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Subject: 9.15  Descending II
From: Jobst Brandt <jbrandt@hpl.hp.com>
Date:    Mon, 20 Sep 1999 14:41:04 PDT

Descending and Fast Cornering

Descending on mountain roads, bicycles can reach speeds that are more
common on motorcycles.  Speeds that are otherwise not attainable, or
at least not continuously.  Criterium racing also presents this
challenge, but not as intensely.  Unlike a motorcycle, the bicycle is
lighter than the rider and power cannot be applied when banked over
low.  Besides, the hard and narrow bicycle tires have little traction
margin, so that a slip on pavement is usually unrecoverable.

Drifting a Road Bicycle on Pavement

Riders have claimed that they can slide a bicycle in curves on dry
pavement to achieve greater cornering speed, as in drifting through a
turn.  A drift, in contrast to a slide, means that both wheels slip,
which is even more difficult.  This notion may come from observing
motorcycles, that can cause a rear wheel slide by applying power when
banked over.  Besides, when questioned, the ability was always seen,
done by others.

A bicycle can be pedaled only at lean angles far less than the maximum
without grounding a pedal, so that hard cornering is always done
coasting.  Therefore, there is no power in the curve.  Although
bicycles with high ground clearance have been built, they showed only
that pedaling imbalance has such a disturbing influence on traction,
that pedaling at a greater lean angle than that of a standard road
racing bicycles has no benefit.

That bicycle tires have no margin for recovering a slip at the maximum
lean angle, has been measured by lean-slip tests on roads and testing
machines.  For smooth tires on pavement, slipout occurs at slightly
less than 45 degrees from the road surface and is precipitous and
unrecoverable.  Although knobby tires have a less sudden slipout and
can be drifted around curves, they begin to side-slip at a far more
upright angle, because they exhibit tread squirm, whereby the tread
fingers walk rather than slip.  For this reason, knobby tires cannot
achieve the lean angles of smooth tires and offering no advantage.

How to Corner

Cornering requires assessing the required lean angle before reaching
the apex of the turn.  The angle with the road surface is the critical
parameter.  This angle is limited by the available traction that the
must assess from velocity and apparent traction.  For good pavement,
this angle is about 45 degrees, in the absence of oil, water, or smooth
and slick spots.  Therefore, a curve banked inward 10 degrees, allows
a lean of up to 55 degrees from the vertical, while a crowned road
with no banking, where the surface falls off about 10 degrees, would
allow only up to 35 degrees.

The required lean angle for a curve must be estimated from traction
and speed expected after braking to the apex of the turn.  The skill
of visualizing effects of speed, traction, braking, and curvature
complex but, something humans and other creatures do regularly in self
propulsion.  The difficulty arises in adapting this to higher speeds.
When running, we anticipate how fast and sharply to turn on a
sidewalk, dirt track, or lawn, to avoid sliding.  The method is the
same on a bicycle although the consequences of error are more severe.

Cornering requires reflexes to dynamics that are easily developed in
youth, but some have not exercised these in such a long time that they
can no longer summon these skills.  A single fall strongly reinforces
doubt, so cautious practice is advisable while recalling them.

Countersteer

Countersteer is a popular subject for people who belatedly discover or
rediscover how to balance.  What is not apparent, is that two wheeled
vehicles can be controlled ONLY by countersteer, there is no other
way.  Unlike a car, a bicycle cannot be diverted from a straight path
by turning the wheel to one side.  The bicycle must first be leaned in
that direction by steering it ever so slightly the other way.  This is
the means by which a broomstick is balanced on the palm of the hand or
a bicycle on the road.  The point of support is moved beneath the mass
to align with the combined forces of gravity and cornering and it
requires steering, counter and otherwise.  It is so obvious that
runners never mention it, although football, basketball, and ice
hockey players conspicuously do it.

Braking

Once the basics of getting around a corner are understood, doing it
fast involves careful use of the brakes.  Besides knowing how steeply
to lean in curves, understanding the brakes makes the difference
between the average and the fast rider.  When approaching a curve with
good traction, the front brake can be used almost exclusively, because
it is capable of slowing the bicycle so rapidly that nearly all weight
transfers to the front wheel, at which point the rear brake is becomes
useless.  Once in the curve, more and more traction is used to resist
lateral slip, as the lean angle increases, but that does not mean the
brakes cannot be used.  When banked over, braking should be done with
both brakes, because now neither wheel has much traction to spare and
with lighter braking, weight transfers to the front diminishes.  To
develop a feel for rear wheel lift-off, practice at low speed.

