
Tube
selection
Selection
of a section for the structural tubing of the frame
is frequently determined by the type of stock manufactured and the design requirement
that the material remain in its elastic range during normal use. The
length of the tube is determined by the frame geometry, the diameter, D, and
wall thickness, t, by the elastic demands and availability.
For the
seat tube, the load is compressive with a value of 119.8 N for the case we are
considering. The steel tube is typically 28.6 mm in diameter with a wall thickness
of 1.4 mm. 













Using
this data, the crosssectional area of the steel is: A = pDt
= 1.26 x 10^{4}m^{2}. Using this area to compute the
wall compressive stress gives: s = F/A
= (119.8/ 1.26 x 10^{4}) Pa = 9.5 x 10^{5} Pa. The yield stress of
the carbon steel is about 1.8 x 10^{8} Pa, and it is seen that the tube is well within
its elastic range with (s/s_{y}) = 5.3 x 10^{3}.
If the unloaded tube
length is 572 mm, the strain at the load postulated is e = s/E = 4.5 x 10^{3}.
The change in length of the tube due to the load is then: DL = Le = 2.6
mm, a reasonable
design value. A thinner walled tube could be selected but it would be
more susceptible to denting. 

