I’d been riding around on a functional but ugly bike for a while. I’d decided I needed something more attractive and a bit sporty. I decided to convert a mountain bike with full suspension.
I spent quite a bit of time picking a frame. I wanted something cheap, but also not too heavy. I settled on an aluminum frame with a RockShox monarch rear shock.
I don’t have pics from the welding. I made up a simply bracket to hold the battery I was using on my last bike. It’s a 7S20P pack made using LG MJ1 18650 cells. It has more than enough power and gives me decent range. You can see some build pictures of it here.
I have done a fair bit of hobby type TIG welding in the past but never on Aluminum. I had to spend a day playing around until I was comfortable enough to start welding on this frame. The results are acceptable and strong but not pretty. That’s how most of my welds end up but so far nothing has ever fallen apart. I tend to use too much heat and get more warping than a more skilled welder would, but the strength is there.
In the above picture, you can see where the welding was done and that it’s a little lumpy. I also welded lugs in the frame to hold the controller. This was my first test fit of everything before I took it apart and painted it.
The bike is built on an aluminum full suspension frame (low end) modified with battery bracket. This is what makes the bike not pretty. I know it, but it was easier than a full frame. Rear end dropouts are reinforced with 5mm steel plates. I have Chinese made front suspension (good enough for street riding) and a rock shox air rear shock (not in love with it).
My current bike tops out at 65km/h running a 36V 1000W on a 72V pack (7S20P pack made using LG MJ1 18650 cells). I bored out the axle and have motor phase wires coming out of both sides. It allows a lot more wire, but I wouldn’t do it again. Too much wiring. It’s basically a rocket and there is nothing around here to beat it. I know many of you have faster bikes, but around here, very few hit 50km/h. In fact, I’m not sure where I could go faster because as it is, the brakes are not meant for a heavy bike running that fast and stopping is useful. I use a 5000W kelly controller, 22″ rear, 24″ front wheel. It’s got analog regen braking (I absolutely love regen braking).
It is rounded off with some super bright LED headlights and a regular horn. Around here the horn is almost as important as the brakes.
I used a Kelly controls KLS-S controller. You can see it mounted here in front of the rear wheel. I ended up potting the controller with potting compound for 100% waterproofing. It’s very prone to a lot of dirt in this position. Never had a problem though.
Mountain bike frames, especially ones with aluminum dropouts, are not well suited to hub motors. When you accelerate, that torque is resisted by the dropouts. This is not how they were designed. On a regular bike, the dropouts never seen any torque. I designed and had some torque plates made up. These ones were cut out with a wire EDM. Laser would be a much faster way to make them, but at the time the EDM place was much closer to where I live.
I needed a way to stick a lock on the bike so I welded up a bracket on top of the battery box. It fits pretty well and is very practical.
This is the finished bike and it tops out at 65km/h. Not bad and certainly fast enough to be fun. I used Zoom brand front fork. They don’t have much in the way of suspension. They look much better than they are effective at damping bumps. That being said, this isn’t an off road bike, and help a little. Next bike will have better suspension.
I had a few problems with rims bending. I was using regular eBike steel rims and they would bend on large bumps. After replacing the rear rim twice, I switched to an aluminum motocross rim. It’s worth the extra weight.
This motor wasn’t all that powerful, so I drilled out the axle and ran thicker wires out both sides of the axle. This gave me about 4 times the cross sectional area of wire. It definitely helped the power, but added a good deal of complexity to the wiring. I would not recommend it. Also wires coming out the end of the axle are not ideal. Here you can see where they got cut off in an accident.
The bike got stuck outside during a rain storm. When I started the bike up, there must have been water in the throttle and it took off on its own. Eventually tipping over and crashing. No major damage, but after that I did pot the controller and do my best to keep it out of the rain. Driving in the rain has never been a problem with any of my bikes, however being parked outside during a rain storm has caused be grief on two occasions.
I welded up a trailer for the bike so I could go and pickup material and stuff. I never ended up using it much though it works OK. I don’t have any complaints with it. I just never needed to pickup that much stuff. It was fun to build though.
My dog now rides in a backpack instead of a basket. It’s a great bike to ride and it’s a complete success. Now if it could only carry a passenger…