Thread: How do you calculate load bearing of metal tubes ??

Originally Posted by Gumnut

Well, using the diagramme above, what we could do is sum the anti-clockwise moments (nil) and clockwise moments, ignoring weight of tube = about 350Nm, and working backwards, and if you wanted to use the sound method described by Jekeslouw, the equivalent load at the end is about 35kg.

Maybe make a prototype, hang half a chaff bag of sand on the end of it, and pick a quiet and quite bumpy road to test it on......

I take it that is using 2g as the force?

I have done exactly these calculations before, I have no idea where they went and I still haven't got around to building that bike rack.

Most people I know used 1" NB pipe which is actually 33.4mm OD. You can step down to 3/4" NB pipe which is 26.7mm OD and commonly comes in wall thickness of:
2.87mm (med wall or SCH40)
3.91mm (SCH80)

My steel catalogues show 25mm OD boiler tube is available with a wall thickness of 2.9mm, ERW tube (electric seam welded) is only available to 2mm wall.

For the medium wall (SCH40) pipe your SMOA (geometric property that is basically resistance to bending) is 1.55e-8 m^4.
Working off a safe maximum stress of 120MPa (good fatigue life even when welded, yield factor of safety roughly 2, breakage factor of safety roughly 3) this pipe can support a bending moment of 140Nm at our chosen stress level.

Your four bikes as a point load are 60kg at 0.25m from the support.
BM = 60x9.81x0.25
BM = 147 Nm.

But that is total and spread over two prongs. Each prong is subject to half that which is roughly 74Nm.

20NB med wall pipe prongs can take a design load of 140Nm (including fatigue, impact loading etc) and only have to take 74Nm.

So yeah the 20NB medium wall pipe is fine and gives you plenty of scope to add padding. Anything bigger and stronger is also perfectly fine. Obviously this analysis is simplified and free, but the results are in line with what has been proven to work.
1" (25.4mm OD) tube may be hard to find with a suitable wall thickness.

I'll be using 1" NB pipe (33.4mm OD) when I get around to building my rack. I need to build a square receiver towbar to fit it first.

would shs be stronger than round for the same OD, wall thickness

Originally Posted by slug_burner

Given that the wheel carrier is made of mild steel and if you are to weld to it or even if you clamped somehow I'd want about the same wall thickness on both pieces.......

Originally Posted by bee utey

Where the 25mm tube is welded to the upright is where trouble will start. The stress concentration at the weld point is the main weakness..........

I was thinking along the lines of sleeving or clamping rather than welding, but I haven't got that far yet.

Originally Posted by Dougal

.........Most people I know used 1" NB pipe which is actually 33.4mm OD. You can step down to 3/4" NB pipe which is 26.7mm OD and commonly comes in wall thickness of:
2.87mm (med wall or SCH40)
3.91mm (SCH80)

The main reason I'm thinking 25.4mm OD is I'm looking at using Thule brand cradles and straps and they fit on 1" (25.4mm) round tube.
Also, Thule make a 'No Sway Cage' which also clamps on the 1" tube and prevents the bikes from swinging back and forth.
It seems that Thule parts are available to purchase as separate items, I'm just waiting to hear back from a couple of places regarding price etc.

Thule Cradle


No sway cage



Thanks for all the replies.

Cheers, Murray

[QUOTE=Dougal;1603340]I have done exactly these calculations before, I have no idea where they went and I still haven't got around to building that bike rack.


Your four bikes as a point load are 60kg at 0.25m from the support.
BM = 60x9.81x0.25
BM = 147 Nm.


Errr,

I do not think that it is appropriate to add up the weights and halve the length.

A more accurate and appropriate method is to work out each bending moment, and sum the moments.

This method would provide for a point load of approx 140kg at 0.25m from support. Whoops, back to year five remedial maths for me - should be half that - approx 70kg, depending on exact placement of clamps.

But maybe things work out different like in NZ......

Originally Posted by uninformed

would shs be stronger than round for the same OD, wall thickness

Yes it is. It is also usually a stronger grade of steel, yeild around 350MPa instead of 200-250MPa for basic pipe.


Murray, I understand the convenience of using the Thule clamps. But I've found things that lumpy often stop you being able to slide a bike on. The standard setup on my mates racks (I'm still using the old hook type) is foam padding to about 2" OD. This fits the biggest variation in bike types. To seperate the bikes and prevent damage I just use split foam pipe insulation cut to 100-200mm lengths on the contact points. It holds itself well on forks, frames and pedals. The bikes are tied back to the main spar of the rack to prevent sway.

Originally Posted by Gumnut

Errr,

I do not think that it is appropriate to add up the weights and halve the length.

A more accurate and appropriate method is to work out each bending moment, and sum the moments.

This method would provide for a point load of approx 140kg at 0.25m from support.

But maybe things work out different like in NZ......

It's correct, here is the full working, but more accurate this time.

4x bikes at 15kg spaced at 50, 200,350 and 500mm from the point of attachment.

Lumped mass (as I did it, but more accurate this time)
60kg centred at (50+200+350+500)/4= 275mm
Moment = 60*9.81*0.275
Moment = 162 Nm.

Sum of individual moments.
Moment = 15*9.81*(0.05+0.2+0.35+0.5)
Moment = 162Nm

Exactly the same result. I'm a mech eng, I get paid to do this for a living. This situation is exactly the same as load centres on forklift prongs. It's the distance to the centre of mass that matters.

Your real moment is going to be less because the bikes will be closer together than that.
Another big consideration is the prong spacing. Some of my mates run theirs at 600mm, which works great for big bikes but I think I'd struggle to get my wife's Intense to sit nicely. I was going to make one at 500mm spacing, the smaller commercial racks are 400-450mm

ok, now that we have the sum of moments sorted.

How did you turn the maximum safe working stress of 120MPa to a bending moment of 140Nm?

Originally Posted by slug_burner

ok, now that we have the sum of moments sorted.

How did you turn the maximum safe working stress of 120MPa to a bending moment of 140Nm?

Stress = Moment*Distance from NA to surface/SMOA