This is the story of an old MTB frame, a newborn daughter, a big love for cycling and how all that eventually led to the creation of a Cargo Bike. It was 2011.
Some years later (and many other Cargo builds) our project of recycling old MTB frames eventually turned into a bigger one. But that is another story.
In any case, even if a long time has passed from that first bike, we are still very passionate about Cargo Bikes as we think they really can make a difference in urban mobility. So we thought..what about sharing our experience, to help as many people as possible to build their own bike? Of course the answer was yes!
Step 1: Preparing the MTB Frame
The first thing to get your hands on to start is an old MTB frame.
The size for the starting frame is determined by the end user's physical characteristics. Choosing a size that suits you will result in a finished frame on which will be comfortable for you.If you have any doubt about your ideal size, here you can find some useful tips to guide you.
Once you have chosen the right frame the next steps are:
- Cut head tube and down tube
- Cut V-brakes mounting bolts
- Cut rack eyelets
- Grinding and Cleaning
Let's start with the description of the head tube and down tube cuts. These are simple operations (mainly cutting and grinding) but still not without risks. Take all the necessary precautions, and especially if you decide to use the angle grinder, always wear gloves, protective glasses and earplugs. The frame must be cut at the sections shown in the picture, trying to stay as close as possible respectively to the head tube and to the bottom bracket.
The second step is the cut of the down tube. If you find yourself with not enough space, proceed with a series of minor cuts to avoid damages to the bottom bracket.
Finally, we can remove the v-brake mounting bolts and the rear rack eyelets (if present). This is where the supporting plate for the disc caliper will be welded.
Once all the cutting operations are completed, we can proceed with a final grind. With the angle grinder or half-round file polish the bottom bracket, and if removed, the v-brake mounting bolts and rack eyelets.
Step 2: Geometry
For the frame we have chosen stainless steel tubes ST37-3 with a constant thickness of 1.5mm. With these tubes it's possible to obtain a really good stiffness of the frame while keeping the weight low. For the head tubes, the choice is connected to the headset that will be used. We have opted for a 1" 1/8 threadless headset, EC34, which requires a tube with an internal diameter of 34mm. Our selection is Dedacciai 34x1.
Before discussing the practical realization, it's important to focus on some design aspects.
Length of the loading bed
The length of the loading bed depends mainly on the needs of the final user. A long surface will offer more loading capacity, at the cost of a heavier and less agile bike. Viceversa a short surface will result in a more responsive bike (especially in some situations like u-turns). We have chosen a short length of 520 mm, suitable for carrying kids and an average amount of goods. This feature is especially good on the narrow italian cycling paths, where zig-zag between obstacles is a never-ending story.
Width of the loading bed
Generally this value is kept lower than the length of the handlebar (max 600mm). In this way is simpler for the driver to understand where is possible to pass through and where not. We have therefore chosen a value of 530 mm, realized using three equally spaced tubes.
Front steering angles
The fundamental parameter of interest is the fork trail. Generally, the trail varies between 50 mm to 63 mm. A high trail value will result in a stabler bike at high speed, less agile at lower speed. We have chosen an intermediate value of 57 mm. At this point knowing the fork rake and the front wheel diameter (20"), it's possible to decide the head angle to obtain the desired trail.
For the trail calculation you can either use a 2D CAD (many freeware available) or an online calculator like Bycicle Trail Calculator. The final design of the bike is reported in the pictures.
Step 3: Tube Notching
The best way of preparing the tubes is by using a CNC bending machine. This allows to save a lot of welding and to obtain a very high accuracy on the angles and on the geometry. The extremity of the tubes that will be welded to the MTB bottom bracket are deliberately long, so that it will be possible to cut them accordingly to the size of the MTB frame. The bent tubes appear as in the picture, ready for the next steps.
An alternative to using the bending machine is to make the frame with rectangular profiles. You can use a fairly simple technique to make the bends in the profile. Once the bending angle α is determined, it will be sufficient to cut a "pizza" slice of width D from the profile and then bend the profile until the gap left by the cut is clodes. The width D of the cut is given by the formula:
D = a * tan(α/2)
The last operation to be done is notching the tubes. This is most commonly done with a rotary hole saw in which a hole saw of the diameter of the tube being attached to is fed into the stock to be notched at a certain angle. The value of the cutting angle is again to be retrieved from the CAD.
Once all the notching is done, our tubes are ready to be placed on the welding jig.
Step 4: Frame Jig and Welding
The next phase of our project is welding. It is a fundamental part, and we need to put special care while doing it since it can really affect the final quality of the frame.
The main difficulties are two:
- The technique by itself requires a lot of experience and good manual skills. Moreover it's important to know very well the welding machine to obtain the proper adjustments.
- Keep in a steady place the parts while welding.
While to acquire the technique the only way is to practice a lot with patience and dedication, to keep the parts in place we can built a welding jig. A jig is a set of constraints and clamps, positioned according to the drawing, where the tubes are layed down and blocked. You can see that our jig has several constraints on the main angles of the frame: horizontal, first and second bend of the loading bed, head tube. This allows to compesate for the bending tolerances, so that we are sure that once the clamps are closed all the angles of the frame are exactly as we want.
