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Model S vs Roadster on the Track

Discussion in 'Model S: Driving Dynamics' started by Cottonwood, Nov 5, 2012.

  1. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    #1 Cottonwood, Nov 5, 2012
    Last edited: Nov 10, 2012
    Saturday, my friend and I took my Model S Signature Performance (MSP) and my Roadster out to High Plains Raceway (HPR) east of Denver, (see High Plains Raceway Track Details), for a day of fun. HPR has 8, 50-Amp RV outlets their so we were able to add charge whenever we were not driving. We had Michael Pettiford, Instructor Bio, as our instructor and as the driver to try for best track times to compare the MSP and Roadster (non-sport).

    Quick Summary of lap time results:
    • All times were for the 1.83 mile, North Course. This leaves the south straightaway at turn 4S, direct to turn 9a, see Track Map
    • All results were with Michael driving, and me right seat timing from the start/finish line. Here are lap time, max speed, and average speed. The max speeds are from the uncalibrated speedometers in each car, the lap times are from my iPhone stopwatch, and the average speeds are calculated.
    • 1:33.7, 113 MPH, 70.3 MPH - 2012 MSP Signature
    • 1:31.8, 107 MPH, 71.8 MPH - 2008 Roadster
    • 1:21.1, 125 MPH, 81.2 MPH - 2005 Corvette Z06, 550 HP

    Even though the Roadster looks like the sports car, the MSP definitely braked and handled the turns better. The MSP had a much more predictable balanced entry into the turns. The Roadster front would not get a grip and had a definite understeer tendency. Because of the better braking on the MSP, Mike could wait a significant distance longer before each turn before he had to apply brakes going into the turns.

    From my previous work with Mike at HPR three years ago in the Roadster, we knew about that PEM cooling was the weakness of the Roadster on the track. This November day was much cooler, and the local service center had just swapped out my Roadster PEM due to cooling issues last summer. I am sure both helped. Three years ago, we could barely get one full, 2.55 mile lap in before the PEM hot warning came on and the power was reduced. This time, we could get two laps or more before the "dinner bell" rang and we had half power.

    With its liquid-cooled PEM, batteries, and motor, I had high hopes for the MSP to hold power. Unfortunately, my hopes were dashed. In 1/2 to 2/3 of the 1.83 mile lap, the little dashed limit line came on the power display, usually reducing power from 320 kW to a little more than 160kW. This shows up in the MSP having a better maximum speed (at the end of the south straight away) before the power was limited, but having worse lap times because of the reduced power in the last 1/3 of the lap. The Roadster tells you the PEM hot is the limiting factor. The MSP just lets you know the power is limited. We don't know if it was the PEM, batteries, or the motor. We did do this at the end of the day with the MSP charge at 100 miles or so rated range. If the batteries are the limiting factor, a higher charge might help. Our second try, keeping everything cool as long as possible was heading to beat the Roadster time, but the car we were catching at the finish line did a goofy move and Mike aborted the lap. We were heading for a better than Roadster time, even with power limiting then. Because the track operators wanted to lock the gates soon after the track went cold, and we had 82 miles to drive home, we did not have enough battery energy left to make one last try. Every 3-4 lap session was equal to an hour of charging at 40 Amps. Even with the power limiting in the MSP, I think we could have beat the Roadster time by a second or two. Another day...

    The 'vette times are with Mike's personal street car (he has other faster racing cars). It was 505 HP stock, but Mike guesses that he is currently getting about 550 HP with various mods. His "street" tires are very sticky, the kind that holds onto the sand and gravel in the parking lot. This time was on his first hard lap. He was definitely faster on the second lap when he developed a brake problem. As we came into turn 13, heading for the finish line, when Mike let up on the accelerator, we both hit the shoulder straps hard with uncontrolled, hard brake application. Mike steered to a safe spot on the outside of the course. With clutch in and parking brake off, we were not moving down a pretty good hill. The brakes were locked and smoke was coming out of both front wheels with that putride, hot-brake smell. After the track tow truck came out and some investigation, Mike discovered that a small piece of plastic had slipped down under the dash and was applying the brakes without any foot pressure. Apparently this had been getting worse as the lap went on and overheated his brakes. We went back to the pit area and that was the end of the day.

    There were two modified Lotus Exiges and one highly modified (more horsepower) Lotus Elise on the track as well. All three were way more balanced in the turns and could slightly outperform the Roadster in high speed acceleration.