Braking in Corners

Why brake in the turn?  If all braking is done before the turn, speed
will be slower than necessary early in the turn.  Anticipating the
maximum speed for the apex of the turn is difficult, and because the
path is not a circular arc, speed must be trimmed all the way to the
apex.  Fear of braking in curves usually comes from an incident of
injudicious braking at a point where both front and rear brakes should
have been used with a gentle touch to match the conditions.

Substantial weight transfer from the rear to the front wheel will
occur with strong use of the front brake on good traction just before
entering the curve.  When traction is poor or the lean angle is great,
deceleration and weight transfer is small, so light braking with both
wheels is appropriate.  If traction is miserable, only the rear brake
should be used, because although a rear skid is recoverable, one in
the front is generally not.  An exception to this is in deep snow,
where the front wheel can slide and function as a sled runner.

Braking at maximum lean

For braking in a curve, take for example a rider cornering with good
traction, leaning at 45 degrees, the equivalent of 1G centrifugal
acceleration.  Braking with 1/10g increases the traction load on the
tires by one half percent.  The sum of the braking vectors is the
square root of the sum of the two accelerations squared,
SQRT(1^2+0.1^2)=1.005 or an increase of 0.005.  In other words, there
is room to brake substantially during maximum cornering.  Because the
lean angle changes as the square of the speed, braking can rapidly
reduce the angle and allow even more braking.  For this reason skilled
racers nearly always apply both brakes into the apex of turns.

Suspension

Beyond leaning and braking, suspension helps substantially in
descending.  For bicycles without built-in suspension, this is
furnished by the legs.  Standing up is not necessary on roads with
fine ripples, just taking the weight off the pelvic bones is adequate.
For rougher roads, enough clearance must be used so the saddle carries
no weight.  The reason for this is twofold.  Vision will become
blurred if the saddle is not unloaded, and traction will be
compromised if the tires are not kept in contact with the road while
skimming over bumps.  The ideal is to keep the tire on the ground at
uniform load.  Besides, if the road has whoop-de-doos, the seated
rider will get launched from the saddle and possibly crash.

Lean the Bicycle, the Rider, or Both

Some riders believe that sticking the knee out or leaning the body
away from the bike, improves cornering.  Sticking out a knee is the
same thing that riders without cleats do when they stick out a foot in
dirt track motorcycle fashion.  On paved roads this is a useless but
reassuring gesture that, on uneven roads, even degrades control.  Any
body weight that is not centered over the bicycle (leaning the bike or
sticking out a knee) puts a side load on the bicycle, and side loads
cause steering motions if the road is not smooth.  Getting weight off
the saddle is also made more difficult by such maneuvers.

To verify this, coast down a straight but rough road, weight on one
pedal with the bike slanted, and note how the bike follows an erratic
line.  In contrast, if you ride centered on the bike you can ride
no-hands perfectly straight over the same road.  Leaning off the bike,
trail of the front fork causes steering on a rough road, especially in
curves, in contrast to riding centered over the bike.

Outside Pedal Down

It is often said that putting the outside pedal down in a curve
improves cornering.  Although most experienced riders do this, it is
not because it has anything to do with traction.  The reason is that
it enables the rider to unload the saddle while standing with little
effort on a locked knee, and this can only be done on the outside
pedal because the inside pedal would hit the road.  However, standing
on one extended leg does not work on rougher roads, because a stiff
leg cannot absorb road bumps nor raise the rider high enough from the
saddle to avoid getting bounced.  Rough roads require rising high
enough from the saddle to avoid hard contact while the legs supply
shock absorbing knee action, pedals horizontal.

Body Contortions

Most of the "body English" some riders display is gratuitous
gesturing, much like the motorcyclists who stick their butt out in
curves while their bikes never get down to 45 degrees (the angle below
which hiking out becomes necessary to keep hardware from dragging on
the road).  In fact, if you are taking a bunch of ess bends rapidly,
you'll have no time to change your position.  Just keep your weight on
your (horizontally positioned) feet, and unless the road is rough,
keep light pressure on the saddle.  On rough roads, the cheeks of the
saddle, (the ones that went away with the Flite like saddles) are
used to hold the bicycle stably between the legs while not sitting.

