a link so ill jsut paste the url http://paultan.org/archives/2006/01/11/top-gear-tests-cornering-ability/ Pratj 11:41, 2 December 2006 (UTC)

Great link. The Prius had understeer (not oversteer), the Malaysian thing had some sort of snap oversteer, and the Smart had terminal understeer. IF they were all at the same speed going into the corner I'd say the Prius was borderline acceptable, the other two, not. Since this is the oversteer page, I think any refernece to the Smart should be on the understeer page. Greglocock 01:22, 3 December 2006 (UTC)

yes, i did wonder why that guy put this on the oversteer page when it was understeer. i didn't think about that Pratj 16:30, 3 December 2006 (UTC)

Phenomenon?
Why does the article describe oversteer as a "Phenomenon" but understeer simply as a "term"? This seems a little overstated to me. —Preceding unsigned comment added by 155.31.230.90 (talk) 22:50, 15 October 2007 (UTC)

Bad figure
The previous figure (the second one) is either fantasy (bad physics) or very misleading. (If a car goes off the road to the inside, it's not strictly lack of friction. It's ultimately bad driver steering.) The top one shows more realistic failure: understeer you see hit the wall head-on, understeer you rear-end the wall.

FWIW, I still think this whole article should be merged with Understeer. Mikesayersskier (talk) 00:15, 18 May 2011 (UTC)


 * Do it already. Just follow the directions here. -AndrewDressel (talk) 02:12, 18 May 2011 (UTC)


 * OK. I'm trying a redirect. We'll see if it works. Mikesayersskier (talk) 03:04, 18 May 2011 (UTC)

Does loss of traction in the rear wheels somehow allow you to turn corners more tightly than you would otherwise be able to with all four wheels gripping the road? Of course not. This figure is completely wrong and should be deleted. -- Sakurambo 桜ん坊  13:01, 28 February 2008 (UTC)


 * Er, yes it does. Ultimately it "allows" you to turn through 180 degrees - try that with all four wheels gripping the road! There are hairpins which can be negotiated in an oversteering slide which would necessitate reversing otherwise. -- Ian Dalziel (talk) 13:51, 28 February 2008 (UTC)

What you're talking about is the orientation of the vehicle. Not its position on the road. Think about it. How could the loss of traction in a vehicle's rear wheels possibly enable it to manoeuvre through a tighter corner than it was able to manage with all four wheels gripping the road? (Hint: the lateral force on the tyres is inversely proportional to the radius of curvature.)

The car will in fact run off the other side of the road. Its orientation might spin through 180 degrees in the process. But its position will definitely not follow the path shown in this figure. -- Sakurambo 桜ん坊  11:14, 29 February 2008 (UTC)


 * Oversteer is not necessarily equivalent to loss of control. Change of orientation without loss of front-wheel grip enables a tighter turn - as in a handbrake turn, for instance. -- Ian Dalziel (talk) 17:27, 29 February 2008 (UTC)

You don't seem to have a very good grasp of the physics involved. In a handbrake turn, the rear wheel traction is lost deliberately by applying the handbrake to stop the rear wheels from turning. Oversteer occurs spontaneously when a car tries to take a corner too quickly and the rear tyres are unable to provide sufficient traction to counteract the resulting centripetal force. -- Sakurambo 桜ん坊  18:08, 29 February 2008 (UTC)


 * Oversteer is oversteer whether it is spontaneous or deliberately induced.
 * I make no claims to being a physicist, but I think you might want to re-read the section on centripetal force before throwing stones out of your glass house - the centripetal force is supplied by the tyres, not overcoming them. Momentum overcoming the centripetal force is what produces the "centrifugal" effect.
 * A car losing control due to an oversteer-induced slide will indeed exit the outside of the corner, not the inside - the diagram may well be confusing in that respect. But a skid turn certainly can be tighter than the normal turning radius of the vehicle - as I already said, rally cars have to induce an oversteering rear-wheel slide to negotiate the tightest hairpins. -- Ian Dalziel (talk) 18:29, 29 February 2008 (UTC)

OK, so are you suggesting that this figure is supposed to be an illustration of deliberately induced oversteer? Do you think that is helpful given the context of the figure's caption and the rest of this article? -- Sakurambo 桜ん坊  10:34, 1 March 2008 (UTC)


 * I think it's an illustration of oversteer below the limit point - understeer and oversteer are components of normal cornering behaviour, not restricted to conditions of excessive speed. That could perhaps be made more clear in the text? -- Ian Dalziel (talk) 11:03, 1 March 2008 (UTC)


 * Trying not to be negative, but I'm not sure if the illustration adds much clarity as it doesn't show what happens at the rear of the car. If the image requires additional a lot of additional explanation is it worth including? Done a quick search on google (also did one for understeer previously) and found the following


 * http://www.worcadvmot10.fsnet.co.uk/Skid_Control_Oversteer.htm - good image which shows steering position


 * http://www.drivingfast.net/car_control/oversteer.htm - which has a decent animation and some supporting pics. Don't know if you can link directly to a flash animation with wikipedia?


