Why Such Sudden Oversteer....?

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Heard anecdotal evidence (ie. instructor at last NSXpo event watching 3 track prepped NSXs crash) and experienced sudden oversteer myself and wondering why the NSX is so prone to it (ie. letting go without much warning). Is it strongly correlated to its mid-engine location?

I have an '88 M3 track car that is much more progressive when exceeding the limits of adhesion, which allows me to four wheel drift it on certain turns at Summit Point.

However, last tues, when moderately accelerating the NSX to about 30 on a wet street and then around a bend, the back end stepped out very, very quickly with not much warning. was able to grab it and not ground loop it (thanks to lots of skid pad time), but it came around much faster than either my M3 or my Z3 would.

Is this type of behavior common with the NSX? (I have only had mine 3 months and still getting used to it).

Thanks!




------------------
- Joel Smernoff
'96 NSX-T
'96 Z3
'88 M3 track toy
'91 Legend winter trasher
 
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Originally posted by jsmernoff:
Heard anecdotal evidence (ie. instructor at last NSXpo event watching 3 track prepped NSXs crash)

Absolutely not true. Amazing how these rumors fly around.

At NSXPO 2001, there was only one "incident", a low-speed off-track excursion by a stock NSX resulting in minor front-end damage. In fact, the track management at Road America (where there are two BMW CCA events every year) said that our event was the safest they have seen in a long, long time. Windy City BMW (our local chapter, whose track events are noted for their rigorous safety procedures) holds a track event there annually, and it was incident-free only one time in the past 22 years.

That being said, all cars are prone to oversteer when the throttle is lifted suddenly, and the NSX is no exception. Mid-engine cars, by their geometry, are easier to spin, due to a low polar moment of inertia (this means that the weight is in the middle instead of toward either end of the car). The NSX isn't normally prone to oversteer, but if you lift suddenly in a turn, it is more likely to spin than some other cars.

The other thing that the NSX is susceptible to is that the OEM tires, which are sticky and handle great, do quite well on wet pavement as long as they have plenty of tread; however, when they are worn close to the tread indicator bars, they become more susceptible to hydroplaning, so additional caution on wet pavement is required. Perhaps your tires are getting worn? This could have contributed to your incident.
 
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It's most likely happening because the driver is driving the NSX like a front engine car (i.e. trail braking through the turns). It's funny how often I see BMW club, Viper or Corvette, etc. hotshoe instructors drive their student's NSX's at track events and totally screw up!
 

Edo

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ALL mid engined cars are more proned to spinning.
Think about it this way. Assume an Ice Skater is turning circles with their arms out. They can then pull their arms inward and spin faster without exerting any extra force on themselves.

Similarly, an NSX with all its weight centered around the middle of the car will spin faster than a car with weight centered closed to the edges (M3)

This is an advantage in the sense that the car is incredibly quick response to steering inputs, and can make very quick transitions from turn to turn due to having all of its mass centrally located.

Physics dictates that "All objects in motion will continue in the same direction unless acted upon by another force"

Using this, we can find that in an M-3 the bulk of the mass is in the front of the car. Hence when you turn the wheels, the heaviest part of the car (The nose) wants to continue going forward. This causes understeer or "Push". In this case turning the steering wheel more will do no good as you've already reached the maximum ability of the tire to change your direction.

In an NSX or other mid engined car, the nose has no weight, so consequently will turn very quickly, but the Mass, located behind the steering wheels, wants to continue going forward (Just as in the M3) Consequently, the nose turns, and the rear wants to continue going forward. This causes oversteer or makes the car "Loose".

What makes a mid engined car so quick through the corners also revolves around the method of settling the car down WHEN it starts to oversteer. In an inherentlky understeering front engine car, the only way to get the car to settle down after inducing understeer is add weight to the front wheels. (IE Letting off the throttle a bit, or braking just a little). In a mid or rearn engined car, since the rear wheels are losing traction you need to stay on the throttle to transfer weight to the rear wheels.

