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Square tube, or round?

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DavidAger View Drop Down
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    Posted: 20 February 2016 at 3:16pm
Hi all,
Just want to say hello.
I'm a newbe to this forum,club an HPVS, or any forums come to that.

I don't have an HPV as yet but hoping to build one!

I've found some plans on AtomicZombie, the tomahawk (SWB low racer)looks good to me except they use square box tube and I would prefer round.
I'm aware that it will be more difficult, but I think I can cope with that.
My main problem is if I use round tube which would be the best size to replace the 1.5mm box tube?
Any ideas please?

Thanks for reading this, and might see you at the first meet, looking for ideas.

David Ager AKA David

Edited by DavidAger - 20 February 2016 at 3:20pm
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AlanGoodman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlanGoodman Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 5:17pm
Welcome David!
 
There are quite a few builders on here so I'm sure somebody that knows what they are talking about will be along soon... Big smile
 
In the meantime it might be worth grabbing a copy of SYWTB, our guide, the latest edition of which was put together by Mike Burrows.
 
 

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DavidAger View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DavidAger Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 5:29pm
Thank you for the welcome Alan, and the info, I'll look forward to any reply.
David
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 6:19pm
Hi David,

As you are probably aware, tube is stiffer in bending and torsion than square box-section for the same weight, but box-section is easier to mark out, mitre and jig, which is probably why Atomic Zombie use it.

It is difficult to give advice without seeing the design you plan to build. If you post a link then we can have a look. All I can say at this stage is that you will make a lighter bike if you can find 18 swg (1.2mm), which is not always easy in the larger diameters but possible and worth the effort. I've used 1 3/4" diameter by 18 swg ERW. My first bike was 1 1/2" x 18 swg box, which worked fine for a short wheelbase bike.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 6:29pm
Ah, okay, found the Tomahawk. I'd advise building a 'straight-tube' framed bike as a first project, they are much easier to make, lighter and easier to ride, not to mention less daunting to ride in traffic.

A 'Z-frame', like the Tomahawk, has a lot more joints and are much harder to weld straight. I have built a number similar machine with round tube (mostly with 1 3/4 diameter x 20 swg T45 but that is very expensive stuff!). I would certainly recommend adding a gusset around the head-tube to the Tomahawk design as this stiffens the frame considerably.




Edited by GeoffBird - 20 February 2016 at 6:37pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlanGoodman Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 6:46pm
Have you ridden any recumbent bikes so far David?
As Geoff says lowracers are a little more tricky to get on with. Harder to get started on hills, a little awkward at junctions. I wouldn't ride one in heavy traffic myself to be honest.
 
Something like the Atomic Zombie Spirit would be easier to ride while getting the hang of it.. http://www.atomiczombie.com/Spirit%20Short%20Wheelbase%20Racing%20Bike.aspx
 
 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DavidAger Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 6:52pm
Thanks Geoff,
I've included the link but must admit I do wonder if it's a bit low for traffic.
I can get 50mm x 1.2mm erw tube and perhaps other sizes but I'm still in thinking stage at the moment so can do with as much info as poss.
Not looked at your bikes yet but I will shortly.

http://www.atomiczombie.com/TomaHawk%20Short%20Wheelbase%20Recumbent%20Lowracer.aspx

David
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 6:59pm
2" x 18 swg wil make a very stiff frame - my 3rd to 5th bikes used either 50mm x 1mm CDS or 2" x 20 swg T45 (all straight tube designs), but this is very difficult to get now. The 1 3/4" just saves a bit of weight and is stiff enough for most applications.

The other thing that concerns me about the Tomahawk is the position of the chain pulleys up by the head tube - they look like trouser-eaters to me :-) . Note in the video how he is riding bow-legged.


Edited by GeoffBird - 20 February 2016 at 7:01pm
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DavidAger View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DavidAger Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 7:06pm
No I've not ridden one yet. I liked the look of the low racers, perhaps I should leave them for the future as Geoff advised.
I can probably make one from just photos so I'm no restricted to plans.I'll have a look at the links that you and Geoff gave me and go from there, so, if you see some one walking around with a tape measure and pad at Hillingdon in April you'll know it's me, if that's allowed of course.

Thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 20 February 2016 at 7:11pm
Happy to send you scale drawings of some of my bikes if that helps David. I even have CAD models of the more recent ones. The difficult thing to get right is chain runs and chain management in my experience. This is useful link - http://www.framebuilding.com/ - Peter is a very helpful and efficient retailer.
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