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Any proof that same tires perform better on 21" wheels than the 19" wheels?

Discussion in 'Model S: Driving Dynamics' started by bhzmark, Jun 7, 2015.

  1. bhzmark

    bhzmark Active Member

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    Where is any authoritative evidence that **given the same tires** the 21" wheels are significantly higher performing than the 19" wheels?

    Are there any direct comparisons using the same tire but different wheels at the drag strip, skidpad, track, or an efficiency/range/rolling resistance comparison?


    It seems to me that there is substantial anecdotal evidence in these forums that 21" wheels perform much worse than the 19" wheels when going over potholes and when trying to maximize battery range. Where is the countervailing evidence that they perform significant better in either acceleration or handling or other areas?


    Note, this is not a question about the tires, but simply wheel size.


    Also, armchair theorizing and speculation isn't very useful -- any real evidence? Does anyone have any evidence to support their position that 21" wheels are substantially better performing than 19" wheels with the same tire?


    Since there is significant evidence that tires vary widely in performance, but none that 21" is better than 19", it is likely that:

    -- the tire choice on either wheel is what primarily determines performance (handling, skidpad, acceleration).


    -- the wheel choice likely only affects aesthetics, pothole resilience and ride harshness.
     
  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Because the sidewall is shorter, the 21" (or 22") wheels can feel more responsive. This works fine if the roads you drive on are glass smooth--I believe there is at least one mile of road like this within a fifty mile radius of my home, but there might be two. It's also true that the tires are almost never the same so there is a perception that the larger diameter wheels give more performance. The main purpose of larger diameter wheels (plus lower aspect ratio tires) is to enrich the tire and wheel manufacturers and sellers. They have spent a lot of money to make people believe they perform and look better. And in some situations they may--it's just that those situations don't cover more than 20% of the actual driving people do on the roads they really travel (and that's probably a generous estimation).
     
  3. 7racer

    7racer Member

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    That's a really hard question to answer as it really just depends.

    I track my GT-R a lot and with the other track rats talk about different tires etc.

    In general, smaller wheels (not the tire but rim) are lighter and easier to spin so the car feels quicker.
    On the heavier GT-R and other heavy cars, a stiffer side wall helps performance (the factory run flats or increase tire pressure).
    Sometimes, stickier tires are only found on larger tires (not always)
    For drag, a soft side wall is usually best. That's why there are drag radials and drag slicks. I have a set of 18" drag radials that I swap out on the rear when I change my GT-R from Track to Drag configuration.

    But this is all really kind moot for a street driven car.
    In general you will get a lighter wheel with the 19"s and better "mileage" as there is less mass to rotate.
    Also, depending what tires are available, there might be limitations on low resistance tires for mileage on 21s vs more performance ones.
     
  4. David99

    David99 Active Member

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    I honestly think that low profile tires are a look thing and have nothing to do with performance. Customers with performance car want them, but it's not helping performance. If that was the case, race cars would have tires like that, but none of them do. Any professional race car, Nascar, F1, 24h Le Mans, have bigger profile tires.

    racetire.JPG
     
  5. Zextraterrestrial

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    For better cornering I run 19s at 36psi

    21s suck!

    Pretty but that's it
     
  6. jcaspar

    jcaspar Member

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    I agree. Given the same tires, I think 19" wheels will perform 21" wheels in virtually every metric except looks...
     
  7. stopcrazypp

    stopcrazypp Well-Known Member

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  8. Zextraterrestrial

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    steering response may be quicker but I think you can get a larger contact patch by lowering psi too and if you do that with a small sidewall you will be running on your rim in corners.

    but I just started racing a couple of years ago when Joules asked if she could get racing shoes so my experience is rather limited ;>
     
  9. MileHighMotoring

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    I think you've pretty much nailed it, OP. 21" will corner a bit better, and provide more feel due to less cushion in the tires. But all that comes at the expense of comfort and rim durability. Frankly the 85D is too quick for me already, lol. I'm looking for a good set of 15" wheels. (Joking, joking)
     
  10. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    The contact patch is slightly wider because the tire is more square. However, what Yokohama is referring to is the more typical practice of having a wider nominal section width in addition to the more square profile. In the Model S' case both 19" and 21" have a 245 mm nominal section width (assuming a non-staggered configuration), so the difference in tread width is minimal by comparison.
     
