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. The profiles and standard parts were provided to us by Movetec. Custom parts for the fitting need to be machined according to your needs 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.

Step 1: Design philosophy

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.

<p>Hello, guys. Looking through the list of materials i found a profile marked as &quot;hole&quot; please tell me what that means?</p>
One of the profiles has a 10 mm hole in it, which makes it different from the rest of the profiles.
<p>Is it really necessary to have exactly this type of profile? Or i can use the one that doesn`t have such hole? is it okay to assemble the mentioned jig using only the type of profile i attached. Thank you.</p>
<p>i made it!</p>
<p>Nice! Looks good.</p>
<p>How can you set the headtube angle independent of the seat tube angle?</p>
the angles used to join the profiles allow rotation around the tightening bolt if installed that way. The profile for the seat tube would be kept at a right angle and the head tube profile rotated. The length of the profile is sufficient even when fastened at a small angle.
<p>Take a look at this image http://www.minitecframing.com/Products/Profile_Fasteners/T-Slotted_Fastener_Catalog_Pages/21.1027_Clamping_Claw.html</p>
<p>Hi! Amazing Instructable. We just finish building it and we cant wait to start welding with it. I still need to figure out how to use it properly. Greetings from Spiele Bike Shop, M&eacute;xico City...</p>
Wow! Glad to hear. Hope it serves you well. It's easy to modify, too.
good <br>
I wil have to re learn my trig. but this is great!
It looks more complicated than it really is. The spreadsheet will do the calculations for you, but I think it's good to know what the connection is between frame geometry and jig setup. Especially if/when you want to modify something.
awesome! <br>do you think this could be adjusted for producing gravity bikes, <br>or other style bikes. it seems very versatile.
That's an interesting question! I know barely nothing about gravity bikes but it seems that getting the rear axle aligned relative to the head tube would be enough and all that this jig can help with. In that case even a simpler jig would suffice. At least you would want to increase the length of the horizontal profiles to accommodate the longer-than-usual wheel base of gravity bikes.
Very interesting! Thanks for sharing.

About This Instructable




Bio: ideas2cycles is a product development platform for novel bicycle concepts. The aim is to bring new ideas to life by using the latest technology and ... More »
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