This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Thu, 22 Jan 1998 18:15:42 PST
Many sharp objects, especially those that lie flat on the road like
nails and pieces of metal, more often enter rear tires than the front
tires. That is because the front tire upends them just in time for
the rear tire to be impaled on them.
For example, nails seldom enter front tires. When dropped from a
moving vehicle, nails slide down the road, and align themselves
pointing toward traffic, because they prefer to slide head first as
they would when laid on a slope. The front tire rolling over such a
lengthwise nail, can tilt it up just in time for the rear tire to
encounter it on end. I once got a flat from a one inch diameter steel
washer that the front tire had flipped up so that the rear tire struck
it on edge. When following another wheel closely, the front tire can
get the "rear tire" treatment from the preceding wheel.
The front wheel set-up effect is especially true for "Michelin" wires,
the fine strands of stainless wire that make up steel belts of auto
tires. These wires, left on the road when such tires exposes their
belt, cause hard to find slow leaks almost exclusively in rear tires.
When wet, glass can stick to the tire even in the flat orientation and
thereby get a second chance when it comes around again. To make
things worse, glass cuts far more easily when wet as those who have
cut rubber tubing in chemistry class may remember. A wet razor blade
cuts latex rubber tubing in a single slice while a dry blade only
makes a nick.
As for pinch flats, aka snake bites, they occur on the rear wheel more
readily because it carries more load and is uncushioned when the rider
is seated. The rider's arms, even when leaning heavily on the front
wheel, cushion impact when striking a blunt obstacle.