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Thread: Mountains and steep downhills

  1. #11
    EVocate DITB's Avatar
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    Thank you for your replies, I think as I understand it, I am quite happy but also looking forward to test this myself.

    I was initially thinking how to factor in headwind, car weight, downhill slope and so on ... but in the end I thought, it is really quite simple:

    No matter how much downhill, tailwind and what weight on board - if 1) the speed is INCREASING and 2) your foot is off the accelerator, then regen should do its best effort to keep that particular speed.

    What I mean is, I hope that the Tesla S regen function is not just a "dumb" fixed resistance, but that it considers whether or not the car is actually accelerating, maintaining speed or decelerating.

    Having to ride the (conventional) brakes on downhills in a modern electric car would be entirely disappointing, but it seems from the replies above (thanks) that this is NOT a problem.

    I assure you that when I get my Tesla S eventually, I will test and report from the hills and mountain roads of Hong Kong

  2. #12
    Roadster#433, Model S#S37 Cottonwood's Avatar
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    Quote Originally Posted by DITB View Post
    What I mean is, I hope that the Tesla S regen function is not just a "dumb" fixed resistance, but that it considers whether or not the car is actually accelerating, maintaining speed or decelerating.
    Its much easier to drive than to describe. As I have told many people, driving the Roadster is like driving a manual transmission car alway in 1st gear. The Model S is very similar but always in 2nd gear. You very quickly get used to feathering your foot on the accelerator to get just the level of regen that you want. If you use cruise control, it does the feathering for you, and holds a nice constant speed.

    I have not met a descent in Colorado yet, that regen could not tame.

  3. #13
    EVocate DITB's Avatar
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    Quote Originally Posted by Cottonwood View Post
    Its much easier to drive than to describe. As I have told many people, driving the Roadster is like driving a manual transmission car alway in 1st gear. The Model S is very similar but always in 2nd gear. You very quickly get used to feathering your foot on the accelerator to get just the level of regen that you want. If you use cruise control, it does the feathering for you, and holds a nice constant speed.

    I have not met a descent in Colorado yet, that regen could not tame.

    I am quite content with that answer, thank you. I have not yet really found anything I DON'T like about this car, apart from a bit of a worry about all the gadgetry - will it last the test of time? I remember when all the main brands of premium cars went very electronic (including Mercedes, Audi and such) - from pure and simple turn the key and start to computers taking over. Stories of how an almost new car would just refuse to operate, because of computer problems.

    Simple and pure is often easier to maintain, so I hope the Tesla S is NOT so over-engineered that it will cost a fortune to maintain ...

  4. #14
    EU Model S P-37 VolkerP's Avatar
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    OK, it's time to put all fears to a stop

    Going downhill "frees" potential energy (m*g*h), where m is vehicle mass, g is gravitational acceleration, h is height (elevation). The energy conversion rate (called P for power) depends on speed of elevation loss dh/dt, which in turn depends on vehicle speed and grade:

    P = m*g* dh/dt

    To solve for elevation loss

    dh/dt= P/m*g

    Using metric system here for easiness of conversion: m=2100kg, g=9.81m/s², P=60kW=60.000kg m / s³, result is dh/dt = 2.9m/s.

    What does that mean? You have to go downhill at a rate in excess of 10ft or 3m of elevation loss per second to max out regen.
    • on a 10% grade, that's 30m/s or 108km/h or 67 mph. At that speed, drag losses are ~18kW, slowing you down, too.
    • on a 20% grade, that's 54km/h or 33mph.
    • on a 30% grade (Filbert Street, SF), that's 36km/h or 22mph. I'd say you can do that if you're bold enough not to touch the brake pedal.


    Here is a picture of the world's steepest residential street, Baldwin street in NZ, at 35%. You should get the idea of really steep from that.
    My conclusion: your will not go down such a grade and be surprised that regen is maxed out. Your gut will scare you way before that happens.


  5. #15
    EVocate DITB's Avatar
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    Great information, VolkerP, thank you for taking your time to clarify it.

    And nice picture! Had I parked my car there, I would have turned the wheels all the way toward the curb though ... !!!

  6. #16
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    Quote Originally Posted by Cottonwood View Post
    As I have told many people, driving the Roadster is like driving a manual transmission car alway in 1st gear.
    Except that if that in almost every manual transmission car, there will be some slack in the drivetrain, so whenever you let go of the gas pedal, there will be a nasty jolt. This does not happen in the Roadster, it transitions completely smoothly between acceleration and regen.

    It feels almost as if the road was conveyor belt, and you control its speed with the accelerator. We're all used to the car responding immediately when you move the steering wheel. The Tesla regen adds the same feeling to the forward movement as well - it's always "connected to the road" in that direction too. Don't know if that made any sense, but that's how I would describe it.

    I was fortunate enough to get the opportunity to drive one for an hour some time ago. I found only one major flaw - you look like a complete idiot when you try to stop smiling and discover that you can't because your front teeth have dried...
    Last edited by eledille; 2012-11-15 at 09:11 AM.

