Street Machine Australia – September 2019

W

HEN engineers talk about depth

of section, we are referring to

the ‘cut-through’ view or section

view of a particular shape or

style of material being used in a construction;

the depth of the section is the longest axis or

side of that shape.

If you pick up a piece of timber stud on a

building site, you can bend it over your knee

if the short side is vertical. But now put the

long side (axis) vertical and all that happens is

that your knee hurts. There’s no bending that

sucker! That’s a simple test that confirms the

importance of depth of section.

The most common structural shape in

the automotive world is the rectangle. Hot

rod chassis are rectangular, and unitary-

construction street machines have rectangular

subframes holding them together too.

Sometimes those rectangular sections are

made from ‘top hats’ spot-welded to floorpans,

where the floor forms the fourth side of the

rectangle. You’ll notice that the long axis of the

rectangle is always vertical, to resist bending

forces as the vehicle operates on the road.

You’ll also notice that the rectangular section

of the chassis or subframe reduces in depth

towards the front and back of the vehicle. That

is because the bending load on the vehicle is

heaviest in the middle and lighter on each end

• just like a bridge.

TABLE 1 : NEUTRAL AXIS

IMAGINE a piece of solid rectangular bar

suspended between two trestles with a load

placed in the middle. A long time ago a clever

scientist realised that when a rectangular

section is subject to a bending moment like

this, the upper fibres are in compression while

the lower fibres see tensile stress. It was

found that there was a neutral point in the

middle of the section where the fibres saw no

stress. This was called the neutral axis (see

Table 1). On either side of the neutral axis,

the fibres see increasing stress the further

from the neutral axis they are.

TABLE 2 : RHS

THEN a clever engineer thought it would be

a good idea to hollow out the rectangle and

get rid of all those fibres that were having a

holiday while the outer ones did all the work.

The result was called a tube! Today we refer

to rectangular tubing as RHS (Rectangular

Hollow Section).

The rectangular tube is a very efficient

structural member because it resists bending

and is good in torsion as well. Take a look at

Table 2. I’ve calculated the bending strength

and weight of some typical RHS sizes to make

the point.

You can see that the 75x50x2mm tube has

70 per cent of the strength of its 3mm brother

and will probably do the job required of it. But

D

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im

FATHOMING THE DEPTHS

DR TIM EXPLAINS HOW DEPTH OF SECTION IS IMPORTANT IN MAXIMISING THE STRENGTH

OF YOUR VEHICLE WHILE MINIMISING WEIGHT

TECH TORQUE

STORY TIM BARTROP PHOTOS SM ARCHIVES