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Sticking to the road when going over uneven pavement

Discussion in 'Roadster: Performance' started by zack, May 9, 2011.

  1. zack

    zack Member

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    I had a scary experience today on a freeway ramp that I was unfamiliar with. I went into the turn not knowing how sharp it was and hit a pretty uneven area (some raised bits) and it really felt like the car was going to break completely loose and I would slide off the road. I must have moved a foot or two left of my intended path. I've left the suspension at the factory setting... if I adjust it more for performance will it hug the road better in these conditions?
     
  2. S-2000 Roadster

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    I think this vehicle invites the driver to push the limits - at least it did for me when I drove one of the validation prototypes in 2008. I took my familiar S-2000 around the same course right after the Tesla, and I was sliding all over the place at what was surely even slower speeds. The Traction Control generally keeps you out of trouble in the Roadster when you'd already be sliding in a standard vehicle.

    My question is how aggressively were you driving when you felt like you were going to break loose? Were you maybe unconsciously pushing it a bit too fast?

    Regardless of your answer, I think this is a good question. I'm hoping that people more familiar with the suspension settings will answer. My sport suspension is still at the factory defaults - assuming it wasn't altered by someone in the first 111 miles!
     
  3. Doug_G

    Doug_G Lead Moderator

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    What sometimes surprises me is when the traction control goes off while decelerating due to one or more big bumps in the road. Even if you're already on the friction brakes it feels like the car lurches ahead, and I'm sure you end up a couple of feet further up the road than you intended.
     
  4. vfx

    vfx Well-Known Member

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    minifreakout to be sure
     
  5. TEG

    TEG TMC Moderator

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    Traction control OR regen?
     
  6. Doug_G

    Doug_G Lead Moderator

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    Traction control WHILE regen.

    If the car thinks the wheels are slipping it will engage traction control, regardless of whether you are accelerating or decelerating. Works really great on snow, but if it triggers only because you hit a big bump, you unexpectedly lose a good fraction of your braking power. Of course you instinctively hit the brakes harder to compensate, but it is an uneasy feeling.
     
  7. S-2000 Roadster

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    This happened to me during pure regeneration deceleration in the rain while taking an off-ramp from I-5. I didn't feel anything, but I noticed the TC light on the dash for the first time ever.

    I think it's really smart that Tesla prevents the wheels from sliding during regen. I'm sure the brakes work as normal, unless anti-lock brakes would kick in at the same time - I don't know, because I was using pure regen.

    In any event, 'normal' vehicles with anti-lock brakes warn drivers explicitly that stopping distances may be increased by the system. Does the Roadster have anti-lock brakes as well as TC during regen?
     
  8. tetsuos

    tetsuos HK #488 (2.5)

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    I have had a few scary experiences with the anti-lock brakes, which would be due to Tesla's interpretation of how they should work. When braking hard to avoid an unexpected obstacle, if there is any uneven portion of the road that causes one of the wheels to come off the road momentarily, Tesla's anti-lock system will immediately engage for all four wheels. This causes strong push back on the brake pedal, significantly reducing braking strength, giving the sensation of the car lurching forward. The reaction time necessary to compensate by pushing harder on the brake pedal results in many feet further braking distance than would be expected.

    This is certainly an implementation issue, as other cars will engage antilock brakes on a more relaxed basis, or without as much push back on the pedal... This is a safety consideration which I have adapted to by reducing my aggressiveness on uneven road surfaces, but there can be surprises on otherwise good roads, such as potholes and buckled sections. I have been wondering if anything could be done about this through software, or if the antilock system is outside the software control...
     
  9. S-2000 Roadster

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    Are you absolutely certain about that? My understanding of anti-lock brake systems is that they only disengage the brake on the one wheel that is locked up, and only temporarily. If more than one wheel locks up, the system might disengage the brake on more than one wheel, but that could easily cause complete instability. I'm totally guessing, but I have a hunch that Tesla Motors would not be approved to sell a vehicle which engaged anti-lock on all wheels at the same time. I hope someone who is an expert can clarify here.

