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



Good information.  I still believe that in response to the initial post
("I didn't realize a narrower rear tyre improves handling.  Maybe that
explains why 125's and 250's can carry more corner speed than the
diesels."), the reason the lighter bikes "carry more cornering speed
than a liter class bike" is the weight.  There is less weight being
pulled out (the centrifugal force) and thus less force acting on the
tire at the same speed as a heavier mass/bike.  Thus, a higher speed can
be held to bring the tire up to its traction threshold.

You are correct in that the tire differential (and the physics of any
front-steer machine, in which the rear wheel follows) causes the
differing arc lengths and the understeer (from the differing arc lengths
and the fact that the power is being applied at the rear tire - e.g.
front-wheel and rear-wheal drive cars)


Matt Heyer


- -----Original Message-----
From: owner-st@xxxxxxxxxx [mailto:owner-st@xxxxxxxxxx] On Behalf Of don
draper
Sent: Monday, November 15, 2004 11:21 PM
To: ST@xxxxxxxxxxxxxx
Subject: [ST] for Matt and Richard - tire width & cornering


 Certainly weight and tire size affect the cornering of a
motorcycle,... but the effect that tire size DIFFERENTIAL - between
the front and rear tires - has is another kettle of fishy lines on
the pavement.

 When any front-wheel-steered, two wheeled vehicle describes an arc
(goes around a curve) regardless of size, the front wheel describes a
larger radius arc than the rear wheel.  If you've ever swerved
quickly around a small rock in the road, you probably were aware that
your rear tire passed it on the opposite side from your front tire. 
In cornering a motorcycle, this doesn't mean much until you start to
feed in the power to accelerate out of the corner, although it does
help the bike fall in. 
 Because the rear tire is on an arc inside that of the front, it's
vector of force is at an angle tangential to the arc and it tries to
push the front wheel straight ahead towards the outside of the
corner; this is UNDERSTEER, which is commonly known as PUSH.  This
causes the bike to drift wide in the corner and limits the amount of
power you can use.  Obviously the greater the difference in radius
between the two arcs, the harder the front will be pushed.  Because
the contact patch in a turn is on the inside edge of your tires, the
wider you go on the rear with the same front, the greater will be the
difference in radius and the worse understeer you'll get.  Watching
racers powering out of corners, you'll see the front tires leaving
black streaks on the pavement,... they're being pushed sideways while
being laid over.  You may not want to try this at home,... or with
your own bike!    =8o) 

 All else being equal, the rider with the narrower rear tire will be
able to come out stronger with more power applied while still holding
his line, 
higher in the RPM range and beating his buddy to the end of the
straight.
 
 Race teams mount the widest rear tire they can get to obtain the
most traction possible when the bike is upright and depend on the
near magical skill and daring of the riders to keep from running the
thing off the corners while leaned over, often achieved by spinning
up the tire until it power slides out to the same arc as the front
(or beyond), enabling the front tire to hold it's line.  This keeps
the bike on the track but scrubs off speed and is OVERSTEER -
commonly referred to as LOOSE - and provides those long, lurid black
lines so beloved of spectators everywhere.  This whole scenario is
what's changed cornering techniques in racing from long smooth arcs
to the 'drive-it-in, get-it-turned, pick-it-up & fire-it-out' method
used most forms of racing today.  Sorry for the long posting!

                            Have fun;
                                        doggydoo   - - - o&\o
   
For added understanding, see Proverbs 16:22.

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