Fun thread
I think the topic has been pretty well covered, but I have a bit of experience specifically with the 2nd gen MR2, of which the early cars ('90-early '92) were well known for snap oversteer. Basically people seem to have problems driving them near the limit because of two issues. The first problem is the inherent relationship between PMOI, driver sensitivity and mid-engined cars (transverse or longitudinal) which I feel Driven5 nailed here:
Driven5 wrote:
... The masses that make up the car are trying to straighten their path from the arc. The further back you place the various masses, the more they try to essentially 'rotate' the back of the car around the front axle of the car, or oversteer. A transverse middy layout has almost essentially all of the drivetrain weight located in front of the rear axle by a relatively short distance. Now a longitudinal layout moves some of the engines weight further forward from the transverse mounting, but also moves the weight of the entire transmission behind the rear axle. So while the force from the engine towards creating oversteer is reduced, the force from the transmission towards creating oversteer is magnified. Notice however that in this case the engine is not spinning around its center of mass, but rather is cantalivered out and being swung in its entirety around an external axis towards the front of the car. The moment of inertia for a rigid system, like a car, is the sum of the moment of inertia of all of the components relative to the axis of rotation of the system. So locating a similar component mass with a similar center of mass location will have a similar effect on the overall moment of inertia of the car. Neither the weight distribution nor the moment of inertia of the car about the cars rotational axis as oversteer occurs should be significantly different between these two mid-engine layouts…Thus realistically if a driver does not possess the skill to control a transverse mid-engine car, they should not assume their lack of skill will be any less of a concern in the same car with a longitudinal drivetrain.
I think the following two clips of earlier posts really get into the driver sensitivity (skill?) portions of that relationship:
BluEyes wrote:
To your physics argument, what you are talking about is PMOI. That does not affect the steer tendency of the car, but rather how quickly it will react to inputs. It will make a car more likely to spin quickly if you cross the line, but also easier to recover from. ...
And:
Driven5 wrote:
... Ultimately I would say that the biggest inherent contributor to the challenge many drivers have with mid engined cars, is that a mid engine drivetrain places the driver closer to the axis of rotation than with a front engine layout. Because of this the driver experiences a less pronounced change in physical position (lateral and rotational) relative to the path of travel than if seated towards the rear of the car. This reduces the average drivers ability to detect the small changes in vehicle attitude leading up to an oversteer situation, often until it's already too late to react. The simple fact is that it generally requires a more sensitive posterior to drive a mid or rear engine vehicle near its limits. ...
The only point I would argue differently is that because a low PMOI allows a car to rotate more easily, it requires a more sensitive driver to detect and correct for the driver's inner ear being (usually) closer to the center of rotation in a middy (especially a transverse middy) and the sensitivity and experience needed to detect and correct for the car's behavior are greater than most other vehicle formats; essentially it makes it harder for most drivers to avoid crossing the "spin point" in the first place. Longer wheelbases and moving the actual masses out towards the ends of the car slow down rotation and give the driver more time to react. In a tightly packed middy everything happens much faster and many times an inexperienced driver doesn't sense the severity of the problem until after a threshold has been reached were the car's oversteer can't be easily corrected for. I think this relationship essentially applies in all (rear engined) middies, and is probably one of the many reasons modern supercars tend to favor long wheelbases.
My second point, as BluEyes pointed out, is that the MR2 has it's own special issue:
BluEyes wrote:
... At least a few other people (and Toyota engineers) felt that the MR2's behavior was caused by the suspension, or at least could be adressed with simple changes:
http://en.wikipedia.org/wiki/Mr2#Revisions_and_Model_Year_Changes*edit: fixed link the right way this time
The early 2nd gen ('90-early '92) MR2 has an additional issue in that it's rear suspension had excessively short radius rods compared to the control arm lengths and the car's bump steer would put it into a significant toe-out situation when the rear suspension was heavily compressed. OK for an AutoX if you know what to expect (and when), but not so good on real roads or in panic situations. Throw the car into a really hard corner (or have it suddenly roll heavily to one side) and the back end would suddenly become very lively and try to help the car rotate even more. The car received a lot of "word of mouth" about the issue because there were far more '90-early '92 cars produced and sold than late '92-'95 cars which had a revised rear suspension. For track use we addressed the problem by stiffening up the suspension to the point where the toe-out problem was minimized and manageable.
An interesting side note was that I was more comfortable with less understeer dialed in than the other drivers whom I worked with. Both of them liked the front end to push a little instead of needing to balance the back end of the car in fast corners. That set-up did hurt lap times, I was consistently just a tiny bit faster; about 1/2 second per lap on a typical track.
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