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Tyre Sizes

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Shldze, Sep 4, 2012.

  1. So, i was trolling around another forum and found this about tyres - topic based around a 600 supersports.

    "You can run whatever compound you like as long as the tyres are geometrically compatible. There are lots of people who will tell you its a no-no etc etc but many are regurgitating something they heard off someone else who's girlfriend read about it on a napkin at the hairdressers... and the only thing that shouldn't be used on a "sports" bike IMO is a 120/60 on a front or a 190/50 on a 6" rear..... You should always use a 120/70 front and it's geometric match, which is a 190/55 on 5.5" rear (600's) or 200/55 on a 6" rear (1000+). Anything smaller is more of a touring friendly commuter set up."

    Thats an interesting point. Bikes are built to a cost and budget. Is a 180 is part of that compromise? Yes?

    Does anyone back this statement saying that a 190/55 or 200/55 would be a better option..for weekend junkies/track day hacks?

    So, next time i need a tyre should i be changing up??

  2. [MENTION=37691]MMMTS[/MENTION] might be able to help you there
  3. run the tyre sizes recommended to you by your manufacturer.
  4. Where's the can of worms warning? 2 things: 2 tyres of same dimensions are necessarily the same and the bike was designed around certain angles more than the width/carcass stiffness etc.

    1) From an administrative POV, just because 2 tyres are marked 180, or 190 etc, doesnt mean they both are. E.g. a dunlop GPA is 190/55, and measures ~188.5 across, the KR449 is a 200/55 and measures ~194 accross, the standard Qualifier that comes on the R6 is a 180/55 and measures ~188 across I think. And they are all dunlops, imagine with other brands too. In fact the 600s in FX are running the 200/55 on a 5.5in rear this year, as I suspect they will in ASBK next year too with the change in tyre rule.

    2) From a design POV, in general manufactures get it mostly right for a particular swingarm angle and rake/trail that the bike has tyre sizes specced for. IMO you should aim to at least maintain those angles after a change then work from there. E.g. the difference in diameter between the Dunlop Qualifer (634) and the KR449 (657) rear tyres mentioned above is 23mm, the fronts are nearly identical in diameter. i.e. after fitting the KR's the rear axle of the bike will sit 11.5mm higher and you need to account for that change. The reason is the whole bike is now tilted forward and all the angles are now different, everything. To get those angles back you could play with forks/shock lengths to get the angles back. That's another story.

    My opinion:
    Does the profile change when you mount a tyre designed for a larger rim on a narrower rim - yes.
    Will that change of profile be beneficial - e.g. larger contact patch at full lean - maybe but not guaranteed, tyres have a particular profile they are designed for which could be achieved, exaggerated or fcked up by being put on a diff size rim.
    Is a larger tyre heavier than the smaller narrower tyre - possible/quite likely, check first.
    Will you notice any difference of any sort - maybe, it's possible you could notice/detect esp when accelerating or through a flip/flop, assuming you are consistent enough in laps/sectors to quantifiably detect a change and not have a gut feel about it. Anything else could just be noise in regular inconsistent lap times.
    Will it make you faster - see what you prefer the feel and feedback of, check your times and choose accordingly.