54 | New Scientist | 17 July 2021Flat out
When cycling over rough ground,
am I more likely to avoid a
puncture by riding slowly or fast?Martin Watson
Ay r, U K
There are two main ways that
a bike tyre can be ruptured –
by a penetrating object or by
a “pinch” puncture.
A penetrating puncture from,
say, a thorn is more likely at high
speed – and also at high tyre
pressure, as the tyre has less
opportunity to deform away
from the object rather than being
ruptured. A pinch puncture occurs
when the bike’s inner tube is
compressed between a hard
object, such as a pothole edge
or rock, and the tyre rim. This is
more likely at high speed and at
a lower tyre pressure. So, to avoid
a puncture, it is best to ride slowly.Rodney Priest
Chippenham, Wiltshire, UK
Punctures on rough surfaces are
best avoided by pumping tyres up
hard and standing on the pedals
through the worst sections.
Lower speed is only relevant
for helping you see an object
ahead in time to avoid it and liftyour bottom off the saddle. This
allows your calf muscles to act
as shock absorbers and so help
prevent punctures that happen
when the downward force of body
weight is transferred through the
frame to the metal rims and sharp
object, thereby pinching the
rubber tube between them.
For five years, I took groups
of inexperienced teenage cyclists
out to ride along stretches of sharp
aggregate that was used to pave
a tar-free track on the edge of
Salisbury Plain. After learningWant to send us a question or answer?
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Full terms and conditions at newscientist.com/lw-termsThe back pages Almost the last word
WORLDSPEC/NASA/ALAMYthe “bum-off-saddle” technique,
they didn’t get a single puncture.Anthony Woodward
Portland, Oregon, US
A sharp object, such as a nail or
thorn, will penetrate the tyre and
tube at whatever speed the bike
is ridden. Pinch punctures – also
known as snake bite punctures
because the holes resemble snake
bites – occur when the tyre forcibly
hits an obstruction. The impact
will be more forceful the greater
the kinetic energy – that is, the
greater the speed – of the bike.
Riding slowly over rocky terrain
will therefore reduce the chance
of a pinch puncture.Peter Groom
Winscombe, Somerset, UK
The speed at which you choose tocycle is mainly about your skill
at balancing, control, safety and
bragging rights. As your abilities
develop, you can go faster with
more control and be easier on the
bike overall, choosing a better line
and using the suspension to reduce
impact force against obstacles.
Going faster reduces contact time
with the trail surface, so possibly
could reduce penetration by
thorns, but not by much.
I went from getting punctures
every time I rode to none at all
not by changing speed, but by
switching to tyres without
inner tubes, where latex fluid
sloshes around in the tyres
and fills any holes.
Anyway, with the Mendip hills
on my doorstep and a mountain
bike in the garage, it is time for me
to stop writing emails and get out!Cross-legged
Is sitting with crossed legs bad
for you? If so, why does it feel
so comfortable (until your leg
goes numb)?Chris Daniel
Glan Conwy, Conwy, UK
The reason that cross-legged sitting
is comfortable is that it allows
the trunk to be held in an upright
position with relatively little effort.
In a standing posture, the pelvis
is rotated slightly forwards, giving
the spine an “S” shape with an
inward curve of the lower back
(lumbar lordosis), an outward
curve of the upper trunk (thoracic
kyphosis) and a further inward
curve at the neck (cervical
lordosis). When we sit with our
legs out straight or on a chair
with knees flexed and feet on the
floor, the pelvis rocks backwards.
This flattens the lumbar curve,
causing the whole spine to take
on a “C” shape, which requires
effort to correct. As the muscles
fatigue, the position becomes
increasingly uncomfortable.
In cross-legged sitting, the
thighs are angled outwards and
rotated at the hips while the knees
are fully flexed and the lower legs
crossed. This has the effect of
shortening the hip extensor
muscles, such as the glutei and
the hamstrings, reducing their
tendency to pull the pelvis
backwards. This makes it easier
for the hip flexor muscles, such as
the rectus femoris and iliopsoas
on each side, to hold the pelvis
upright. This makes a good sitting
position easier to maintain.
Sitting in any fixed position
for too long isn’t advisable.
Crossing the legs can strain the
joint ligaments and cause blood
to pool due to constriction of the
veins. When blood is allowed to
flow again, it can be painful for a
few seconds, but this might be a
small price to pay for the benefits
of good posture.This week’s new questions
Seeing the light A rainbow is formed from a continuous
spectrum of wavelengths of light, so why do we perceive
it as seven separate bands of colour? Peter Holmes, Ulverston,
Cumbria, UKLifeless planet What would Earth be like if life had never
existed? Derek Johnson, Moreton-in-Marsh, Gloucestershire, UKIf life had never existed,
what would our planet
be like today?“ After I taught students
the ‘bum-off-saddle’
technique for cycling
over rough ground,
they didn’t get a
single puncture”