Someone help me understand how VSC works...

Discussion in 'Gen 2 Prius Main Forum' started by kdmorse, Dec 24, 2006.

  1. kdmorse

    kdmorse Member

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    And no, I'm not talking about Traction Control - I know the difference.

    Just so you know where I'm coming from...

    Before my 2006 Prius, I drove a 1995 Saturn Station Wagon, and I drove it for 10+ Years. While it came with fairly crappy tires (The sort of firestone's that got recalled for spontaneous blowouts) - it was always very good to me, and was a very solid, surefooted car. Once the firestones went bald, I put on some of Michelins best, and had been driving on them for the past 5 years. This was also a car with no VSC, early generation anti-lock breaks, and very minor first-gen traction control. And it's what I got used to, got to know the feel of, and could handle in all kinds of weather. I could easily recover if the car began to slip or slide, could easily control turns with a little bit of slip in them, as there was nothing more to it than simple physics, inertia, and friction.

    Now, I love my Prius, but it doesn't have the best traction in the world. And compared to what I'm used to, it feels very, very slippery when the roads are wet. (OEM Tires). And there hasn't been too much bad weather since I bought it in January, so I don't have that feel for how it behaves when it starts to slip that I had in my old car. But what happened yesterday freaked me out but good, and *could* have been quite bad.

    The roads were wet, and I was making a right turn into a shopping center of of a 55MPH highway. I slowed down for the turn, and would not have said I was driving too fast for conditions - however the fact that the car slipped as badly as it did means that I was driving too fast by definition. I can't actually tell you what my speed was at the moment I tried to turn. As I entered the turn, the car began to slide a little. At this point, I pretty much lost all control. The car beeped at me like mad, lights blinked, and it didn't seem to pay any attention to my attempts to correct it's direction, and I swear it braked on it's own in a way that pulled me to the left, preventing me from completing my turn. ie - it was actively preventing me from turning right, as if it thought turning to the right was the bad thing that was happening. Of course, I had already completed half the turn, so this meant heading into the oncoming lane as I was only able to complete about 45 degrees of a 90 degree turn.

    Anyway, having no directional control for a split second (which ain't fun), heading straight towards the curb on the other side of the road, I did the only sane thing I could think of, I slammed on the breaks. The car slowed dramatically, and I was then able to regain control, and I returned to my rightful lane. Fortunately, there was noone exiting the shopping center at that exact moment, because if they had been, I would have driven straight into them.

    Yes, yes, I'll take that turn (and all others) much slower next time... Under the circumstances - I'll take them little-old-lady slow.

    But my whole question here... is does anyone know exactly VSC is supposed to do for you, how exactly it works? I swear it kicked in after completing half a turn, and then tried it's damnedest to keep the car going in a perfectly straight line, which under the circumstances was about the worst possible thing it could do. I'm quite certain that under those circumstances had my old Saturn started to slip, I could have corrected it and made it round the turn with only a little slide at worst. (Although I'm not sure my Saturn would have slipped at all under those circumstances). Perhaps if I understood the VSC a bit better, I won't end up fighting it for control of the car should something like this ever happen again.

    Any advice on understanding it?

    (Yes, get better tires, thinking about it)
    (Yes, drive slower, thinking about it :))

    -Ken
     
  2. john1701a

    john1701a Prius Guru

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    <div class='quotetop'>QUOTE(kdmorse @ Dec 24 2006, 05:02 AM) [snapback]366238[/snapback]</div>
    To restore grip, the best choice is to press on the accelerator instead. With a front-wheel drive car, you'll actually be able to pull out of the skid. It's pretty cool, after you get past the counter-intuitive nature. Watch the "Cars" DVD. The "turn right to go left" suggestion doesn't make any sense at first either.

    Pressing firmly on the brakes is the next choice. Slamming on them is only a last resort.
     
  3. tumbleweed

    tumbleweed Senior Member

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    I have only seen the VSC activate a couple of times and both times I did it on purpose, in an empty parking lot just to see how it would react. Both times it simply used brakes and backed off the throttle a bit to keep the car from sliding but the car continued turning at the same radius.

    I have never had the experience you describe so I can't really comment on it. You said "As I entered the turn, the car began to slide a little." did it start to slide in the front, the rear, or both ends?
     
  4. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    gee... dont know how fast you were going??? well... ya didnt get into a wreck and you admit to driving too fast for conditions, so sounds like VSC did its job.

    the lack of details really makes any other comments difficult. from your description, you were either trying to make a turn at 40 mph, or were in very slippery road conditions.

    what was the road conditions??
    weather conditions??
    temperatures??
    any black ice.??

    sounds to me that you entered a completely unexpected handling situation and VSC saved you.

    your feelings of panic are normal. in emergency situations only a small fraction of us would handle it with any kind of cool. as soon as VSC takes over, that feeling of having no control is intense.

    try using VSC when you are ready for it. that will give you much better insight on its benefits. a few weeks after on snowstorm (illustrated on the Monday night football game bet Seattle and Oakland) i had the perfect opportunity to try VSC. going down an empty hill in Tacoma. temps in the low 30's, road about 80% glare ice, reasonably steep slope. i got up to about 35 mph. then braked, and at the same time did two lane changes over two lanes and back to the middle.

    i pulled the maneuver off without any real issues at all. in a car without it, i would have gone completely sideways with the back end leading.
     
