Thick walled vs solid

**mattsdragons** · 27th November 2013, 13:16

Trying to figure out new axels for BB but this is also a general point

I will use my calculations as an example

I have 2 options:
20mm SOLID Stainless Steel round
OR
20mm x 5mm WALL Steel round

Obviously 20mm solid stainless will be strong but when we are getting into 5mm wall at quite a small radius which is also slightly lighter, which would you pick?

**maddox10** · 27th November 2013, 14:44

The hollow one will be stiffer than the solid one.

**Eventorizon** · 27th November 2013, 17:54

Originally Posted by maddox10

The hollow one will be stiffer than the solid one.

How can that be true?

**geeza** · 27th November 2013, 15:25

If you are using tube straight of the shelf it will not be as round as bar will be so you may run into trouble with your bearings. But I agree with the above

**mattsdragons** · 27th November 2013, 15:52

Only be shortish sections bought from a local engineering firm so hopefully I wouldn't run into too many issues

**Liftoff** · 27th November 2013, 19:24

Diameter for diameter the solid shaft is stronger than the hollow tube....marginally.
But weight for weight the hollow tube is way much stronger, so if say you increased the diameter of the tube slightly to 25mm you could get the same torsional rigidity of the 20mm shaft for half the weight.
The central part of the shaft does not carry the same load as the outer surface, much less infact, thats why you're better off with a tube

**Eventorizon** · 27th November 2013, 21:34

I understand and totally agree with you. The original statement is incorrect: a hollow tube is not stiffer than a solid one. But since he needs the weight saving it is better to go hollow.