Lighter, stronger and cheaper?

Joined
Jun 22, 2022
Messages
4
Being in the land down under, 1.5" SHS is very hard to come by ... so I've looked at the metric equivalent.

40mm x 1.6mm is a little heavier, so I started looking at alternatives.

I used BeamBoy (see image below) to compare the strength (resistance to bending) and came up with this table:



50mm x 25mm RHS seems to be a better option to me ... it's 3.3% lighter than 1.5" SHS of a similar wall thickness, but bends about 30% less.

More significantly for me, it's 7% lighter than 40mm SHS while bending 18% less.

Because SHS/RHS pricing is related to weight, it's cheaper too ... probably saving about $2 ... woo hoo!

The only downsides I can think of are:
  • looks ... would rectangular tubing look as nice as the square stuff?
  • horizontal bending ... I've assume vertical forces are significantly greater than horizontal forces (the front boom being an exception here ... RHS on it's side?)


BeamBoy calculations for 16 gauge 1.5" SHS:

 
Joined
May 31, 2013
Messages
3,624
Location
South Benfleet, Essex, England, UK
You are not alone and non-square steel box has been used before to good effect.
Go for it! :)
 
Joined
Oct 19, 2012
Messages
2,008
Location
Wakefield, UK
I've used 40x20 to make a few machines. The bulk of any force is vertical so having rectangular makes sense. Like yourself I looked at beam deflections before choosing.



 
Joined
Jun 22, 2022
Messages
4
Thanks guys, good to see that the theory works in practice (Albert Einstein: "in theory, theory and practice are the same ... in practice, they are not").

Good to see some examples too ... very nice!
 
Joined
Jun 22, 2022
Messages
4
@maddox ... interesting. Is it enough to be annoying / concerning?

I've been meaning to use my bathroom scales to see how hard I can push, and then use BeamBoy to calculate front beam deflection for various options (including 50x25 on it's side).

Some quick Googling came across an article comparing upright vs recumbent pros and cons when cycling uphill. It included a chart (below).

A 200N-m moment at the end of an 800mm long front boom of 50x25x1.6 RHS, according to BeamBoy, would deflect by 12mm each way if upright, but only 4mm each way if flat. For comparison purposes, a 40x1.6 SHS boom comes in at 5mm and a 1.5" #16 at 6mm.

Now, I'm not sure I've modelled this correctly, so do your own research, but I'd say the relationship between them would be pretty good (i.e. an upright 50x25 RHS would deflect about twice as much as the 1.5" SHS, but the flat 50x25 RHS would be stiffer than the 1.5").


[From: https://kraatsgb.nl/recumbent/pedal.html]
 
Joined
Jan 16, 2011
Messages
594
Location
Vilvoorde / Flanders / Belgium
It annoys me a bit, as it will fatigue the aluminium of the expensive commercial frame of Grey Ghost.
But any calculation shows it will be a few decades before I will break something.
 
Joined
Oct 19, 2012
Messages
2,008
Location
Wakefield, UK
I simply turned up the preload on the suspension units until my power strokes were insufficient to compress the suspension. I lost a little in comfort but it was just that - a little. The force my legs supply is much less that that of a pothole. For sure it's not an ideal place to put the suspension but I liked it sufficiently from an aesthetic view to compromise slightly on the engineering side.
 
Joined
Oct 20, 2022
Messages
9
I simply turned up the preload on the suspension units until my power strokes were insufficient to compress the suspension. I lost a little in comfort but it was just that - a little. The force my legs supply is much less that that of a pothole. For sure it's not an ideal place to put the suspension but I liked it sufficiently from an aesthetic view to compromise slightly on the engineering side.
it is really pretty.
 
Top