HELP WANTED - how to make d shaped tubing?

**desbromilow** · Jul 26, 2020, 04:29 AM

hopefully someone here knows how to make D cross section tubing for bike frames.

trying to replace frame tubes in a vintage motorcycle which has D shaped tubes in the rear diamond...
am currently squashing circular tube in a press with shaped dies and packed sand in the inside, but the shape is still not right.

also looking for advice on bending the same tube in a tight radius with the centre of turn being on the flat side of the D

thanks,
Des

**olderdan** · Jul 26, 2020, 01:39 PM

Hi Des, I have the same need for something similar for the rear stand on my 1919 V twin AJS so I will be hoping also for ideas.

**tonyfoale** · Jul 28, 2020, 01:37 AM

What about cutting a strip out of the length of some round tube and then welding in a flat strip to get your D.

**desbromilow** · Jul 28, 2020, 02:59 AM

Originally Posted by tonyfoale

What about cutting a strip out of the length of some round tube and then welding in a flat strip to get your D.

Tony,

we have tried that, but the shape isn't a perfect "D" with sharp corners, it's a D shape with rounded corners...in the example we're trying to rebuild, the tube transitions from perfectly round to D shape over a 5" length, and then turns a couple of tight radius turns to step out for the belt rim.

We've been trying to make D shaped tubing by pressing round tube into an offsided ellipse, then a second operation to flatten the offside to create the D... it sorta works, but is nowhere as neat or consistent as the manufacturer managed back in 1909.

We've had plenty of goes at this, and found a number of ways which don't work for various reasons, but finding the way which works consistently is eluding us.
I was starting to look at drawing as an option, but then realised it won't work due to the transition...

Given that bicycle frames were/are using tubes of rounded square, D's, and other shapes, I was hoping someone here had experience in how they are made, or has recreated pieces from round in the past.

Thanks,
Des

**hemmjo** · Jul 28, 2020, 06:14 AM

How long are the pieces you need? I am imagining a set to dies you push the round pipe through. You could have the shape you want cut in a thick piece of steel. You may need more than one die, as the tube would be to be gradulally transformed. You could have the die plates cut from steel plate at a shop with a laser cutter. The cut edges will not be smooth, you would have to use a die grinder to smooth and polish them. Also chamfer the input side to get the pipe started. Then use a hydraulic press to push the pipe through the die with lots of lubrication. If you only need a short section of the D then you could weld the round to it to get the length you need.

I have never done this it is just a thought.

**tonyfoale** · Jul 28, 2020, 07:23 AM

Des,

Is weight an issue?
If not, maybe you could machine them from the solid (CNC perhaps) and then drill out the circular part as deep as possible.
I have made tapered and aerofoil tube but never had cause to make D shaped and bent tube.

**desbromilow** · Jul 29, 2020, 07:57 AM

Originally Posted by tonyfoale

Des,

Is weight an issue?
If not, maybe you could machine them from the solid (CNC perhaps) and then drill out the circular part as deep as possible.
I have made tapered and aerofoil tube but never had cause to make D shaped and bent tube.

Tony,

weight is an issue (within reason) ..

I am curious to your references to making airfoil shaped tubing, and tapered tubing (I assume round, but tapered) - can you describe how you did those?

Thanks,
Des

**Unkle Fuzzy** · Jul 29, 2020, 09:13 AM

If not too large a piece the original may have been hydro-formed in a die.

**desbromilow** · Jul 29, 2020, 05:12 PM

the pieces are typically around 400-700mm in length, about 40-65mm in circumference with a wall thickness of 0.8-2.2mm. If I had a time machine, I'd run back to 1910 and visit a ton of factories just to record how they made certain things....

**Toolmaker51** · Aug 3, 2020, 01:34 AM

The initial D shape I'd begin by drawing through a succession of shaped rollers.
For the record; round, square and rectangular tube is formed out of flat stock.
For the bend, there a friendly tube bending [exhaust] shop nearby. I'm willing to bet a linked mandrel will do this.
Half the bend desired is called 'hard way', where apex meets the flat of D. Hard way refers especially to angle iron, C-channel, flat stock etc, when the bend stretches outside of perpendicular edge.

Homemade tools made with Harbor Freight tools
Post #15 Harbor Freight Bender mod for on-edge [hardway] forming.