  5. jburns

    jburns Senior Senior Member

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    <div class='quotetop'>QUOTE(john1701a @ Dec 24 2006, 08:43 AM) [snapback]366264[/snapback]</div>
    Other cars yes but if the front wheels start to slip at all, which they probably will if you are already drifting sideways and hit the gas, the traction control will kill the power. I am very unimpressed with the emergency handling of the Prius. The VSC and traction control become less and less effective the more serious the situation until they actually can prevent you from regaining control of the vehicle.
     
  6. Tideland Prius

    Tideland Prius Moderator of the North
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    <div class='quotetop'>QUOTE(kdmorse @ Dec 24 2006, 03:02 AM) [snapback]366238[/snapback]</div>
    Your experience seems to mimic mine on two occasions when we had that snowfall on the west coast at the end of November. On two separate occasions at two different intersections, I understeered and nearly hit the oncoming car (in both cases, there just so happened to be a car in the oncoming lane).

    The car did what you described. It understeered (which I think is what you're describing when you said the car seemed to want to go "straight" on instead of turning right), VSC came on (the usual beeping and flashing) and the car corrected itself. I almost gave that girl in the other car a heart attack. She was stopped at the stop sign and I could see her eyes getting wider as I plowed straight towards her car.

    So, how does VSC work.

    It has a yaw sensor to determine whether you're going off your intended path (under or oversteer). It will retard the throttle and/or apply the brakes to each wheel individually to regain traction.

    The best thing I can suggest to you right now is to go to an empty parking lot and re-enact your situation. However, you can substitute speed with steering angle. IOW, instead of going the same speed as the scenario above, just yank the wheel further than usual. That will induce the same characteristics. That way, you don't have to speed and thus can use a smaller parking lot.

    Just yank the wheel, let the car understeer and just see how VSC works.
     
  7. cwerdna

    cwerdna Senior Member

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    See my post at http://priuschat.com/index.php?s=&show...st&p=354141.

    All cars have their handling and grip limits so it's still possible to spin or go out of control or spin out w/stability control. The Prius isn't a sports car nor does it have very wide or grippy tires.

    To the OP: BTW, in the two cases where you used the word "breaks", you should've used "brakes".
     
  8. donee

    donee New Member

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    Hi All,


    I do not have VSC, but do have a Prius.

    VSC works by sensing the car's "yaw". Look down at the top of the car, and imagine the car turning left or right around the an axis through the center of mass (gravity). That is Yaw.

    The VSC assumes that when the car is Yaw'ing in oposition to driver command, the wheels are slipping sideways. This may or may not be the case with the stock Prius.

    The unmodified Prius has oscilatory yaw response due to chasis flex during high chasis loads. Many of us have replaced the stock floor stiffening plate with the BT Tech plate to help with this situation. Back to the unmodified Prius. The car can yaw dramatically, even with the tires firmly planted. The body flex is steering the tires, and the tires are not slipping, but the steering is yawing the car. VSC will act on it.

    I had similar a similar thing happen to me even though my car does not VSC. I was turning to the left on the inside lane of a two lane road, onto a two lane road. All of sudden the car just squirmed into the outside lane, and there was no skiding. Luckily it was early in the morning and there was no other traffic. There was no skiding because of course VSC did not activate (don't have it). I decided that day to order the BT Tech chasis stiffening plate.

    As to the comment that the Prius is not a sport's car, well, neither is Saturn SL2 (my previous car) or the OP's SL wagon. But neither of these cars have this problem. All of these cars are similar in weight to the Prius and my SL2 actually used the exact same tire size as the Prius. Because its nearly the same weight as the Prius, and it had 15 inch rims. These identical tires with the same weight on them will slip at the same side load.

    Now, what happens when you put an intellegent controller (human or computer) in control of the car during rapid yaw events, and the car starts to squirm. The human driver gets all confused and induces PIO (pilot induced oscillation). The VSC does the same thing, only its steering with differential brake inputs, not steering wheel angle. The basic problem here is the intelligent controllers are used to rapid yaw being due to sliding, not chasis deflection steering. Reacting to the rapid yaw as if its sliding, is the wrong thing to do when the actual cause is Prius body flex misalligning wheel angles. Automatic controls can easily become oscillatory, or drive the system to an out-of-control state (steer the wrong way) when the basic structure of the machine has a non-linear response. Steer left should cause the car to turn left, not right after some load limit is reached (what happened to me).

    With the BT Tech plate the car is much more linear in response to steering inputs and does not squirm back and forth under load. I recommend you get one. I actually skidded around a wet curve on the way home last friday, and pulled into a recovery drift with up throttle. No problem. This would have been impossible with a stock Prius. Thank You BT.
     
  9. kdmorse

    kdmorse Member

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    Thank you all for your input.. a few responses...

