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Results of testing front toe-in vs energy usage

Discussion in 'Model S: Driving Dynamics' started by zwede, Apr 22, 2014.

  1. zwede

    zwede 2013 P85+

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    The weather has been very stable the last few days so I decided to test the influence of front toe on energy consumption. To my surprise there is a very real and rather large effect when changing toe.

    Car: P85+ on the factory Michelin PS2 tires (21").

    Drive cycle: 7 mile commute, typical suburban drive. Speeds of 40-50mph. Ambient temps 65-80 dgrs (morning/afternoon. Temps stable day-to-day).

    Started the test by driving several days with front toe set at 0.07 dgr IN. Energy consumption: 310-320 wh/mile.

    Changed front toe to 0.30 dgr IN (max toe-in according to spec). Energy consumption: 340-350 wh/mile.

    Changed back to 0.07 IN and consumption is now back to 310-320 wh/mile.

    That's a solid 10% difference in range. Next I will re-check rear toe and see if there's some additional efficiency to be found.

    The engineer in me is finding this to be a great deal of fun. :)
     
  2. Zextraterrestrial

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    Thanks to you I adjusted my rear toe. Actually just the right rear toe, this was the tire that was wearing the worst out of my rears, and the factory paint mark was off from where the bolt was resting. Besides the car feeling much more stable under cornering acceleration + straight line hard slipping accel + drifting on damp, it also seems to be driving with less resistance too. Whr/mi #'s are seeming a noticeable bit lower.

    I think that my overall toe might still be a tiny bit out, but at least each side matches now. running a 6' level across the rear wheel towards the front (at ~ the chrome strip height) and each side lines up pretty parallel to the chrome strip on the car.

    (& it was easy, thanks)


    so, one more question. how are you measuring individual toe accurately?
     
  3. zwede

    zwede 2013 P85+

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    To do it accurately I use 4 jack stands, 2 on each side of the car. I run a string between each pair so I have a string along each side of the car at the height of the wheel center.

    Each string needs to be close to the rim without touching and exactly parallel to each other. Can't run them at the same distance from the rim front and rear as the track differs. Have to measure between the strings. Once I have that set up, I measure the distance from the string to the front and read edge of the rim. Then take the diff and divide by the distance front-rear of the rim. Do an Atangent ("reverse" tangent) on the number and I get the angle.
     
  4. AmpedRealtor

    AmpedRealtor Active Member

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    Reducing or eliminating toe-in will increase the likelihood of tramlining.
     
  5. zwede

    zwede 2013 P85+

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    True. So far I haven't noticed any lack of high speed stability though. I know BMW likes to dial in quite a bit of rear wheel toe-in for autobahn use. As everything, it's all a compromise. I'll take the extra 20 miles of range over a bit of high speed stability, though. :)
     
  6. zwede

    zwede 2013 P85+

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    I've done more experimentation on rear toe. It absolutely hates any toe-OUT. Efficiency nose-dives. What surprised me was that even rather high toe-IN doesn't hurt it. I had it set at 0.4 dgr IN and still managed 299 Wh/mile city driving (not bad for a P85+ on the 21s). Now have it at 0.15 IN and will see if this is the sweet spot.

    I'm not quite done yet, but I'm fairly sure that it wants more toe-IN rear than front. The front seems happy at zero toe to about 0.15 IN, but doesn't want much more than that. The rear needs at least 0.1 IN and is fine up to 0.4 IN.
     
  7. TomT

    TomT Technical Maven

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    One way I test efficiency quickly is by doing a coast down and timing how long it takes to go from, say, 50 to 10 in neutral. You want to do this on a fairly level surface and average doing it in both directions to compensate for wind and slight differences in levelness. Since rolling resistance becomes much more significant at lower speeds, you don't want to start at an insanely fast speed. Conversely, you don't want to go to a very low speed or to a stop since any wind will then become significant... I've tested a number of cars and tires this way over the years and have gleaned some useful data.
     
  8. zwede

    zwede 2013 P85+

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    Finding a road where I can do that would be hard. Although traffic is not heavy outside rush hour, there's always traffic here. But the traffic lights are all synchronized so I have an 8 mile loop I test on. I always hit the exact same lights so it is very consistent.

    Btw, I set front toe at zero (instead of 0.15 IN) and there is a slight improvement. I think I'll leave the front at zero. I like the way it turns in with zero toe. Feels better.
     
  9. KevinMS

    KevinMS Member

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    @zwede, thought I was reading one of my own post for a moment! Jack stands, string, et al! Using kite string and a digital caliper, the toe angle can be measured/calculated very accurately.

    Recently, I rotated the 19" GY Eagle tires on my '12 MS with 20k miles. On the rear tires, I found the inside rib completely worn away with cords showing. The rest of the tread appeared like it could go another 20k miles. So, I measured the total toe on the front and rear to see if I had the same problems as other early cars. The front was good at 0.07 deg. toe out, but the rear had 0.78 deg. toe out! I adjusted the rear toe in by 1 deg (0.5 per side) and now have a total toe of 0.22 deg. in. The adjustment made a huge difference in energy use. Went from an average of 310 w*hr/mile to 270 w*hr/mile on the typical commute to work!
     
  10. zwede

    zwede 2013 P85+

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    Kind of spooky how consistently wrong both our cars were. I also found 0.8 dgr toe-OUT in the rear after my service center aligned it. Did you have your SC align it or was the 0.8 dgr from the factory?

    I did find a "shortcut" for setting up my toe measurement. From the string to the rear center caps I use 45 mm, in the front 64 mm. Doing this the strings are exactly parallel.
     
  11. Zextraterrestrial

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    #11 Zextraterrestrial, Apr 30, 2014
    Last edited: Apr 30, 2014
    I have/had the exact same toe out...pretty sure almost the entire time since all of my tires have been wearing the inner edge horridly + I rotate tires monthly. I assumed the first set (5 months 7500mi - pushed them to >9k on inner corded fronts ) was due to all the test rides I was giving but probably not
     
  12. KevinMS

    KevinMS Member

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    I assume that my car came from the factory with the rear wheels toed out. Alignment was not mentioned at the one year service. Was planning to take it to SC for alignment after replacing the back tires. But since I've aligned it, it's been running great! Today, the round trip to work averaged 258 w*hr/mls! Never had numbers that low before! For string alignment, I set the strings parallel, then center them on the car by measuring off of the center caps, too. :)
     
  13. KevinMS

    KevinMS Member

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    #13 KevinMS, May 1, 2014
    Last edited: May 1, 2014
    My shortcut for adjusting the toe is to place a dial indicator at the edge of the rotor face (hub level) and trig out how much I need to adjust it to get the toe angle desired. I set it up to be 0.000" at that angle. Break the cam loose and adjust to zero. Using this method, you only have to adjust it one time. Then remeasure toe angle to verify.
     
  14. zwede

    zwede 2013 P85+

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    Kevin: You did place the string 19 mm further out from the front hub, right? Otherwise the string is not parallel.

    Also, be sure to double check after driving several miles. I found mine changed a bit after the suspension settled.
     
  15. KevinMS

    KevinMS Member

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    Yep, double or triple check that the side strings are parallel, centered on each axle and not touching tires/car body. I don't target a specific distance from the caps. So, don't remember the difference in track width.

    Probably a good idea to recheck after suspension settles. After putting the wheels back on, I take it off of jack mode, dropped the suspension to standard and rolled the car back & forth a few feet to relax any residual stress in the tire/suspension from jacking. Then recheck toe.
     

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