    It was a very fun day. I learned that the MSP has more performance than I want to apply on the track and way more than I ever want to use on the road. Michael taught me some very good driving skills and is an excellent instructor. I strongly recommend him to anyone who wants to do some serious learning about how to drive your car hard and safely.

    The big surprises were that the MSP power limits quicker than the Roadster under VERY hard driving, and that the MSP braking and handling exceed the Roadster.

    Maybe we can get more Teslas out on the track another time. Its starting to get cold in Colorado, and I will probably put the snow tires on my MSP in the next month. We should start planning something for next spring. Maybe, we could even talk HPR into installing a HPWC. :wink:

    Update: Here is a picture of me coming out of turn 13, entering turn 14 at HPR:

    HPR-1.jpg

    Screen Shot 2012-11-06 at 5.50.39 AM.png
     
  2. AnOutsider

    AnOutsider S532 # XS27

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    Not TOO surprised on the latter, but definitely surprised by the former. I thought, especially with the "performance" option, this would have been better. Either way, great writeup, and thanks!
     
  3. smorgasbord

    smorgasbord Active Member

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    Great report and a fun read - thanks!
     
  4. 7racer

    7racer Member

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    Cottonwood,

    awesome, awesome post! LOVE IT! And hats off for taking both of your cars to the track. Most people never get to push there cars anywhere close to the limits.
    In regards to the Roadster. Do you have the adjustable suspension?

    The roadster understeers like a pig. I would like to dial in some oversteer but with all the weight in the back, I am afraid of getting snap oversteer like in a back heavy Porsche. Did your instructor have any ideas or play with the suspension set up?
     
  5. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    Sorry I had to miss the fun, Cottonwood. It would have been instructive to see how my Sig S Standard performed with 19" factory wheels and tires. I wonder if the power limiting threshold might have been reached later than in your P85...
     
  6. Doug_G

    Doug_G Lead Moderator

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    That's a good point about the adjustable suspension. The stock suspension (and the adjustable suspension if you don't actually adjust it) comes with a heck of a lot of understeer dialed in. No doubt Tesla is worried about people doing power oversteer and/or snap oversteer into the ditch. I've dialed back the understeer via the front sway bars and it makes a huge difference. (If it wasn't my daily driver I'd consider adjusting the rear too.)

    Another thing I've noticed - understeer is a LOT less of a problem running A048's than AD07's.
     
  7. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I have an original, non-sport Roadster without an adjustable suspension.

    Are there adjustments in the adjustable suspension that will change under/over steer?

    If the power limiting is coming from a hot PEM, an idea that I had while driving home, is to set the regen mode to "light." That will take the regen heat load off of the PEM; the friction brakes have plenty of stopping power for the track. It may also reduce some heat load on the batteries from recharging them. Obviously, the battery will be consumed a little faster, but if we can get a faster lap time, its worth it! :biggrin:
     
  8. DaveVa

    DaveVa Sig Perf #236 VIN #484

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    I am very surprised given the water cooled nature of the PEM, batteries and motor. I wonder if the cooling system was operating correctly (e.g. Radiator flaps full open for heat exchange). If I recall, the beta Model S did far better than the roadsters in an earlier electric rally. Would be interesting to see if Tesla responds to your posting.
     
  9. Doug_G

    Doug_G Lead Moderator

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    Yes, loosening the front sway bar, or tightening the rear, will reduce the understeer. Highly recommended for track use.

    Also yes, setting regen to low would be a good idea on a track, for two reasons. As you say it will reduce the PEM heating (your brake rotors will get hotter instead), but more importantly it affects how the car handles in a corner. I find with the Roadster I have to move my foot very rapidly from the brake to the throttle and then give it just enough juice to keep the acceleration neutral; otherwise it messes up the weight transfer to the front, which makes the understeer worse. It also means you can't trail brake.
     
  10. arg

    arg Member

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    I was wondering whether it might have got better had they carried on driving. Letting the car cool down before trying for a best lap sounds like the right thing to do, but maybe it confuses the control systems - it may take some time to transition from energy-saving minimal cooling at idle to flaps-open maximum cooling (and do we know how/if the AC compressor interacts with the powertrain cooling on the model S?).
     
  11. Johan

    Johan Took a TSLA bear test. Came back negative.

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    @Cottonwood: Thanks for a very interesting read!

    I also would agree with previous posters that 1) this seems a little disappointing 2) It might have gotten better if driving was continued, and maybe also if the SOC was higher to begin with?

    With regards to battery cooling: In this racing setting is correct to talk about "cooling". But actually it's a system for battery thermal management. If it's very cold outside the system will heat the battery until it reaches an optimal operating temparature, and if it's extremely cold I suppose the system would keep heating it continously if need be?

