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Are Turns Circles?

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Please check out this link:



This post has bugged me from even before I took the SBS level 1 course because the computer programmer turned court room motorcycling expert has proclaimed it impossible to make a 90 degree corner at greater than 23 mph. And I believe that there's a corner matching his description that I routinely enter at 25mph and exit at about 35 mph. Not having been a star at the Level 1 course, and because I do not generally hang off, I believe that others who can hang off and ride better can enter and exit at even higher speeds.


This post, I believe, has been much abbreviated, as this or a similar thread was quite controversial and had many differing opinions. Any feedback would be appreciated. My seat of the pants impression of aggressive lean-in and late apex turns is that the radius of the turn is greater than the 40' as described in the link. But my mind can't wrap around how the entry into the turn is not tangent line into a constant radius curve which would lead me to run into the outside curb. In other words, if a 40' radius circle is the greatest radius curve possible centered over the corner, how can we make a 50' radius curve for example, centered elsewhere when it seems that it would require greater turning forces in order to do that?


Another way to look at it. How, for instance, is it that if the 50 degree lean angle generates the maximum 1.2 g's of turning force my tires are capable of, that my tire tracks seem to describe an abrupt change in direction which seems to require much more g force? How is that abrupt transition possible?


I believe the issue is not with how we ride, but in how the limitations are artificial and incomplete in this physics example.


Please help.





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Intersections pretty much never have such an accute 90-degree point that way it is shown in his drawing. Even city blocks have somewhat of a curve which will increase the largest possible radius by a lot. The way they have that drawing, a norml car couldn't even make the turn without driving up on the curb or into the other lanes.


Usually there is also a row of parking or shoulder or something between the 12-foot lane and the curb, so rarely is the lane actually against the curb. This would also drastically increase the largest possible radius.


The 23mph limit shown in the link seems to be based a cornering ability of 0.9g. Of course more is possible. But intersections usually have large grease spots and other vehicles traveling in a lot of different directions, so it doesn't seem like a great place for practicing your corning.


The speedometers on most bikes are not accurate. Usually they are 10% optimistic. I've owned 8 bikes, they were all like that. So 25 indicated = 23 in reality. Not a big difference but it becomes one at higher speeds when you figure that 80 indicated is actually only 72.

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He also points out a 41 degree lean angle. I don't know where that comes from. Pedrosa leaning off the bike does over 60 degrees. I'm not saying that people take street corners like Pedrosa, I'm saying that there is A LOT he's not taking into account. Raised rearsets, lean of the rider, rear tire spin, etc. It's a VERY generic diagram.

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The drawing is correct enough for demonstration purposes, but there's one basic assumption that's faulty; that a rider's speed is constant throughout the turn. With that idea, he's technically correct that that it'd be extremely difficult if not impossible to maintain a constant speed of 35mph around that turn. He's also correct that the rider he's talking about had a faulty idea as well - using 35mph as an entry speed is one thing, using it as a minimum speed is something else entirely.


Now, going through that turn with an AVERAGE speed > 23mph is entirely possible. Just ask any Irish Road Racer, some of the roads they race on are only 12 feet wide.


I say the drawing is correct enough because the assumption is a city intersection which means curbs. A riders line would have to take those into account for ground clearance reasons so a radius of 40 feet is a perfectly fine assumption. I haven't taken time to verify the math.

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I've finally got around to reading this thread and now I'll tell you why this isn't real in any way shape or form. Firstly, when the bike begins to turn, it of course starts to turn slowly (but it still steers), as it cannot get to it's required lean angle immediately whether that needs to be 41 degrees , 20, or whatever of lean angle. A corner taken on a motorbike, i.e. the line it takes, doesn't look like a nicely drawn radius (with a compass) when you look at the actual path the motor bike takes.


Secondly, the line and lean angle thats required differs dependant on the mass of the bike and the rider on the bike. This is one of the reasons little small GP bikes like 125's can have massive corner speed, yet a sports tourer has to go say 40% of that speed, even if they can hold exact same lean angle.


So I think in summary his theroy is completely mute!




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