So in an M3: When you are going too fast, you need to lift or brake
in an NSX : When you are going too fast, you need to throttle a little bit to clear the corner without spinning.

As Driving fast is about giving a car as much gas as possible without exceeding its limits, you can see the inherent advantage in having to throttle up to keep the car steady through a corner.

I personally find the NSX's limits to be VERY easy to read and have no problems with snap oversteer. The NSX gently Drifts when pushed to its limits and has never spun unexpectedly.

As a side note, most race car drivers don't really like understeer. On the other hand, all street cars are built to understeer, and most people do not have the necessary experience or training to control an oversteering car.

This may explain why you feel more comfortable in your M3...or at least feel that it is easier to drive. Understeer=safe
 
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Edo' commenst are pretty good
biggrin.gif


With his permission and the forum's permission we would like to add this discussion on our Sacto web page under track preparation section.

Do we have your permission?

Hrant
 
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Originally posted by Edo:
Using this, we can find that in an M-3 the bulk of the mass is in the front of the car.

Edo, great info and tips on how to properly behave in each car.

BTW, I thought the M3 (E36) had near perfect (almost 50/50) front-to-back weight distribution, no?
 
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Hi Joel,

I also own a 88 M3 that I'm running in BMWCCA club racing in JS.

In general the NSX is more sensitive to steering input and the M3 is more forgiving. You just need more seat time (at the track) with the NSX. Once you get accustom to the car it will give you a big reward.
 

Edo

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Weight distribution, and Polar Inertia (mass location) are 2 different things.

For example..a Corvette C-5 has a near 50/50 weight distribution. They accomplish this by having the engine in the front and the Tranny in the Rear. (2 heaviest items in the car at 2 opposite ends). While this is great for a car that handles relatively stable due to its 50/50 distribution of weight, it does not necessarily make for a car that makes quick transitions. This goes back to our Ice Skater analogy. If you put heavy items near the edge of a rotating mass it rotates slower.

The Key is not so much the weight distribtuion front to rear as it is WHERE the weight is located in regards to the car.

Also keep in mind, Less Mass = better handling. Think about a body of water for example. A Large body of water does NOT react quickly to forces acting on it, even if the force acting on it is extremely high.

Same with a car. A car with less mass will react quicker to steering inputs, and direction changes.

Another odd thing, which might be a bit hard to understand is the weight to mass relationship. On Earth weight and mass are *almost* interchangeable. (Please anyone correct me if I am wrong with any of this)
But think about it this way, weight distribution wouldn't mean a thing in Space where mass matters and nothing has weight
So consequently, an M-3 which has a 50/50 weight distribtion on Earth, would be very difficult to move if you pushed the direct center of the car in space. You would have to move to the front of the car where most of the mass is located to move it. So regardless of the weight distribtion the key point is where the mass lies. This becomes less of an issue on the Earth's surface at 1G but, while cornering if you ignore the 1G force pushing the car down on the Y-Axis, the X and Z axis motion are the same as in space.

Also, apparently a slight rearward weight bias is desireable on race cars..Not too sure why.

Also, I'm willing to bet that the M-3's 50/50 weight distribtion takes into account its suspension. Slight changes in suspension heights and mounting points can greatly affect weight distribution. I am almost willing to bet money that if you took an M-3 at the exact center of the chassis and tried to balance it, it wouldn't work. ( I could be wrong on this assumption. No flames please)


If anyone is interested I'd be happy to go into greater detail as to the physics aspects of a car's handling and response...
Just let me know what aspect you want explained further.


[This message has been edited by Edo (edited 28 March 2002).]
 

Number9

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Originally posted by 8000RPM:
Edo, great info and tips on how to properly behave in each car.

BTW, I thought the M3 (E36) had near perfect (almost 50/50) front-to-back weight distribution, no?


Not after you throw out the spare and the jack!
wink.gif
I think Edo just meant that there is a large concentration of mass at the front of the car away from the center of rotation, whereas in the NSX the engine mass is pretty close.
 
