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Thread: Interpreting tire sizes, etc.

  1. #1
    What's that noise...? basketcase's Avatar
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    Interpreting tire sizes, etc.

    While shopping tires for the RT a question or two has occurred to me.

    1. What is the secret to interpreting the sizes?

    In other words, what is the real difference between a size 90/90 - 18 and a size 100/90 - 18?

    Next, I can find Avon Venoms in a 100/90-18 and in a 120/90-18, but both are sold as front tires. The rear tire specs for the RT call for a 120/90-18.

    So here is the question -

    2. If a tire is advertised as a front tire but the size is what is called for in a rear tire, is there some compelling reason not to go with it to match a set?

    Part of me says "The tire will never know the difference."

    But then, one never knows unless he asks...

    Thanks in advance,
    Rick
    '98 BMW Z3 Roadster, '00 R1100RT

    If you insist on exercising a right to burn our flag, first be so kind as to wrap yourself in it and then douse yourself with gasoline just before you strike the match.

  2. #2
    Cam Killer marchyman's Avatar
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    interpreting the sizes?

    Metric tires are specified by width and aspect (height specified as a percentage of width) in milimeters, with a wheel diameter in inches.

    100/90V19, for example, is a tire 100 mm wide and 90% of 100 mm (90 mm) tall. It's speed rating is V (up to 149 MPH) and it fits wheel 19 inches in diameter. A 90/90H18, on the other hand, is 90 mm wide and 90% of 90 mm (81 mm) tall. It's speed rating is H (up to 130 MPH) and fits wheels 18 inches in diameter. Sometimes an "R" is thrown in next to the speed rating to indicate a radial tire.

    Front and back may be specified for two reasons. One is tread pattern, the other is tire construction. In both cases the reasons have to to with the forces on the tire. The force on a rear tire is primarily due to drive. The force on a front is primarily due to braking. Drive and braking are opposite directions. Some tires will have two arrows, one for when mounted on the front and another for rear mounting.

  3. #3
    Old man in the mountains osbornk's Avatar
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  4. #4
    What's that noise...? basketcase's Avatar
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    Thanks for the feedback. After sleeping on it I realize I set my question up wrong. Where I really need the most feedback is on the front/rear issue.

    I am running Avon Venom's front and rear on the SE and am shopping tires for the BMW, but I am not finding a Venom with a "rear" designation for the beemer.

    The thread pattern are exactly the same on the F & R Venom's for the Wing.

    Now, per the manual the RT calls for 90/90-18 (front) and in a 120/90-18 (rear).

    But what I find in the Venom's is 100/90 and 120/90 but with both sold as a front tire.

    So the main question is at the point of running a tire that is sold as "front" on the rear of the bike.

    If marchyman is correct about differences in construction then it is ill advised. However, if there is no difference except the size, I don't see a problem.
    '98 BMW Z3 Roadster, '00 R1100RT

    If you insist on exercising a right to burn our flag, first be so kind as to wrap yourself in it and then douse yourself with gasoline just before you strike the match.

  5. #5
    Registered User lkchris's Avatar
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    You need to come to the conclusion that Avon doesn't make tires for your bike and move on to a company that does.

    Just search for tires for your bike at their website to verify this.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  6. #6
    What's that noise...? basketcase's Avatar
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    Quote Originally Posted by lkchris View Post
    You need to come to the conclusion that Avon doesn't make tires for your bike and move on to a company that does.

    Just search for tires for your bike at their website to verify this.
    Hmm. I don't think the tire knows if it is on a BMW or on rice burner, and Avon does make tires in the front and rear configurations that fit the bike.

    At the moment it has a set of almost gone Metzler's on it.

    Metz 100/90 on the front (specs call for 90/90)
    Metz 120/90 on the rear.

    My draw to the Venom's is that they are H rated (longer service life) and I have had great service out of them on the Wing. But there is not a Venom set for the RT.

    However, the Avon Roadrider is available (for F & R) in the exact sizes noted in the manual. The Roadrider is a V rated tire.

    With all that said, what I've been able to gather reading and asking around is that on most tires the construction is different for the rear tire in the form of a different compound and more of it. Some tires use the same thread pattern and others are different for F & R.

    Probably I'll end up with a set of Roadriders.

    Thanks again to all.

    '98 BMW Z3 Roadster, '00 R1100RT

    If you insist on exercising a right to burn our flag, first be so kind as to wrap yourself in it and then douse yourself with gasoline just before you strike the match.

  7. #7
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    Actually, the metric figures like "100/90" don't necessarily reflect a 100mm tire with 90% aspect ratio - one can get 100/90 tires from different mfgrs and different styles of tires within the same mfgr, and the widths will not all be the same - there is often a variance of a couple mm and sometimes more. You would hope that everyone would use the same meter stick, but apparently they don't always. The 100/90 is generally correct in that the height/width ratio is pretty accurate to the x/y numeric ratio, even if the first dimension isn't exact. Because of this, you may find Brand X tire in 120/90 size will still fit on your RT's rear rim, but Brand Y will rub considerably.

    Just in case you weren't confused.
    BMWs in my garage: 1982 R65LS, 1978 R100/7

  8. #8
    Registered User kgadley01's Avatar
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    Just don't put a tire that says front on it on the rear. I did this and only got 4,000 miles out of the tire. the next tire looked exactly the same but said rear on it. it lasted 14,000 miles. live & learn...

  9. #9
    What's that noise...? basketcase's Avatar
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    Quote Originally Posted by boxermaf View Post
    You would hope that everyone would use the same meter stick, but apparently they don't always. The 100/90 is generally correct in that the height/width ratio is pretty accurate to the x/y numeric ratio, even if the first dimension isn't exact. Because of this, you may find Brand X tire in 120/90 size will still fit on your RT's rear rim, but Brand Y will rub considerably.

    Just in case you weren't confused.
    No confusion here. I just couldn't recall the specifics of how the measurement applied to the size to describe the tire.

    And the sizing variance issues pop up with clothing, shoes, helmets -- anything. In fact, I have a Z1R leather jacket that fits well, but when I bought the particular jacket I tried everything on the rack and bought the smallest one of the lot -- even though they were all the "same size."

    Back to the matter of tires, I typically like to put the same model of tire on both front and rear for consistency of feel, road feedback, and handling. Sometimes, I have found, owning a BMW makes that a bit of a challenge...
    '98 BMW Z3 Roadster, '00 R1100RT

    If you insist on exercising a right to burn our flag, first be so kind as to wrap yourself in it and then douse yourself with gasoline just before you strike the match.

  10. #10
    keelerb
    Guest
    I have put wider tires on some bikes, but find the airhead likes what the factory put on it. Damn, those guys were on to something.

    I have had good luck with Metzlers, INCLUDING THE BRAZILIAN MADE ONES, on my airhead. I know others may disagree, as it is their prerogative, if not indeed their DUTY to do...

  11. #11
    Manfred
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    I put Avon RoadRiders on my airhead shortly after I bought it. Very good tires. I currently have a Bridgestone Spitfire on the rear (having put lots of Yamaha miles on them), but I think I will go back to the Avon.

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