Tire/rim physics.

Discussion in 'Car Discussion' started by angrypenguin, Sep 14, 2018.

  1. angrypenguin

    angrypenguin Eat Lead Mothertrucker!! Advanced User Premium Member

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    Before you read the below, please note the situation I am bringing up does not involve "plus" or "minus sizing", as this is not a usual topic that is tabled!

    I'd like to bounce this idea off of folks and get an opinion here. Tire Size Calculator

    Pls enter these two numbers under the "tire comparison" tab.

    Size 1: 245 45 19
    Size 2: 245 45 20

    What I'm trying to figure out is

    (assuming tires used in both situations are the same tire, same load rating, same speed rating)

    1) Which tire is noisier
    2) Which tire is more comfortable.

    Let's look at a couple of interesting facts here between these two tires, one being 19" and one being 20"

    The width of both tires is 9.6"
    The sidewall width is both 4.3"

    So to me, it doesn't appear that one can make a blanket statement of saying "the 20" will ride harder", because, on the face of the math above, this is not true. My only thought here though, is which tire has more contact patch with the road? My immediate thought process is that a 1" wide tire (let's go to extremes), has a lot less in contact with the road than a 30" tire, so does that mean then that my two tire sizes above - the 20" has a lot more in contact with the road and thus would be noisier?

    My question above is based on the assumption that both cars are the same weight, which is impossible, but if we strip out all the variables, all things being equal, does a tire have more in contact with the road if it's a bigger tire?
     
    Last edited: Sep 14, 2018
  2. CJR238

    CJR238 -CMS Article Admin- Administrator Advanced User Premium Member

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    1) Which tire is noisier = 245 45 20
    2) Which tire is more comfortable. = 245 45 19

    Steel rims or aluminum?
     
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  3. DeerHunter

    DeerHunter PSL +15 Advanced User

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    Assuming we ignore the difference in rolling diameter (one of these will affect gearing and speedo accuracy):

    The 19" wheel will have weigh less, as will the 19" tire. Unsprung weight has a large effect on ride comfort, so the smaller set will be more comfortable.

    Tires are imperfect balloons. Within reason, size of the contact patch is determined by inflation pressure and vehicle weight on each corner. All else being equal, wider tires have a wider/narrower contact patch (with approx. the same area). Increased performance stems from the extra stability of the wider contact patch (on a wheel of appropriate width). So, in theory, your 19" and 20" combinations will have pretty much the same contact patch size**. The 19" combo, due to lower unsprung weight, will allow the suspension to follow road variations more easily and extract more real-world performance, meaning the smaller set will perform better (yes, I know you didn't ask this question, but it's a thought experiment).

    Noise is mostly determined by tread design and tire compound, with unsprung weight having some effect. Given this, the 20" set will be slightly noisier.

    ** Porsche put 21" wheels on the back axle of the 911 GT3RS because the 1" bigger wheel apparently allows for a slightly larger contact patch. Shows that theory can only take us so far (although I would imagine the difference is fairly minor).
     
  4. Deacon

    Deacon TXCTG Advanced User Premium Member

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    I hate metric tire sizing. Ratios are not absolute. Imagine if pant sizes, instead of waist and inseam measurements, were waist measurement and inseam expressed as a ratio to waist measurement. Unnecessarily confusing, and you have to do the math most of the time between different sizes to figure out what you’re dealing with. However, for your purposes you really shouldn’t need a tire calculator. The width is the same and the sidewall height ratio is the same, which means they’re exactly the same except overall diameter of course increases with a larger wheel.

    For the same given tire diameter, larger wheels will necessarily mean less sidewall, which means (in theory) less sidewall to flex, requiring the suspension to be either softer or more sophisticated to still provide comfort. That’s generally why people say that larger wheels are less comfortable. Of course, the difference between 17” and 22” wheels is much greater than 19” to 20”.

    Of course, what counts as comfortable varies depends on who owns the butt in the seat. ‘70s Cadillac or ZL1? The Germans have mostly managed to figure out how to balance to two fairly well.

    In your case, you’ve give tires of the exact same width and sidewall depth. The tire itself will have something to do with it, but otherwise you’re also increasing the overall tire diameter when you keep the same width and ratio but inflate the wheel size. That leads to other difficulties in gearing, braking, and automatic transmission shift points, without even considering unsprung weight (which can vary based on tire and wheel combos chosen).

    Which tire is noisier is kind of a silly question. Noise has very little to do with rolling revolutions per mile, especially when they’re that close in size. It has a whole lot more to do with tread design and compound. For the exact same make and model of tire, in theory the greater overall diameter of the version on the 20” wheels would yield fewer revolutions per mile, which could means the edges of the lugs would strike the ground slightly less often, but that assumes the design is stretched to make the same number of lugs go farther along the tread of the tire, and that’s not how it works. So...probably no difference.

    In the end, it’s not clear what exactly you’re intending to accomplish with this exercise.
     
    Last edited: Sep 14, 2018
  5. CJR238

    CJR238 -CMS Article Admin- Administrator Advanced User Premium Member

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    Is it ever?
     
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  6. angrypenguin

    angrypenguin Eat Lead Mothertrucker!! Advanced User Premium Member

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    To answer a few questions above.