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Some very good physics theories here about the NSX. As i am a track rookie learning how a mid engine car behaves.

Edo that was a good write up, I am gonna print it out and use it as my track bible. My buddy with the E46 M3 would also appreciate the writeup.

Anymore advice from veteran racers out there?
David
 
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Originally posted by jsmernoff:
...However, last tues, when moderately accelerating the NSX to about 30 on a wet street and then around a bend, the back end stepped out very, very quickly with not much warning. was able to grab it and not ground loop it (thanks to lots of skid pad time), but it came around much faster than either my M3 or my Z3 would.

I think the answer is not to screw around with your NSX on a wet road!
 
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Originally posted by Edo:
Weight distribution, and Polar Inertia (mass location) are 2 different things.


That is the key sentence in 2 excellent posts. Polar Inertia is what needs to be understood.

And dont early apex
smile.gif


[This message has been edited by justin hall (edited 28 March 2002).]
 
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To tell you the truth, I kinda dissapointed by Fred Puhn's book. Maybe because I've been reading too many books and his books doesn't cover anything new.

I would recommend Race Car Engineering and Mechanic, by Paul Van Valkenburg.
 
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OK guys, editorial copyright chest pumping or pimping ego issues aside <g>, is the info from this public forum restricted from being copied when proper acknwoledgment is offered?
 

Number9

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Originally posted by Edo:
Using this, we can find that in an M-3 the bulk of the mass is in the front of the car.

(Please anyone correct me if I am wrong with any of this)
But think about it this way, weight distribution wouldn't mean a thing in Space where mass matters and nothing has weight
So consequently, an M-3 which has a 50/50 weight distribtion on Earth, would be very difficult to move if you pushed the direct center of the car in space. You would have to move to the front of the car where most of the mass is located to move it. So regardless of the weight distribtion the key point is where the mass lies.
[This message has been edited by Edo (edited 28 March 2002).]

Good posts, Edo. Two minor nits - since the weight distribution of the M3 is about 50/50, the "bulk of the mass" of an M3 isn't in the front of the car, though there does happen to be a mass concentration there relatively far from the axis of rotation, hence the higher polar moment.

Regarding the space example, pushing at the engine would most likely cause the car to rotate - but applying a force at the centroid would cause translational movement. End of nit.

BTW, Hrant, get permission from Edo to copy, reproduce, display yada yada his post since he owns the copyright. Fair use only gets you so far...

[This message has been edited by Number9 (edited 28 March 2002).]
 

sjs

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It sounds like your near spin was mostly throttle induced which is significantly different than much of the material covered. Once you're loose they are much the same, but if you are asking about the cause I would guess it is simply a case of having the car approaching the limits of adhesion while still accelerating. You have only so much traction available. You can spend it to accelerate, brake, or turn. Of course those limits are much lower on wet pavement, and as already noted, the amount of tread becomes critical. When you accelerate, your rear tires have less traction for turning than your fronts. (Although the get some back due to rearward weight transfer.) With the power of the NSX it takes very little throttle to spin the tires on wet pavement, much less if most of your traction is already spent on cornering, and before you realize what happened it can be stepping out on you.

Although the other posts here were very good, I’m not entirely comfortable with some of the points made so far. For example, it seems that the importance and impact of polar moment was not thoroughly explained.

Keep in mind that the skater analogy applies after traction is lost and the spin started, and even then I think it’s cheating a bit. Pulling the arms in while already spinning does not so much “allow” them to spin faster as force them to. I’m also not sure you covey the right meaning or mental image when you say that a car with more mass in the center will spin faster. I’m having trouble putting it into words myself (and I’m no engineer or physicist) but perhaps more discussion of polar moment will help.

