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how does tyre profile affect speed?

Discussion in 'General Motorcycling Discussion' at netrider.net.au started by nibor, Mar 10, 2009.

  1. yes

  2. no

  1. im sick at the moment, and thinking hurts when im healthy :LOL: so i put it to you NR:

    i have a slight concern regarding the possible variation in my speed. fair chance this has something to do with an avid hate towards speed cameras, and my ability to attract their attention.

    i normally run a 140/70 tyre on the rear. the bike i just purchased came with a 140/80. my understanding is the second value is a percentage of the first - the tyres i just got have a 10% larger wal height in relation to the tyre width.

    thus the radius of my rear is increased by 14mm, and will increase my actual speed compared with my percieved speed.

    what i want to work out is how much the speed increases.

    understanding that C = 2ΠR, an increase in R increases C by a factor of 2Π.
    so irrelevant of the original Circumference, the new circumference has increased by 14mmx2xΠ, or roughly 88mm.

    but to work out an increase in speed, we need to know the original circumference. im taking a punt assuming i can say with a 17 inch diameter rim (215.9mm radius), and an original tyre wall height of 70% of 140mm, which is 98mm, my old tyre had a total radius of 313.9mm.
    this results in a circumference of approximately 1971.292mm.

    thus the increase in circumference is almost 5% (4.4% i think), and as such any speed i read on my speedo, i should add 5% to be safe.
    this doesnt take into account any inaccuracy of my speedo already...

    does this sound right mathematically so far to you guys?
  2. I've found most speedometers (cars and bikes) to be conservative by between 5 and 10% anyway.

    Your maths is probably fine; but worthless unless you know the accuracy of your speedo to begin with.

    Recommend you blu-tack a GPS to your tank and go for an experimental ride :)
  3. wanna buy me a GPS? :grin:
  4. Have you thought about getting a bulk purchase discount on VTR250's?

    I think your maths are ok, but I'll let others confirm that.
  5. im just trying to make sure there are enough spare parts floating around the market to keep me going for a while :p
  6.  Top
  7. useful link that. bookmarked!
    confirms my theory though.
  8. looking at the page for 10 more seconds i noticed the other calculator further down.
    entering the data, and it came up with this:


    looks like my maths hasn't let me down either :)
  9. hey its good huh? i printed out the calculations and kept it in my car if cops ever wanted to hassle me about the wheels and Tyre fitment :cool: a lot cheaper than an engineer certificate!
  10. eh?

    Isn't your speedo connected to the front wheel? You've only been talking about your rear wheel... therefore there's no change in measured speed. :?

    Or do you have one of the gearbox jobbies?
  11. Thats the first thing that popped in my head, but not sure if you bike is set up that way.
  12. First, work out which end is the right one. Dont bother looking or anything stupid like that. Pull a burnout, does the speed rise? No? Just to be sure, now pull a wheelie, give the front brake a tap, and go through a couple of gears. If the speedo still reads zero, you can now be almost sure. Double check to be safe. Is it the front? Then your speedo hasnt changed.

    To check your speedo anyway, forget the tyre calculations from profile etc, they are never spot on. Set your tyre pressure. Put some chalk on your tyre and the ground, then whilst sitting on the bike, roll forward 10 revolutions and mark the ground again. Divide by 10 and that's your rolling circumference. Now do your calcs with your gear ratio, primary drive ratio, final drive ratio, and circumference. Get up to speed, and note the revs, back calculate to see how far it's off.

    Then pull a wheelie.