Introduction: How to Build a Bicycle Frame

By Chronova Engineering
About: I am a postgraduate researcher in Biomedical Engineering at the University of Southampton. I am interested in engineering and science and like making stuff in my free time.

Ever since the Global Cycling Network posted their series on building a bike frame, I was inspired. The process is actually relatively simple – in fact, the most time-consuming part is making the tooling and jigs, although you can buy or hire most of these too. I have made two bicycle frames; the first being a road frame and the second being a gravel frame. This tutorial will cover my process for the latter. At the time of writing, I have ridden a total of approximately 9000 km on my homemade steeds: nothing has broken, and I still love the ride quality.

Why make a frame?

1. Financial value. The cost of the tubing and braze-ons came to £380. The braze costs an estimated £30. There are of course other costs associated with the brazing equipment, paint and materials for jig construction. Including this, the price of making this frame is still less than £600 - and that's for high-end tubing (Columbus Spirit/Life).

2. Custom. I wanted a frame that was fast, racy and stiff, with clearance for large tyres. I had a specific geometry in mind: seat tube length, top tube length, head angle, head tube length and downtube diameter. There is no mass-produced frame on the market that meets my stringent requirements.

3. Enjoyment. There are few things more satisfying than riding a frame made from scratch, by yourself. The process was fun and educational.

This tutorial will focus more on the building process of making a bike frame than the design stages. There's a lot to consider when it comes to frame geometry and so much is personal preference so I recommend you read around about geometry design too.

Supplies

  1. Tubing
  2. Frame jig (can be made)
  3. Mitering templates (printable)
  4. Hole saws (optional)
  5. Hacksaw
  6. Pillar drill / milling machine
  7. Lathe (optional)
  8. IPA
  9. Brazing equipment (brazing torch, oxygen, propane/acetylene and glasses)
  10. Brazing rod (SIF Bronze No.1)
  11. Silver solder
  12. Flux powder (different fluxes for the silver solder and brass braze are required)
  13. Citric acid
  14. Wire brush
  15. Assorted files
  16. Dremel (recommended)
  17. Headtube & seat tube reamers (optional)
  18. Abrasive cloth (P120, or similar)
  19. Paints (primer, topcoat & lacquer)
  20. Airbrush and compressor
  21. Corrosion inhibiting spray (i.e. ProGold)
  22. Protective stickers (optional)
  23. Seat tube spacer (optional)

Step 1: Design Frame Geometry

Before discussing brazing the tubes, it is worthwhile considering geometry. Arguably the most significant parameter determining the way a bike handles is trail. This is the horizontal distance from the steering axis to the point where the tyre contacts the ground. Trail is mainly affected by fork rake and head angle. It is therefore important that the frame is designed to match the forks you intend to use. Due to safety reasons, I advise against making your own forks for a first attempt.

There are several different software packages available for designing your frame geometry.

The one I mainly used was BikeCAD: https://www.bikecad.ca/ , which allows you to create and save designs in your web browser.

A popular alternative is RattleCAD: http://www.rattlecad.com/

If you're not a fan of CAD, there's no reason not to design your frame with pencil and paper too.

Step 2: Buying the Tubes

Once I had decided on the geometry, it was time to decide on tube material and diameter. Steel is an amazing material and, in my opinion, underrated in the bike industry. Many steels have a higher specific stiffness than aluminium and even titanium. Steel has excellent fatigue resistance and a very high tensile strength. I mainly chose a high-grade triple butted, fine grain Niobium doped steel: Columbus Life. I spent ages selecting specific tubes from the plethora of options ‘Ceeway Bike Building Supplies’ offered. My tubes arrived quickly and they even came with a free sticker!

Step 3: Set Up the Jig

This step will be different for every bike and jig, so there's little I can advise on this stage. My own jig was homemade (by my very helpful dad) but you can buy or borrow these too.