Of course polar moment can be very significant in an autocross slalom or even tight esses where you have relatively abrupt transitions in direction, but otherwise it is probably less significant than overall balance if you’re in control. Where a high polar moment really comes into play is how forgiving the car is in terms of catching it after things go wrong. With a car like the 944 or C5, the F/R balance is very good as noted above, but when it does get loose, catching it can be tough. Rather than a skater, imagine a simple barbell. Slide the weights near the center then grasp the bar in the middle with one hand and spin it first one way and then the other. Now slide the weights out to the ends. It will be noticeably harder to start it spinning, but really tough to stop it and get it going the other way. This is where the laws about a body in motion can bite you fast and hard. That too is over simplified because a car has tires at the corners rather than a literal fulcrum at the center, but it gives a reasonable picture of why once you start to spin in a high polar car, a slight over-correction can quickly escalate into a several oscillations and a very hairy spin. That’s the worst danger of polar inertia.

I think overall F/R balance is more important than you give it credit for in terms of turn-in and controlled cornering. Although being nose-heavy certainly is a factor, that an M3 understeers coming into turn is as much due to suspension tuning as weight distribution. Even the NSX and rear engine Porsches are designed to understeer in normal conditions. The problem with a tail heavy car is that when you lift or brake in a turn, a significant amount of weight transfers to the front. Since available traction is closely linked to the weight on a given tire, that means you just shifted some of your traction from the back to the front. If you were approaching the limits of adhesion already, the back is going to break loose as the front digs in and serves as your new pivot point. The more tail heavy the car, the more weight there is to shift and the faster it snaps. That sounds a bit like the skater analogy, but I don’t think it’s really the same. (Obviously, the suspension plays a role in how much weight transfers to the front, as does brake balance if you’ve made the mistake of braking with the wheels turned.) It’s important to note that on street cars of all configurations this often starts as understeer, prompting the lift or brake. (remember the recent video of the S2000?) A tail heavy car like the 911 can understeer from too little weight up front (and therefore traction) unless the driver knows how to set the suspension with a little lift or tap of the brake just before turning in. That’s one reason many people complain the old ones are rather vague at turn-in, the nose is so light.

I’d also recommend caution on the statement that with a front engine car you need to lift or brake if you’ve come into a corner too hot. Depending on how and where you got into trouble, the same weight transfer can still cause the back end to come around. If you’re in a bad understeer situation and really have no choice but to slow down or dive headlong off a cliff, it is sometimes possible to straighten the wheels briefly so you can use a little brake, then turn in again. The sooner you recognize that you aren’t going to make it, the better your chances of pulling that off.

While the standard answers about how to get out of trouble in a front engine Vs. mid/rear engine car are certainly not “wrong”, there are so many variables in terms of how/why you got loose, road conditions, corner shape and camber, suspension setup, etc. that the actual best response in a given situation can easily be the reverse.

OK, rant away!
smile.gif



[This message has been edited by sjs (edited 28 March 2002).]
 

Edo

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ok, Phew..sjs that was a good one. One thing I wanted to add, was regarding "recovering" a vehicle that has already lost traction. Many people are used to countersteering and throttling fairly heavily (lots of countersteer) In a C-5 where the bulk of the mass is located at the edge of the rotating mass, the response time in regards to your countersteering is slower, but consequently is easier to judge (More time). In an NSX, once it spins, it wil recover quickly, but will also whip the car the other direction due to #1 Low polar inertia, and #2 what I call the Pendulum effect. The heavy portion (the rear) swings one way, then when over correcting swings the other directions and voila. Spin.
If you watch people at the track who spin in mid engine cars its typically a spin one way and then whip the other way sort of spin. Whereas in a Front engined car like a C-5 the spin is just to one direction.

This is just *some* of the particulars that I'd wanted to mention and does NOT in any way even come close to covering some of the dynamics of the whole thing, just as Sjs said.

HRant, You are free to copy and paste/link anything that I have written to any forum anywhere.



[This message has been edited by Edo (edited 28 March 2002).]
 

sjs

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Edo, again I mostly agree, and that's what I was getting at when I said:
...it gives a reasonable picture of why once you start to spin in a high polar car, a slight over-correction can quickly escalate into several oscillations and a very hairy spin. That’s the worst danger of polar inertia.

