After completing the white Raleigh Twentie pedal bike which was modified to a lightweight and modern specification, I noticed the remaining parts and possibilities.
It seemed that Twentie’s design suited her tastes better than the Unibega Mixed she currently rides. Compact geometry and low step-over height naturally increase operability, and she rarely uses all of the current gear, I thought that a simple three-speed change might suit the way you interact with bicycles.
At the same time, there were several parts that did not perfectly match the idea of a white bicycle. For example, I got a classic Raleigh shopping rack (front and rear), but it looked great, but it was made of heavy steel, and it did not match the direction of weight reduction of the white build. He also had a Wyman 19mm wide front rim, which he discovered early in the project, but after adopting a Sun CR 18 wheel and a 28mm thin tire, he could not procure the corresponding rear rim. Hongjo Fender decided this decision. This fender only supports tires up to 28mm wide.
It made more sense to build a utility-oriented Twenty version than to leave these parts in vain. In other words, a thicker tire or rack can be installed, and an electric assist can be incorporated to accept increased weight. At one point, we considered creating a custom fork for a standard 100mm front hub motor. Later, however, he learned that Swytch manufactures kits for folding bicycles. With an 85mm spaced hub and an 8mm axle, the kit was almost perfectly compatible with Raleigh Twenty’s genuine forks. This is the decision. No need for fork width or custom machining - - a simple and undo way to electrify the second Twenty.
In order to proceed with the modification work, I want to set up a thread as a kind of production record and a place to look back. Experiment, record and perhaps think about everything too much - so feedback is welcome!
Does the front hub motor have a torque arm to clamp the fork blade? Or does it depend only on the axle flat of the dropout? Even with a torque arm (better), it is the same situation as adding disc brake caliper mounts to forks that are still not designed. Just reverse the direction of the input moment.
It’s interesting to hear about the torque arm - it’s exactly what I was thinking about yesterday. As the modification progressed, I became concerned about how to handle torque from the front hub motor - especially when considering the fork design of Rory Twentie. The dropout of this frame is quite simple, and it is not a brazen pressing or forging dropout, but a structure that simply flattens the end of the tube-shaped fork blade. There is no problem with the standard front axle, but when the torque from the motor is added, you can’t help worrying about how long this design can endure.
The kit included a Swytch universal torque arm. This is a two-piece structure, one fixed to the flat part of the motor shaft with a clamp, and the other wrapped around the fork with a hose clamp. It seems to be intended to accommodate various forks, and the reason is that it adopts slotted arms with flexibility in the mounting method.
However, there are some concerns:
The slot of the arm causes movement in the upper and lower directions, and it appears that the axle may be slightly twisted before the arm bites completely.
In the instructions, it is installed in a state without preloading, and it seems that the torque arm reacts after the axle starts to move.
Depending on the mounting position, the arm may devour into the fork blade when loaded. Modern forks with high strength may not be a problem, but this old design is not ideal. We have not yet decided which direction to proceed. We are considering modifying the torque arm, and we are considering the possibility of welding the two parts to eliminate play, and a proposal to disperse the load into a fork using a rigid clamp that may be made by itself. The hose clamp does not rely on preventing vertical movement (especially when there is no preload) and is not suitable for dispersing the load in the fork direction (the manual shows the method of installing the arm on the fork front of the drive side).
When I made a front hub-driven bicycle for my daughter, I used Grin (ebikesca) torque arm 4. It is a good idea to refer to the company’s website because there is a lot of information about torque arms etc.
