Gymkhana

Discussion in 'The Perfect Line and Other Riding Myths' started by Vulfy, May 6, 2012.

  1. Motogymkhanaman

    Motogymkhanaman Been here awhile

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    Hi Shaddix, welcome to the world of Moto Gymkhana!

    The minimum turn radius with full lock and maximum bank angle is the source of much confusion amongst new riders. Take a look at the following video of Noboru Yoshino on the GP8 course. You will notice that although the bike is changing direction very rapidly as it rotates around the pylon it's not actually going very fast.


    <iframe width="420" height="315" src="http://www.youtube.com/embed/bXoqgwAz6v0" frameborder="0" allowfullscreen></iframe>


    What riders are looking for is the exact combination of speed and bank angle that will give them the maximum direction change in degrees per second. This will vary according to the particular capabilities and constraints of the bike and rider combination.

    The optimum rotation rate is about 144 degrees per second which equates to a full 360 degree rotation in about 2.5 seconds. Get a friend to time you making a 360 degree rotation and that will give you some sort of baseline to work from. Make only one change at a time such as the amount of throttle or your body position and try another roatation aginst the clock, if you are faster than your original time then that was a good change, but if you are slower you will have to try something else instead.

    All improvements in Moto Gymkhana riding come from riding against the clock, so it is worth your while getting into the habit of timing everything you do.
  2. shaddix

    shaddix Banned

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    It looks easier if you're able to accelerate out of every corner. Just drop it in and gas it out, even if you're falling to the earth you will have completed your turn before you go down and throttle will pick you back up. I'm sure it isn't that easy in practice... Supposed to be sunny this weekend and I'm going to try my hand at this gp8.
  3. Storm Shadow

    Storm Shadow Thread Ninja

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    A constant lets low side it and recover it at the last moment.

    send on a small touch screen by a guy with fat fingers
  4. Vulfy

    Vulfy Been here awhile

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    ... here is an interesting theory in my head, while I'm preparing for tomorrow's session. Its a bit geeky, so if you fall asleep after first paragraph I'm not blaming you. Plus I'm not quite sure how solid my calculations are.

    Motorcycle is rear-drive. Rear wheel pushes the entire motorcycle, while front does the steering, same as rear drive car.

    We can not rotate motorcycle around rear wheel, if the rear wheel is stationary. Rear wheel always needs to move forward and push, in order for the front wheel to rotate around (I'm talking about riding circles).

    When the motorcycle swings around the cone, the rear (powered) wheel, only moves in a short arc, while front wheel draws a much wider arc around the cone, covering a much greater distance.

    So motorcycle still needs to put in that same amount of power to swing the front around, whatever distance the arch is, as it would if it was covering same distance in a straight line.

    Lets assume that we have a motorcycle that is 7 foot long, from rear contact patch to front patch.

    Because rear wheel still needs to move, and can not pivot just around itself, we can assume that an 7 foot long motorcycle will be drawing a circle around a cone with radius of around 9 feet. Basically the radius can not be equal or smaller than the length between contact patches. So 9 feet for a 7 foot length is just a safe assumption.

    That is if we are really good, and we are managing steering, lean and power in such a way, that rear wheel moves the smallest distance around the cone, while still providing enough power to push front around.

    So 9 foot radius gives us circumference of around 57 feet.

    Given that a good rider can do a 360 rotation in 2.5 seconds, means that he has to cover 57 feet with his front wheel in 2.5 seconds. Converting that to miles / hour speed we get...

    Around 16 miles per hour.

    Thats the speed of the FRONT wheel, NOT rear. So if your speedometer sensor is mounted on front wheel, and you are doing around 15-16 miles per hour in a turn that is about 18 feet wide, you should be in the ballpark of good Gymkhana riders.

    ... Beuller?
  5. Vulfy

    Vulfy Been here awhile

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  6. Vulfy

    Vulfy Been here awhile

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    So basically my whole argument behind this, is that "slow" and "fast" are terms that need definition.

