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250 gearing for motard - Yami WR250.

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  • 250 gearing for motard - Yami WR250.

    Hi All,

    I need some advice on gearing as I’m trying to figure out what size rear sprocket I need.
    What is a reasonable cruising RPM for a wr250 dirtbike with road tyres?

    Currently my WR has stock gearing and geared stupidly low for off-road work – at the moment I’m doing 5000rpm @ 60kph and 8250rpm @100kph (Calcs below).
    I was thinking in the order of 6000rpm when cruising at 100kph……

    This would mean I need a 14 front and ~38 rear sprocket.
    Can anyone see an error in my calcs (if you can figure out what I have done)?

    WR250F
    * Primary Drive Ratio = 57/17 (3.353)
    * 1st gear ratio = 31/13 (2.385)
    * 2nd gear ratio = 28/16 (1.750)
    * 3rd gear ratio = 28/21 (1.333)
    * 4th gear ratio = 25/24 (1.042)
    * 5th gear ratio = 0.814
    * Front sprocket (stock) = 14
    * Rear sprocket (stock) = 52

    Tyre - 120/80-18 dunlop
    = 18” + (80% of 120) = 457 + 96 + 96 = 649 div 2 ……radius = 325mm.
    Circumference - 2 PI r = 2 * 3.1415 * .325m = 2.04m per revolution.
    Calcs
    Speed (kph) = RPM *60 (revs/hour) / 5th gear ratio / primary ratio * chain sprocket ratio * wheel circumference /1000 (kms)
    60kph = XXX*60 / 0.814 /3.353 * 14/52 *2.04m/1000
    XXX = 4970rpm. @60kph.

    100kph = YYY*60 / 0.814 /3.353 * 14/52 *2.04m/1000
    YYY = 8280rpm. @100kph.

    110kph = ZZZ*60 / 0.814 /3.353 * 14/52 *2.04m/1000 =
    ZZZ = 9110rpm. @110kph.

    I want to be riding at roughly ~ 6000rpm @100kph.
    Therefore
    XXX = 6000rpm. @100kph.
    100kph = XXX*60 / 0.814 /3.353 * AA/ZZ *2.04m/1000
    AA/ZZ= 0.3716 - ie 14/38

  • #2
    http://www.12oclocklabs.com/gearcalc.htm

    Seems right to me!

    [email protected] is 99.032 kph
    Project: Fix my tard! Now with extra reliability!
    SupermotoWA on Bookface
    You should go here and here, and definitely here.

    Comment


    • #3
      most tards i've seen as well as my own running on 17s are 14/45 for top speed around 140 and revving pretty hard at 100

      the 18s would be larger rolling diameter and therefore circumference i'd assume you would need >45 to keep it peppy.

      38 would give you good top speed and the right cruising rpm at 100 but make the ride pretty damn sedate.
      Do you remember the good old days before the internet?

      when arguments were only entered into by the physically or intellectually able.

      Comment


      • #4
        Do you have a black WR250 by any chance?
        Were you riding it around Curtin Uni on Tuesday?

        Comment


        • #5
          Originally posted by filbert View Post
          most tards i've seen as well as my own running on 17s are 14/45 for top speed around 140 and revving pretty hard at 100

          the 18s would be larger rolling diameter and therefore circumference i'd assume you would need >45 to keep it peppy.

          38 would give you good top speed and the right cruising rpm at 100 but make the ride pretty damn sedate.
          Thanks Filbert.

          The WR has a very big jump in gearing from 4th to 5th gear, so you keep it peppy in the gears 1-4 and then 5th is a good cruising gear.
          Is this the case for other dirtbikes?

          so i think something like a 40 tooth rear sprocket will be a good compromise.

          At the moment im in 4th gear by 40/45kph.

          Comment


          • #6
            Yea 5th 6th and more cruising gears on my 525. She will idle along at 100.

            Comment


            • #7
              www.gearingcommander.com

              Had all your answers
              Change the gearing in it's active online sheet and it'll give you the info you're chasing
              Commander Keen and Shady 7/8 are doing the Kiwi Shitbox rally 2016 as the Dropkick Dropbears- donate here to help us change cancer!


              Originally posted by Martin Luther King Jnr
              One has a moral responsibility to disobey unjust laws.
              . .

              Comment


              • #8
                Originally posted by Lit View Post
                Do you have a black WR250 by any chance?
                Were you riding it around Curtin Uni on Tuesday?
                That may have been my brother on the WR250x.
                That bitch took my dollar...

                Comment


                • #9
                  Originally posted by Lit View Post
                  Do you have a black WR250 by any chance?
                  Were you riding it around Curtin Uni on Tuesday?
                  Nah wasnt me - have a blue wr250f. and dont go to curtin

                  Comment


                  • #10
                    Originally posted by jarrod View Post
                    That may have been my brother on the WR250x.
                    Haha, what a homo. Why ride the foot paths when there are perfectly good kerbs to jump.

                    Comment


                    • #11
                      Originally posted by Shadow78 View Post
                      www.gearingcommander.com

                      Had all your answers
                      Change the gearing in it's active online sheet and it'll give you the info you're chasing
                      Awesome - thanks Shadow
                      Exactly what i wanted....seems my calcs were correct afterall

                      Comment


                      • #12
                        what i was hinting at is that your calculations might be correct and you will end up with the rpm you want at 100km/h but is it the right rpm?

                        i would personally be aiming for peak torque at 100km/h not just for fuel economy but also for a bit of throttle response when there is something to pass etc, if you just choose an rpm that sounds good and not revving too hard you may end up below peak torque or in a gutless part of the power curve that means gusts of wind drop 10km/h or more and passing is an effort.
                        Do you remember the good old days before the internet?

                        when arguments were only entered into by the physically or intellectually able.

                        Comment


                        • #13
                          Originally posted by Lit View Post
                          Haha, what a homo. Why ride the foot paths when there are perfectly good kerbs to jump.
                          yeah, cheeky c*nt shouldn't have even been on the bike... he's bloody 15.
                          That bitch took my dollar...

                          Comment


                          • #14
                            Originally posted by filbert View Post
                            what i was hinting at is that your calculations might be correct and you will end up with the rpm you want at 100km/h but is it the right rpm?

                            i would personally be aiming for peak torque at 100km/h not just for fuel economy but also for a bit of throttle response when there is something to pass etc, if you just choose an rpm that sounds good and not revving too hard you may end up below peak torque or in a gutless part of the power curve that means gusts of wind drop 10km/h or more and passing is an effort.
                            Oh righto - how would i know where in the RPM the peak torque is without a dyno run?

                            Comment


                            • #15
                              go by feel, try it in 1st or 2nd gear, somewhere in the rpm range is where "the grunt" is, if at 4000rpm you crack the throttle and it stutters then try higher if at 7000 it lurches forwards without hesitation or wheelies slightly then it's probably there, if you roll the throttle on and it just picks up speed without any real feeling like being shoved along say at 9000rpm then you're probably above it.

                              otherwise try running into the upper rev range in 1st or 2nd until near redline where it stops feeling like it's pulling hard and just gains rpm this is peak horsepower, at that point shift up a gear whatever rpm it is at when you hit that next gear is somewhere near peak torque.
                              Do you remember the good old days before the internet?

                              when arguments were only entered into by the physically or intellectually able.

                              Comment

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