 * http://www.iihs.org/ratings/esc/images/oversteer.gif - from an ESP website


 * Not sure if any of these could be adapted or linked to? I'm no graphic designer so would really struggle! If there's no objection, may put these in the external links section if we can't adapt one for ourselves. Driver sam (talk) 15:32, 10 June 2008 (UTC)

I agree with the statement that the illustration is bad. I immediately reacted to it when I first noticed it.

I believe that many who read this article does so in the context of autoracing or sports driving, and then this figure is completely inaccurate, even misleading.

It is possible for the vehicle to take on the curve presented in the figure when oversteering, but only when NOT traveling at (or close to) the limit of its cornering envelope, i.e. only at moderate to lower speeds (compared to what the vehicle in question is capable of in the current situation); e.g. if the rear wheels of the car somehow instantly loose traction, e.g. by explosion or the unlikely event that only the rear wheels would get affected by oil on the road, or by driver induced oversteer (e.g. if the driver felt like attempting a drift but despite his attempt managed to steer the car off the road like depicted).

However this would obviously not happen if the car would attempt to take the corner at its cornering limits, oversteer implies loss of grip, and any loss of grip entails loss of cornering speed -- the car would slide out of the corner on the other side or in any case progressively slide towards that side if it did not spin around.

I think at least it should be stated the circumstances for the figure, i.e. that the car is not a race car, and that it is not going around that corner as fast as it can. But regardless of extra information it would still be ambiguous -- if a figure needs so much explaining it might be better with a completely different figure if it only serves to be confusing.

Ideally there should be two figures; one for taking the corner at the limit and one for taking it at a moderate speed.

Bottom line: The figure is misleading and completely wrong in all auto racing contexts and most sports driving context and is ambiguous at best in other cases.

The figure should be removed until there is a more clear non-ambiguous alternative, two illustrations, or at least proper text to explain the current figure properly. Posix memalign (talk) 18:03, 30 January 2009 (UTC)


 * Oversteer does not imply loss of grip. Oversteer can result in loss of grip, but once grip has been lost there is no kind of steer. The diagram does show the effect of oversteer on cornering. It is confusing because the path is extended off the road - that would never happen normally because a correction would be applied. It is important, though, to distinguish between oversteer and an oversteer-induced slide. -- Ian Dalziel (talk) 06:46, 31 July 2009 (UTC)

Either this is hardly readable and makes no sense OR...
...or is slight-to-moderate oversteer good, while understeer is positively lethal?! Of course, I may be projecting my own experiences of totaling an FWD jap due to understeer, while a slightly oversteer-prone RWD beamer has saved my butt quite a few times. Aadieu (talk) 07:43, 19 February 2010 (UTC)
 * If you find understeer lethal then you are ignoring the vast weight of evidence by experts and engineers alike. Your BMW has understeer, at least if it is a 3 series or 5 series or an X5. I have the measurements to prove it. Any reasonably powerful RWD can be provoked into oversteer, more people get into trouble with it than not. Greglocock (talk) 22:49, 19 February 2010 (UTC)

Understeer and Oversteer should be one topic
The concepts of understeer and oversteer have been around for many decades, and they're much simpler in concept than the two wiki pages indicate. The US Society of Automotive Engineers (SAE) has published a standard for terminology J670 that covers the topics in a few sentences. The definition in J670 from 1976 has been kept in the updated version (2008?) and is also in the new ISO 8855 standard on vehicle dynamics terminology. They're also in any textbook I've seen covering vehicle dynamics. Certainly Gillespie's "Fundamentals of Vehicle Dynamics" textbook (referenced in the Oversteer page) presents the concept succinctly.

Summary of the Definitions
The concept is that a car making a turn with non-slipping tires has front-wheel steer equal to the arctangent of the quotient of wheelbase divided by the turn radius. This is called the Ackermann steer angle. If the lateral acceleration increases due to increased speed on a constant-radius track, or decreased radius at a constant speed, or any combination, there is a gradient of steering wheel angle vs. g's that would be obtained with Ackermann steering (Ackermann's name is always used for the reference neutral steering).

The real measure is the difference in gradient (real - Ackermann). If the difference is greater than zero, it's understeer. If less than zero, it's oversteer.

Different companies and organizations have slightly different test procedures for establishing the gradient. It's normally done under quasi-static conditions in which either steer or speed are increased very slowly. Results do depend on the test, so just giving a deg/g number is not sufficient; you also have to indicate the type of test used.