Make sure you check key things like wheel clearance at this stage. It's also important if you have curved stays to ensure they curve in the correct plane, leaving a sufficient tyre gap. As shown in the first photo, I machined a heat sink for the seat tube. This prevented excessive warping of the seat tube during brazing (a mistake well learnt from frame 1).

Step 4: Mitering the Tubes

Before you begin mitering the tubes, it's a good idea to mark where the internal butting (the thicker part) ends and base your mitering position on that. It's important to leave some butting at both ends so your frame will be stronger, so try to position your cuts with this in mind. (See diagram)

You can mitre the tubes using either a mill and angled hole saw (as shown in the pictures) or using a file. From experience, I'd say the hole saw approach is faster but I still touched up the mitres with a file anyway, just to get them spot-on. (The mitres don't need to be perfect but the more accurate they are, the easier brazing will be and the joint will be stronger). You can either angle the hole saw or clamp the tube at the correct angle. Regardless of if you are filing or machining, always be sure to support the tube inside a block so it won't be squashed in the vice (see picture 2). I made my blocks by mounting some blocks of nylon to a vertical slide and using a boring bar mounted between centres in a lathe to machine the semi-circular groove of the same radius as the tubing. Alternatively, they can be purchased from Paragon Machine Works (https://www.paragonmachineworks.com/frame-building...

Regardless of the method you choose, it's wise to download and print off a template for the curve, which would be difficult to judge by eye. I recommend this website: https://cq.cx/tubejoin.pl

Step 5: Drill Mounting Holes & Dimple Stays

There are lots of additional components which require mounting holes in your frame; these include bottle cages, bosses for chain stays and cable routing for electronic gears. Remember to drill holes in your headtube and bottom bracket such that no tube is sealed. Air will need to freely flow in and out of the tubes to prevent the build-up of pressure inside the tube during brazing. To allow flow of air into and out of the seat stays, I found it easier to drill small holes in the seat stays themselves, rather than the seat tube. Make sure the holes are big enough to inject the rust inhibitor into the frame later on.

You'll also need to make a slot in the seat tube, with a hole at the end to reduce stress concentration. On my bike, this slot is approximately 30 mm long, 3 mm wide and with a 5 mm hole at the end. I mounted the tube in the milling machine and used a slitting saw to cut the slot but I am sure a hacksaw and file would suffice.

At this stage you can dimple the stays if more tyre/chainring clearance is needed. I did this by rounding the end of a piece of steel and using this as a punch. It is possible to squash or dimple the stays after brazing but it is easier to do at this stage.

Step 6: Brazing

Clean surfaces are essential to producing a nicely brazed joint. First, remove oil from the surfaces around the joint, including inside the tubes, using a combination of sandpaper and isopropyl alcohol.

Prepare your brazing gear. I used SIF Bronze No.1, flux and oxy-propane but with an oxy-acetylene torch (number 7 nozzle). I highly recommend using shade 3 welding glasses: it makes it a lot easier to see. You will need oxygen support for brass brazing; don't bother with MAPP gas unless silver brazing. There are a few companies such as ‘Hobbyweld’ that sell rent free oxygen. I should mention, if you're using stainless steel tubes, TIG welding or silver brazing are the only options.

Next up, practice! This can't be overstated. I advise buying a couple of spare tubes made from a cheaper steel to get used to the brazing process before you jump into the final frame.

When you're ready to braze, follow these steps:

  1. Mix up some flux paste (flux + water) and apply it liberally around the joints.
  2. Gently heat the braze, then dip it into the flux powder if more flux is required.
  3. Tack each joint first, adding a spot of braze to the top and bottom of each joint. Then, once these are all done, do the same on the left and right. (Remember the braze shrinks when it cools, so try to maintain symmetry.)
  4. Clean the tack brazes before reapplying the flux paste for the fillet brazing. The cleaning process I used is explained in a later step in this tutorial.
  5. Optional: remove the frame from the jig. You may also need to add heat sinks to the bottom bracket and seat tube.
  6. Braze around each joint smoothly and quickly to avoid overheating and warping the frame. Remember to keep switching sides when brazing a joint to reduce warping.

