CITY MODEL 1 • VERSION 1
Metals vary in density. Density is weight per unit of material. Steel weighs 7.8 grams/cm3
(grams per cubic centimeter), titanium 4.5 grams/cm3, aluminum 2.75 grams/cm3.
Contrast these numbers with carbon fiber composite at 1.45 grams/cm3.
Metals are subject to fatigue. With enough cycles of use, at high enough loads, metals will
ventually 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 Ebike. You cannot and will not stay on the Ebike, 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 Ebike, the steel fork may be severely bent and the frame undamaged. Aluminum 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 in tension and the down tube 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 Ebike 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 fiber.
See Section B, Understanding composites below. The relative ductility of metals and the lack of
ductility of carbon fiber means that in a crash scenario you can expect some bending or
bucking in the metal but none in the carbon. Below some load the carbon fork may be intact
even though the frame is damaged. Above some load the carbon fork will be completely broken.
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. An Ebike 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 be-
comes 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 immedi-
ate failure of the part.