Once the tubes are positioned, we need to verify the joints to be sure that all the tubes fits nicely. Once we are 100% sure we can proceed to welding, firstly just in few spots to fix the positions and then all the frame.
A this point we just need to weld the MTB frame that we have previously prepared. Before doing that we need to decide the value for the rear drop, which is the vertical distance between the bottom bracket and the rear axel. A high drop will lower the bike centre of gravity making it more stable, with the risk of touching the ground with the pedal during a turn.
A rule of thumb for a good drop on a 26" frame can be obtained by:
- D = 230mm – crank_length.
Therefore, considering a standard crank length of 170-175 mm, the resulting drop value is around 55-60 mm. A simple jig for the rear drop is showed in the picture.
To notch the horizontal tube of the MTB frame, since it has already been cut as close as possible to the head tube, we just need to stay with the hole saw as close as possible to the edge.
At this point we can put back the frame on the "rear-drop" jig and measure the horizontal distance between the cargo rear head tube and the MTB horizontal notched tube. Subtract from this measure half of the diameter of the MTB bottom bracket and shorten the two cargo lower tubes by this value. In our case we shortened the two tubes by 70 mm, since we had 90 mm of distance and a bottom braket with 40 mm of external diameter. We can now notch the two lower tubes, positioning the saw axel exactly on the cutting plane. This will result in a simmetrical notching, that will penetrate exactly in half of the bottom bracket.
Time to weld the MTB on our front loading bed. To maintain the alignement between the two we can once again take advantage of the rear drop jig. At this point our frame looks more or less done!
Step 5: Disk Brakes
Even if the disc brakes are a newcomer in the bike industry, there are already several standards available. One of the first standards to be adopted was IS, acronym for International Standard. In this standard the disc caliper is attached to the frame/fork with two bolts that are 51mm apart (center to center). The bolts aim at the wheel.
For the rear caliper, the two mounting eyelets are positioned on two circumferences with radius 78.1mm and 39.9mm. The circumferences are centered on the wheel axel.
For the front caliper the eyelets are instead positioned on two circumferences of radius 49.7 mm e 87.3 mm.
The jig that we designed to position the eyelets in the correct way is based on those contraints. It is positioned on the dropout and has two mounting holes at the correct distance from the wheel axel. With the help of the holes, it is possible to screw a plate on the jig.
To realize the caliper support we decided to use a laser cutting machine, but they can also be realized in a more "artisanal" way with a 4-5 mm stainless steel plate.
For the front fork the technique is the same, of course with a different geometry
Step 6: Steering and Kickstand
The last part of the project is represented by the kickstand and steering rod.
The steering rod transmits the rotation impressed on the handlebar to the front wheel. Among the various systems we have chosen the single rigid arm. The choice is dictated mainly by the simplicity of the system and by its small footprint.
The main charateristichs that the single arm system needs to have are:
- Stiffness, to avoid whobbling effects of the front wheel
- Simmetry, in order for the bike to behave the same in left and right turns
To obtain the right stiffness we decided to use a quite beefy circular profile (16x1.5mm), while for the simmetry requirement we operated on the geometry directly in the CAD.
The last design parameter is represented by the ratio between the angle of the handlebar and the corresponding steering angle of the front wheel. From experience we have found that a ratio slightly higher then 1 makes the steering more responsive.
With this solution we were also able to limit the maximum handlebar angle, so that during a tight turn the outer hand doesn't need to move too forward with the risk of losing balance.The optimal ratio that we've found was 1.2, as showed in the picture (wheel angle of 40° obtained for steering angle of 33°).
Once the shape of the rod is decided, it's time to choose where to place the mounting bracket on the fork. To determine this parameter we can place the front wheel all turned to the left, placing the bracket so to have a minimum space between the rod and the wheel.
To fix the rod we used an M8 female joint head. To secure them to the rod we took advantage of the geometry of a hexagonal cap screw M8, the diameter of itd head being exactly 13mm. It fits perfectly inside the tube and it is easy to weld.
The last part of the build regards the kick-stand. Unfortunately this small component it's actually one of the trickiest!
Our kick-stand moves rotating on two pivots grafted on two bushings welded to the frame, and when it is in its parking mode is positioned with an inclination of about 5 ° to the ground. This angle is sufficient to ensure a stable stop, and prevents the center of gravity of the bicycle to move too far forward. By using a higher angle you'll risk that the kick-stand gets stuck when pushing the bike front, making it difficult to use.
The other kick-stand feature is the length of the leg, that we have designed so as to lift the front wheel off the ground of about 20-30mm. It is a fair compromise, allowing a good stop over rough terrain or slightly downhill, while avoiding to lift the bike too much, making a stop very tiring especially with front load.
At this point our frame is complete, ready to be assembled with all the components.
Step 7: Painting, Assembling and Riding
Finally, our frame is ready to be painted! We chosed a sturdy powder coating. In the pictures you can see some examples of different colours and builds.
Happy cargo to everyone!