# Homework Help: Working out the apropriate i.d. of a hollow tube

1. Apr 7, 2012

### JJParr

Hi,

Ive just joined as ive spent the last few months falling onto this site to try and help me in various bits and peices. I am however stuck on something.

For part of my dissertation (automotive) I am introducing a brace into a subframe of a competing drift car. To save weight this is brace is going to be a diagonal hollow tube from one corner to the other. (the current subframe setup is basicly a square).

I need to justify the size of the tube I use including wall thickness. The tube will be mainly loaded in tension and compression which is what I will be focusing on.

Whats known/fixed

Length
Material/material properties of low carbon steel

I am worried that I am overcomplecating things in my head. Is it just a simple pressure = force/area for a solid bar, working that out and then trying to get that strength out of a tube? Everything I search for seems to come up with torsional loading or bending.

Any help will be hugely apreciated.. Not necciserally an answer, just a point in the right direction.

2. Apr 7, 2012

### Mindscrape

I'm not super mechanically savvy, but I think you could start with young's modulus, bulk modulus, and shear modulus to justify your bracing choice.

3. Apr 7, 2012

### engineerme

(FOR HOLLOW PIPE)

if you have modulus of elasticity for that particular steel you can work out the extension:

extension= (Force x Length)/(Material area x modulus of elasticity)

(material area= total area-void area)

you will have to play around with the dia. and void area (area of inside tube) for wall thickness, to show different extensions. Works well if you use metric system, plug it all in meters.

We got for 50kn extension force, 75mm dia tube, void area = 60mm, Mod of E = 100Gpa, resulted in an extension of 0.188mm.

Material area = 1.59x10-3m2 (total area-void area)

e=50x103N x 0.6m/ 1.59x10-3m2 x 100Gpa

e= 188.7x10-6m or 0.188x10-3m or 0.188mm

This works for pipes, but not sure for other shapes. Hope this helps

Dunk

Last edited: Apr 7, 2012