This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Date: Fri, 05 Dec 1997 16:29:59 PST
Drag racers first recognized the traction benefits of slick tires,
whose benefit they could readily verify by elapsed times for the
standing start quarter mile. In spite of compelling evidence of
improved traction, more than twenty years passed before slicks were
commonly used for racing cars, and another twenty before they reached
racing motorcycles. Today, slicks are used in all weather by most
street motorcycles. In spite of this, here at the end of the
millennium, 100 years after John Dunlop invented the pneumatic tire
for his own bicycle, bicyclists have not yet accepted smooth tread.
Commercial aircraft, and especially motorcycles, demonstrate that a
round cross section tire, like the bicycle tire, has an ideal shape to
prevent hydroplaning. The contact patch, a pointed canoe shape,
displaces water exceptionally well. In spite of this, hydroplaning
seems to be a primary concern for riders who are afraid to use smooth
tires. After assurances from motorcycle and aircraft examples,
slipperiness on wet pavement appears as the next hurdle.
Benefits of smooth tread are not easily demonstrated because most
bicycle riders seldom ride near the limit of traction in either curves
or braking. There is no simple measure of elapsed time or lean angle
that clearly demonstrates any advantage, partly because skill among
riders varies greatly. However, machines that measure traction show
that smooth tires corner better on both wet and dry pavement. In such
tests, other things being equal, smooth tires achieve greater lean
angles while having lower rolling resistance.
Tread patterns have no effect on surfaces in which they leave no
impression. That is to say, if the road is harder than the tire, a
tread pattern does not improve traction. That smooth tires have
better dry traction is probably accepted by most bicyclists, but wet
pavement still appears to raise doubts even though motorcycles have
shown that tread patterns do not improve wet traction.
A window-cleaning squeegee demonstrates this effect well. Even with a
new sharp edge, it glides effortlessly over wet glass leaving a
microscopic layer of water behind to evaporate. On a second swipe,
the squeegee sticks to the dry glass. This example should make
apparent that the lubricating water layer cannot be removed by tire
tread, and that only the micro-grit of the road surface can penetrate
this layer to give traction. For this reason, metal plates, paint
stripes, and railway tracks are incorrigibly slippery.
Besides having better wet and dry traction, smooth tread also has
lower rolling resistance, because its rubber does not deform into
tread voids. Rubber being essentially incompressible, deforms like a
water filled balloon, changing shape, but not volume. For a tire with
tread voids, its rubber bulges under load and rebounds with less force
than the deforming force. This internal damping causes the energy
losses of rolling resistance. In contrast the smooth tread transmits
the load to the loss-free pneumatic compliance of the tire.
In curves, tread features squirm to allow walking and ultimately,
early breakout. This is best demonstrated on knobby MTB tires, some
of which track so poorly that they are difficult to ride no-hands.
Although knobby wheelbarrow tires serves only to trap dirt, smooth
tires may yet be accepted there sooner than for bicycles.