frogbikes.com
Frog Bikes
Owner’s Manual
3
rd
Edition
2014
A. Understanding metals
Steel is the traditional material for building bicycle frames. It has good characteristics,
but in high performance bikes steel has been largely replaced by aluminium and,
in some cases, titanium. The main factor driving this change is interest by cycling
enthusiasts in lighter bikes.
Properties of Metals
Please understand that there is no simple statement that can be made that
characterizes the use of different metals for bikes. What is true is that the way the
metal is applied is much more important than the material alone. One must look
at the way the bike is designed, tested, manufactured, supported along with the
characteristics of the metal rather than seeking a simplistic answer.
Metals vary widely in their resistance to corrosion. Steel must be protected or it will
rust. Aluminium and titanium quickly develop an oxide film that protects the metal
from further corrosion. Both are therefore quite resistant to corrosion. Aluminium is not
totally corrosion resistant and particular care must be used where it contacts other
metals as galvanic corrosion can occur.
Metals are comparatively ductile. Ductile materials bend, buckle and stretch before
breaking. Generally speaking, of the common frame building materials steel is the
most ductile, titanium less ductile, followed by aluminium. 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/cm3, aluminium 2.75 grams/cm3.
Contrast these numbers with carbon fibre composite at 1.45 grams/cm3.
Metals are subject to fatigue. With enough use, at high enough loads, metals will
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 fork may be severely bent and the frame
undamaged. Aluminium is less ductile than steel, but you can expect the fork and
frame to be bent or buckled. Hit harder and the top tube may be broken, the down
tube buckled and broken, leaving the head tube and fork separated from the main
triangle.
When a metal bike crashes, you will usually see some evidence of this ductility in
bent, buckled or folded metal. It is now common for the main frame to be made of
metal and the fork of carbon fibre. See Section B, Understanding Composites below.
The relative ductility of metals and the lack of ductility of carbon fibre means that in a
crash scenario you can expect some bending or bucking in the metal but none in the
carbon. Below a given load the carbon fork may be intact even though the frame is
damaged. Above a given load the carbon fork will be completely broken.
The Basics of Metal Fatigue
Common sense tells us that nothing lasts forever. The more something is used, and
the harder it is used, and the worse the conditions in which it is used, 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. A bicycle in a garage does not fatigue.
Fatigue happens only through use. 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.
Parts can be designed with such strength that fatigue life is almost infinite but this
requires a lot of material and a lot of weight. Any structure that needs to 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 bicycle with an infinite fatigue life, it would weigh far
more than any bike sold today. So we all make a trade-off: the wonderful, lightweight
performance we want requires that we inspect the structure.
What to look for:
• ONCE A CRACK 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 a 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 bike, lubricate your bike, protect your bike from
salt and remove any salt as soon as possible.
• STAINS AND DISCOLOURATION CAN OCCUR NEAR A CRACK. Staining may be
a warning sign that a crack exists.
SIMPLE RULE 3: Inspect and investigate any staining to see if it is associated
with a crack.
• SIGNIFICANT SCRATCHES, GOUGES, DENTS OR SCORING CREATE
STARTING POINTS FOR CRACKS. Think about the cut surface as a focal point
for stress (in fact engineers call such areas “stress risers” – areas where the
stress is increased). Perhaps you have seen glass cut? Remember how the
glass was scored and then broke on the scored line.
SIMPLE RULE 4: Do not scratch, gouge or score any surface. If you do, pay
frequent attention to this area or replace the part.
• SOME CRACKS (particularly larger ones) MAY MAKE CREAKING NOISE AS YOU
RIDE. Regard such a noise as a serious warning signal because a well-maintained
bike will be very quiet and free of creaks and squeaks.
SIMPLE RULE 5: Investigate and find the source of any noise. It may not a be
a crack, but whatever is causing the noise should be fixed promptly.
APPENDIX B
