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The Adverse Range with 21" Wheels

Discussion in 'Model S: Battery & Charging' started by skilly, Jan 1, 2015.

  1. skilly

    skilly Member

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    Its clear to me that the 21" wheels reduce range. Whats unclear to me is why - I have asked a bunch of times in different threads but no one (that I have seen) has come out with a definitive answer. Yes the tires are more sports oriented so we can expect more traction/grip as a result. So, in the beginning one can expect a 5%+ delta between the two - that eventually reduces to a minimal impact though. So, with that in mind is the delta then more attributed to weight because the wheel cast is larger than the 19" wheel? If so, then wouldn't it make sense that a wheel in 21" that had an unsprung weight lighter than the 19" would reduce (and perhaps eliminate) the delta between the two wheels?

    Has anyone ever "weighed" in on this subject with comparisons or thoughts around this?
     
  2. omarsultan

    omarsultan Active Member

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    I have not looked in a while but the Tesla website used to say its a 3% hit. They primarily attributed it to the higher rolling resistance of summer tires (softer).

    O
     
  3. Danal

    Danal electricmotorglider.com

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    Might also be some effect due to increased Polar Moment of Inertia on 21" wheels.

    The further the mass in the wheel is from the center, the longer the "lever arm", and therefore the more energy required to accelerate (spin up) the wheel.

    I used to autocross a car with two sets of wheels. A "pretty" set for street driving, with great big wheels and little thin tires, and a racing set (Panasport, not even street legal), that had quite small metal wheels and great big sidewalls, resulting in the same tire diameter (due to class rules). Due to all that rubber, total weight difference between the wheel sets was quite small. Often, I drove to an event on the street wheels, with a jack and the racing wheels in the car. The first few minutes after switching wheels, it felt like a 10% or more horsepower boost... primarily because the metal mass was closer to the hub. It wasn't just getting the weight out of the car... out of curiosity one time I put the street tires and jack back in the car. Still quite a boost in power feel.


    So, it would take some weights that we don't have (but can probably get), and a little math... anyone have the total weight of the 21" and 19" wheels, separate from the tires? And the weight of the tires? And, ideally, the weight of the RIM ONLY on the 19" and 21" wheels? Or we can just estimate that...
     
  4. skilly

    skilly Member

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    I was at the Factory SC and they didn't hesitate to attribute it to weight. They didn't explain their answer - just very quickly attributed it to that. Assuming tire wear and the same contact patch, you would think at some point the delta would be non-existent if it were all to do with tires...so weight does make more sense to me. This certainly holds true in racing scenarios.

    If it is unsprung weight causing the negative impact, nothing is heavier than a cast wheel (except of course a steel one). Cast wheels have some design that are better than others (they are constructed with unsprung weight in mind - such as the 996 Porsche Turbo Wheels); however, I don't think that they could come close to a mono block aluminum wheel. Looking at various other forums on the topic, it seems that they conclude that the difference could be as much as 2% or 3% with an unsprung weight reduction. This would also improve performance and handling but may find more feedback at the drivers wheel.

    So, if this were true, and you were a fan of the larger wheel characteristics, it would make sense to move to a mono block 21" all aluminum wheel to get that larger wheel look without having a negative impact on range. It may even improve range...

    - - - Updated - - -

    very good point! Although, I think tire technology has come a long way - they don't necessarily have to be heavier to accommodate that lower profile - case in point, F1 is even moving to a larger 19" wheel from the race standard 17" today. Those guys are so weight conscious that they will silk screen logos on the drivers fire suit over a patch in consideration of weight! Im really interested to see what we know as a group...even gross weight of the 19/21 with tires I haven't been able to find yet.
     
  5. ArtInCT

    ArtInCT Always Learning

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    Also, the wider rear tires present quite a few more square inches of wind resistance at speed.
    This adds up especially at higher speeds.
    "You never get something, without giving something up"... this saying is very true in automobiles.
     
  6. skilly

    skilly Member

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    I would agree if this were an open wheel car - and if it were spokes and their effect in a wind tunnel we would have consider the difference between the two versions of 19" wheel offerings....and they don't distinguish between the two as far as ratings. That said, for the larger tire, the contact patch would also be larger...marginally. Again, it seems to point back to unsprung weight.
     
  7. smac

    smac Active Member

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    I wonder how it effects underbody airflow though. I saw some of the DIY aero-mods for the Roadster included diffuser fins on the inside of the wheel arch, and they seemed to make a difference.

    But in general I agree with the view it's the unsprung (and more importantly rotating) weight, along with tyre compounds, that's causing the biggest degradation.

    I'd have preferred 20" wheels as a factory option, with all seasons or efficiency orientated summer rubber. Smaller efficiency penalty, but big gains in pothole protection and looks compared to the 21s or 19s respectively.
     

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