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Wheel and tire upgrade, shim question

Discussion in 'Prius c Main Forum' started by Jason254, Aug 25, 2014.

  1. Jason254

    Jason254 Junior Member

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    Hello,

    I am going to upgrade my Prius c to new wheels and tires. I plan on keeping my old steel 15" wheels and eventually putting 185 60 r15 winter tires on them. The new wheels are going to be 15x6.5 and the tires are going to 195 60 r15's.

    From my research the car can handle this size of tire and rim. But will it need shims? And if so, what is the minimum thickness? 5mm? 10mm? (I can't image it would need greater than 10mm)

    thanks in advance!

    -Jason
     
    #1 Jason254, Aug 25, 2014
  2. xliderider

    xliderider Senior Member

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    It all depends on the offset on the new wheels whether you need any spacers or hubcentric adapters.

    SCH-I535
     
    #2 xliderider, Aug 25, 2014
  3. Jason254

    Jason254 Junior Member

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    I want to get Konig Helium's. They need hub centric rings for sure. The offset is 40mm. From these two threads it looks like I won't need shims.

    Installing Aftermarket Wheels on the c | PriusChat
    Test Fit of Wheels on a Prius c | PriusChat

    But I figured I'd post to make sure. I want to run these rims with low rolling resistance Continental TruContacts 196 60 r15's. They weigh 2lb's more than the oem tire, but the rims are at least 4lb's less than the stock wheel, maybe 5lbs. I'm hoping the weight reduction will offset the mpg loss of the wider tire.
    But
     
    #3 Jason254, Aug 25, 2014
  4. mahout

    mahout Active Member

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    The extra 2 lb at a diameter of 24.12", or 1,1 ft, 1.1radius is a torque of 2.2 lb-ft more per wheel/tire while the 4 lb savings at 6 inches, or .5 ft is reduction in torque of 2 lb-ft. added for 4 wheels the new tires are nearly 3 lb per wheel because the new wheel/tire is nearly half an inck greater than OEM. The net increase for all 4 wheels will come close to 12 lb-ft and the OEM engine only has 90 lb-ft or so maximum. If indeed the new tire wheelnly increases torque load by 4 lb-ft you can expect a loss in both power and mpg. But its far closer trtha\n a lot of tire/wheel changes we see.You might want to be sure those wheels are 4 lb lighter.
     
    #4 mahout, Aug 26, 2014
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  5. Jason254

    Jason254 Junior Member

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    Thanks for the response Mahout!

    The average weight reduction the Konig rim vs the steel oem should be about 4lbs, maybe as much as 4.5lbs. I could go with 195 55 r15 tires to reduce wheel size, but the selection isn't as good. But I did find some decent Yokohama's at discountiredirect, though they are a bit more expensive than the continentals.

    I went on the tire websites directly and got more accurate numbers than what was on tirerack, though the goodyear site did not list weight for the oem tire. I did however find what I think is the right weight for the goodyears on amazon.

    Rims
    Konig helium weight 11.1
    Steel oem weight approximate 15.5

    Tires
    (OEM) Goodyear assurance fuel max 175 65 r15 diameter 24.0" weight 16.6
    Continental TruContact 195 60 r15 diameter 24.2" weight 17.4
    Yokohama YK580 195 55 r15 diameter 23.5 weight 18.7

    Mahout, can you help me with the math comparison? I'm a bit lost trying to follow. The rims are about 4 to 4.4lbs lighter.

    The continentals are .2" greater in diameter and .8lb's heavier. I think this would be about 1.5lb-ft per wheel with the new wheels? For a total of 6lb-ft? But I'm really kind of guessing here.

    The Yokohama's are actually .5" smaller in diameter, but weigh 2.1lbs more. I no idea what the hit would be, but I think it's worse than the Continentals.

    thanks in advance,
    -Jason
     
    #5 Jason254, Aug 28, 2014
  6. mahout

    mahout Active Member

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    Again, its not the wheel weight that matters most but the tire weight because their radius is much greater so the weight at a greater radius is harder to turn. Its not a reason to go lighter but its nice.
    The simplest way to look at the energy required to turn a wheel is
    diameter, in inches x weight, lb, divided by 2, so
    175/65x15 T = 199.2 inch-lb
    195/60x15 T = 210.5 inch-lb
    195/55x15 T = 219.7 inch pounds.
    Between these 2 upgrades the Conti looks it will handicap your engine by the lesser amount. Converting to ft-lb the 'loss' is 17.5 ft-lb vs 16.6 ft-lb, or 0.9 ft-lb per wheel with a total of 3.6 lb-ft total for 4 wheels . Saving 4 lb per wheel should gain you about 2.5 lb ( 4x7.5in/12 inper foot) so in your case there should be a loss of 3.6 lb and a gain of 10 ft-lb for a net improvement of 6.2 lb-ft.that your engine has to work against so your mpg and acceleration have a good chance to improve in spite of the loss in gearing going from 24" tire to 24.2". check the revs per mile though to be sure bas thats the true comparison rather than diameter.
    So in your case the lighter wheels help a lot. Sure they are 11.1 lb?
    good improvement I say. This time racing is not the only way that lighter wheels matter. When the saving in wheel weight is 3 times at least the increase in tire weight at the same diameter lighter wheels matter.
    That help?
    cheers
    PS at 40 mm offset spacers are of no use. And never exceed 6mm, 1/4", spacers.
     
    #6 mahout, Aug 30, 2014
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  7. Jason254

    Jason254 Junior Member

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    I went through and I was able to follow the math. I would save conservatively about 1.5 lb-ft per wheel for a total of 6 lb-ft. That's awesome! Not only should handling on the highway improve but my mpg/acceleration should too.

    I am pretty sure I won't need spacers with the rims. It would be better if I don't as I believe the turning radius will be better without them. But even if I do it won't be a big deal to add them as I don't believe the hit will be all that bad.

    As for rim/tire measurements:

    I am 100% sure on the weight of the rims:
    Konig Helium 72B HE Matte Black Custom Rims Wheels - Konig Custom Wheels Rims

    I am also 100% sure on the weight and size of the continental tires:
    http://www.continentaltire.com/www/download/tires_us_en/themes/car_tires/passenger_coupe_minivan/truecontact_datasheet_en.pdf

    I am about 95% sure on the weight of the oem goodyear tires:


    And I am 100% sure on the size of the oem goodyear tires (not sure why they don't list weight on their site...):
    Assurance® Fuel Max® Sizes & Specs | Goodyear Tires

    thanks for the follow up Mahout! It was much appreciated!

    -Jason
     
    #7 Jason254, Aug 30, 2014
  8. xliderider

    xliderider Senior Member

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    You can do all the theoretical math you want, but the mpg and acceleration will take a hit. Hopefully, it won't feel like you are dragging an anchor around when you step on the gas, but handling will definitely improve. :)
     
    #8 xliderider, Aug 30, 2014
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