COMMENT & ANALYSIS
ecently, I was cycling back from my
badminton club. About halfway home,
a gentle rasping sound began. It
happens quite a lot – leaves or a twig get stuck
somewhere in the rear wheel spokes, and then
once every rotation the uninvited hitchhiker
brushes against the reflector or the brakes.
I was hungry and in a hurry, and although
I peered back at the wheel every time I
stopped at traffic lights, I couldn’t spot
the twig and wasn’t bothered enough
to have a proper look. The soft swish,
swish, swish sound followed me
home. It was only when I was
inside the bike cage at my flat
that I gave it my full attention.
What I found made me gasp out
loud. And, because this is 2017,
it also made me reach for my
phone and start taking pictures.
A 4cm-long nail had pierced the
rear tyre and had gone straight
through, re-emerging close to the
rim. The tyre was still at full pressure,
and the swishing sound had come from
the spiky end of the nail grazing against the
brake block. Some bike wheels now have
swanky insides, lacking an inner tube
or filled with gloop that’s supposed
to automatically bung up puncture
holes, but not this one. The nail had
gone through both tyre and inner
tube, twice, and had re-sealed so
quickly that
I had cycled three miles on it
without noticing. After poking
at the tyre for a bit in disbelief, I
used my keys to lever the nail out.
Four seconds of the sort of hiss usually associated with
an extremely angry viper, and the tyre was as flat as a
pancake. When I extracted the inner tube, it had two neat
round holes in it, almost as if fangs had done the deed.
My first thought was to wonder how much further
I could have cycled without it deflating. And my second
thought was about how it had sealed so well. My best
guess is that the inner tube was pushed inward to create
a skirt around the nail, and that the pressure inside
the tyre pushed the rubber against the metal to seal
the gap. I’ve got a road bike, so the pressure inside
was close to seven times atmospheric pressure- enough to form a strong seal. That might
explain the nail going in, but I can’t quite see
how it would work on the way out.
I can see the foundation of the solution,
because this type of seal is pretty common.
Rubber is formed from stretchy polymer
chains locked together with cross-
links. In tyres and inner tubes, this
matrix is reinforced with carbon
black to make it more durable
(that’s why tyres are always
black, even though natural
rubber is white). Rubber will
deform when squeezed, but it
won’t get smaller – it just squishes out
sideways. So if you apply pressure, a
rubber matrix will squish into all the
tiny gaps in nearby surfaces, forming
a perfect barrier. But how exactly did
that work, twice, inside my inner tube?
The most frustrating thing about all
this is that even though I’m a huge fan
of experiments to help understand a
situation, I can’t imagine being able
to recreate this one. I could cycle
over beds of nails for months, and
never get a double puncture like
this again. I could look right at
that nail, but I couldn’t see
what was just on the other
side of the tyre, forming the
seal. So this one is unfinished
business, to be stored away
in the pile of not-quite-solved
scientific puzzles. But you never know when a new nugget
of understanding will boot one of those mysteries out into
the open again. Tyres, I’m watching you... ß
ILLUSTRATION: KYLE SMARTR
HELEN CZERSKI ON ... TYRE PUNCTURES
“THE NAIL HAD GONE THROUGH BOTH TYRE AND INNER TUBE,
TWICE, AND HAD RE-SEALED QUICKLY”
Dr Helen Czerski is a physicist and BBC science presenter. Her book,
The Storm In A Teacup, is out now