In more transient maneuvers, such as racing turns or stability control tests, the names understeer and oversteer are also used by engineers to indicate that the steering needed for the maneuver is more (understeer) or less (oversteer) than some reference.

What Next
I'm new to wikipedia editing, and have no clue about how to merge pages. To make sure this is seen, I'm pasting it into the talk pages for both topics.

If they are merged and redone, I can contribute if needed to ensure that the descriptions match the SAE and ISO definitions.

Mikesayersskier (talk) 18:36, 12 June 2010 (UTC)


 * I'm in total agreement not only that the articles should be merged, but that they should begin with explanations similar to the summary definitions given above by Mikesayersskier. The article currently begins with an explanation that seems wrong to me: "The car is said to oversteer when the application of cornering force (lateral force) also applies a rotational torque (or moment) to the car in the direction of the turn." Isn't there always a rotational torque in the direction of the turn when the car is, um, turning? Otherwise it's not turning.


 * Also, FWIW, I agree with posts above that the current diagram is misleading and confusing. ColinClark (talk) 17:45, 9 December 2010 (UTC)


 * In a neutral car, there is only a rotational torque in the direction of the turn when turning in. It's simple physics... once the vehicle has achieved a rotational velocity (i.e. rotating about its CG) it needs no more torque to continue rotating. If you inaccurately consider the tires to be a rigid connection to the ground, then this concept would seem incorrect. Instead consider that the car is floating in space and that the tires merely provide all of the input forces that cause it to turn around a track. F = ma and Torque = inertia * rotational acceleration still apply.


 * There is an opposite torque (out of the turn) on turn exit. An oversteering car will exert a moment that tries to rotate the car into the turn during steady state cornering. Understeer behavior is the opposite. Bdc101 (talk) 17:54, 9 December 2010 (UTC)


 * Ok, that makes sense. But I'm still not convinced that's a real definition of oversteer. That definition would seem to say that either all cars oversteer on corner entry and understeer on exit, or that oversteer and understeer only occur during steady state cornering, but we know that none of those are the case. At the least, I've never heard it described that way anywhere else.


 * To take a step back, though: everything in a Wikipedia article needs to be sourced and the explanation/definition in the introduction is not. I think the article would be best served by citing the SAE and ISO definitions mentioned by Mikesayersskier.


 * For a layman's or simplified description I actually prefer the one in the "Physics" section of the understeer article ("If the slip angle of the front wheels exceeds that of the rear, the vehicle is said to be understeering. If the slip angle of the rear wheels exceeds that of the front, the vehicle is said to be oversteering."). That's the one I've most commonly seen.


 * What do you think about merging the understeer and oversteer articles? You can't really talk about one without the other, and I think it would be easier to maintain and keep one article consistent. ColinClark (talk) 05:52, 10 December 2010 (UTC)


 * Colin, thanks for the informed reply. I absolutely agree that they should be merged as they each describe half of the spectrum of vehicle stability. I also agree that the definition should be fully sourced. But the definition you state (I believe it's from Fred Puhn's book?) is, in my opinion, the worst definition of stability that there is. Slip angle is not a widely understood concept, and the definition only applies to the ideal test vehicle which has four identical tires, alignments, no Ackermann geometry, etc. It's absolutely not a layman's explanation, and it serves to confuse when applied to non-ideal vehicles (or someone's personal vehicle if that is their primary interest for learning this stuff).
 * I always explain oversteer and understeer using forces and moments for this reason. It disregards the state of the tires and alignments, and instead focuses on the car as a whole, non-rigidly constrained to the ground, obeying F=ma. It applies to any vehicle in any state. But it depends on how simple or complex we want our definition to be. Do we want anyone to be able to understand it or does the reader need to have an initial understanding of vehicle dynamics, tire dynamics, F=ma, etc. before they can understand this article? I don't know if I should answer that question. Bdc101 (talk) 18:12, 10 December 2010 (UTC)


 * I agree with merger: I went to conpare the two pages and found that the intro para's read very different and in trying to understand and compare the two concepts (since they are essentially symmetrical) which is counterproductive.

RECOMMENDATION and SOLUTION: MERGE. 122.148.41.172 (talk) 09:24, 23 December 2010 (UTC)

Bad Citation
I agree with parts of an above discussion that the "critical speed" section is unclear and seems dubious. The only readily available citation, http://www.me.umn.edu/education/courses/me5286/vehicle_dyn/Vehicle%20Dynamics%20Lecture05.pdf, does not link to an existing page, and should be replaced with something that can be read. 174.29.64.173 (talk) 00:58, 5 August 2010 (UTC)