From: Jobst Brandt
Date: May 11, 2001
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
when cornering hard. Because narrow bicycle tires inflated hard have
little traction margin, a slip on pavement is usually unrecoverable.
Drifting a Road Bicycle on Pavement
Riders have claimed they can slide a bicycle on dry pavement in curves
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 motorcyclists, who can cause a rear wheel slide by applying power when banked over.
Besides, when questioned about how this is done, the proponent says
that the ability was observed, done by others.
A bicycle can be pedaled only at lean angles far less than the maximum
without grounding a pedal, so hard cornering is always done coasting:
hence, there is no power in hard cornering. 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 is why road bicycles are built the way they are,
no higher than is useful.
That bicycle tires have no margin for recovering a slip at maximum
lean angle has been tested in 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 both precipitous and
unrecoverable. Although knobby tires have a less sudden slipout and
can be drifted around curves, they begin to side-slip at a more
upright angle as their tread fingers walk rather than slip. For this
reason, knobby tires cannot achieve the lean angles of smooth tires and
offer no cornering advantage on pavement.
How to Corner
Cornering requires estimating the required lean angle before reaching
the apex of the turn where the angle with the road surface is the
critical parameter rather the angle with the vertical, as is evident
from banked curves. Lean angle is limited by the available traction
that must be assessed from velocity and appearance of the surface.
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 at least 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.
Banked curves have a greater effect than just adding to the maximum
lean angle, because with a steeper banking, more of the centripetal
cornering force goes into increasing traction directly into the
banking, up to the limit of a vertical wall where only the maximum
G-forces limit what speed a bicyclists can attain. In contrast, an
off-banked curve makes cornering progressively more difficult until
the bicycle will slip even at zero speed. This effect is more
naturally apparent to riders who exceeded these limits early in life
and have added the experience to expected natural phenomena.
The skill of visualizing effects of speed, traction, braking, and
curvature is complex, but is 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
Cornering requires reflexes to dynamics that are easily developed in
youth, while people who have not exercised this in a long time find
they can no longer summon these skills. A single fall strongly
reinforces doubt, so cautious practice is advisable if returning to
bicycling after a long time.
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 steering 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, in line 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.
Once the basics of getting around a corner are developed, doing it
fast involves careful use of the brakes. Besides knowing how steeply
to lean in curves, understanding braking 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 nearly
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 transfer diminishes. A feel for how
hard the front brake must be applied for rear wheel lift-off can be
developed 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 before the apex. Anticipating maximum
speed for the apex is difficult, and because the path is not a
circular arc, speed must be trimmed all the way to that point. Fear
of braking in curves usually comes from an incident of injudicious
braking at a point where braking should have been done 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 cannot be large and therefore, weight transfer will be
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, a front skid is generally not. An exception
to this is in deep snow, where the front wheel can slide and function
as a sled runner while being steered.
Braking at maximum lean
For braking in a curve, take the example of a rider cornering with
good traction, leaning at 45 degrees, the equivalent of 1G centrifugal
acceleration. Braking with 1/10g increases the traction demand by one
half percent. The sum of cornering and braking vectors is the square
root of the sum of their squares, 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.
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 bearing with uniform force on the
road while rolling over bumps. Ideally the tires should bear on the
road at constant 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 bicycle 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 over uneven road. Getting weight off the
saddle is also made more difficult by such maneuvers.
[I have some disagreement with this advice. Pulling the knee in suddenly can increase traction when cornering and possibly help to prevent an incipient fall -- John Allen]
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. While leaning off the
bike, trail of the front wheel causes steering on rough roads.
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 it has nothing 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, cushioning his weight on his ankle. 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 the ankle cannot absorb large 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 with pedals
and cranks horizontal.
Most of the "body English" riders display is gratuitous gesturing,
much as with 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, in a series of tight ess bends, there's no time to do any of
this. It's done by supporting weight on the (horizontally positioned)
pedals, and unless the road is rough, with a light load 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, which
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, where pedaling at
that lean angle is not possible, nor does pedaling accelerate as fast
as braking decelerates.
Although the railroad term switchback arises from early 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, on roads these are hairpin turns. In such turns trajectory
asymmetry is most conspicuous, because braking can be hard enough to
raise the rear wheel when entering but one cannot exit with such
acceleration. For this reason, riders often find themselves with
extra road on the exit of such turns, having slowed down too much.
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 monitors 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 confrontation,
stopping image processing of the road surface for 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 mentally to become a passenger of that vehicle. Always look
ahead of the vehicle, observing it only peripherally.
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.
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
hitting a bump or a "Bott's dot" is better than altering 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 25 mm 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 gives a restricted lean angle.
Mental speed is demanded by all of these. However, being quick does
not guarantee success, because judgment is even more important. To
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 higher 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 than rim glue melting as all pressure sensitive glues do with
heating. As glue softens, tires slip on the hot rim and pile up on
the valve stem. This is the usual indicator that tubular tire wheels
are too hot. The next is that the tire arches off the rim in the area
just before the stem.
This is a serious problem for both tubulars and clinchers because most
clincher tires, given enough time on a hot rim, will blow off if
inflated to the recommended pressure, that gives good rolling
performance (hard) while tubulars roll off 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 below 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 to normal. The
blow-off pressure is the same for small and large tires on the same
rim, it being dependent only on the rim width. Also,
tires with a smaller air volume become hot faster than larger ones.
There is no way to descend 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 unknown
to most people when they were last offered.
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