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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.

Figure 8: Diagram made from CAD software showing the location of the main triangle, rear triangle and stress points. The stress points are highest at the vertices of the frame while the lowest are the centers of each triangle side.
Bike frame showing the location of the main triangle (located towards the front of the bicycle) and the rear triangle (located near the rear of the bicycle) and their stress points. The stress points are highest at the vertices of the frame while the lowest are the centers of each triangle side. 

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

  • Stress - internal distribution of forces within a body.
Figure 9: Effects of stress on a material.
Stress diagram showing the relationship between internal forces (F) of the frame it is experiencing and its cross sectional area (A).
Stress , σ = Force Cross - Sectional   Area = F A o
  • Strain - the deformation of a body caused by an applied stress.
Figure 10: Effects of strain on a material
When stress continues, the pressure causes the frame to stretch, causing strain. L0 is the original length of a bar being stretched. L is its length after it’s been stretched. Δl is the extension of the bar, the difference between these two lengths.
Stress , ε = Elongation Original   Length = Δ L L o = σ E = stress Young ' s   modulous

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

Young ' s   Modulus ,   E = stress strain = 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.

Figure 11: Diagram of the different thicknesses in butting techniques of a straight gauge, single butted, double butted, and triple butted frame butting. Notice the thickness of the tube in the center and at the end points.
Straight gauge butting has the same amount of wall thickness throughout the tube. Single butted has one end thicker than the rest of its tubing. Double butted has both ends thicker than the rest of the tubing. Triple butted has step like thickness at the ends and its walls are thinner towards the middle.

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.

Figure 12: Bike frame diagram showing the different sizes from A to H, a good ratio to best suit your anatomy.
Frame diagram: A is the angle between the fork rake and the wheelbase. B is the seat tube angle of the rear triangle. C is the head tube length. D is the top tube length.  E is the chain stay length. F is the bottom bracket. G is the offset. H is the wheelbase.

Bike Fit Measurements

Figure shows a slight bend in the knee at the bottom of the pedal stroke, displaying the Saddle Height bike fit measurement.

Saddle Height

Figure shows a flat or gently curved back with a slight bend in the elbow and knees in-lined with the pedal spindle, displaying the reach and set back bike fit measurement.

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.