Correct Tire Size?

Discussion in 'Dyna Models' started by cmvsm, Jan 28, 2011.

  1. cmvsm

    cmvsm Member

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    I'm getting ready to change my wheels to some PM Torque's. My question is that I currently have 16's on the rears, but the new PM rear wheel is an 18. Current tire size is 150/80/16. What would be the correct sized replacement for the rear, only in an 18? Dealer says their shouldn't be a problem fitting it to the bike, but that doesn't make it so.

    Not that it makes any difference, but the front is a 100/90/19.

    Any suggestions on size and brand would be appreciated.

    Just looked at a 150/70/18. This would give me a little over 1/2" lower of a ride than I have now. My current clearance is roughly 4.6", so I'd go down to roughly 4".

    Anyone know if I can go to a wider tire or higher aspect ratio to keep the ride height closer to what I have now in terms of clearance without having to change anything else out?
     
    Last edited by a moderator: Jan 28, 2011
  2. Hoople

    Hoople Account Removed

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    A 150/60/18 is about the same overall height as a 150/80/16. An exact same height tire would have an odd-ball aspect ratio that will never be available..

    When you change brands, the manufacture can measure slightly different but they should be close. Overall height is without weight on the tire.
    [Aspect ratio times thread width] times 2 plus rim size should equal tire hight, so double check this work.
     
  3. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

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    The manufacturer's website for the tires you are looking for should list the diameter of the various options available. Try to get close to what you have now so you do not change the geometry too much. The other consideration is the load rating of the new size. As a quick calculation:

    150/80-16
    150/70-18

    (2 x width x aspect ratio)/25.4 + wheel diameter = approx. tire/wheel diameter

    (2 x 150 x .8)/25.4 + 16 = 25.45"
    (2 x 150 x .7)/25.4 + 18 = 26.27"

    So nominally, the 150/70-18s should be about 3/4" (actually .82") larger diameter than the 150/80-16s. On the bike that will be about 3/8" (.41") taller at the back. You will never notice it.

    For some real numbers, here is info from the AVON Venoms:

    Tire size load/spd index rec rim(in) alt rim(in) min/max width(mm) width(in) dia(mm) dia(in) depth

    Venom AM42 (rear) 150/70B18 70V MT4.25 3.50-4.50 148 5.8 665 26.2 5.7
    Venom AM42 (rear) 150/80B16 77H MT3.50 3.00-4.25 150 5.9 641 25.2 8.7

    On the width, the main issue is how much clearance you currently have to the belt. Too much width and you will have issues with interference with the belt.

    TQ
     
  4. cmvsm

    cmvsm Member

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    Great info. Thanks guys. One other thing. I was looking up the max rim and wheel width for my bike, and it seems that I could go to a 160/60/18 without modification to the bike. Using TQ's calculations, this tire puts me at very close geometry to the 150/80/16 that I have now.

    Any pros or cons to the 160/60/18 vs. the 150/70/18?
     
  5. Jack Klarich

    Jack Klarich Expert Member

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    Bigger foot print Read more contact with the road, HMMM not a bad thing:s
     
  6. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

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    Check the load rating. Do the calcs assuming you have full wet weight of the bike plus your weight and passenger if you ever do 2 up. Evaluate 60% and 75% on the rear and see if either of those hits the load rating of the tire.

    TQ