A Simple Bicycle Frame Jig

This is an instructable for building your own bicycle frame jig. It was designed for bonding, laminating monocoque frames, brazing and welding. The jig stands heat from welding and brazing and it can also take a lot of abuse and still remain straight and true. Any tubeset can be used regardless of shape or diameter. The Jig can be clamped to a repair stand or vice. It will also stand upright on the floor without support (of course, it will tip over if pushed).

We used Minitec aluminium profiles that can be sourced in USA/Europe and SI units/metric threads. Custom parts for the fitting need to be machined or you can order them from ideas2cycles. The jig is based on five interchangeable 1000 mm long profiles attached to each other orthogonally using angles. PowerLocks and clamping claws could be used for the joints as well. The head tube and seat tube fittings are held in place by angles also. The bottom bracket fittings and rear dummy axle are based on threaded rods.

This step describes the thought process when designing the jig. Since the construction is quite flexible, motivating some of the choices made could be useful for the modifiers. Skip to the next step if you just want to get on building the jig.

Why aluminium profile?
It is light, strong, stiff, robust, heat resistant, machinable, weldable, corrosion resistant, easily available and accurate. On the downside, it can be considered a bit expensive compared to wood or traditional metallic square profiles.

Why 1000 mm long?
For the sake of simplicity. One could manage with shorter ones and they don't all have to be equal in length. The wheelbase of regular bicycles is around 1000 mm, so with the front fork left out and the frame slightly rotated, 1000 mm is a suitable width. The height of the jig could be reduced, but we wanted to use integrated seat tubes on some of our designs and wanted to leave some room for that.

Why is the rear axle fixed and positioned like it is?
Making the bottom bracket fixed would have required space for adjustment at both ends of the jig. Fixing the rear axle saves space. In addition, attaching the profile holding the rear axle flush with the supporting (horizontal) profiles situates the centre line of the frame nicely relative to the planes of the rear drop outs.
The 250 mm position (from the bottom) for the rear axle is chosen because it allows ample room for bottom bracket height adjustment for all conventional frame geometries, but keeps the bottom bracket low for designs with long integrated seat posts.

Why an orthogonal configuration?
The seat tube and head tube do not have to be parallel to each other. The profiles can be fastened at an angle either using the same 45 GD angles or with clamps (called Cross connector) from the same supplier. This is made easy in the newer version of The Jig from ideas2cycles. However, in most cases the orthogonal layout is sufficient and simpler to assemble because it requires no angle measurements, which can be cumbersome. Keeping everything square is far easier. It does require some calculations, but at least they are exact. The result is two coordinate systems: global and local. The global is the jig's coordinate where everything is orthogonal. The local coordinate is that of the frame, which can have tubes in any direction and the horizontal is defined by the ground when the bicycle is standing on its wheels. In the end, however, it comes down to getting the four points (head tube, seat tube, bottom bracket and rear axle) correctly positioned relative to each other. Frame geometry is traditionally thought of in the local coordinate, but the fittings can only be moved in the global coordinates. Therefore a transformation has to be made.

How is the coordinate transformation made?
The second picture shows the jig in its own (global) coordinate system. The blue triangle represents the known values of the frame (local coordinate):

BB = (Vertical) bottom bracket drop
CS = Chain stay length
Effective CS (CSeff) = Chain stay length parallel to the ground (horizontal)
ST = Seat tube angle relative to the horizontal.

The frame’s coordinate is simply rotated counter clockwise around the bottom bracket by the amount of the seat tube complementary angle. Hence, the seat tube runs vertical as do the profiles in the jig. The rear axle is fixed in the jig at a height of 250 mm from the bottom edge. We need to solve the position of the bottom bracket relative to the rear axle in the global coordinate, i.e. solve dx2, and dy1+dy2. We know the bottom bracket drop, chain stay length and seat tube angle. You might have designed your frame using the effective chain stay length to begin with, but in case you did not, it can be calculated using the Pythagorean Theorem. All equations are presented in the third picture.
The horizontal position DX of the profile holding the bottom bracket fitting can be measured using a tape measure. The vertical position DY can be measured with a calliper taking into account the radius of the fitting (Dbbfit/2). See fourth picture.

How is the center plane of the frame determined?
The slots in the 45x90 angles allow adjusting the center plane. For ease of use, we are recommending to slide the seat post and head tube fittings to one extreme of the slots. When considering the rear spacing, it becomes clear that the outermost slot of the angle has to be used. However, to avoid an unnecessarily long dummy axle the inner edge of the outer slot is best. The inner edge of the outer slot is 54.5 mm from the base of the angle which, together with the 8 mm diameter bolt leads to a center plane 58.5 mm from the aluminium profile surface. The profile with the dummy axle is hence 58.5+45 = 103.5 mm from the center plane. With a maximum rear spacing of 135 mm, this leaves 103.5-135/2 = 36 mm for the drop out thickness and nuts. One might want to move the center plane further away from the profiles if room is needed for example for a welding torch or wide rear axles like those used in fat bikes.

