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Speed to set Air Suspension to "Low"

Discussion in 'Model S' started by JClu, Nov 15, 2016.

  1. JClu

    JClu Member

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    With the Smart Air Suspension, at what speed do you set it to transition from "Standard" to "Low" height? 55, 60, 65, or higher MPH?

    I don't care so much about the aesthetics (lower looks nicer). I understand lower suspension makes the car more aerodynamic and efficient overall, at the potential expense of vehicle damage (if you hit a pothole or any item on the road etc). I've heard some people even suggested lower suspension causes more tire wear-n-tear. Is there any truth to that? :confused:
     
  2. NOLA_Mike

    NOLA_Mike Grouchy

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    Among other geometry changes, the rear camber changes with suspension height (the lower the suspension the more negative camber) which results in more wear on the inside edge of the rear tires. It also, theoretically, results in better handling at higher speeds but that may only be noticeable near the limits?

    Having said that, I set mine to 60 MPH to lower. That's where I've had it since they introduced the option to set it almost 2 years ago. And I got 27,000 miles on my first set of 21" PS2 tires (which I think is fantastic for summer compound low profile tires). When I replaced them, the steel belts were just starting to peek thru on the inside 1" of the rear tires. :)

    I figure I only hit 60 MPH or more on the highway (I don't want it constantly raising and lowering around town).

    Mike
     
  3. JPUConn

    JPUConn Member

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    Mine is set to 50 mph and ~6k miles later my 19" tires show no sign of uneven wear. I did notice the other day when my wife was driving and I had set the suspension to low that the camber of the rear wheels was noticible. Maybe I'll change it to 60 mph or shut it off all together
     
  4. HebrHmr

    HebrHmr Member

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    How does lowering links impact this negative wear? For example, if I was set to standard with the use of lowering links actually putting me at a "Low" height would I receive the benefits of being lower to the ground without the camber wear associated with the suspension in that position? Is the negative camber caused by the ride height of the vehicle or the position of the suspension/length of travel?
     
  5. NOLA_Mike

    NOLA_Mike Grouchy

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    Calling @lolachampcar - he's the expert!

    From what I understand the rear camber is not "adjustable" without changing parts.

    Mike
     
  6. NOLA_Mike

    NOLA_Mike Grouchy

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    Here's a thread on rear camber if you are interested: Upper Rear Link Build (Reduces rear camber to improve tire wear)
     
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  7. green1

    green1 Active Member

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    I chose 80km/hr (50mph) for my lowering. my reasoning was that if I'm going 80 or higher I'm probably on an actual highway instead of a side-street.

    That said, the total height difference from "very high" to "low" isn't as much as I had thought, and I am starting to wonder if there's really any point to the air suspension anyway. Air suspensions have notoriously been a maintenance nightmare as they age, and I don't find myself manually setting it much, and the "advantages" to low on the highway seem marginal at best. The Tesla one isn't known to increase comfort any either, so I'm thinking that if buying new, it might be something that's just not worth it.
     
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  8. HebrHmr

    HebrHmr Member

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    I'm not suggesting adjusting the camber per se. I'm just curious if its correlated to ride height or suspension setting. If the camber is directly related to the suspension setting then running it at standard with lowering links would give the effeciency of running at low without the increased tire wear.

    If it's related to ride height then lowering links would just make it all worse.

    I'll read through that thread as well.
     
  9. green1

    green1 Active Member

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    It's ride height. All the lowering links do is fool the suspension in to thinking your car is at a different height than it actually is. So for example you're at normal height and it thinks you're at high so it lowers you. (the "link" is between the actual suspension, and the sensor that detects what height you're at)

    As a result, yes, lowering links will make it worse absent any other correction. There are after market parts available to correct rear camber (which may be useful for this purpose) but from what I gather the stock suspension isn't adjustable in that regard.
     
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  10. Model S M.D.

    Model S M.D. Ludicrous Radiologist

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    I like this thread, I have mine set to 60 mph (i.e. highway driving only) for fear of wearing out the tires early...
     
