Can I change my tire aspect ratio?

Discussion in 'Wheels' started by cdn-bigfoot, May 30, 2010.

  1. cdn-bigfoot

    cdn-bigfoot Junior Member

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    I have a Dunlop Screaming Eagle tire on my rear solid Deuce style rim. It is a 180 60B17. My bike is a 2006 Dyna Street Bob with an upgraded rear wheel. The belt guards have been removed because of the tight tire clearance with the 180 tire. It measures 7 1/4" wide and comes within a half inch of the drive belt.

    I am having a lot of difficulty finding those exact same measurements in a new tire. I am able to find lots of tires with a 180 55/17 measurement. Will putting this different tire size make a difference? I expect that going from the 60 aspect ratio to the 55 would mean a shorter sidewall. Or could it mean the tire is wider?

    Any advice would be appreciated.






    Here's a shot of my tire from last year. It has now worn to almost bald in the centre 3 inches of the tread.

    [​IMG]
    [​IMG]
     
    Last edited by a moderator: May 30, 2010
  2. Hoople

    Hoople Account Removed

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    As long as the 1st number (180mm) is the same, the tread width should not change. Rather than define or state an absolute mm wall height by manufactures, it is given in a ratio of height to width.
    Therefore 55% of 180 equals 99mm of wall height.
    Therefore 60% of 180 equals 108mm of wall height
    Both tires should have the same tread width.

    This is how I always understood the tire sizing thing. It would be easy to check to be sure.

    Note: 7.25" is actually 184.15mm. You really should be measuring closer to 7.10" which would be 180.34mm
     
  3. cdn-bigfoot

    cdn-bigfoot Junior Member

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    So would I be OK going with the 180 55/17 then?

    I would have a much greater choice of brands on J&P Cycles.
    I was thinking of going with the Metzeler ME880 Marathons. I have a Metzeler on the front. Tread seems to last a lot longer than the Dunlop. Only thing with the Metzeler is I am concerned about a fair amount of fine cracking in the sidewall and in between the treads.
     
  4. NEWHD74FAN

    NEWHD74FAN Experienced Member Retired Moderators

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    Actually most of the tire manufactures have internal specs of the actual inflated diameter of their tires...they used to publish them on their websites, but I guess they keep that data hush-hush nowadays in this liability/sue happy society we have grown "accustomed" too...! Dunlop used to include that critical data in a spreadsheet for each tire model, might provide the information if you talk to technical support/CSR...
     
  5. Hoople

    Hoople Account Removed

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    I know it will be small but if you change the aspect ratio expect to see a speedo error. Expect to see a reading higher than your really going. And you thought the geometry you learned was never going to be useful... :p:p

    Going from a 60 to a 55 should reduce your radius by 9mm or ~ .355"
    giving you a reduced overall tire height by about 3/4".
     
  6. cdn-bigfoot

    cdn-bigfoot Junior Member

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    That could mean less speeding tickets? :D

    Bang on with the 9mm. So .355" being close to 1/3 inch reduced ride height since the axle actually only sits on 1/2 the tire height (radius).

    Can I assume then that the 180/55/17 should be a good replacement for the 180/60/17 and fit on my bike?
     
  7. Hoople

    Hoople Account Removed

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    Well BigFoot you know how it works. The only way you will really know is if you try it. On paper it looks to be workable and practical. 5% is not a big difference and since your going from 60 to 55 and not 55 to 60, I would say you will be fine.

    Tell you 1 thing... I just put on a set of those Metzeler ME880 Marathons and those babies stick like Glue... Nice!

    Be sure to check the balance of the rim without the tire mounted first, then key the paint dot to the true heavy spot.. .






    Something else I just thought of. When I went with the Metzeler 880, the same size tire came in 2 different load ratings.. Get the highest number you can. Better 2 up capacity and safety. I have a 79 rear and I like that number because my Sweetheart has a little junk in the trunk.

    examples:
    76 = 882 lbs
    74 = 825 lbs
    79 = 963 lbs
    63 = 600 lbs

    The 880 also has some high cold presures. Some are in the 40's with just 1 up.
    Be sure to check what they need to be.
     
    Last edited by a moderator: May 30, 2010
  8. cdn-bigfoot

    cdn-bigfoot Junior Member

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    I've got a Metzeler 880 Marathon on front right now. 120 70/21 62V on a billet rim from a Victory Hammer. It has ridden great and worn relatively well. I'm just a little concerned with some small cracks in it.
    [​IMG]
    The Dunlop "Screaming Eagle" on the rear has squared off and gone bald faster.
    I would like to go with a Metzeler 880 on the rear but can't find my size. My rear wheel is the same as the 2010 WG. So I need a 180 60/17 or even a 180 55/17. I may have to go with a Pirelli Night Dragon 180/60HB-17 TL. I'm hoping since Pirelli makes the Metzeler they will be a good match.
     
  9. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

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    You will be fine with the 180-55/17s. Will affect the accuracy of your speedometer a tad, but probably not enough to notice.

    TQ
     
  10. Hoople

    Hoople Account Removed

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    I was talking to a a Very knowledgeable Metzeter rep at the last Daytona Bike week. I hammered this guy with some really deep questions and he walked the walk without any lip service. He told me the Night Dragon actually had a higher concentration of Silica in them and they grab even better than the 880's. But he also said they won't last as long.
    I may be wrong but I would try and keep the same brand of tire front & rear. I can't tell you why, it's just a 2 cents opinion & something I feel...

    I have to admit you have 1 Beautiful Bike. Man that thing is Gorgeous. You have a better looking ride than me and I am giving You advice? Somethings not right with that! :p:p