    Regardless of how the individual wheel brakes operate, the brake pedal vibrates during anti-lock action, so perhaps you are reacting. I don't mean any disrespect, but do you have any experience with anti-lock brakes? I only have one other vehicle with anti-lock brakes, the 2000 S-2000, and it has only ever engaged the anti-lock system on uneven pavement - specifically, when driving over the "road turtles" that we have here in the U.S., combined with sand or grit on the paved surface to make things worse. So, from what is being described in this thread, nothing really seems any different with the Tesla Roadster than with other anti-lock systems. Granted, the interpretations here seem to include some wild theories, but the subjective reports don't seem that alarming to me.
     
  10. Doug_G

    Doug_G Lead Moderator

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    Being in Canada and driving on snow a lot, I have a lot of experience with antilock brakes, although not in the Roadster (since I don't drive it on snow). No matter what car, it is a most unpleasant experience when it triggers. The pedal vibrates and you feel a loss of braking power, so you stomp down harder on the brake.

    The purpose of antilock is to maintain steering control, NOT to slow down faster. In fact antilock usually increases your stopping distance. But while it's engaged you will stay in a straight line, and you can even steer around obstacles.

    Generally you would stop fastest if you can simultaneously lock all four wheels, i.e. if you suddenly stomp real hard. But in the real world only one or two wheels lock up, and the car starts spinning. It's better to slide a few more feet and be able to steer than to slide through an intersection sideways.
     
  11. smorgasbord

    smorgasbord Active Member

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    Antilock brakes almost always reduce your stopping distance. On snow or gravel, where stuff can build up in front of a locked wheel and create a dam effect that increases friction, it is possible that antilock brakes would increase your stopping distance, but it's more the exception than the rule. On all other surfaces, wet or dry, anti-lock brakes will reduce stopping distances.

    The difference is due to static friction versus sliding friction. Static friction is enough higher than sliding friction that using less brake force to maintain static friction will reduce stopping distances. Where the differences between static and sliding friction are greatest, such as wet roads, you'll get the most benefit. Where the differences are less, such as gravel, you'll get less benefit. And where you can get the dam effect to occur, you can even get the sliding friction to be slightly greater than the static friction (due to larger contact area) - and that's the only case where anti-locks won't stop shorter.

    And, as Doug points out, anti-lock also gives you steering control of the vehicle, which can be more important the stopping distance in avoiding an accident.
     
  12. Doug_G

    Doug_G Lead Moderator

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    On dry pavement, sure, it'll reduce stopping distance in most cases. But I can't recall the last time I triggered ABS on dry pavement. About the only time I ever get ABS active is on snow, which is to say it's a reasonably common occurrence.

    On snow it definitely does not improve stopping distance, and usually does the opposite. But it allows you to retain control, which 99% of the time is what you really need.

    The one time I didn't like ABS was an black ice, when I was sliding towards another car at a mere 3 kph with nowhere to turn. We were sliding for several seconds and I was straining on the brake pedal the whole time. We did stop in time, though.
     
  13. Lloyd

    Lloyd Active Member

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    Best braking is just before wheel lock up. Once the wheels lock, your stopping distance increases dramatically.
     
  14. S-2000 Roadster

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    Two things: If the wheels that steer your car are sliding, then they no longer function for steering. You can turn them any direction you like and the momentum of your vehicle will continue to carry you in a straight line.

    More importantly, if you get into a situation where anti-lock brakes are causing more harm than good, then you actually need to let off of the brake and modulate your pressure to maintain that balance right before wheel lock up, and Lloyd says. You have to react quickly to your car in these sorts of situations because there is no technology that is going to do that for you. I can't remember how many times I've let off the brake some to maintain control of my vehicle. In other words, if you find yourself "straining" on the brake pedal then you're not really controlling your car.