  11. HebrHmr

    HebrHmr Member

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    So, vanity/performance/efficiency OR tire life but not both. I'm already going to be running staggered Michelin PSS's so I can't afford any more loss in tread life.

    I guess I'll take a look at @lolachampcar 's kit. Thanks for the educated response!
     
  12. green1

    green1 Active Member

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    you can have both, it just requires more work.

    You've already bought lowering links, consider the rear adjustable camber bolts part of the package to do it right.

    It also depends on how extreme you're lowering. Keep in mind that "low" isn't as low as it once was. Tesla removed low after a couple of battery fires, and when they brought it back it wasn't as low as before. So if you lower only a bit, you're no lower than the car was designed to accommodate in the first place.
     
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  13. ig_epower

    ig_epower Member

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    Here's my take on this interesting topic. 1) At highway speeds and even paralegal speeds, there is no noticeable difference in fuel economy or even handling between the normal and low setting. 2) If inside tire wear is indeed a side-effect of using the lower setting, I would then prefer not to lower the car at all.

    In my situation, I like to raise the car when I need the ground clearance because it is not safe to scrape the bottom of this type of car. If the look of the lowered car is nicer - then I would want to lower my car when it is parked (kind of like the old Citroen cars if anyone knows what they are).
     
  14. green1

    green1 Active Member

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    FYI: the standard suspension sits at a level higher than the normal level of the air suspension, mitigating this issue (unless you have a specific use case in mind that needs even higher)

    I'm really starting to see that the air suspension may not really have enough advantages to justify it's cost and future maintenance. (I originally wanted it for the efficiency which has basically been debunked, and ability to raise for obstacles, which I rarely use, and the standard suspension may be enough anyway)
     
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  15. demundus

    demundus Member

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    There is another option to the wear issue, or so I was told by Tesla. After taking my car to a special shop for alignment, they had to call Tesla to ask questions, Tesla told them the wear issue is known and they have a special tune they deploy on the air suspension to help fix this. Not sure how true this is, but I do know each bag is configurable... so not far fetched.
     
  16. ig_epower

    ig_epower Member

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    Yes - I have a specific need for the air suspension. I have a Torklift accessory hitch and I mount a platform in the back where I haul my electric golf cart. Raising the suspension reduces any platform drag on the road when I hit small bumps and depressions. The platform is supported entirely by the hitch (and car) and has no wheels unlike a conventional trailer with wheels.
     
  17. lolachampcar

    lolachampcar Active Member

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    I'd like to toss some ideas or comments into this thread if I can.

    First
    Rear tire wear seems to be a function of ride height (camber gain mentioned later), toe in and tire side wall stiffness. Accelerated wear occurs when a narrower part of the rear tire is bearing a lot of load and when that part of the tire is scrubbing against the direction the car is traveling.

    Modern suspension is designed so that, when the side of the car is loaded and the wheel is pushed up into the fender well, the tilt in of the top of the tire (negative camber) is increased. Outward side loading of a tire that is tilted inward at the top (negative camber) forces the entire contact patch of the tire onto the road. Cornering hard pushes the outboard rear tire hard towards the outside of the corner and negative camber uses that load to force more of the contact patch onto the ground.

    FMVSS 126 is the Federal Motor Vehicle Safety Standard which applies to electronic stability control. There is a maneuver in 126 that requires a right, left, right yanking of the steering wheel with pauses build in to simulate an emergency avoidance maneuver. It tries to excite the rear of the car like a pendulum and drive the car to swap ends. A lot of European car manufacturers have taken to using rear negative camber to help catch the rear of the car in these types of situations.
    FMVSS No. 126 Electronic Stability Control Systems - NHTSA

    Start with rear negative camber (top of tire tilting inwards putting lots of pressure on the inside shoulder of the contact patch) and add toe in.. Toe in is when the front of the rear wheels are pointed slightly inward from dead straight. This is done, in part, for two reasons. First, it acts akin to feathers on a dart as your car goes down the highway. It tends to keep the rear directly behind the fronts. Second, modern suspension tends to have the rear tires toe outwards just a tad when under hard acceleration (upright moves forward, toe link leading the upright => upright rotates toe outward). The last thing you want is the rear tire steering outward when going around a corner under hard acceleration. We would use this on some race tracks but boy you had to be ready for the car to aggressively rotate under power :)

    The last element that exacerbates this problem is tire side wall stiffness. 21s have stiffer side walls than the 19s which means the 21s can not give as much to help dissipate that high inside shoulder loading.