    A bike might look almost stationary when its going around the cone, so it looks "slow", but if we look at just the front wheel, we see that it swings around the cones, quite "fast".


    Duncan, am I even close to anything reasonable here, or just whistling out of my ass? :D
  7. Vulfy

    Vulfy Been here awhile

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    Today's session.

    <iframe width="640" height="360" src="http://www.youtube.com/embed/P8B1E9eoILI?rel=0" frameborder="0" allowfullscreen></iframe>


    FUN!
  8. Sckill

    Sckill Been here awhile

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    Vulfy - Thanks again for setting everything up today and getting the video up so quickly.

    Sent from my SAMSUNG-SGH-I727 using Tapatalk 2
  9. Sckill

    Sckill Been here awhile

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    "Ballast" wants to know where you get your funky music from. Thanks again for the new nickname for my fiance. :D

    Some more pictures from today:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
  10. Harvey Krumpet

    Harvey Krumpet Long timer

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    :clap
    Hey Vulfy & co, I have not gone back to look but I think your all looking great out there, improving immensely. In the latest vid you all look very smooth & controlled. Impressed I am!

    Shirley the "ballast" could not weigh more than a couple of bags of sugar? She could fit in the top box. I would be interested to read how it feels pillioning on a course. Not a lot of time to relax.
    :freaky
  11. Sckill

    Sckill Been here awhile

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    Abby frowns upon the bags of sugar comment.

    For the rest she says:

    1. Get used to the feeling of falling over. It's probably not good for a new passenger.

    2. She had to look over my shoulder in the direction of the turn. That small weight shift really helped the bike to turn.

    3. Put more weight on your legs instead of sitting so that it's easier to keep your balance.
  12. Vulfy

    Vulfy Been here awhile

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    Heh, sorry about the the whole "ballast" thing :D

    As far as the music, that was all by Ratatat.

    Harvey, yeah we are having a blast doing this. These guys are picking it up really fast, I'm actually a little bit jealous. It took me months to wrap my head around navigating around the course smoothly. :norton

    I'll try to film a bit more next time, as I doubt I will have my rear rotor fixed for next weekend, to ride the course hard. Should be fun to capture the entire day, from setting up the course, to them practicing and warming up tires in figure 8, to tail chasing, and maybe even some timed attacks. Hopefully we will get a few more people next time as well. The more the merrier.

    2-up riding was really impressive for me to watch.
  13. liquid_ice

    liquid_ice Been here awhile

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    Yesterday, In Amsterdam we had a ladies-only gymkhana experience :clap
    I think there were about 15 ladies with bikes.

    To bad I wasn't there, but I understood that they had a great time.
    Pictures are on it's way :evil
  14. Vulfy

    Vulfy Been here awhile

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    Yeah! Pictures pictures pictures.
  15. ohgood

    ohgood Just givver tha berries !!!

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    wow , cool !
  16. Motogymkhanaman

    Motogymkhanaman Been here awhile

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    Vulfy - Excellent explanation, but I'll take your word for the figures. I have the new VBOX Sport data logger so what I will do is get one of our better riders to do a couple of rotations, one with the logger GPS sensor mounted over the front wheel spindle and one with it mounted over the rear wheel spindle. Although the actual circles picked up by the data logger will be smaller than the circles described by the bike, due to the bank angle, I'm sure we can get some figures that are broadly correct.

    Of course when we are looking at how fast the bike can be turned we always have to factor in the tyre slip angle which adds a layer of complication to the sums!
  17. TheWall

    TheWall 0 miles and counting

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    Is GPS precise enough to provide meaningful data? As I understand, GPS accuracy (without using WAAS) is on the order of a couple of meters. Since the front and rear wheel contact patch is roughly two meters apart on a typical motorcycle, I would think the margin of error is such that it would be hard to tell much difference in the radii of the circles traced by the front and rear tires. Nevertheless, it's all theory until you actually give it a try. I'll be interested in the results regardless :)
  18. shaddix

    shaddix Banned

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    Couldn't you just lean the bike over with the steering locked and walk it in a circle with someone following behind putting marks on the ground
  19. Vulfy

    Vulfy Been here awhile

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    Please note that everything below is written with a U-turn or a full 360 turn in mind. Going into a shallower turn is a bit different. Here I'm talking about braking at the cone, and doing a U-turn around it.

