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Why do the largest tires have a harsh ride?

Discussion in 'Model X' started by MitchJi, Sep 4, 2015.

  1. MitchJi

    MitchJi Active Member

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    According to Consumer Reports, in their MS review, with the 21" tires the car stopped quicker and handled better but the ride was less comfortable. One of the reviewers specifically said that if he bought a MS he would not get the 21" tires.

    Why?! The biggest advantage of larger bicycle tires (29") is a smoother ride on rough surfaces due to theit larger circumference.

    NOTE: I am posting this in the MX Forum because I think that it is an issue that people who are configuring their cars might want to be aware of.
     
  2. Yggdrasill

    Yggdrasill Active Member

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    The circumference is pretty much unchanged with larger rims. You just have less rubber. Less rubber absorbs the bumps to a lesser extent, and feels less comfortable.

    Also, when it comes to the Model X, the wheels have a larger circumference than the wheels on the Model S, so 22" wheels can be approximately as comfortable as the 21" wheels on the Model S, and the same with 20" vs 19".
     
  3. brkaus

    brkaus Member

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    The tire size is actually the rim size. The outside diameter of the tire remains the same.

    Larger rim, less flexible tire sidewall to absorb bumps.

    Better handling of larger rims is because of less flexing.
     
  4. blanche

    blanche Member

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    I should point out ride "harshness" is relative. The 19" feel like I'm floating along on a barcalounger with wheels. I would never recommend them to anyone. The 21" aren't tight enough for me, although I suppose it will have to do. It just depends what your previous cars were I guess.
     
  5. Johan

    Johan Took a TSLA bear test. Came back negative.

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    As others have said: the wheel wells are the same size so when the wheel (the metal part) is larger the tire (rubber and air) is thinner. In addition to the suspension there is considerable cushioning going on in the soft part of the tire (i.e. air and rubber). When there is less air and rubber the ride gets harsher.
     
  6. gregincal

    gregincal Active Member

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    Basically just echoing what other people said, but pointing out that you read one thing and remembered another. I'm pretty sure if you go back and read the article again you will see they reference 19" and 21" wheels, not tires. In cars you tend to talk about wheel size, in bicycles you tend to talk about tire size, but they aren't the same.
     
  7. ArtInCT

    ArtInCT Always Learning

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    On the larger wheels, the tires' sidewall height is diminshed in order to maintain the outside tire and wheel combo diameter. The smaller sidewall reduces the tires ability to act as a spring. With the thinner sidewall, there is less "give" in the tire. So as a result the ride can feel firmer (harsh). Also since the tire's sidewall is thinner on the larger wheel, the rim itself is prone to more impact which can and has resulted in more bent and damaged wheels in the 21" fitment.

    This is also somewhat known as D Plus fitment. On the Tesla S the normal D is 19" and the 21" is D + 2" (19" + 2" = 21"). So the tire's sidewall on the D + 2 is about 1" thinner top and bottom for a total of 2" less tire. Potholes do NOT like D + 2 fitments. That particular fitment is not very popular in the snowbelt where Springtime potholes are the norm.

    As you go higher in D + n the sidewall becomes thinner and the ride becomes harsher and the handing in cornering should become better.

    It is always a game of what do you get at the expense of what do you give up.

    D + 1 would be a 20" wheel and tire combo on the Model S and would be a compromise between the two other fitments.
     
  8. Nubo

    Nubo Test Mule

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    [offtopic] Bike tire sizing is an insane mish-mash of ISO standards superimposed on conflicting, misleading and obsolete national systems, on top of which manufacturers lie about dimensions and then the public develops slang. :)

    Tire Sizing Systems [/offtopic]
     
  9. David99

    David99 Active Member

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    After trying both 21 inch and 19 inch rims there is no doubt I will stick to my 19 inch. Much quieter and the probability to get a flat from pot holes (which are very common where I drive) is much lower. The larger rubber area helps.

    It is also a myth that larger rims would lead to better handling. They feel harder but that's not making them better. If you look at race cars you will always see pretty small rims.
     