Thank you for the photo. You should avoid that structure using a hose clamp. When the drive torque is applied, the torque arm is pulled backwards and the hose clamp is transformed into an oval shape. Then, by bouncing back to the original circle, fatigue accumulates and eventually breaks. Also, the loosening of the fork blade scrapes the coating, causing fritting wear. At a minimum, it is necessary to insert the fork blade firmly at the end of the arm, but to reduce pressure to a degree that does not cause stress due to pressure. A thin rubber sheet (old tire tube) between the clamp and the fork is also useful. This method is often used to reduce impact loads at structural joints such as shock absorbers, struts, and engine mounts, and can be said to be an appropriate measure for torque arms. However, even if proper installation is performed, the problem that the moment of bending is transmitted to the fork remains similar to the disk brake caliper mount. However, because the front drum brake uses torque arms, it is worth investigating whether it adopts a more rugged fork as a result. Virtually all “rental” bikes adopt front drum brakes, so let’s check it out. Note: Wheels larger than 20 inches generate greater torque and bending load, even if the braking thrust to the ground is the same. 20 inches eases it somewhat.
I think that the photograph of the Swytch manual is a little difficult to understand - - the fork seems to be turning back. However, this shows the drive side, and I think the torque arm is mounted in front of the fork blade. In my case, it actually makes sense: the wiring of the motor comes from the non-drive side, and the fender stay gets in the way behind the fork. Therefore, attaching the torque arm to the driving side front seems to be the clearest solution.
Although the 8mm axle hub motor itself is unusual, the torque arm solution for the 8mm axle is even more unusual. Some say that the torque arm is unnecessary for less than 750W - this Swytch motor is only 250W - but Raleigh’s fork is not particularly rugged, so I think it’s wise to take precautions.
To be honest, using a hose clamp on a bicycle feels a bit like a place. I am thinking of TIG welding a 4mm mild steel tab on a fork blade and making a custom 3mm 304 stainless torque arm. Basically it is like Grin V6, but spline processing is omitted. It’s a little difficult, but with SendCutSend it’s easy to do.
The Swytch kit is not scheduled to be shipped until September, so there is plenty of time to detail. Meanwhile, I am looking for a way to avoid using Swytch’s general-purpose pedal assist sensor (which I don’t like much), and I will definitely upgrade the brake.
You should avoid welding to a fork blade without consulting the frame builder. In particular, fatigue strength may weaken the blade. There is a possibility that it will not, but it is a area that needs attention. Much of its strength may be derived from cold processing by mechanical pulling. In general, alloys that hold high strength after welding are selected for the bicycle tube welded. For example, chromium molybdenum steel, which has high quenching properties and is re-cured by air cooling without rapid cooling. However, you should avoid welding without consulting the frame builder. However, there is a good chance that the fork is not a chromoly but simply a “high tensile steel” that is less quenching.
Brazing is different. The tube is not heated to melt or annealing temperatures, thus maintaining the strength of the tube. Brazing is fully viable and low risk. Brompton folding bicycle frames are all assembled with fillet brazing. Steel frames with lugs also use brazing to join tubes and rugs.
Hose clamps are only first aid. Find the right clamp bracket. These may be found in the torque arm parts of the coast brake or drum brake attached to the frame tube.
Thanks for the detailed metal engineering discussion. After reconfirming the tube specifications (probably 1020 or 1018) and thermal impact area data, we decided that TIG welding or brazing a small tab on a Twenty fork would be fine. Local TIG welding heat only softens a few millimeters around the bead - - enough strength to withstand the 250W hub.
However, with silver brazing, the peak temperature of the metal is reduced to about 650 ° C, and the problem of the thermal effect area can be avoided completely, and the shear strength of the junction maintains more than 250MPa. This is about 20 times the maximum stress 12MPa the torque arm tab receives. With the MAP-Pro Bernzomatic torch, 4mm mild steel tab and 45% silver rod, the work can be done quickly and orderly, virtually zero stress on the fork.
Therefore, after fixing the tab with silver brazing, the 3mm thick 304 stainless steel torque arm of integrated molding will be bolted within 40 mm at the bottom of the blade. This ensures rigid play zero installation while maintaining the original spring characteristics and fatigue life of the fork.
Based on your opinion, we have come to a solution that ensures sufficient strength while minimizing invasiveness. Thank you very much.