Subject: 9.15 Descending II From: Jobst Brandt <[email hidden]> Date: Fri, 11 May
2001 16:35:42 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 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 motorcycles, that 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 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 point 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 are 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 severe.
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
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.
Braking
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
transfers 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.
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
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.
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 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,
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.
Body Contortions
Most of the "body English" 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, 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, 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, where pedaling at that lean angle is not
possible, nor does pedaling accelerate as fast as braking decelerates.
Hairpin Turns
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.
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
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 to mentally 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.
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 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 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 gives a restricted 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. 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 both for tubulars and clinchers because most clincher tires, given enough
time on a hot rim will blow off if inflated to recommended pressure. 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 opening of the rim width. Also, tires with a smaller air volume become hot
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 unknown to most people when they were
last offered.
Jobst Brandt [email hidden] Palo Alto CA