  7. #17
    Senior Member smorgasbord's Avatar
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    Quote Originally Posted by strider View Post
    No one on the planet has been able to make regen on the brake pedal work properly. There is always a jerk when the car transitions from regen to friction brakes.
    Reports are the the Fisker Karma does a really good job at blending regen and friction braking and at providing good braking feel throughout.

    Regen on Roadster does work really well. You need to relearn how to drive smoothly, though. With ICE and automatic transmissions, we're used to completely lifting off the accelerator to coast. On Roadster and Model S, you don't press down as far. On Roadster 2.x and Model S, you have a power gauge (kW) that you can use to help you find that coasting spot. You can always spot new Tesla drivers because their brake lights keep coming on even though there's no reason for them to be braking - and they're not, they're just regening when they should be coasting. Tesla should put a brake light indicator on the dash to help you learn when you're regening too much.

    The downside to regen on our rear wheel drive cars is that typically you want braking on the front wheels, due to weight redistribution. I think that's why Tesla both limited and feathered the regen on Model S. Roadster gets away with more and more instant regen because there's so much weight on its rear wheels (you may have seen Roadster falling off 4-point lifts not properly equipped) that it's OK to brake heavily on them. But, Model S has a better for driving 53/47 weight distribution, so braking hard on the rear wheels only would not be safe. I'm expecting that future AWD Teslas will again have heavy regen since they can do so on the front wheels, which is the safe way to brake.

    So, if you can figure out how to combine regen and friction brakes, you do get some advantages. You could use front friction and rear regen to get balanced braking at all times. You can have an easy coast mode that doesn't require the driver to find the sweet spot on the accelerator. You can get better track performance, too (at Laguna Seca, the trick the Roadster lap time record holder used was to 2-foot drive the car, which enabled him to keep the accelerator at coasting while the other foot nailed the brakes).

    But, I agree that on Roadster on the street, the regen/brake setup is wonderful. The only downside is that I don't use the brakes enough and so when I do need them they squeak embarrassingly. Even after a day of hard braking at Laguna Seca, they were back to squeaking within a week.

    Back to hills, I live at an elevation over 2000' and drive into Silicon Valley every day. It's about 5-6 miles down and I gain about 3 miles of range going to work, even with a full Standard charge. The road is twisty, but if I choose the right speeds I don't need to brake at all unless there are other cars on the road. I do, however, have to press the accelerator some since full regen on Roadster would make the trip too slow for my tastes (but made for an interesting regen experiment I did a year ago and posted about on TMC).

  8. #18
    Roadster#433, Model S#S37 Cottonwood's Avatar
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    Quote Originally Posted by smorgasbord View Post
    But, I agree that on Roadster on the street, the regen/brake setup is wonderful. The only downside is that I don't use the brakes enough and so when I do need them they squeak embarrassingly. Even after a day of hard braking at Laguna Seca, they were back to squeaking within a week.
    I've found a good compromise that gets most of the regen energy, gets me places a little faster, and keeps the brake disks wiped.

    The energy that is recovered on flat ground is 1/2 * m * v2. Because the remaining energy to be recovered when stopping is proportional to v2, if you use friction braking for all of the last 1/2 of the speed reduction, then you have only given up 1/4 of the regen available, 1/3 of the speed gives up 1/9, 1/4 of the speed gives up 1/16, etc. Also the time to cover the total distance is disproportionately used when dwelling at lower speeds; that's why you always wait until the last second to brake on the track.

    So, my method is to try to let up on the accelerator pedal so that I have to start applying friction brakes during the last 1/2 to 1/4 of the speed in deceleration to a stop. This loses little regen, saves me time, and wipes the brakes. Your mileage may vary.

    Coming back to this thread, variations of this work on hills as well.

  9. #19
    Senior Member strider's Avatar
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    Quote Originally Posted by smorgasbord View Post
    But, I agree that on Roadster on the street, the regen/brake setup is wonderful. The only downside is that I don't use the brakes enough and so when I do need them they squeak embarrassingly. Even after a day of hard braking at Laguna Seca, they were back to squeaking within a week.
    Switch to the Carbotech 1521 pads. No squeaking and nearly no dust while still providing great stopping power when you need it.
    Twilight Blue Roadster 2.5 - #1098 / Grey Model S Performance - #1459

  10. #20
    Senior Member smorgasbord's Avatar
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    Quote Originally Posted by strider View Post
    Switch to the Carbotech 1521 pads. No squeaking and nearly no dust while still providing great stopping power when you need it.
    I've thought about that, but am waiting to see if Tesla does a brake system upgrade and how well that gets reviewed. Ideally, someone will do the kind of test that was done for aftermarket HIDs versus Tesla's HIG upgrade, but for Carbotech's versus Tesla's upgrade, if and when they actually have one.

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