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RE: [ST] for Matt and Richard - tire width & cornering



The idea of a tire holding only so much force is obviously dependent on
chemistry (the material properties of all substances depends on this,
but the material science can be excluded from this debate).  The
DIFFERENCE in the two bikes cornering ability comes down to how they
differ from each other with the same tires.

Traction is proportional to the normal force or gravity pulling down on
the object.  The thing trying to break this traction while cornering is
the centrifugal force (just like my pushing a box along the floor is
what is trying to overcome the friction between the box and the carpet,
which is proportional to the box's weight and dependent upon the
coefficient of friction between the two materials).  Obviously, the
heavier bike has a greater normal force, and thus more traction; but at
any given cornering speed, the heavier bike is also going to be exerting
more force centrifugally.  

Now we really may need to break out our college engineering text books
to "prove" the matter, but my experience and knowledge leads me to the
additional weight being more detrimental because of the increased
centrifugal force than beneficial because of the increased normal force.

To demonstrate, take a passenger with you on your next ride (I have
ridden VERY spirited with and without my fiancé on the back of the
bike).  Her weight (oh - she'll kill me for even referring to her
weight...) is more detrimental in cornering due to the increased force
fighting the turn, than it is beneficial in increasing the normal force.

Matt Heyer


- -----Original Message-----
From: owner-st@xxxxxxxxxx [mailto:owner-st@xxxxxxxxxx] On Behalf Of
Thomas Emberson
Sent: Tuesday, November 16, 2004 11:23 AM
To: ST@xxxxxxxxxxxxxx
Subject: Re: [ST] for Matt and Richard - tire width & cornering

Heyer, Matthew A. wrote:
> Yes - you are correct in traction being proportional to the weight
over
> the surface area - same as a normal force acting upon a point.  A
> diagram would help me in my point immensely here.  
> 
> What I'm saying is that the heavier object has the greater centrifugal
> force during the turn.  And the only thing opposing the centrifugal
> force is the tire and it's traction to the road.  With a said tire
> having a coefficient of friction of "x", that tire can only oppose so
> much force before it breaks its traction.  As speed increases in a
turn,
> so does that pseudo centrifugal force that is at odds with the tires
> traction.  Let make up a number and say that a bike weighing 500 lbs
> going through example turn 1 at a speed of 100mph causes that force to
> be 100 units.  Another bike that weighs 300 lbs going through the same
> turn at the same rate of speed might only cause a force of say 70
units.
> If the tires limit is 100 units before loosing traction, that lighter
> bike can go up to 100 units and thus can travel faster through the
same
> arc (thus increasing that outward force) until it reaches that "100
> units" number.  So the lighter bike can travel through the same corner
> on the same tire at a higher rate of speed due to its lighter weight
and
> consequent lower outward force than a heavier bike at any given
> cornering speed.

We are close to talking the same language. The traction of the tire is 
proportional to the weight imposed by gravity pulling the bike down. So 
you have 2 forces, one centrifugal and one gravity. Traction is 
proportion to that of gravity that is the weight of the bike. Thus a 
bike weighing 300lbs is going to have less traction than that of the 
bike weighing 500lbs, when you take traction as meaning the amount of 
raw grip the tire has.

So with you numbers 500 lbs bike has 100 units of traction as, thus it 
can take 100 units of centrifugal force, the 300 lbs bikes will have 
*70* units of traction and less gravity acting on the tires, thus only 
70 units of centrifugal force can be applied.

The big difference is the 100 units of traction is going to cause more 
heating and chemical stress in the tire than the 70 units. If the 
chemical nature of the tire can only handle 85 units is sheer force 
before the heat and molecular bonds get torn, the 300lbs bike is going 
to kick some serious 500lbs bike a$$ as the 500 lbs bike would 
experience tire failure.

If that magic tire number was say 200units instead of 85, both bikes 
would be equal.

Assuming you can separate Chemistry from Physics, tires are all about 
chemistry and little about physics, unless Rossi can truly change 
9.8m/s^2  :-)

(see, I got some moto content in there :-) )

- -- 
Thomas Emberson


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