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Old 10-27-2013, 09:05 AM   #67
Joined: Oct 2013
Oddometer: 17
Originally Posted by lnewqban View Post
Yes, with more or less weight on the contact patch, the area of contact grows or shrinks some, but it cannot change as much as the vertical load or force applied on the patch.
That means that if you double the load, the area does not double.
Furthermore, the area has much less influence on traction than the vertical force.
Reduce area in half and you may reduce traction in 10%; reduce vertical force in half and you reduce traction at least 50%.

The lateral force also deforms each instantaneous contact patch sideways, forcing the next one to land a little off the trajectory (a micro side-slide among successive instantaneous contact patches) and forcing the rider to over-steer (or under-steer if the rear slides more) to compensate.

In real life, the vertical force can be drastically reduced while the lateral force remains the same (bad because traction is reduced but not lateral force) when rolling over road imperfections that the suspension cannot follow impeccably: right after the crests are the killers because the tire and the weight that it supports float for fractions of seconds, due to the inertia with which the weight of the bike and tire fall or rise.

To make things worse, a nervous rider could interfere with the little self-adjustments that the steering geometry does to adapt to the sideways forces that appear when rolling over road imperfections at substantial angles of lean.

In that way, for bad suspension and bad road, the contact patch constantly goes from deformed to symmetrical to deformed.
As a deformed tire is more rigid than a normal one, the inherent suspension of the tire also constantly goes from bad to good to bad (which worsens the overall suspension).
Some tire's manufacturers claim that their tires grow the area of contact patch as those lean (see schematic below); however, there is nothing they can do about the internal stress of deformation and uneven distribution of vertical forces.
The contact patch of a tire leaned 45 degree is feeling and resisting slide from a lateral force as big as the weight that it is carrying (over 200 lbs for a light sport bike with a 50/50 weight distribution).

As I see it, for a constant lateral force (steady turn, radius and speed) and the actual variable vertical force and area and rigidity of the patch, the less weight and inertia the better to stay within the margins of the traction that is necessary to counter-act that force.
While leaned, the closer to a wheelie attitude, the less critical these things that affect front traction become.

...........Very sorry, Beendog, I couldn't understand your last question.

This is the best explanation as to why our tendency should be to keep the bike as upright as possible, and everything else is a secondary consideration. Thanks very much lnewqban I understand this to my satisfaction now!

The second question, you are probably overthinking it is all. Is it better to give up the wheelie attitude and be going 10mph slower, or have the wheelie attitude going 10mph faster? Really depends on what speed the corner dictates I would surmise, and would be answered case-by-case, so I doubt any discussion of the preferences of the overall machine would make this clear.

Thanks again. That whole "having a bit of throttle stabilizes the bike" never satisfied my curiosity as to WHY that is the case. But having the bike as vertical as possible + the wheelie attitude affecting the suspension and contact patch is a far more detailed and satisfying explanation.
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