    1) Both rims are aluminum
    2) To answer your question, I need to decide between a 2019 S450 with 19" rims (size is as per my first post) or a 2019 Lexus LS500 which comes with the 20" rims. I know the LS500 rides harder and is noisier (both have the same RFTs), but what I'm trying to figure out, is why. The Lexus is around 400 pounds heavier, and my thought process is perhaps I budget the $600/rim (rim, not rim + tires) and buy 4 19" rims on the LS and then maybe I can align the LS's ride more like an S.

    That said though, my other issue which led to this post is....why does the 20" ride harder? Both have the same width and sidewall depth, but the explanations above re: unsprung weight and a 19" performing better as it can adjust better to road imperfections are both things I did not think of.

    Love ya all!
     
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  7. Deacon

    Deacon TXCTG Advanced User Premium Member

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    On the list of reasons for comparing the smoothness of the ride between an individual S-class Mercedes and a Lexus, 19” vs 20” is buried pretty deep, a distracting red herring leading you on a goose chase down the wrong rabbit hole. Putting 19” wheels and tires on a rough riding Lexus is not going to turn it into an S-class.

    I don’t know which particular years, packages, and Tim options you’re looking at. That will make much bigger difference. Besides, you’re comparing apples to...something not as good.

    The S-class is the gold standard of suspensions for that kind of car. Air suspension and active dampers are standard. They even have a “Magic Body Control” optional version. All made by Germans who have a lot of experience perfecting sedan suspensions and charge accordingly.
     
  8. angrypenguin

    angrypenguin Eat Lead Mothertrucker!! Advanced User Premium Member

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    You're right on re: the Germans, but unfortunately the Magic Body Control option is not available for any 4matic S class sedan, in the world. The only active type of suspension that you're referring to (for 4wd sedans) is an option on BMW 7 series X drives, and the Audi system on the A8, which has not yet been perfected so is not available on any A8 (2019) in the world.

    Trust me when I say I've spent a large amount of time understanding what is on what car. To be frank, the suspension system is actually even more advanced on the LS than the S, but that doesn't mean much because as indicated upthread, the S still rides better.
     
  9. Deacon

    Deacon TXCTG Advanced User Premium Member

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    “Advanced” doesn’t mean much. The results are what matters. ALL S-class models come with air suspension with active dampers as standard. Besides, you’re almost certainly elbowing your way into six figures with an S450 4MATIC.
     
  10. ThreePedals6.2

    ThreePedals6.2 Motor Head Moderator Advanced User Premium Member

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    @angrypenguin Dont take this the wrong way, but your thinking about this way too hard.
    Test drive these cars. And then test drive them again. Try cars that you may not even think you like right now.
    Comfort is too subjective. And I understand that you have pain issues. But wheel size with a 45 profile either size , I can almost guarantee that you wont notice a difference.
    Go out and test drive away. That is the only thing that will give you your answers.
     
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  11. PointerCone

    PointerCone M3 Kng Advanced User Premium Member

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    As a general rule, which holds true in most cases, the lower the profile of the tire, the worse the ride comfort. There's less ability of the radial to "flex" to assist the suspension. Great if you're racing, BUT not good for frost heaved roads nor the highways and byways of North America. Note too that there's a LOT more opportunity for pot hole damage to rim (and tire too) with a low profile tire. They look good and are great for racing or high performance spirited dirving, BUT for everyday use, you need some semblance of a sidewall and low profile do not provide that.
     
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  12. angrypenguin

    angrypenguin Eat Lead Mothertrucker!! Advanced User Premium Member

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    Both cars are exactly the same price now and yes both are six figures but there are pros and cons to both so trying to have my cake and eat it too
     
  13. Deacon

    Deacon TXCTG Advanced User Premium Member

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    If they’re the same price (wtf options boxes are you ticking on the Lexus??) I can’t see any reason to go with anything less than the S450.
     
  14. angrypenguin

    angrypenguin Eat Lead Mothertrucker!! Advanced User Premium Member

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    Definitely taking your advice and will go for a drive again for both. I think that's the only way to settle this. Appreciate the advice!
     
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  15. angrypenguin

    angrypenguin Eat Lead Mothertrucker!! Advanced User Premium Member

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    I get 12000$ off of the S due to employee benefits but I prefer the massage and seating position of the Lexus. All mercedes cars have their steering wheel offset and thus are not center and it drives me bonkers. But it is quiter and rides better. That and the ALP is an easier install on the S.

    Decisions decisions.
     
  16. Deacon

    Deacon TXCTG Advanced User Premium Member

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    I think you have your answer.
     
  17. colloyd00

    colloyd00 vMax Advanced User Premium Member

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  18. DeerHunter

    DeerHunter PSL +15 Advanced User

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    I think I saw you say something about run-flat tires in your follow-up post. They're the work of the devil - the stiff sidewalls required for RF capability destroy any chance of decent ride comfort. Also, if you do get a flat and limp to a tire shop, two things will happen: The tire will be used up (i.e. not repairable), and; they likely won't have your tire in stock in any case. You can increase ride comfort (and most likely handling too) significantly by ****-canning the runflats and getting normal tires plus a can of sealer/inflatant (or an AAA membership). If you do it before or right after delivery, you could probably recover 70-80% of your new tire cost by selling the OEMs as "take-offs".
     
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  19. angrypenguin

    angrypenguin Eat Lead Mothertrucker!! Advanced User Premium Member

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    I know of this for sure. But in this case both cars come with rfts and I need to swap out both for normal tires

    I HATE run flat tires!!!
     
  20. samq45

    samq45 Premium Member Advanced User Premium Member

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    And all seasons.....
     
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