    At this point, its interesting to find out the actual speed difference between the wheels, front and rear.

    Finding out just the turning radius of each wheel is easy enough. As you said you can just trace the wheels as you duck walk it around at full lock. Leaning the bike is a bit harder due to the weight, but can be done with a few people.

    However, as I said, the interesting thing to see is the speed difference between wheels.

    You can see how top riders turn around, their rear wheel is drawing an extremely tight circle around the cone, so it travels very little distance, in a turn.

    However the front wheel has to keep up with the turn rate of the rear wheel, but at a larger radius arch, which will make it travel much faster compared to rear wheel.

    What I'm fascinated about, is that the perceived "speed" of a motorcycle traveling through space vs turn rate around the cone.

    If we look at the motorcycle as a whole, it looks to travel slowly around the cone. Simply because its rigidly connected to rear wheel, which travels short distance when turning around the cone. So the ENTIRE silhouette of the motorcycle looks to be moving slowly through space.

    However the swing of the front wheel is much faster, and that is a speed of rotation, which is somewhat high, as it needs to keep in line with rear tire, but at a much wider arch.

    I think the whole point of this thought exercise is for beginner riders to realize that going "slow" does not necessarily mean going slow.

    Going slow in a straight line is not the same as going "slow" around the cone. The bike might look "slow" going around the cone, but throttle input, and the feeling ON the bike, is very different to going slow in a straight line.

    I think for beginner riders, it is important to understand this distinction. I know I struggled a lot more trying to slow down the motorcycle too much around the corner.
    I was fighting it against the gravity too much, when instead I should have probably relied more on momentum I was carrying, and concentrating more on the proper inputs through my body position and handlebars.

    Basically all I'm trying to say is, don't go too slow into a turn, where you are struggling to keep the bike from falling over. You need a bit of momentum, you need RPMs to pick the bike up, you need brakes to balance out your power input, and you need to feel the bike, and let it do what it needs to do.

    Bike needs to fall into the turn, but not fall because of lack of power, but because of momentum and the mass of the bike against the brakes. There is always a point in a turn, where you will feel the bars twisting from your hands, into the turn. That means that the transfer of weight under braking is twisting the front wheel into the turn, tightening the turn. If we don't fight it, do it smoothly and in control, the turn will naturally tighten, and at the scariest moment when we feel as if we are falling over, the bike WILL pick itself up and straighten out, usually already at the exit of the turn

    But all of that will happen ONLY if you have enough power and momentum going into the turn, otherwise you will either flop on your side, or you will panic and straighten the bars out, and widen your turn considerably.

    So yeah... don't go "slow".
    At least that is my personal experience so far.
  20. Motogymkhanaman

    Motogymkhanaman Been here awhile

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    Excellent explanation there Vulfy. We have a saying that "If you try to go fast, you will end up going slow".

    What we mean by this is that a lot of riders have only ever had experience of banking their bike whilst travelling at a reaonable speed and few, if any, have experience of the handlebars turning significantly whilst doing so. Because of this a lot of new riders try to maintain their balance by travelling at too high a speed as they turn around a pylon. This leads to the build up of a significant tyre slip angle and so they naturally make a very large arc around the pylon. Distance is the thief of time, so these big arc's all add up to the rider taking a much longer path around the pylons and obstacles than they need to.

    Getting riders to believe that the bike will lean over quite happily at low speeds so long as it is changing direction quickly, is one of those key Moto Gymkhana riding skills that we need to encourage.