What's this?

There are three programs you can run here-- framebuild.py, copecalc.py and riderfit.py.

How to run them

You need Python, specifically Python 3 (any version that starts with a 3 is fine, e.g. 3.7.1 is good) which is freely available for all widely used operating systems. You also need a rough idea how to run programs from the "command line" or "command prompt". You will also need to be able to open and print out .png files if you want to use the mitre templates.

copecalc.py

This generates mitre templates, similar to this online tool.

The idea is you generate a template for the join you're trying to make, print it out, cut it out with scissors, wrap it around the tube and trace the curved edge onto the tube with a pen or something. Then cut/sand/file the tube to that line.

This version can join round tubes to round tubes and also elliptical to round. The elliptical tube can come out at any angle but it has to have either its semi-major axis or its semi-minor axis aligned with the parent tube. This is all you need if what you're making is fairly symmetrical. This can be useful for chainstays if they're elliptical (or just generally squashed, which is close enough to ellipitcal) at the front.

It can also do offsets-- if you want the cut tube not to aim to the centre of the parent tube but off to one side. This is useful for seatstays.

It can also generate a template for a tapered parent tube, assuming a linear taper. You specify the ratio of diameter change to length change.

You can also adjust the resolution at which it prints. It defaults to 100px per inch which is the same as the one on metalgeek. I print from the GIMP which has an Image Settings tab where you can set the pixels per inch.

The "twist" option is slightly obscure-- it just offsets everything to one side to alter the "clock" of your mitre by the specified number of degrees. This can be useful for mitring fork blades or segments because you can position the template relative to a centre-line on the top and get the desired clock change for your fork offset.

framebuild.py

This takes a configuration file like default.ini, in which you specify the dimensions of all your tubes and your desired geometry.

It outputs a couple of diagrams, a bunch of mitre templates, all the measurements you need to cut the tubes to the right lengths, and a few other useful metrics.

riderfit.py

This takes a rider configuration file like default_rider.ini and optionally also a frame configation file. If you provide a frame configuration it just means it can draw a diagram of the rider actually sitting on the bike.

To get a sense of what size bike you want for a particular rider first put her measurements, and those of the handlebar and stem and the seat tube angle into the configuration file. Then decide on how much bar drop and what sort of shoulder angle you want based on how aggressive a position you want. The program will output the stack and reach you want to aim for. The shoulder angle is the angle between the rider's upper arm and his torso, and should be a maximum of 80 degrees for the most aggressive position.

Designing your bike

The following is all assuming a lugless frame-- i.e. welded or fillet brazed. If you're making a lugged frame, I guess the lugs you have already define the angles, and you need to solve for the lengths. This program does output the angles, so maybe you could tune the lengths to get the angles you need.

Choose some numbers

1. Pick a standard wheel size.
2. Decide what stack and reach you want (possibly using riderfit.py) -- these are the two measurements that depend the most on the size and proportions of the rider and what sort of riding position you're looking for.
3. Decide on a seat angle-- usually 73 or 74 degrees.
4. Decide on a head angle based on your understanding of the black art of steering geometry and what fork you're going to be using.
5. Pick a chainstay length taking into consideration whether you want to fit mudguards and things.
6. Decide on a BB drop. Most bikes use 70mm. Reduce this if you want more ground clearance, maybe for a cross bike. Increase it for a lower centre of gravity, or if you want it to be easier to put your feet on the ground. Don't go too low (too much drop) for a fixed-gear bike in case of pedal strike around corners.
7. The [Extensions] section is based on safe margins so you aren't welding too close to the ends of the tubes, have room for a seat-clamp etc. Probably better not to reduce them by much but you can make them bigger. I've seen Surly touring bikes with an increased head tube upper extension presumably so they could get a higher stack while keeping the top tube horizontal.
8. Think about where you stand on aesthetic issues-- do you want a horizontal top tube? Maybe a top tube and seat tube the same length for a classic Italian "square" look? Do you want to avoid a really short or really long head-tube?

Fine-tune them

1. Copy default.ini to mybike.ini
2. Set up any numbers you have pretty much decided on in the Basic Dimensions and Angles sections.
3. Run python3 framebuild.py -c mybike.ini
4. Look at the "Other Metrics" section and see what stack and reach you ended up with.
5. Adjust basic dimensions and repeat until you get the stack and reach you want while maximizing your aesthetic considerations. To increase reach, grow the top tube length. To increase stack, increase the top tube angle or the seat tube length.
6. Check side_view.png to see what your bike's looking like and to check the wheels actually fit etc.
7. Check chainstays.png for tyre and chainring clearance. Note that it will look tighter in the diagram than it really is because the chainstays are drawn as a constant width tube. In reality they will have an oval section about where the tyre hits.
8. Repeat steps 2 to 6.