  11. bhzmark

    bhzmark Active Member

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    That article is about high to mid profile tires. and the benefits of going from your grandfather's buick's 65 aspect ratio to 55 aspect ratio. 55 still isn't low profile.

    the 19" wheel is already low profile at 45. the 21" wheel takes it to 35 aspect ratio -- into rubber band territory.

    Just because 55 is better than 65 it doesn't follow that 35 is better than 45. There can be too little sidewall -- or at least diminishing marginal returns -- especially when it exceeds the marginal cost (in $, ride comfort, longevity, range)

    - - - Updated - - -

    And as I think you've pointed out before, while the contact patch is wider, it is also shorter and thus not overall bigger. While wider is theoretically better for sideways handling, at least in some cases (who know about this exact case?) the shorter patch can mean the tire is much worse in rain and much easier to hydroplane.
     
  12. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    That's correct. Contact patch area is a function of load/pressure.
     
  13. 7racer

    7racer Member

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    some of this has to deal with regulations....
     
  14. green1

    green1 Active Member

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    #14 green1, Jun 8, 2015
    Last edited: Jun 8, 2015
    It should also be noted that it's been said the 21s weigh more, so more unsprung weight, which hurts performance.

    If you want looks, get the 21s
    If you want practicality, get the 19s
    If you want performance get 19s with performance tires.
     
  15. jerjozwik

    jerjozwik Member

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    #15 jerjozwik, Jun 8, 2015
    Last edited: Jun 8, 2015
    motoiq did a handling guide a while ago, and made a whole section dedicated to the effects of wheel / tire size. good read for anyone that is an armchair automotive engineer.

    The Ultimate guide to Suspension and Handling Part 1, Wheels and Tires


    also, since this is a wonder thread... im interested to see the relationship between a larger tire at 45psi vs a thinner tire at 45 psi. i spent some time finding light weight wheels for my track miata. the wheel itself was 15.5lb. sadly as soon as the 40 profile tire was added the wieght jump up to 45lb per wheel tire combo + all the air inside. sure, the p85d does not pack light wieght wheels but i do wonder if a smaller lighter wheel is still lighter after tire and all the air are added to the mix. vs a larger heavier wheel with... less air? ¯\_(ツ)_/¯
     
  16. Xenoilphobe

    Xenoilphobe Active Member

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    21 inch rims are similar to high heels on women. They look great, but don't perform any better. In fact i feel like I have to slow down to protect the rims in my P85+, much like high heels on rough terrain. The P85 has 19's and I never worry about damaging the rims..
     
  17. Canuck

    Canuck Active Member

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    I've never tried high heels on any terrain, let alone rough terrain. But I'll take your word for it... ;)
     
  18. Xenoilphobe

    Xenoilphobe Active Member

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  19. Uncle Paul

    Uncle Paul Member

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    The origin of bigger wheels giving better performance began when racers fitted larger wheels to allow for larger disc brakes to be installed.

    To keep the gear ratio's the same, and the speedometer accurate they developed a plus 1 or plus 2 system, where the larger diameter wheels were fitted with smaller sidewalled tires. The end result was the capacity to run larger brakes while maintaining the same total tire diameter.

    These larger diameter tires reduced the sidewall flex inherent in thicker tires, and produced quicker response, better turn-in. The reduced flex also produced a cooler running tire with less pressure buildup and longer life.

    This, getting rid of the flex, does not matter significantly for street tires, but is critical for performance applications on the track. Normal bumpy traffic situations might call for 19" while the cooler running 21's might be better fitted for cars with higher speed capabilities. High speed running on such a heavy car can really heat up a flexy sidewalled tire. I notice they allow the P Models a higher top speed. Maybe that is one of the reasons.

    Tesla has to decide which specifications their standard alignment, tracking and geometry will be optimized for. Believe this is why they encourage 19" on the lower potential models, and 21" for their Uber models with Insane performance capability.
     
  20. David99

    David99 Active Member

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    I believe it is save to say that it's more the opposite. 21 inch tires/wheels cause lower range because of a higher rolling resistance and don't last as long as 19 inch. Even Tesla's website will point that out in their interactive range calculator.
     

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