Gocycle GX - Page 32

Gocycle
®
Owner’s Manual for GX, April 2019
© Karbon Kinetics Limited. All Rights Reserved. 32
Metals are comparatively ductile. Ductile means bending, buckling and stretching before breaking.
Generally speaking, of the common bicycle frame building materials steel is the most ductile, titanium
less ductile, followed by Aluminium and magnesium.
Metals vary in density. Density is weight per unit of material. Steel weighs 7.8 grams/cm3 (grams per
cubic centimetre), titanium 4.5 grams/cm
3
, Aluminium 2.75 grams/cm
3
, magnesium 1.8 grams/cm
3
,
carbon fibre composite at 1.45 grams/cm
3
.
Metals are subject to fatigue. With enough cycles of use, at high enough loads, metals will eventually
develop cracks that lead to failure. It is very important that you read the basics of metal fatigue below.
Let’s say you hit a curb, ditch, rock, car, another cyclist or other object. At any speed above a fast
walk, your body will continue to move forward, momentum carrying you over the front of the bike. You
cannot and will not stay on the bike, and what happens to the frame, fork and other components is
irrelevant to what happens to your body.
What should you expect from your metal frame? It depends on many complex factors, which is why
we tell you that crashworthiness cannot be a design criteria. With that important note, we can tell you
that if the impact is hard enough the fork or frame may be bent or buckled. On a steel bike, the steel
fork may be severely bent and the frame undamaged. Aluminium and magnesium is less ductile than
steel, but you can expect the fork and frame to be bent or buckled. Hit harder and the frame
components or wheels may be broken or buckled, leaving the head tube and fork separated from the
frame.
When a metal bike crashes, you will usually see some evidence of this ductility in bent, buckled or
folded metal.
7.4.5 The basics of metal fatigue
Common sense tells us that nothing that is used lasts forever. The more you use something, and the
harder you use it, and the worse the conditions you use it in, the shorter its life.
Fatigue is the term used to describe accumulated damage to a part caused by repeated loading. To
cause fatigue damage, the load the part receives must be great enough. A crude, often-used example
is bending a paper clip back and forth (repeated loading) until it breaks. This simple definition will help
you understand that fatigue has nothing to do with time or age.
So what kind of “damage” are we talking about? On a microscopic level, a crack forms in a highly
stressed area. As the load is repeatedly applied, the crack grows. At some point the crack becomes
visible to the naked eye. Eventually it becomes so large that the part is too weak to carry the load that
it could carry without the crack. At that point there can be a complete and immediate failure of the
part.
One can design a part that is so strong that fatigue life is nearly infinite. This requires a lot of material
and a lot of weight. Any structure that must be light and strong will have a finite fatigue life. Aircraft,
race cars, motorcycles all have parts with finite fatigue lives. If you wanted a Gocycle with an infinite
fatigue life, it would weigh far more than any bicycle sold today. The wonderful, lightweight
performance we want requires that we inspect the structure.
7.4.6 What to look for
• ONCE A CRACKS STARTS IT CAN GROW AND GROW FAST.
Think about the crack as forming a pathway to failure. This means
that any crack is potentially dangerous and will only become more
dangerous.
SIMPLE RULE 1: If you find
crack, replace the part.
• CORROSION SPEEDS DAMAGE. Cracks grow more quickly when
they are in a corrosive environment. Think about the corrosive
solution as further weakening and extending the crack.
SIMPLE RULE 2: Clean
your Gocycle, protect your
Gocycle from salt, remove
any salt as soon as you can.