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Frames

Frame Design

The Frame is the Foundation of the Bike

Modern bicycles utilize a diamond shaped frame composed of two triangles which allow the frame to be strong, stiff, and light weight.  Frame designers utilize CAD software to analyze bicycle frames. In this example, stress levels are shown by colors – blue is the area of least stress and orange is the area of most stress.
Example of CAD software used to analyze bike frames, the main triangle, rear triangle and stress points. 

A materials strength relates to its behavior when subject to stresses and strains

  • Stress - internal distribution of forces within a body.  Stress diagram showing where variables, F and Ao. F comes out from the sides, and Ao is the middle
S t r e s s ,   σ = F o r c e C r o s s - S e c t i o n a l   A r e a = F A o

  • Strain - the deformation of a body caused by an applied stress. 
Strain diagram showing variables 
S t r a i n ,   ε = E l o n g a t i o n O r i g i n a l   L e n g t h = Δ L L o = σ E = s t r e s s Y o u n g ' s   m o d u l u s

Young's Modulus is a measurement of the stiffness of a solid material.  

Y o u n g ' s   M o d u l u s , E = s t r e s s s t r a i n = σ ε

Frame Butting

Metallurgy of the tubes, wall thickness, cross-sectional shape, and size all influence performance. The goal of designing a good bike frame is to put the appropriate amount of material where the bike frame undergoes the most stress—at each end of the tubes. This process is known as butting. The result is a tube with a thin-walled central section and thicker-walled ends.

    frame butting techiques

Different butting techniques:  Notice the thickness of the tube in the center and at the end points.

Frame Geometry

Fitting a Bike to be an Extension of Your Body
The length of the tubes and the angles at which they are attached determine bike size and fit. Each person has a unique set of measurements—a bike must fit the upper and lower body geometry to be a good fit.  To use your muscles effectively, a bike must properly fit your anatomy.
Bicycle frame diagram   

Bike Fit Measurements

   Person demonstrating the saddle height 

Saddle Height

Person demonstrating the reach and setback

Reach & Setback

A few of the key measurements for bike fit include:

  • Saddle Height:   The saddle should be positioned such that there is a slight bend in your knee at the bottom of your pedal stroke. 
  • Reach:   The handlebar should be positioned such that there is a slight bend in your elbow and your back is flat or gently curved.  
  • Setback: When the pedals are level, your knees should be in line with the pedal spindle. 

Frame Composition

Using the Properties of Elements to Construct Better Frames
Composition: The elements used to create a bike’s frame affects its weight, cost of production and vibration damping.

What is your bike's frame made of? 
Most are comprised of metallic alloys, or combinations of different elements. These alloys are often stronger, lighter, and more durable than pure metal elements.Three of the most common materials used for bike frames are STEEL, ALUMINUM, and CARBON       
 
Chromoly steel frame

Chromoly Steel Frame

Iron - 97.6%  
Chromium - 1%  
Manganese - 0.5%  
Carbon - 0.3% 

Pros:

  • Strong, proven material 
  • Easy to cut, bend and weld 
  • Takes severe impacts without cracking 
  • Easily repaired 
  • Least expensiv e 

Cons:

  • Heavy 
  • Corrodes easily 
Aluminum frame

Aluminum Frame

Aluminum - 97.3% 
Magnesium - 1% 
Silicon - 0.6% 
Copper - 0.3% 
Iron - 0.3%

Pros:

  • St rong, low density material 
  • Weighs less 
  • Preferred for competitive frames 
  • Excellent corrosion resistance 

Cons:

  • Stiff, creating an uncomfortable ride 
  • Metal fatigue affects reliability 
  • Skilled welders needed for repair 
  • More expensive than steel 
Carbon fiber Frame

Carbon Fiber Frame

Carbon Fiber and Polymer Mix: The properties of carbon fiber-reinforced polymers vary according to weave pattern, fiber density and matrix polymer.

Pros:

  • Non-metallic composite  
  • Stronger than steel, lighter than aluminum 
  • Resistant to corrosion 
  • Absence of design limitations 

Cons:

  • Will crack in severe impact 
  • Costly, much more than aluminum  
  • Very difficult to repair