How
Slippery Is It?
Folks new to winter cycling often are amazed that anyone can remain upright on a bike
on snow and ice.
After you have been on the bike for a few days, you come to realize that the situation
is quite manageable. Snow, while certainly more slippery than bare pavement, is somewhat
like a shallow covering of sand, which requires somewhat more gradual turns, but otherwise
is not particularly slippery.
Ice on the other hand presents a totally different situation. Lots of winter cyclists
end up with a death grip on the handlebar worrying about every little steering movement.
After a while you get use to the amount of slip and slide movement that the bike exhibits,
and learn to "dance with the bike", controlling those small slips and learning
to avoid the big ones. Then you get overconfident, and crash.
So how slippery is it? What kind of conditions do ICEBIKERS have to especially watch?
The Society of Accident Reconstructionists (SOAR) puts out a publication called
"The Source". In the "Winter 1998" issue they published a study done
by John E. Hunter who conducted an extensive study of the frictional values of car tires
involved in collisions on snow/ice covered roads. The Friction Values (coefficient of
friction) were taken at the road tire interface and represent values available to a
normally treaded tire representing the equivalent of "good equipment."
We've excerpted the key findings below. The Classifications and descriptions are Mr.
Hunter's.
Classification
|
Description
|
Friction Available
|
Dry
Asphalt
|
This value is commonly used as the reference value for
rubber tires on dry asphalt. Concrete is typically lower. |
0.68 to 0.85
Average value of 0.72 |
Partial
Frost
|
Light or partial coating of frost on the road surface.
Visible to the driver as intermittent frosting appearance. |
Partial Frost had a resistance level
similar to the lower range of wet asphalt.
Average value of 0.63 |
Frost:
|
General white coating covering entire lane. Visible to
the driver and completely recognizable as frost. |
Frost was .10 less than Partial Frost.
Average value of 0.53. |
Heavy
Frost:
|
Almost ice conditions. Heavy white coating and very
visible to the driver |
Heavy Frost had a value close to the
higher ranges of ice.
Average of a 0.39. |
Tracked
Snow:
|
Snow compacted by vehicles. |
The test results varied in range.
Average was a 0.35 |
Untracked
Snow:
|
Snow not compacted by prior vehicles. |
The individual readings were similar to
Tracked Snow.
Average of 0.35 |
Snow
& Ice:
|
Generally known by motorists as compact snow and ice, or
"hard pack". |
Snow and Ice was nearly identical to the
frictional resistance found for Black Ice, 0.25 to a high of 0.41
Average of 0.32 |
Black
Ice:
|
Icy layer generally covering asphalt, difficult to see
by the average driver. Often found on overpasses and elevated structures. |
The ranges for Black Ice varied from a
low of 0.25 to a high of 0.41
Average of 0.32 |
Sunny
Ice:
|
Ice that has been exposed to the heating rays of the
sun. A water layer was not generally observed. |
Sunny Ice yielded low readings,
Average of 0.24. |
Wet
Ice:
|
Ice covered with a layer of water. Generally seen when
the temperatures reach 32 to 33 degrees, or near the melting point. |
Wet Ice, similar to sunny ice,
Average of 0.24. |
Glare
Ice:
|
Ice that was the smoothest surface observed. Similar to
wet ice except the water layer was not observed. looks like glass. |
The lowest value measured was Glare Ice.
Average of 0.19. |
Friction - Friend or Foe?
Where the rubber meets the road friction is absolutely essential. Without it, you can't
steer, you can't accelerate and you can't stop.
In ICEBIKING, friction can be thought of a a consumable. You are always consuming some
of the available friction between your tires and the road surface. More friction is
consumed to accelerate, steer, and brake, but some friction is there at all times. Once
the available friction is used by any one or all of these components, the vehicle will
lose traction and operator will lose control. This is true of automobiles as well as
bikes.
The principal difference is that a bicycle is a steer balanced vehicle, which requires
that we reserve some of our precious friction for steering, or we will simply fall over.
In addition, a single track vehicle is subject to sideways deviations induced by uneven
roadway surface, ruts, or ice bumps. Dual track vehicles are seldom bothered by this,
(except when following deep ruts).
Therefore, ice ridges, such as found on hard-packed snow or icy roads cause more
problems for cyclists, because a quick lateral wheel diversion induced by an ice ridge
must be "paid back" with in a couple of seconds by at least as large a steering
movement back towards the original track1. The
longer this is delayed, the more the cyclist tips, and the larger the steering movement
required. We simply MUST reserve some friction for these corrective steering movements, as
well as arresting the initial wheel diversion.
Practical Application:
Most ICEBIKERS would agree with the findings in the SOAR study in the relative ranking
of icy surfaces, with the exception that the two categories Tracked
Snow and Snow and Ice (italicized above) would be on our "watch
list" as these conditions present special dangers for a single tracked vehicle such
as a bicycle. Even though not as slippery as wet ice they can be more of a problem to
handle.
You might bear in mind that you may have less than 1/4 the traction
on glare ice as you do on bare pavement, and much of ICEBIKING is done with 1/2 to 1/3 the
traction of summer riding. This is why ICEBIKERS are always raving about studded tires.
This requires shallow turns at lower speed. Upright turns (where the cyclist offsets
the body to the inside of the turn while holding the bike in a more upright position) keep
more of your tread and studs on the ice and are a common low speed turning technique on
real slick surfaces.
Still, most winter cyclists find few unmanageable problems in staying upright during
routine operation. When playing around or taking risks, crashes do result. But you are
usually aware you are pushing the envelope in these situations. The longer you have been
at it, the closer to the edge you can go. But when you do go beyond, it is usually
spectacular. And this tends to happen just as you think you have the situation mastered.
We are left with the following two truisms:
There is MORE traction than most NON ICEBIKERS
think.
There is probably LESS traction than most ICEBIKERS think. |
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