    <div class='quotetop'>QUOTE(john1701a @ Dec 24 2006, 08:43 AM) [snapback]366264[/snapback]</div>
    I believe that's exactly what I tried first, as it's exactly what I would have done in my previous car. I think the regular traction control kicked in and stopped me though, and I think that's when VSC started yelling at me. (Very hard to tell, it may have kicked in independently). With all that going on, I had the immediate feeling that attempting to pull out of the skid was *not* working, and shifted to brakes instead. Because of the approaching curb, I didn't really have time to experiment.

    The brakes however performed perfectly, no complaints there. They slowed the car dramatically and quickly without locking up, and as soon as they restored proper traction the car turned to the right as requested.

    <div class='quotetop'>QUOTE(tumbleweed @ Dec 24 2006, 11:34 AM) [snapback]366281[/snapback]</div>
    Not quite sure how to answer that question. Best as I can describe it, once I was about 45 degrees through the turn, the center of mass of the car wanted to continue traveling in a straight line (at that 45 degree angle), while the car continued to rotate beyond 45 degrees. It seems that the instant the two directions were not the same, VSC kicked in.

    <div class='quotetop'>QUOTE(DaveinOlyWA @ Dec 24 2006, 12:03 PM) [snapback]366304[/snapback]</div>
    Wet. It had been raining all day, but it had stopped raining some earlier. Just wet pavement, not large puddles. Weather was 55F-60F, so no chance of ice.

    I'm not convinced of that, but will withhold final judgment until I understand it better.

    <div class='quotetop'>QUOTE(donee @ Dec 24 2006, 05:24 PM) [snapback]366373[/snapback]</div>
    I'm still working my way through your post - but there was most certainly a left to right shake while this was occurring, as if the car was not 100% sure which way it wanted to correct, or was correcting back and forth in both directions alternatively. It felt very much like the sort of feedback loop that can occur when two separate systems (one computer, one human), are trying to compensate for each other, each end of overcompensating, and nobody wins.

    I will have to go find an empty parking lot on a wet day... And I will reconsider the BT plate.

    Thanks,

    -Ken
     
  10. Robnerika

    Robnerika New Member

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    This is my first post...hope I'm not violating any protocols....Just bought an 07 with package 2 two days ago. Don't have the car yet. I had heard about the BT tech plate from my boss who has one installed and strongly recommended it. The Prius we bought has the VSC. I'm wondering if adding the plate would help the VSC, work against it, or be unaffected by it? Logically it seems that it would stiffen the chassis and make it less likely that the VSC would be engaged if the yaw was due to chassis twisting. Would you be willing to share your thoughts on that?

    BEK



    <div class='quotetop'>QUOTE(donee @ Dec 24 2006, 05:24 PM) [snapback]366373[/snapback]</div>
     
  11. donee

    donee New Member

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    Hi BEK,

    Yes, that was the jist of my post. If the car does not unintendedly yaw, except when the tires start to slide, the VSC should work only when the yaw is due to sideways tire slide. The result should then be the VSC working for the situations it was designed.

    The BT Tech plate turns the car back into what drivers of stock small cars are used to, except for the additional rolling action due to the tall design.

    The VSC cannot be designed to fix the chasis flex. It only senses yaw and steering command. If the two do not match, it takes action. Keeping the car stiffer, will make the yaw match steering input more closely in driver controlled manuevers. Which should result in VSC not being triggered unless the tires are sliding sideways. With the body squirminess, the car noticably yaws even when tires are not sliding sideways - the situation the OP was in. If a person can notice this, the VSC sensor can.
     
  12. Robnerika

    Robnerika New Member

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    Thanks! :)

    <div class='quotetop'>QUOTE(donee @ Dec 25 2006, 05:02 PM) [snapback]366549[/snapback]</div>
     
  13. Bill Merchant

    Bill Merchant absit invidia

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    Stop thinking about it, kdmorse, get better tires!

    I think the BT Tech brace is great and highly recommend it, but the first place where slipping occurs is where the rubber meets the road. I'm sure you've read MANY threads here about the driving quality of the OEM tires. On clean, warm, dry roads without curves, they may be OK. On wet roads making 90° turns at greater then "granny speed", they do not yield the performance you desire.

    The BT Tech brace will help the mediocre OEM tires grip slightly better, but ComforTreds, HydroEdges, or SVXs will grip better in wet in any case. Get better tires and the BT Tech brace and you'll think your car is on rails. It becomes very hard to activate VSC.
     
  14. Devil's Advocate

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    I trigger the VSC often, usually under heavy breaking when the front tires hit bumps in the road. The effect is that you temprorarily appear to lose braking as the VSC thinks that the bouncing tire (which at that point is locking up) is actually sliding. I am not very impressed with the VSC's response time, but it does do what it is supposed to, correct driving conditions so that the tires stay rolling and on the road.

    I can also get the VSC to trigger on dry freeway entry ramps when taken at speed. Its kind of cool to hear the beep beep beep all the way around the ramp. With a little finesse you can actually get a little drift going but not enough to bring the back end around or push the fron t end out from under you.

    The key is to know that the VSC one, is somewhat slow to react and two, is not the most intelligent as to your intentions with where you want the car to go.
     
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