When you say:
In an NSX, once it spins, it will recover quickly, but will also whip the car the other direction due to #1 Low polar inertia, and #2 what I call the Pendulum effect.

I'm not exactly sure what you mean. If you are saying that high polar inertia gives you more time to correct initially, well I guess so, but the price of too much correction too fast, is much more severe and then things happen really fast. I'll definitely take the lower polar moment and rely on my reaction time, even at my advancing age.

The pendulum effect of course is the price of moving the engine behind you. Like having one weight further out on the dumbbell bar than the other, exacerbating the effect of polar moment when that’s the back end of a car. In moderation it's a bonus to someone who knows how to use it, but makes driving at the limit much more difficult for those who don't. I find the NSX to be very good in this respect, having gotten crossed up plenty of times, taken a few excursions, and looped at speeds I prefer not to think about, but never the multiple oscillation spin. (I tend to play a little too hard)


[This message has been edited by sjs (edited 28 March 2002).]
 
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Not having tons of experience on racing, I can't comment on all of the correctness or errors of the above, but being well versed in Newtonian Physics (our cars might be fast but not fast enough for relativity to matter), I can comment on some of the above. Weight is merely the product of gravity acting on a mass. Moment of Inertia is the rotational equivalent of mass. Center of gravity is usually referencing how high the Centroid of your Moment of Inertia sits off the ground. The Centroid is the point of balance in all dimensions. If you spun a mass in space, not matter which way you spin it, there is a point that would stay at the center, the centroid. The skater spins faster by bringing her arms in because she is conserving rotational momentum. When a given object of fixed mass has most of it's mass located close to the centroid, it takes less energy to make it spin at a given rotational velocity. Move the bulk of the mass further from the centroid, and it requires more energy to spin that mass at the same rotational velocity as it has a higher Moment of Inertia. I can go on & on about Physics, but I will stop here for now. Anyone wishing more dialogue on Physics, ask your questions, I'll do my best to explain.

Fritz

P.S. Gravity, it's not just a good idea, it's the Law.
 
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Mid-engined cars have certain odd manners. This can be experienced when driving over a steel-grated bridge, the NSX will wander from side-to-side.
 
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Originally posted by nsx4fun:
The skater spins faster by bringing her arms in because she is conserving rotational momentum. When a given object of fixed mass has most of it's mass located close to the centroid, it takes less energy to make it spin at a given rotational velocity.

Very nice...all traction put aside, the reason a mid engine car is prone to spin easier is because the majority of the weight is closest to the centroid?... According to your illustration the closer the weight gets the faster it spins. That would explain why its more difficult to 'correct' a mid engine spin because the event takes place quicker?...less time for reaction.

Originally posted by nsx4fun:

Anyone wishing more dialogue on Physics, ask your questions, I'll do my best to explain.

Fritz

Keep going..i don't know on what..but physics is nice.


------------------
jack of all trades, master of some.
 

sjs

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Originally posted by true:
According to your illustration the closer the weight gets the faster it spins. That would explain why its more difficult to 'correct' a mid engine spin because the event takes place quicker?...less time for reaction.

Edo and I tried to touch on that, but the first statement seems prone to convey the wrong idea, the second helping a bit but it still comes up short. Yes it takes less energy to spin the center-heavy object, but that does not make it easier for the driver to lose control (spin) in the first place except for the second point about less time to react. However, the amount of correction required for the center-heavy car is far less than for one with a high polar moment, and the consequences of correcting too much are far lower. And, the tires and suspension are better able to cope with that lower level of correction. So overall, the center-heavy/low polar moment car is easier to control. However, the NSX being mid engine means that besides having a low polar moment it is also tail heavy and there for more prone to Edo’s pendulum effect than a car with otherwise similar weight distribution but biased forward. (The RX7 comes to mind, but it has low a polar moment and better overall distribution overall).

The fact that a car has it's wheels at the extreme four corners (in terms of mass) seems like an important factor that hasn't been thoroughly incorporated into the analogies here. Can you help to analyze their impact on the overall physics?
 
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