Some custom made parts are needed in addition to the standard parts. A full BOM is included as a picture.

These are the parts you need:

Part number Description Quantity

20.1033/0 Profile 45x45 F 1000 mm 5

21.1397/2 Angle 45x90 GD-Z 3

21.1133 Angle 45 GD 6

Bolts, washers and square nuts are included in the angle package.

These can be acquired from any hardware store:

M10 threaded bar - 300 mm

M10 nut - >6 pcs

10 mm washer - 2 pcs

M8 threaded bar - 200 mm

M8 nut -1-2 pcs.

The following parts need to be custom made or ordered from ideas2cycles:

- Stepped headtube fitting for zero-stack and external cup standards 1 pcs

- Conical fitting for all other headesets and seat tubes 2 pc.

- Bottom bracket fitting 3 pcs.

The dimensions depend on the respective standards you are using. The fittings kit from ideas2cycles covers all standards. Additional M10 and M8 nuts are handy when figuring out rear drop out positions and BB fitting respectively.

The assembly process is very simple and the jig is put together with ease on the floor using just a 5 mm allen key. It can be mirrored if you want the drive side facing outward. There is a spreadsheet included that calculates the DX, DY, and DZ values based on the following inputs: bottom bracket drop, (effective) chain stay length, seat tube angle in degrees, diameter of the bottom bracket fitting, rear spacing (120, 130, 135 mm or whatever), thickness of the drop outs and the position of the center plane of the bicycle frame (58.5 mm in these instructions). The output values are used as follows:

The horizontal position DX of the profile holding the bottom bracket fitting can be measured using a tape measure. Measuring at both ends or measuring diagonals assure that everything is square. A square angle can be used to verify. The first picture explains this best. The profiles can be fastened at an angle, too, if you want for example a steeper seat tube compared to the head tube (as is common for time trial bikes). This is done using either the same angles, which allow for rotation around one of the profiles, or then buy clamps from the same manufacturer. I.e. the profile can rotated to any angle you want and then clamped down.

The vertical position DY can be measured with a caliper taking into account the radius of the fitting (Dbbfit/2). You might want to add another M8 nut to keep the BB axle in place without the BB shell.

The rear spacing DZ can be easily calculated and measured in many ways. One way is to tighten two M10 nuts against each other on the dummy axle and measure the appropriate distance from the profile surface and compare this with the position of the center plane. The best way would be to measure from the inside of the drop out to the surface of the profile, but it can be a bit tricky with some drop outs. The spreadsheet contains a DZ number that can be used to calculate the position of the outer side of the drop out from the back surface of the profile holding the axle. This is shown in the second picture and is easy to measure using calipers. Note: The newer version sold at www.ideas2cycles.com has a different way of defining the dropout positions.

The last step is to position the profile holding the head tube adapter(s). This does not actually require too much dimensions or measuring. If everything else on the frame is properly lined up, then the head tube can not go much wrong. An effective top tube (center-to-center) dimension can be used to preliminary position the head tube and adjust from there if necessary. Note: The steering angle can be different from the seat tube angle and this is made easy in the newer version sold at www.ideas2cycles.com.

Using the jig is fairly straight forward, but we'll show the basic steps using an ideas2cycles prototype bicycle frame.

All you need to know is the bottom bracket drop, chainstay length, seat tube angle and the diameter of your machined bottom bracket fitting. Insert these values to the spreadsheet or calculate the distances in the global coordinate in some other way.

As a result you'll get the horizontal distance between the profile holding the rear axle and the profile holding the BB fitting and seat tube fitting. This dimension is called DX and can be measured with a tape measure at both ends for sufficient accuracy (see picture).

The position of the BB fitting on the middle vertical profile is determined with the help of DY, which is the distance from the bottom of the profile to the top of the BB fitting. This can be measured with calipers (see picture).

There are many ways of figuring out the positions of the rear drop outs. The DZ dimension helps to position the rear drop outs correctly with respect to the center plane of the bicycle frame. The position of the drop out should be verified from the inside as well. The position of the outer drop out relative to the inner drop out should be measured inside-to-inside to avoid mix ups and inaccuracies due to drop out thickness.

The profile holding the head tube fittings can be positioned by taking measurements from your CAD model, but usually other dimensions (such as point of top tube/head tube intersection) or form factors (e.g. when using lugs) determine the position of the head tube. One should always double check the position of the head tube using other routes.

Other frames built using this jig can be found on www.ideas2cycles.com