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Taking an on-ramp turn too fast?

Discussion in 'Model S: Driving Dynamics' started by Apoclyps, Jun 2, 2014.

  1. Apoclyps

    Apoclyps Member

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    #1 Apoclyps, Jun 2, 2014
    Last edited: Jun 2, 2014
    Yesterday something unexpected happened. I was driving on surface streets approaching a freeway on-ramp (the circular kind). Typically, as I begin my entry onto the on-ramp, I release the acc. pedal (going about 45 mph) and ride through to the bottom. This time, half-way down the ramp (with continual turning), the car starts to shake and I noticed one of the dash lights blink momentarily (the swerving looking one). I looked at the manual, and it is the "electronic stability control system" which applies the brakes "to the appropriate wheel to prevent slippage". This has not happened before, and it happened twice yesterday as I entered the freeway on 2 different occasions (2 different on-ramps). (oddly, this same light blinks when I drive over speed bumps too fast).

    Back wheels were just replaced (Goodyear OEM 19")..and my fronts still look good (yeah...my mistake on tire rotation. had them rotated at 6K, I think they were rotated at 12.5K during annual service, and at 18K the back wheels were pretty much gone). I would think that if any slippage occurred, it would be on the back wheels (or maybe I am not understanding the physics)

    Has this happened to anyone else? Is it just a problem with my tires? my driving? Or something else? or no problem at all.
     
  2. nrcooled

    nrcooled P#8946 VIN 03225

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    There are a LOT of factors that could cause loss of traction around a corner. With that being said, since you were under steady-state acceleration the most likely culprit (my best guess) would have been understeer. From my experience the Model S stability control does not correct for understeer. I have cooked a corner or two in the Model S and with the front tires screaming in protest there was no stability control intervention.

    The only time I have ever been able to get stability control to kick in is with traction control off and getting the rear tires to lose traction through copious amounts of accel pedal application and kicking the rear end out.

    BTW - The above scenarios were performed on a skidpad and NOT the street.
     
  3. Chipper

    Chipper Active Member

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    Have you checked air pressure?
     
  4. pbleic

    pbleic Member

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    This is basic physics. You should always maintain a constant speed during a curve; this will require keeping your foot on the accelerator. When you maintain a constant speed, you are actually accelerating towards the center of the curve; this is known as centripetal acceleration. If you slow down, you have less force towards the center and your car will have a tendency to skid away from the curve. Motorcyclists learn to brake BEFORE a curve, and accelerate into and out of a curve to avoid skidding out. It is VERY dangerous to brake a motorcycle (or car, for that matter) in the middle of a curve.

    Here is a good explanation: http://www.khanacademy.org/science/physics/two-dimensional-motion/centripetal-acceleration-tutoria/v/race-cars-with-constant-speed-around-curve
     
  5. Doug_G

    Doug_G Lead Moderator

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    Did you provide smooth inputs, or was your steering jerky. Jerky motions unsettle the suspension and reduce the grip of the tires.

    Also all tires have a limit to their ability to provide grip. If you're turning at 100% of what is possible, and then try on top of that to accelerate or decelerate, you will exceed the limits of the tires and will start to slide.
     
  6. William13

    William13 Member

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    To answer the question. Yes, I have had this happen. The stability control is applying brakes. Loss of contact with the ground / poor traction likely the cause. I say press harder on the accelerator pedal. The S will keep you going. No worries.
     
  7. EarlyAdopter

    EarlyAdopter Active Member

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    You mention your back tires "were just replaced." How many miles on the new tires? It can take a few hundred miles to break in new tires before they achieve their maximum grip. Something to do with molding releasing agent in the new rubber.
     
  8. Apoclyps

    Apoclyps Member

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    Thanks all for your replies. Yes.. i've heard in racing terms to pedal through the turn. I guess I never really experienced this as every time I hit an on-ramp, there is a line of cars there...so i trickle down.

    The rears probably have a few hundred miles on them now, and all tires are properly inflated (since I was at the SvC having the rears installed).
     
  9. cairnz

    cairnz Member

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    I get the traction control lights up a lot of times, even when it's not expected. For example going uphill on wet roads, if surface changes friction for a bit, it can suddenly lose grip and lights go flashing for a second. Turning increases the likelyhood of this happening. When the 19" tires were new, this was a little more likely to happen than after they've been run for a while (5k miles). You shouldn't worry, as long as you're driving normally. It's just the car reacting too fast :)
     
  10. strider

    strider Active Member

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    I wonder if the SC system is being confused by the different sizes of the tires since the fronts are worn and the rears are not? I don't know why it would only show up on ramps but since one of the inputs to the SC system is rotation rates of the wheels, seeing different rates front and rear may cause it to think that wheels are slipping? Does model S have a "learn new tires" button like the Roadster?
     
  11. dsm363

    dsm363 Roadster + Sig Model S

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    It doesn't yet have a learn tires feature like the Roadster.
     
  12. HankLloydRight

    HankLloydRight Fluxing

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    This is the first thing they teach in any basic driving course. Always set your speed for a curve BEFORE entering the curve, and accelerate out of it.
     
  13. pbleic

    pbleic Member

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    Exactly right. Centripetal force is no different for cars and motorcycles.
     
  14. NoMoGas

    NoMoGas Member

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    Agree 100% Also, not "strange" at all to have it kick on over speed bumps. If you take them to fast the tires lose traction for second. Absolutely what is supposed to happen.
     
  15. Curt

    Curt Roadster Signature #55

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    That should be "like the 2.x Roadster". The 1.5 still doesn't have that feature, either.
     
  16. Doug_G

    Doug_G Lead Moderator

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    I'm guessing the car figures it out, on its own, without you having to press a relearn button.
     

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