    I don't know if it was black ice, but I found myself on a downhill road headed into a 90-degree right turn with a VW bus stopped in the opposing lane trying to get up the hill. Every time I braked to slow down, the car started sliding straight into the opposing lane, and every time I let up on the brake the car regained steering control and turned right to stay in my lane. This particular instance was a challenge because I was in someone else's car that was quite heavier than my own. But, still, as the driver, you have to be able to feel the effects of what you're doing to the car. Again, if anti-lock brakes engage and circumvent your control, then you should let off the brake instead of just straining harder, and you will hopefully find the right input to control your car.
     
  15. Doug_G

    Doug_G Lead Moderator

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    Yeah, of course I did try backing off momentarily, but it changed nothing. If you're on black ice you have virtually no traction. It forms at very cold temperatures, below -20C, when strong winds blow around snow already on the ground. Passing cars press it into the asphalt, polish it up, and it ends up looking totally black. At first it has no effect on traction, but once it builds up to the point where it smooths out the road surface, traction drops to nothing. The only clue is that the pavement might look too dark. If you don't see it, you'll find out when your wheels slip out. Great fun watching the world spin around you.

    We also get ice storms - supercooled rain that freezes and sticks on impact. You'll know it's there, but it can be so slippery that you can't even stand on it. A friend of mine once had his car slide out of his driveway into the ditch after he had parked it and walked away.

    Technology can only help so much with this stuff. Fortunately, right now sunburn is the bigger problem...
     
  16. smorgasbord

    smorgasbord Active Member

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    Black ice is nasty. It will confuse most antilock braking systems, because they don't have a couple of wheels with traction to use as a reference. I've been wondering if it would be possible to use the accelerometer in the Roadster as a another input to the system - if all wheels are locked yet the accelerometer shows the car is moving perhaps they could do something.

    But then again when you have no traction you have no traction. Black ice is nasty.
     
  17. S-2000 Roadster

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    Ok, I really doubt we've seen -20°C here on the coast of Puget Sound. People frequently speak warnings about "black ice," but we surely don't get those temperatures here. It's clearly outside my experience.

    One winter during college, I returned to my parked car to find a nasty letter under my windshield wiper. The author recommended that I take "Parking 101" because my car was bumper-to-bumper with their car, and it apparently pissed them off to no end. Unfortunately, they were gone by the time I got the note, otherwise I would have suggested that they take "Physics 101" because parking on ice means that the car doesn't necessarily stay where you put it. Obviously (to me), my car slid down hill on the ice after I left, yet they assumed that I had purposely parked on top of them.
     
  18. Doug_G

    Doug_G Lead Moderator

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    Yeah, apparently people who live on the East coast of Canada use the term "black ice" in weather conditions where you couldn't possibly get it. I remember hearing the term on the radio while driving into town from the Halifax airport. I have no idea what they meant.
     
  19. daniel

    daniel Active Member

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    An accelerometer senses acceleration. That is, any change in speed or direction. It cannot tell the difference between a stopped car, and a car moving at a constant velocity and direction. Maybe combined with the GPS the car's computer could tell your wheels are locked while moving.

    I had a slow-motion accident once on black ice: I was moving 5 mph and the brakes (with ABS) did nothing at all. On a perfectly frictionless surface, which black ice is close to being, nothing but an obstacle will stop the car. Normally, though, the ABS on my Prius works like a charm: I press hard on the brakes and the car compensates and I retain control. The book says it will take LONGER to stop. I believe this means that ABS on ice takes longer to stop than on dry pavement with no ABS engaged. It does not mean that on ice, ABS takes longer than no ABS. On ice, I stop quicker with ABS than without. But once ABS kicks in, I stop slower than I would on dry pavement.
     
  20. Doug_G

    Doug_G Lead Moderator

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    Probably not quickly enough. Given multipath interference issues on downtown streets, I would not trust GPS for controlling the car.
     

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