Materials and Structure

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.

From: Bowes, Russell and Suter,
"Mechanics of Engineering Materials," Wiley (1984)

Using this data, the cross-sectional area of the steel is: A = pDt = 1.26 x 10^-4m². Using this area to compute the wall compressive stress gives: s = F/A = (119.8/ 1.26 x 10^-4) Pa = 9.5 x 10⁵ Pa. The yield stress of the carbon steel is about 1.8 x 10⁸ 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.