Try to think about how the joints will deform when cooling and braze the joints in such an order as to minimise warping; this is called 'the braze sequence'. For example, try to avoid brazing all the joints around the bottom bracket area before moving onto the other joints.

Depending on the exact type of frame you are making or the design you your jig, there is no hard and fast rule on the braze sequence. I brazed the dropouts, chainstays and bottom bracket first so I could then easily mount the subassembly in the milling machine to mitre the flat mount brake-bosses, see pictures 7 and 8. (Dimensions for the position of the bosses are accessible online).

Step 7: Silver Solder Smaller Fittings or Stainless Steel

For smaller components, the lower temperature of silver brazing can be advantageous. As previously mentioned, this is likely your only option for stainless parts too. You might need to get imaginative about ways of clamping fittings in the right place on the tube. However you do it, only apply a light clamping pressure otherwise the area might deform when heated.

I recommend the Silver-Flo 55 Brazing alloy https://www.axminster.co.uk/silver-flo-55-silver-b...

If you're adding bottlecage mounts, attaching reinforement pads made from some of your earlier offcuts is also a good idea to reduce the stress on the tube when bouncing over pot holes while carrying a heavy water bottle.

Step 8: Flux Removal & Cleaning

Post brazing, the flux had become hard and very difficult to remove without damaging the tubes. In the end, I used boiling concentrated citric acid to dissolve the flux off. Scrub it with a wire brush to remove all of the flux.

Before continuing, you'll need to thoroughly rinse the frame with clean water, ensuring all of the acid is washed away. This is very important because the acid encourages corrosion.

Step 9: File & Polish

Tidy up all of your joints by filing down the excess braze. Start with a medium sized file, being sure to abrade only the braze, avoiding the actual tubes. Rifler files or a Dremel are handy for the fiddly bits, where multiple tubes meet.

Follow the files with some abrasive cloth or sandpaper wrapped around a stick.

Step 10: Boring Headtube (optional)

You may need to enlarge the inner diameter of the headtube. I mounted the frame in the milling machine to bore the headtube to the correct size to fit a headset but you could also achieve this by buying a reamer of the correct size.

Step 11: Painting

It is possible to paint a frame in one weekend, especially if the weather is warm. I bent a hook from an old spoke and hung my frame on the washing line. Before anyone goes hanging their pride and joy on the washing line though, please check the line doesn’t need replacing: another lesson well learnt…

I used a small airbrush from I bought from Amazon; it was slow going but possible to paint a frame with it.

Regarding the lacquer, 2-part types are more durable, but most contain highly dangerous isocyanates. As such, I used Jawel's 2K Non-Isocyanate lacquer: https://www.jawel.co.uk/index.php?main_page=index&...

To paint the frame:

  1. Clean the entire frame with IPA.
  2. Apply the primer.
  3. Apply several thin layers of topcoat.
  4. Add any decals to the frame.
  5. Apply the lacquer (I applied 2 coats).
  6. If you are more patient than I am, I recommend waiting 1 week for the lacquer to harden before riding. (Reduces scuffs and damage.)

If you aren't keen on the hassle of airbrushing, there are other options. These include Spray.Bike or outsourcing the painting to a professional.

Step 12: Finishing Touches

There are just a few things left to do before assembling your new bike:

  1. Spray the inside of the tubes with a corrosion protection spray, such as this one: https://www.condorcycles.com/products/progold-ste...
  2. Leave the spray to cure for a day or two, as it tends to dribble out of the frame until it's fully dried.
  3. Add protective stickers to the chainstay and various other damage-prone areas to prevent damage to the paint.
  4. Optional: You may need a spacer for your seat tube if there's a large difference between the tube and seat post diameters. I machined mine on my lathe but they can be purchased online.

Step 13: Assemble and Ride!

Build up your bike then go out for a ride! Enjoy!

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