The path through a curve is not symmetrical for a bicycle, because it
can slow down much faster than it can regain speed.  Thus the
trajectory is naturally asymmetric.  Brakes are generally used to the
apex (that is usually not the middle) of the curve, while pedaling at
that lean angle is not possible, nor does pedaling accelerate as
strongly as braking decelerates.

Hairpin Turns

The often heard term switchback arises from mountain railroading where
at the end of a traverse, a switch is turned to back up the next
traverse, after which another switch is turned to head up the next.
The term for roads is a hairpin turn and these are the ones where
trajectory asymmetry is most conspicuous, because braking can be hard
enough to raise the rear wheel when entering but one cannot accelerate
similarly.  Many riders find themselves with extra unused road on the
exit of such turns that exemplify the difference between entry and
exit.

Vision

Where to direct vision is critical for fast cornering.  Central vision
should be focused on the pavement where the tire will track, while
allowing peripheral vision, with its low resolution and good
sensitivity to motion, to detect obstacles and possible oncoming
traffic.  Peripheral vision is monitoring the edges of the road and
its surroundings anyway, so the presence of a car in that "backdrop"
does not require additional consideration other than its path.

If central vision is directed at the place where an oncoming vehicle
might appear, its appearance presents a new problem of the
confrontation and stops image processing of the road surface for a
substantial time.  Because the color or model of car is irrelevant,
this job can be left to peripheral vision in high speed primitive
processing, while concentrating on pavement surface and composition.

When following another bicycle or a car downhill, the same technique
is even more important, because by focusing on the leading vehicle,
pavement and road alignment information is being obscured giving a
tendency to mentally become a passenger of that vehicle.  Always look
ahead of the vehicle observing it only in peripheral vision.

Riders often prefer to keep their head upright in curves, although
leaning the head with the bicycle and body is more natural to the
motion.  Pilots who roll their aircraft do not attempt to keep their
head level during the maneuver, or in curves, for that matter.

The Line

Picking the broadest curve through a corner may be obvious by the time
the preceding skills are mastered, but that may not be the best line,
either for safety or because the road surface is poor.  Sometimes it
is better to hit a bump or a "Bott's dot" than to alter the line,
especially at high speed.  Tires should be large enough to absorb the
entire height of a lane marker without pinching the tube.  This means
that a minimum of a 25mm actual cross section tire is advisable.  At
times, the crown of the road is sufficient to make broadening the
curve, by taking the curve wide, counterproductive because the crown
on the far side will restrict the lean angle.

Mental Speed

Mental speed is demanded by all of these.  However, being quick does
not guarantee success, because judgment is even more important.  Not
be daring but rather to ride with a margin that leaves a feeling of
comfort rather than high risk, is more important.  Just the same, do
not be blinded by the age old presumption that everyone who rides
faster than I is crazy.  "He descends like a madman!" is one of the
most common descriptions of fast descenders.  The comment generally
means that the speaker is slower.

Braking Heat on Steep Descents

Although tandems with their high weight to wind drag ratio have this
problem more often, steep mountain roads, especially ones with poor or
no pavement require so much braking that single bicycles blow off
tires from overheating.  For tubulars the problem is not so much over
pressure but rather glue melting as all pressure sensitive glues do
with heating.  As the glue softens the tire slips on the hot rim and
piles up on the stem.  This is the usual indicator that tubular tire
wheels are too hot.  The next is that the tire lifts off in the
compressed area just before the stem.

This is a serious problem both for tubulars and clinchers because most
clincher tires, given enough time on a hot rim will blow off if
inflated to the hardness that gives the best rolling performance (hard)
while tubulars roll of from lack of adhesion to the rim.  The faster
the travel, the more descending power goes into wind drag and the
better the rims are cooled.  Going slowly does not help, unless speed
is reduced to a walking pace.

On steep descents where rims stay too hot to touch for more than a
minute, reducing tire inflation pressure is a sure remedy.  However,
tires should be re-inflated once the rims cool down to normal.  The
blow-off pressure is the same for small and large tires on the same
rim, it being dependent only on the opening of the rim width.  Also,
tires with a smaller air volume heat up faster than larger ones.

There is no way of descending continuously and steeply without
reducing inflation pressure, unless there is an insulator between the
tube and rim of a clincher.  Insulating rim strips are no longer
offered because they were an artifact of dirt roads that often
required riders to descend so slowly that all potential energy went
into the brakes and almost none into wind drag.  These rim strips were
cloth tubes filled with kapok, their insulating purpose being a
mystery to most riders when they were last offered.

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