Talk:Oversteer

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[edit] 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)

Here's a similar case. Accelerate around a corner on a gravel road, in a RWD. As you accelerate you'll need less and less steering wheel angle, as the tail starts to come out. Finally, the rear end will break away, and you'll counter steer, ie steer left while turning right. So, you started with positive yaw velocity gain, must have gone through zero, and ended up negative, just like this equation says. Greglocock 22:30, 31 December 2006 (UTC)

[edit] 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)

[edit] 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)