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Fat tire on rear of supersport bike.

Discussion in 'Off Topic [BG]' started by Tony Flow MMMM, Nov 24, 2013.

  1. Tony Flow MMMM

    Tony Flow MMMM

    Dec 4, 2012
    [del]Ok if you don't KNOW then don't put your opinion in, [/del]
    1st of all the amount of work that goes into keeping these bikes super light, it's seems counter productive
    2nd, it's rotating mass which makes it even worse
    3rd the contact patch of a motorcycle is not greatly increased if any at all by going to a wider tire.
    4th I cannot find any reason for this and I have looked hard. Other than it looks cool and maybe absorbs bumps betters yet I know there has to be a reason it they wouldn't do it. So I'm not here to say its stupid and they should change it(some people do go smaller) I'd just like to know the physics aspect of it, literally cannot find anything about it online, maybe one of you might have an idea?

    Oh it also turns in slower and kinda lessens turning potential...
  2. FanOfAlice


    Nov 8, 2006
    Im sorry don't KNOW for fact, but I vaugely remember it having to do with the build up and dispersion of heat. If you notice in general as the bikes get bigger the rear tires get wider due to more power and respectivly more rear wheel spin.
  3. Tony Flow MMMM

    Tony Flow MMMM

    Dec 4, 2012

    Hmm, I'll have to look into this, but thanks,
    Sorry I didn't mean it come off as I don't want any input, I just know most non motorcycle riders would think "because it's a bigger bike" I have an old school race bike has a bigger engine engine and is heavier yet we ride on 140mm tires.

    Also most people relate car idea to motorcycle such as a bigger rear tire adds more traction thinking the contact patch gets wider,

    And also many people love to stretch lower and add a huge rear tire thinking in the mindset of cars where this would increase handleing but quite the opposite is true,

    Ok anywhore thanks!
  4. Hi.

    Big tires are for looks only.
    All the info I have read about the subject has been on print, so I can't quote any on-line sources. Apologies on that.

    The skinny of it is though:

    Since the Force=Pressure*Area, it's pretty obvious what happens when the area is increased.

    Also, if You look at the contact area shifting in relation to the cl of the bike, you'll pretty soon discover why even the largest sport bikes compete with "skinny" 180mm IIRC rear tyres.

    :) is that really a word?

  5. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Another "I don't know but..."

    The contact patch area is equal to the weight on the tire divided by the pressure. That's got to be the starting point for sizing tires on cars, but I don't know how it translates into the performance (ride, handling, etc.) of m'cycles.
  6. Chromer


    Nov 28, 2012
    It looks like a drag bike. For bonus style points, remove the front brakes.

    And yes, it entirely ruins the cornering and handling, especially when combined with an extended swingarm.
  7. f.c.geil


    May 12, 2011
    It's partly for looks, but mostly because so many inexperienced riders are on them. Having a fat rear tire tames the acceleration (spinning rather than hooking up and throwing the moron off) so that a rookie can better control the launch. They actually handle and accelerate worse than their skinny tire fathers did.

    At least, that's the way I've always understood it.
  8. Tony Flow MMMM

    Tony Flow MMMM

    Dec 4, 2012

    I love using that word lol.

    I guess the only thing that were seeing a misunderstanding on is I think a 180mm is a fat tire.

    I'd consider 140 about normal.

    M just trying to figure out why
  9. Unprofessional


    Mar 5, 2012
    Drag racing from stop light to stop light.
  10. Hi.

    Since it's a 'ho season it only seems appropriate :).

    I usually go feet first, so anything below 230 is skinny to me ;).
    My current rides are on 110's and 140's though.
    I do have a 280 stashed away for that special project...

    Once upon a time it was AFAIK considered that the width of the tire in mm equaled about the amount of Hp it could transfer to the pavement with 10% slippage in optimal conditions.

    Sure the tyre technology has improved since then, but change the Hp to KW and it still about holds true.

    So with that in mind 140mm tyre is more than adequate for all stock applications.

  11. For sport bikes (NOT drag bikes) the big tire is there to handle the extreme lean angles. I remember seeing Kevin Schwantz accelerating around a corner at the French GP...leaned over, front tire cocked high in the air, rear tire spinning under power and sliding sideways--IN THE RAIN at well over 100 mph! Without crashing (that time, he either crashed or won all his races).

    So a nice rounded tire is essential to lean waaay over. You can't lean or turn on a tire very easily with a flat tread, it's got to be round. Now if the track were perfectly smooth (say pool table smooth), a skinny round rear tire might work. But on a real-world track, there's cracks and bumps and imperfections, maybe the occasional piece of small gravel or debris. When really leaned over, a small skinny tire would get knocked around and skid out from under the bike easily. The bigger tire will help handle the rough surface, especially when drifting sideways under power.

    At least that's the theory they told me. I never could ride fast enough to try it out.
  12. jim777

    jim777 Tarantula Lobbyist

    Aug 7, 2006
    South Jersey
    This has always been my thought as well - it's there for better launches. Most of the bikes I see around here with those tires are Hayabusas with extended frames as well. I wouldn't do it to my CBR Blackbird, but I'm not street racing my bike either.
  13. Tony Flow MMMM

    Tony Flow MMMM

    Dec 4, 2012

    Thanks for y'all's help, I could talk all day about physics especially ones that I use day to day but don't completely understand.

    I still can't wrap my head around this, I know if you have a less rounded tire it's harder harder to push it into a corner, maybe this is because due to their high speeds, it is actually more stable, think less twitchy mid corner as it takes more of a variance in turn to charge radius of turn.

    Soo the next part I still don't understand is then why a skinny front tire, and what are the effects of havering a front tire so much smaller than rear. Seems pretty obvious they would want to roll on different radiuses..
  14. etoncrow

    etoncrow (aka Greg Harman, the curmudgeon with a conundrum)

    Dealership Harley mechanic for 7 years; registered professional mechanical engineer for thirty years. This post pretty much states the simple of it. While unsaid but implied the fat tire also decreases street manuverability and wants to roll under in sharp leans. Pretty much a cosmetic application feeding the size matters crowd.
  15. Jeff Bonny

    Jeff Bonny

    Nov 20, 2000
    Vancouver, BC
    I guess fat is a relative term but you don't see anyone road racing sport bikes on anything much wider than a 200 rear. It's all about grip. On a 200+bhp bike with 100+ ft/lbs of torque a few extra ounces of rubber is an acceptable trade-off for increased grip. Loosing your rear end sucks.
  16. f.c.geil


    May 12, 2011
    Pure moonshine, the exact opposite of reality. As one with several years of road (not street) racing experience, I can state fairly authoritatively and completely unequivocally that a skinny tire turns much more easily. For optimal handling, the front and rear tires should also be the same diameter and nearly the same width. Frankly, fat tires handle for s4!t, on the road or track.

    Look at an actual race bike to verify this. Not one has a fat rear tire.
  17. Tony Flow MMMM

    Tony Flow MMMM

    Dec 4, 2012

    Can we establish what you mean by fat?
  18. Maybe it's to make sure that Marc Marquez actually has some rubber left at the end of a race:

    I know all racers powerslide from time to time, but he's the first I've seen who does it nearly ALL the time.
  19. I think you're the one drinking the moonshine. Here's Eddie Lawson, world champion. I could dig up thousands of pictures of GP bikes and they all look the same.

    Attached Files:

  20. Here's another.

    And the front and rear tires aren't the same diameter either.

    Attached Files: