Author Topic: tire arc deviation disscussion  (Read 4556 times)


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tire arc deviation disscussion
« on: June 04, 2012, 01:23:19 AM »
i posted this on another forum and thought it would be a good point here too as we were discussing it for gymkhana

when i work nights, i just pull strait in the plant. we keep it clean here but it's all but impossible to keep the dust down. i noticed tonight that i could see very clearly my tire tracks. you might say, why is this relevent in the GK board slede? well i'll tell ya. i pull strait in where most back in off the street because it's easy for me on a bike to just make a u turn and park. i measured it and the alleyway is a little over 17 feet wide. it's not really a problem to just swing it around. it has been a point of pride for me for a long time that if my bike is rolling my feet are on the pegs, no duck walking. it's always looked hooky to me, like you really don't know what your doing if you have to walk a bike. i've been doing this long before GK got started here and that's probably why i was able to ride the first course reasonable well the first time i tried it. the point i'm trying to make is, i looked at my tire tracks on my usual u turn and they where nowhere close to the same track. i measured my bike, 68" from CP to CP. my ARC was just under 15' the tire tracks where over 26" apart all the way through the arc, that's side to side. front wheel furthest out, rear wheel over two feet to the inside. now this may be obvious to most everybody else but it just hit me looking at this that is the whole reason for the foot stomp. people want to put there feet down because it feels like the bike is turned sideways and leaned over, because it actually is. i took some pictures and will upload them as soon as i get home but i just had to get this out while it was on my mind. if nothing else it will be another tool for teaching/explaining why it doesn't feel exactly right to turn full lock and lean a bike in. i'd love to here all of you're ideas on this as well.

sometimes you need to see the obvious to realize it.
A timid person is frightened before a danger; a coward during the time; and a courageous person afterward. -- Jean Paul Richter


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Re: tire arc deviation disscussion
« Reply #1 on: June 04, 2012, 03:25:12 AM »
Great illustration of why it is that the turn radius of the rear tyre is far more important to us than that of the front. Every bike has slightly different maximum/minimum rear wheel turn radii, so it will pay to find out what yours actually are so that you can use that information the next time you are blasting round a Moto Gymkhana course.

First step is to get down to your local carpark (one with the parking bays marked with white lines) and line up the front and rear wheels along one of the lines. With the engine off, turn the bars to full lock and walk the bike around until it is parallel to the line you started from but facing in the other direction. With a tape measure, measure the distance between the line you started from and the rear tyre and make a note of this distance. This is the maximum turn radius.

Now comes the tricky bit (you might need some friends to help) as you have to repeat the exercise only this time with the bike slightly leaned over. Once again measure the distance and you should see that it is a fair bit shorter. Repeat the exercise over and over using greater and greater angles of lean (hence the friends). When you can't physically push the bike around any more (due to bits of the bike dragging on the ground) you have discovered the bikes minimum turn radius. Now this is only ever a theoretical minimum, because when you are riding the bike the slip angle will increase thus making the tyre run a slightly wider radius due to the difference in speed between walking and riding.


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Re: tire arc deviation disscussion
« Reply #2 on: June 04, 2012, 12:45:11 PM »
Ooooh, you guys are so dang'd smart.  I just love this kind of analysis!  Where's my chalk, I'm going out to twist my bike around and measure stuff!
Lean into it...