  10. jerry33

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

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    Ultra low aspect ratio tires are mainly about profits to the tire industry. The industry has spent a lot of money over the years to convince people that lower profile is better. However in almost all cases, a person will be better served by higher profile tires.
     
  11. goneskiian

    goneskiian Active Member

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    As others have pointed out, the car wheel/tire combos are the same circumference.

    A better bicycle analogy would be comparing a 700c road bike wheel/tire combo to a 26" mountain bike wheel tire combo. Very similar circumference but pretty drastic ride quality differences.

    Cheers!
     
  12. Local host

    Local host Member

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    The same review also mentioned that the larger wheels allowed for more road noise, where the 19" wheels were a quieter ride.
     
  13. MitchJi

    MitchJi Active Member

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    For the MS it's not a myth. It's based on stopwatch times swerving through cones and measured braking distances.

    Thank you everyone for all the useful information. You have answered my question and you might have helped a few people make configuration decisions that they will be happier with.
     
  14. bxr140

    bxr140 Member

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    To my knowledge that's never been done with like-for-like tires.

    Of course a sporty tire on a 21 is going to perform better than an all season on a 19.
     
  15. jerry33

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

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    Correct. I think many would be surprised at the results doing a like to like comparison.
     
  16. MsElectric

    MsElectric Active Member

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    Physics, and less side wall...

    You also get about 5% less range, louder ride, quicker tire wear, and more expensive tires. Oh and also due to the shorter sidewall, you are more likely to damage wheels when you graze curbs. Due to less sidewall to absorb the impact you are also more likely to have damaged wheels and tires due to potholes.

    For some these drawbacks are worth it for the "looks" and what some consider to be sportier/livelier handling.
     
  17. CarlK

    CarlK Member

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    #17 CarlK, Sep 6, 2015
    Last edited: Sep 6, 2015
    And don't forget tire is just part of the package. You can have a hard suspension and use higher profile (more compliant) tires to compensate for that or have softer suspension matched with low profile tires that can still provide sharp handling. That's the reason why people are advised to have air suspension when ordering the 21". Some cars, Porsche notably, have driver selectable suspension hardness settings to fit different needs.

    Also want to clarify some misconception of reason why F1 cars use tires with tall sidewall even though most other types of race cars are not. The tire rule has been there for decades with the reason to limit performance not to enhance it. It limits rim size to 13" so teams could not fit large brakes and would not make the car to go too fast. Teams compensate that for handling by making chassis extremely hard and tires to work as springs. This rule is in the process of being revised since there are separate rules on brakes already. It will make them to catch up with the more modern look.
     
  18. ptsagcy

    ptsagcy Member

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    Echoing what others have said, my previous car was a BMW 650 with low profile tires. Almost every time I hit a pothole at highway speed it bubbled a tire. I went through 6 front tires during the 3 year lease (thankfully I had tire and wheel insurance). I would never own low profile run flats again. While the non-run flats don't have as stiff a sidewall, they are still prone to damage because of the reduced cushioning between the rim and the road.
     
  19. jerry33

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

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    True, but it indicates that low profile isn't the end-all-be-all of performance. The main thing that low profile tires do is stabilize the tread at very high speeds (over 200 km/h). For those of you who recall 70% aspect ratio V rated tires, they got around this by having a floating bias ply layer in the sidewall to stiffen them up (ride wasn't great). A short sidewall reduces the amount of flexing making the tires safer at those very high speeds.
     
  20. lolachampcar

    lolachampcar Active Member

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    I would add that it is not an apples to apples comparison linking side wall to performance. There is a lot of difference in compound between the 19s and 21s. I'm running Pilot Super Sports on 20" rims which have more side wall than Tesla 21" PS2 yet my 20s have better grip. Part of that is better compound in the PSS' and part of it is 10 lbs per wheel less in unsprung weight (my 20" rims are 10 lbs each lighter than the Tesla 21s). The CR generalization works if you are only talking about Tesla's offerings but it falls apart when you consider other/better tire options irrespective of rim diameter.
     

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