    Anyway, I don't believe the system is coupled with the cabin AC. The fact that there are radiator flaps that open up during hard driving would IMO be proof that the battery cooling system works pretty much like a traditional car's radiator cooling system. That is it transports excess heat away and the liquid gets cooled while passing through the radiator, where the excess heat is given to the air that passed over/through the radiator. This temperature exchange of course is better with more passing air = higher speeds. This would also mean that it's unlikely that you could "overcool" the battery using this system, since the fluid won't get much colder than the ambient temperature. That's why there should be a "Racing mode" user setting which puts the battery cooling liquid circulation to max and opens the radiator flaps to max as you toggle it, before the battery gets hot and power is limited. Also, this setting should completely disable regen braking. Yes, it will cost range (no regen, more energy to the battert liquid pump, more air resistance due to fully open flaps) but that's not the issue while racing.
     
  12. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    At this track, (see HPR Track Map below), because the start/stop line is parallel to and in front of the pit area, the procedure we used was to do a moderate (but hard relative to normal driving) partial lap, then start accelerating through the three turns (13, 14, 15) leading up to the start/finish line, start the lap time at the start/finish line, do the hard lap, record the lap time at the start/finish, then do a moderate, partial lap, returning to the pit area. The moderate lap should have gotten the fluids flowing and flaps open for the cooling. If that did not, then there was a minute of so of hard driving before the power limit typically happened between turn 4S and 10. The huge problem was coming out of turn 11, going up to turn 12. Turn 11 is a full 180˚ turn, and probably one of the slowest parts of the lap. Where you really want power is accelerating out of this turn up hill to turn 12 and 13. Reduced power here wastes many seconds getting back up to speed. I would think that the cooling system could respond in the minute or so as the temps were rising in the moderate, warm up lap and the beginning of the speed lap. The decision to open the cooling flaps should be based not only on absolute cooling needs, but the rate at which temps are rising.

    Screen Shot 2012-11-06 at 5.50.39 AM.png

    What details are known about the cooling flaps? Location, size, speed, etc.
     
  13. drees

    drees Active Member

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    Higher SOC should definitely help. With higher pack voltage, less current will be required to produce the same power which should result in less heating of drivetrain components.
     
  14. CroDriver

    CroDriver Member

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    This is not great news, but look at it from the other side: I had a BMW M6 and it overheated too after a few laps of track driving.

    An "Oil temperature too high, please slow down" message would pop-up on the screen.
     
  15. doug

    doug Administrator / Head Moderator

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    Any photos or vids from this track day?
     
  16. William3

    William3 Member

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    #16 William3, Nov 6, 2012
    Last edited: Nov 6, 2012
    This whole thing is really sad. I thought they had totally solved this with the Model S, or that it was at least better than the Roadster. Wow.
     
  17. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Anyone with some more engineering insight or maybe inside knowledge who can shed light on what it actually means when you get the " little dashed limit line" on the display. I mean, I know it means that power is effectively limited to half, but what is the actual trigger for this to occur? Is it a temperature sensor that monitors the battery management liquid as it exits the pack? As it enters the pack? A sensor in the radiator? Are there multiple sensors within the pack? Are there sensors adjacent to the actual battery cells (wall-to-wall) or are they in the liquid surrounding the cells? Or are there other symptoms that the car is sensing - like something more electrical that indirectly indicates that the battery pack is not doing well, such as perhaps: a sudden voltage drop? voltage variations? variations in the current flow from the battery?

    Also, do we know if the limiter is the actual battery pack getting too hot? Couldn't it just as well be the inverter (we know that one of the things that set the performance apart from the regular 85kWh is the beefier inverter)? Could it be the actual motor getting hot?

    I agree with other posters that this is somewhat disappointing. @Cottonwood: Have you contacted Tesla about this? You have a very early car, perhaps there is something wrong with your car (For your sake I hope not of course, but in a way that would be a better explanation than if this is as good as it gets).
     
  18. Doug_G

    Doug_G Lead Moderator

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    We know Tesla was running betas at Laguna Seca and doing good lap times with them.

    Makes me wonder if the production cars have more conservative limits in their firmware...
     
  19. Tommy

    Tommy Member

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    Would the additional weight of the passenger have tipped the needed energy demand to cause the power to be lowered. I recall seeing most cars driven hard on a track being done by a single driver with no passenger.
     
  20. MikeK

    MikeK R#129, TSLA shareholder

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    Well, that is disappointing that the MSP limited performance so early. :-(
     

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