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Over size tyre @ 10 to 35 pounds of pressure! whats the diff

Discussion in 'Technical and Troubleshooting Torque' started by drewzor, Mar 28, 2008.

  1. Ok I have made my plans clear in another post that I was going to mod a CBR125. A key mod was to over size tyres which seem to light up the netrider community.

    "it will effect the handling", "you will adversely effect the bike", "they make the tyres that thin for a reason"

    Blah.. obviousley none of these people have tried an over size tyre on their bike.

    I did..
    I first took it out unknowingly with only 10pounds of pressure.
    It was sloppy to lean into turns but stuck to the road and made for very smooth riding, you just had to man handle it a little more then usual.

    Then not knowing how much to put in I pumped it up to 35pounds
    so that it was standing tall. It now handles sharp but you can feel every bump in the road. The bike wiggles under hard accelleration and doesnt feel as stuck to the road as it did before.

    I should have my camera back next week and I will film just the tyre under both conditions for consideration.

    The tyre (i think) is 140/60/17 and should be a 120/70/17

    My conclusion is over size = over pressurized, but as for these major impacts on handling... I dont think so.

    I will test
    0-100kmph times
    short uphill twisty run times
    stop distances

    Your 2cents please? any suggested tests when I have camera mounted?
  2. i have a 190 on the rear of the sv (long story, but was the only tyre i could get at the time :cry: ) but i reckon just run normal tyre pressures, if you must up the tyre size, haven't noticed any slow down on tip in, though i have chicken strips learner would be proud of, only issue i had with em, was through coming through a some long fast sweepers, when you have it hard leaned over and really on the gas, the rear just feels all loose and i wonder if it's might be from the flex in the tyre wall.
  3. Re: Over size tyre @ 10 to 35 pounds of pressure! whats the

    You should run at least the recommended pressure for the original size tyre.

    The danger is that each rim is designed to run a maximum width of tyre, depending on the rim width (between the inner edges).

    Find what size your rim is, then see what the manufacturers recommend for their tyres.

    If you run too low a pressure you will have sidewall flex which can cause some very intersting handling issues around 80 - 100+ kmh.

    On my Moto Guzzi just running a 130/90 x 16 on the rear instead of the 120/90 produced the most amazing "wobble" when cornering. The bike would lift up and down about 1 - 2 times a second; it was very unnerving and nothing stopped it until I changed to the correct size.

    I tried a lot of other solutions to no avail, including changing the front tyre, steering head bearings, adjusting the steering damper, tightening swingarm bearings and more. It was like that when I bought it, and no one I asked ever gave me the correct solution...

    I only took it up to about 39 psi from the recommended 36, and maybe 42 or more might have done the trick, but I am a keen advocate for standardocracy unless you know the rim can easily take it.

    All the best

    Trevor G

    PS No wise person ever adjusts their tyre pressures to make up for inadequate suspension, or to give a soft ride.
  4. The two tyre sizes you quote should have (pretty much) the same rolling radius so I wouldn't expect much change to anything to do with acceleration or braking.

    If you're happy with the way it handles, then fine. It's your ride.

    But I would advise being a little wary as you approach the limits of lean, because you don't want to suddenly find out that the chicken strips on the front tyre now disappear before the ones on the back :p .