Originally Posted by eric1234,Apr 14 2010, 05:34 AM

(edit: I see that later posts recognize and discuss static and dynamic friction. The following may not be all that helpful - I wrote it in reply to the earlier, quoted excerpt. Disregard as appropriate... ;-))

No, not quite. VSA reduces the amount of torque - to prevent the torque from overpowering the wheels. It is too much torque - overpowering the wheels - it is this overpowering that will reduce wheel grip.

This works in an identical fashion to ABS.

To start at the basics of physics: All of the grip is generated between the wheels and the ground - "grip" is really friction.

There are two kinds of friction:

1) Static friction and 2) Sliding friction. Static friction is ALWAYS greater than Sliding friction.

What does this mean: Put an object at rest on your desk and try to push it. You can apply a certain amount of force BEFORE it will begin to move. This is the Static Friction generated between the surface of your desk and the object. Lets say you can push against the object with 1lb of force before it begins to move. Static Friction can be measured by the amount of force required to get an object at rest to START moving.

Once the object begins to move, the amount of force to keep it moving at any given speed is LESS than 1lb. This is sliding friction - the amount of force required to KEEP an object moving at a given speed once it starts moving. It is ALWAYS less than static friction (I think - maybe there are some very, very rare exceptions. - Every rule has an exception or two).

What does this have to do with VSA or ABS?

Well, when a tire is rolling - it is in constant contact with the ground -> this provides the larger Static Friction. This is the MOST friction (grip) available.

When a tire is skidding (or spinning) there is relative motion between the tire and any given point of contact on the ground - in other words, it is in the sliding (reduced friction/grip) domain.

The whole point of VSA and ABS is to ensure that tires DO NOT make the transition from Static to Sliding Friction. They reduce the power to the tire to PREVENT grip from being reduced - because once grip is reduced, all sorts of (to the driver) unpredictable things may happen.

ABS does this by using speed sensors at each wheel to sense the deceleration of each wheel under braking and releasing brake pressure on a specific wheel that is at the threshold stage between Static and Sliding friction. ("Just about" to start skidding)

VSA does the same thing by using vehicle accelerometers and yaw sensors to determine those conditions when the vehicle as a whole is about to lose traction at one or more wheels (such as for the rear-end to break free). In response to such a determination, the ECU will cut power to the rear wheels, and apply one or more brakes at OTHER wheels to steady the path of the vehicle. Again - all of these (re)actions are designed to prevent the tires making the transition from Static friction to Sliding friction. In other words, all of these (re)actions are designed to enhance / optimize / prevent the grip from going down.

None of them "reduce" grip.

Best of luck,
Eric

Originally Posted by eric1234,Apr 14 2010, 06:03 AM

So, I've been thinking about this some more....

First: I'm not a track-trained driver, just an engineer and patent attorney who is also an auto enthusiast.

So, what I think happens here (in a nutshell) is that ABS/VSA/Traction Control are all designed to prevent the transition from static to dynamic friction. As such, they must all act at some point BEFORE the transition. Let's just say at 95%.

What that suggests is that a very skilled driver - can push further - let's say to 99%, and do better than the VSA. Such a 'magical' driver would, indeed, be hampered by the interactions of ABS/VSA.

BUT - that said, how many drivers are capable of this??? (I really don't know, to be honest - it's just a rhetorical question). Certainly, as a vehicle is operated closer and closer to the threshold, uncontrollable variations begin to come into play. What about that pebble in the road? It is going to reduce contact patch area and friction. If you were operating at 99.9%, might one pebble become the difference between success and disaster?

I would think so... The (rhetorical) question then becomes one of practicality: Should drivers (presuming they can) operate in the threshold above VSA activation? I certainly know I can't - but I imagine some of you guys that are trained and experienced can. But what happens when there are 2 pebbles right next to each other? 3? 4?

Maybe VSA is a good thing, even for experienced drivers, as it can reduce the effect of unpredictable events?

I do know that I've had VSA pop on a few times when I (with my 'standard' street skills/experience) didn't expect it to. The S2000 does seem to have a pretty tail happy tendency. Hope I've added some value or insight to the discussion...

Eric

Originally Posted by trinydex,Apr 14 2010, 06:42 AM

i would say that vsa is there to both prevent (actuate early) and to mitigate (after a catastrophic loss of grip). so i wouldn't say it 'relies' on actuating early, i think it'd do fine actuating after a catastrophic loss of grip, if you were to travel from pavement to grass, dirt, icy side of road (a likely story in any bad accident) then the vsa would still help to correct your yaw and prevent too crazy of a spinning and other undesirable effects. similarly if you were to be clipped in the tail end, instead of whipping around into an uncontrollable spin striking other people in traffic, the vsa could help damp the yaw and keep you going in the same direction as everyone else (at least).

Originally Posted by trinydex,Apr 14 2010, 06:36 AM

1. a pure lateral load can make you surpass available grip, you can easily surpass grip threshold without using the throttle and applying torque to the rear wheels. vsa does turn on in such a scenario. applying brakes in such a scenario would reduce the available grip.

2. braking absolutely reduces available grip. traction circle.

Originally Posted by SlowTeg,Apr 14 2010, 09:02 AM

If the tires are at 50% grip or 99% grip it doesn't matter, the grip is the same.