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Discussion Starter · #1 ·
After some advice from the tubing experts. I'm planning on building a welded steel hardtail MTB for pump / BMX track use. I want to make it in the style of an old BMX a Torker or SE look, with twin top tubes that then flow through to be the seat stays, formed in a loop at the drop-outs and then become chain stays. I've not figured out what tubing the rest of the frame will be made from, but figure the twin tubes will be that hardest parts to work out. Anybody got any advice to offer, I'm fairly clueless at this point, I've built a good few lugged road frames, and a few aluminium MTB's, but quite new to steel. (my next questions will be about down and seat tubes!!!)

Cheers.

This is an idea of what i'm vaguely aiming for looks wise, but in 26". Tire Bicycle tire Wheel Bicycle frame Bicycle wheel rim

https://uploads.tapatalk-cdn.com/20150711/1b6d5cf00453aa5d4c303e8ed54ccbef.jpg
 

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Discussion Starter · #3 ·
Thanks, I’d been looking at that as a strong contender.

The BMX’s I’d seen had 14(ish)mm toptube/seatstays and 28.6mm seat tubes. So going to 19mm will keep roughly the correct proportions to the 34.9 seat tube (I want to be able to use a 31.6 dropper).

Can anyone tell me how to calculate if it’s theoretically possible to bend a 3/4 0.035” tube to the angle or radius I need? If it’s not, I may have to fabricate a drop-out to mimic the looptail look I’m hoping for.
 

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Thats a very tight bend for 3/4" tube.

You might be better to shape a piece of plate steel as your drop-out and do conventional separate C/S and S/S. Use the shaping to create the curve. It's also a stronger form this way.

Eric
 

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Discussion Starter · #5 ·
I’ll get some tube and experiment. If it doesn’t work, I’ll probably machine a drop-out that incorporates the loop shape, and some sort of adjuster for chain tension.

Cheers.
 

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Product White Line Bicycle frame Triangle


While this is not a frame of the style you are wanting to construct (Gravel Bike) it is similar in concept to what you are considering. This one has a 35mm seat tube that reduces down to 28.6 for a 27.2mm stem. The seat tube at these diameters are plenty stiff enough, especially due to the short length to the intersecting stays/top tube. The downtube used here is a 35mm Aero profile, and again, plenty with an intersecting tube to shorten the effective flexing length. Note that comment with truss frames, flexible lengths. In your case you will not have any, so if your frame is to have a full span length and you want to have next to zero front end movement to rear axle, then I suggest you use 38mm or more. The most noteable feature of twin top tubed bikes is that they can be more 'whippy' when using small dia tubes. This frame has 12mm Stays/TT's, to help with a bit of suppleness over gravel. In your case, using 3/4" (19.5mm) will substantially address that issue. It will look 'Meaty'. A subtle bridge placed between the TT section of tubes will help tie the 2 tubes from flex as well.

By 'Whippy', I refer to riding the bike slowly in a straight line and giving the handlebars a quick sharp left/right movement and observing the instant out of plain movement and seeing how quickly the frame snaps back into line. A flexy frame will noodle this action, a stiff frame will not respond at all. This may be of value to you for your bike's end purpose.

Eric
 

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Discussion Starter · #8 ·
Thanks for the input Eric, I totally understand you on the whippy nature of frames. I don’t really ride pumptracks and jumps myself, so am not really sure how much flex will be required, thinking about it, I presume stiffer to be better, so that most rider input goes to moving the bike, not deforming it. I shall have a look at bigger diameter downtubes, I’ve no idea what’s available, or in what wall thickness?

As for seattube I was thinking of a Columbus Zona external butted. It’s 0.7/0.5/0.9 and will accept a 31.6 post. It’s quite thin walled, but will only be maybe 14/15” long.

I was planning on bridging the top tubes at the midpoint with a very short tube, and again at the seattube intersection with some sort of flat plate.
 

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While I've never built a BMX bike, I was under the impression that they're almost always built with thicker walled stuff. Like .9mm wall or thicker almost everywhere. I'm wary of that Columbus seat tube, but maybe I don't need to be. I'd suggest the Vari-Wall seat tube, but they're out of stock yet again (don't hold your breath on that one). The other option is to weld a collar to the top of some 1.375 straight gauge .035" wall 4130. The collars come in both 30.9 and 31.6 internal diameters.

But like I said, I've never built a BMX bike, so take that with a grain of salt.
 

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Discussion Starter · #10 ·
Again, thanks for the input. It’s all greatly appreciated. It’s definitely a mountain bike, just inspired by that old school BMX look.

Wary of the Columbus tube due to its wall thickness?
 

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My experience of 35mm seat tubing is that it is much more resilient to flex over 28.6mm that wall thickness is pretty much irrelevant. I like the bigger diameter, it improves the feel of the bike I general IMO. If the Columbus tube fits what you want, go for it. The bike I illustrated has a Nova 35mm Down tube, but is sleeved. If you have the access to machining or can get a machined sleeve made and are confident in brazing it in, fine. I’m o/k with doing odd parts but that may not suit someone else though.

Do what you have the skills to do.

Eric
 

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since it's for a MTBMX, it will have a much lower BB drop. would that make the bend not so tight?
The BB drop is not really the problem. It is the radius used to achieve that look that is very difficult to do in 4130 tube with a 3/4" dia. The tube is likely to collapse/kink/wrinkle when bending is attempted.

Eric
 

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Discussion Starter · #14 ·
I’ve not made many bike frames, but plenty of various types of fabrication and tons of machining over the years. I can see that the Columbus tube isn’t perfect, but I think the pro’s of it being 1 piece, outweigh the cons of joining together 2 tubes, or soldering a sleeve in. Still, I’ll mull it over a while longer, best to have all my ducks in a row before I start shooting!

I’ve drawn some parts of the frame out in full scale to check tyre clearance etc,, with 2 x 19mm stays, clearance for some kind of brace, and a 33mm seat tube. It’s a very very wide frame at the seat stay cluster.

I’m thinking of notching the inside of the twin top tubes to try and loose some width, nobody wants to bang the inside of their knees!!!
 

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Looptail and twin top tubes, dope. Most bends I've seen that tight are done on something stout like a Di-Acro. Falconer recently did a looptail that is similar to what you want. Hunter is another builder who does similar loops at the seatstay wishbone. They both have Flickr accounts with pictures of their processes, worth spending some time there.
 

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Definitely consider the 35mm seat tube with a topper. It makes for a very clean set up and if you use the relieved paragon topper it takes very little clean up post weld. They also provide a great landing for the seat stays

Tool Metal Steel Tool accessory


Hardwood Metal Cylinder Publication Wood stain


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Discussion Starter · #19 ·
Ah, nice. So I could use some 1.375” x 0.035” tube with that welded to the top. Thanks. Or maybe not thanks, now I have even more options to decide from!! Ha ha!
 

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Knee clearance is an issue.

As you will be bending at the seat tube, there is the need to make an 'adjustment' in this area to make all the connections to come together. As you are thinking of using a plate to connect the seat tube instead of direct attachment, the flow will look good from the side view, but you will be wide at the 2/3 point for the knee. About 50mm at that point is good. You will have to draw it out as a top view and see what it will come out at. If you connect directly to the seat tube, you should be ok. With the plate, well.....

Thanks for liking those Chainstays. They are cool and fun to make.

Eric
 
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