    So much for negative camber and what combines with it to eat up tires.

    Second
    Next is why Tesla's have this problem. Coil spring cars ride higher than air cars. At full load, the coil cars must still be at or above Tesla's minimum ride height. Air cars just add more air to the springs to keep the already starting low ride height. The net result here is that camber gain (more negative camber as the wheel goes up into the wheel well) makes for more negative camber on air cars than coil (as air sits lower). Tesla decided to use one set of suspension geometry for both air and coil to control cost and limit build variations.

    Lower your car even more and, you guessed it, even more negative camber.

    Putting numbers to the above, when I first looked at this (2013) my wife's coil car had -1.2 degrees of camber in the rear while my P85 on air had -2.2. That increased to -2.7 when I lowered the car.

    How I attack these problems.
    If it is a play car where I will not be putting a lot of miles on it, I normally just wait to just under half way through the shoulder tread depth and rotate the tires across the rear. Yep, that means running the inside on the outside which does not agree with the idea of an asymmetrical tire. My thoughts are that I would rather have three grooves that are twice as deep (once the tire is worn) on the inside then four that are half as deep. I have spoken with several tire engineers who tell me there is no tire construction reason I should not do this.

    A daily driver like the Tesla is different. There is no camber adjustment on the back of MS so I made longer upper arms (by 0.210") to remove some rear camber (down to -1 front and rear). I also run significantly less rear toe in (0.1 degree total rear toe in on the rear). My rear wheel drive cars would tend to hunt more on the highway especially when the road was grooved. Range increased as well with less rear toe in. My all wheel drive car is 100% stable with little rear toe likely due to the front wheels pulling the car down the highway. I also am now running 20" rims because they weight less than Tesla's 21" option, cost half the Tesla option and I can use Pilot Super Sports which cost less, wear better and have more grip. Really, I'm not fibbing on the tires :)

    I made suspension bits for my own car. Doing so for someone else involves unacceptable liability. I did provide my work to one of the forum vendors who made some arms. There did not seem to be enough interest and Tesla changed the rear arm design so often (bushing changes) that making arms commercially available did not seem to fly.

    I hope the above helps. I do not begin to know everything so please feel free to correct me.

    Bill
     
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  18. green1

    green1 Active Member

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    I was under the impression that the hitch had a tongue weight rating of about 200lbs, I'd be a little nervous with an electric golf cart plus platform on that.
     
  19. NOLA_Mike

    NOLA_Mike Grouchy

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    Bill, thanks as always for the thorough and detailed response.

    As a side note, you can now get Michelin PSS for the Tesla 21" staggered wheel sizes. They even come with the foam inside (although I'm not sure I like that as you can't repair them with a patch).

    Screenshot 2016-11-15 15.56.07.png
     
  20. HebrHmr

    HebrHmr Member

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    I haven't bought them yet, but I was offered 1.5" lowering links by a shop doing a significant amount of other work for me when the car arrives.

    It sounds like even with an unmodified suspension a rear camber adjustment wouldn't be a bad idea

    Do they still manufacture them/update their design?

    I'll be running 20" Michelin PSSs as well. It definitely seems like lowering without a camber adjustment would be crazy and if I can't find one that even un-lowered I should reverse the direction and rotate the tires around 15k miles or so.

    Thanks again @green1 and @lolachampcar for educating me and if anyone knows of someone selling the parts required to make an adjustment like this please let me know.
     
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