Note that side_view.png is drawn completely to scale, but is projected flat onto the paper. The main triangle is in the plane of the paper anyway, so if you measure pixels there, or even print the diagram out and measure it with a ruler, it should match up perfectly. But the rear triangle measurements will appear a bit shorter than the lengths you're going to actually cut things to because of the projection. The lengths output by the program are the right ones.

chainstays.png is drawn in a reference frame in which the chainstays are flat on the paper, so that should have the right lengths.

Tapered Head-Tubes

framebuild doesn't support tapered head-tubes but copecalc does. You can work around this as follows:

1. Design your frame using the diameter that the HT has at the top.
2. Once you've cut the actual tapered HT to length, mark the positions of the DT mitre on it.
3. Measure the diameter of the HT at the top and bottom of the mitre, and work out the diameter change per unit length.
4. Run copecalc with the -e option to make your template.
5. Replace the HT diameter in your ini file with the diameter at the top of the mitre. Run framebuild again and this will give you the correct DT length from the inside of the ST mitre to the inside of the HT mitre.
6. Replace the HT diameter with the diameter at the bottom of the mitre, run framebuild again, and this will give you the correct DT length from the outside of the BB mitre to the outside of the HT mitre. You don't strictly need both these measurements-- the one in step 5 is enough-- but it's good to double-check.

When you change the HT length and rerun framebuild the angle of the DT will actually change very slightly because it's solving to keep your lower extension at the value you asked for. But the difference is not significant and the cut lengths you get from changing that diameter will be close enough.

Flexural Rigidity and Deflection Estimates

For each tube the program works out the Flexural Rigidity and an estimate of the deflection of each tube under a 100kg load at its centre. For these calculations it isn't sophisticated enough to understand butted tubes properly so it works with the thinner wall thickness in the middle.

It's not that you expect to get that load in particular in normal use but it provides an interesting point of comparison. You probably want the deflection estimate to be similar for all frames-- around 3 or 4mm seems to be about right for a nice lively feeling steel frame.

For example, a long low slack MTB with a 750mm down tube deflects around 4.5mm under this theoretical test load in spite of having a diameter of 35mm, compared to 4.08mm for a road frame with a 620mm DT which only has a 28.6mm diameter.

Building it

Once you're happy with the design, you can use the mitre templates and the distances between mitres to cut up the tubes. Note that the down-tube has two mitres at the bottom end-- where it joins the BB shell and where it joins the seat-tube. The program gives you the offset between the two mitres, so you just trace both templates onto the tube with that offset between then.

The first thing to do with the tubes is mark a centre-line, for example by putting the tube inside a piece of angle iron and using the edge as a ruler. You will match up the centre-line with the guidelines on the mitre templates to make sure that when you have mitres at both ends of a tube that they aren't rotated with respect to each other.

The program also gives you reference points to mark on the tubes, measured from the top, showing where the top and bottom edges of the joining tubes go. Mark these on the tubes with a pen and then when you're fitting things up you can put everything in the right place.

Note that double-butted tubes are supplied with a longer butt on one end. The idea is you cut that end off to get the length you need. So you will want to put one mitre right at the short end (which has a bit of red paint on it in the case of Columbus tubing), and then measure from there to where the other mitre needs to go.

The program outputs mitre templates for the seat-stays on the assumption that they're offset from the centre of the seat-tube by their diameter. Otherwise they would bump into each other. With the templates generated from the program and typical seat-tube and seat-stay diameters the tops of the two seat-stays will fit next to each other very well.

Chain-stays usually have a flattened bit in them where the tyre goes. If you're using 700C wheels this probably means you want to cut the chainstays to length from the front since the flattened bit will be in the right place for that wheel size.

Before cutting anything for the rear triangle, decide how you're going to attach the dropouts you have, measure them, and set cs_length and ss_length in the [Dropouts] section. Run the program with the new data to get the final lengths.

Why not just use BikeCad?

BikeCad looks pretty awesome. I started with the free version, but either it didn't give me all the dimensions I actually needed to build the bike, or I couldn't find them. I also wanted to double-check things anyway before actually cutting into my shiny tubes.

I could have bought the pro version at this point... But then I could have just bought a frame, or a whole bike, and where would be the fun in that?