Manual Kona 2017 (page 39 of 42) (English)

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KONA OWNER’S MANUAL

bicycles, steel has been largely replaced by aluminum

and some titanium. The main factor driving this change is

interest by cycling enthusiasts in lighter bicycles.

Properties of Metals

Please understand that there is no simple statement that

can be made that characterizes the use of different met-

als for bicycles. What is true is how the metal chosen 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 rust will attack it. Aluminum and

Titanium quickly develop an oxide lm that protects the

metal from further corrosion. Both are therefore quite

resistant to corrosion. Aluminum is not perfectly corro-

sion resistant, and particular care must be used where it

contacts other metals and galvanic corrosion can occur.

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 aluminum

Metals vary in density. Density is weight per unit of mate-

rial. Steel weighs 7.8 grams/cm3 (grams per cubic cen-

timeter), titanium 4.5 grams/cm3, aluminum 2.75 grams/

cm3. Contrast these numbers with carbon ber composite

at 1.45 grams/cm3.

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. 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 bike crashes, you will usually see some evi-

dence 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 ber. See Section B, Understanding

composites below. The relative ductility of metals and

the lack of ductility of carbon ber 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.

2K17_Kona_Owners_Manual.indd 39 29.1.2016 15.27

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Kona 2017 Manual

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