main tube choice for a fat bloke

[bambuko]

I am still searching local dumps for a suitable donor ...
so in the meantime I've been pondering the choice of the main tube for a fat bloke like myself.
I weigh 115 kg (253 lbs) and I am concerned about it collapsing under my fat a**e :cheesy:

In the manual Brad advises:
...If you are a heavy rider (over 250 pounds), then you should ask for the next size after that, which will likely be 14 gage tubing, or .078 inch wall tubing...

In another thread (http://forum.atomiczombie.com/showthread.php?t=3110), Brad says:
...I would be highly concerned about the frame using round tubing without suspension with any rider...
OK, so my idea of building a Voyager without suspension is a No, No and I respect this advice.

That leaves me with the choice of the tube - Brad again:
...Square tubing is so much stronger for this kind of thing...
well, it is and it isn't :rolleyes4:

Here some ERW tubes compared:
http://lh6.ggpht.com/_ozT1G3WqXg4/StIjVYdyDVI/AAAAAAAAIUw/wCaGO741hDQ/s144/tube%20choice.jpg (http://lh6.ggpht.com/_ozT1G3WqXg4/StIjVYdyDVI/AAAAAAAAIUw/wCaGO741hDQ/s800/tube%20choice.jpg)

Now, I am not a**l about building light, because as far as I am concerned, it is much easier and efficient to loose some weight myself :devil:, than to try and build the bike very light (using space age materials), but there is no point selecting "wrong" tubes - light, but stiff bike means thin wall, large diameter tubes, rather than small diameter with thick walls.

And as for square ve round :cheesy: ...
1 3/4" square tube will be definitely stiffer (and easier for diy guy) than 1 3/4" round tube of the same wall thickness, but it will also be heavier (but not enough to concern us).

I know, that most of you prefer to build it and see if it bends :) so this table might not appeal, but at least it got things clear in my mind - I am ordering 2" round tube with 1.5 mm wall.

ps apologies to all of you imperially minded for using mostly metric system.

[PeterT]

[QUOTE=bambuko;

ps apologies to all of you imperially minded for using mostly metric system.[/QUOTE]

No need to apologise for those trapped in imperial measurements, it's not their fault their Governments are so archaic, well actually it is, cos they voted them in :jester:

Waiting to see how your ride progresses, and remember to post lots of pics :cheesy:

PeterT

[John Lewis]

I came from the imperial era. Had it bashed into me at school.:jester:
Didn't do much good when we changed to metric. I'm at home with both.

I think we often tend to overbuild as we are not structural engineers. Had a mate designed and and built a plane. Later structural analysis showed it good to 12g. Way more than needed so he could have saved some weight. Still better to be safe than sorry I think.

I used round muffler tube on my SWB and a tadpole trike and it is plenty strong. I used 40 mm x 1.6mm on the Marauder and the Wolf. My really heavy 250+ lb mate has tried them and they didn't complain even though he bottomed the suspension on the Marauder.

John Lewis

[Freth]

To be honest ... a 1 3/4" chromo tube in 0.49 thickness will allow you to build a main frame without the suspension and will adequately handle about 300 lbs. The chromo allows some flex for riding comfort ... but requires TIG welding.

[darwin-t]

I'm building one without suspension using parts from an earlier failed design using 1" x 2" 14 gauge square tubing and weigh 205 lbs. Am I wasting my time?

The tubing is sitting vertically (2" tall and 1" wide). The seat is also very near the rear wheel, so the bike is quite a bit shorter than the Voyageur. The main tube is 45" long and the wheelbase is 63".

I can't feel and flex in the frame if I bounce up and down on the seat.

thanks.

[bambuko]

thanks guys for your comments,

Peter, John,
Like yourself I am not bothered about imperial ve metric issue (I have spent all my working life using both), but I know that for some people, metric is a step too far, hence my apology - not for using metric, but for not providing info in both type of units.

John,
You are right, that we tend to over-engineer. After all, if it hasn't failed, it probably means it could have been made lighter :1eye:.
But I would be weary about making my judgements based on my mate (however fat, ugly and heavy :) ) testing something for five minutes.
My current ride is bog standard (not strengthened) and it flexes a lot, but it hasn't failed ... yet.

Freth,
You are of course right about using either 4130 (or here in UK T45), but they both require TIG or gas welder and possibly post-treatment.
I have made conscious decision to stick to MIG and ERW.

darwin-t
My particular concern over tube choice is related to LWB recumbents designed like Voyager (or Long-Easy from Bentech (http://www.bentechbikes.com/)), specifically because of long single tube design. What makes them beautifully simple, makes them also vulnerable to fatsos like myself :rolleyes4:
I think your design is possibly? over-engineered :sleep1: but I shall leave it to practitioners (rather than theoreticians, like myself) to make comments.
I think you want some flex in a frame, so making it too rigid, without suspension might make a ride harsh?
But we will not know, until you finish and report to us (please)

Anyway, all this still remains theory, because my hunt for suitable donor is not very successful so far - I live in rural (and not really very wealthy) area, so people don't discard things the way they might in the big town :(

[likebikes]

I built out of 1x2 inch 14ga rectangular tubing and it's too heavy for me at 190 but it still flexes over rough roads and has held up very well. I'm currently building with the 16ga recommended by Brad on the Hi Roller but I was tempted to go with 16ga 1x2 on this project. I think asthetically, it looks better and with vertical loading, it should be very strong.
I hate to disagree with the master but I read that round tubing has it all over square strength wise. I'm trying to find the article and I'll post it up.

[bambuko]

...round tubing has it all over square strength wise...
that's simple :)
Given the same material, same tube length you can compare for example:
1.5"(38.1mm) square tube with 16 swg (.064" or 1.626mm) wall will have exactly the same second moment of inertia as 45.2mm round tube with the same wall thickness.
Nearest commercially available tube would be 1.75" (44.45mm).
Or to put it simply - 1.5" square tube will be as stiff as 1.75" round tube (both 16swg)
The only difference is, that round tube would be about 7%-8% lighter.

However ...
unless you are building no-compromise racing machine (and we are not, are we? :rolleyes4: ) practical considerations (ie ease of building) are more important.

Plus overall weight is much more than just the tube choice.

[bambuko]

...I'm trying to find the article and I'll post it up...
Found it for you :cheesy:
how about this one:
http://www.ihpva.org/HParchive/PDF/45-v13n2-1998.pdf
"The Stiffness of Tubes of Various Configurations and Materials" by John Zabriskie

[likebikes]

You're certainly right about sq tubing being the easiest to work with but with time and hopefully my skills improving.... (not happening fast enough! ) I'd like to work with the more asthetically pleasing shapes and techniques.