The Frame… and basically everything else

Quite honestly, if you see a weld in a certain spot, or something set at a certain angle, it was because it felt right, there truly was no conventional design process when making this. Due to having been on a robotics team where weight plays a huge role in design, there was just about exclusively aluminum in the scrap bin. At first I was very worried about heat affected zones because of all the welding I was going to have to do, but this was an issue that was worked around through various methods.

The main basic rear fork unit also makes up the bottom of the overall frame, and is just wide enough to hold a wheel with a sprocket attached… but what about the brakes?? With no space for breaks on the inside of the drop outs, it would have to be moved to the outside. Fortunately because a live axle was used, this is possible. At first I was worried that the lean into sharp turns would result in scraping and a possible crash, but due to an incredibly short wheel base at higher-than-expected-to-achieve speeds, this was never a problem. Hopefully the picture of the bottom half of the frame on a Bridgeport table shows how small this thing is. For a more specific reference, the diameters of the tires are about 8’’.

The seat stays (cathedral-like arch that goes over the rear wheel) are butt welded to the larger rear fork such that there is near no force in the horizontal plane, and nearly all of it is vertical, so as of now, there are nearly no heat affected zones at risk of being under too much stress. The front fork is the upper half of a kids mountain bike suspension fork I found in a dumpster, the springs and lower sections were thrown out due to their excessive length. The head tube (the shiny tube mounted to the frame) was cut off the bike frame, it was turned on the lathe to remove any remaining material, and finally had tabs welded to it. Obviously steel cannot be welded to aluminium, and beyond that, this is not a connection I would leave up to non heat treated aluminium. For that reason, a steel assembly would be bolted on with two 1/4’’ bolt running through both sides. There is no concern for the strength of the bolts, as nearly all my weight is right ontop of the rear wheel/ axle. If you turned the side profile of this bike into a statics problem, there is little load at the steering to worry about.

Finally, we can talk more about brakes. These brakes were not my first choice of stopping power, but they sure were cheap, as in free. Additionally, they are extremely small, only 3’’ diameter for the whole assembly. One might notice 2 spots where it may seem there was a lot of heat applied on the black brake housing, and you would be right to assume so. I may have committed an engineering atrocity by welding to a hardened steel bearing, but I was on a tight budget, and super glue only worked well until the brake started heating up. Surprisingly, it works well, and there was only a slight clicking noise. If this were done correctly, I would have welded a press fit sleeve onto the side of the brake housing so the bearing could press fit and be removeable.

In this next photo, an extra motor controller I from my desk kart was installed. A 350 watt scooter motor was removed from a junk electric scooter from robotics. It is being over-volted from 24 to 36 volts. With a simple seat made from foam, some random canvas, a piece of plywood and a staple gun, this thing was ready to roll. Foot pegs were made from thin aluminium rectangular tubing, but later were upgraded to steel round bar, after the first ones bent. The battery is a 36 volt 4.4 amp-hour unit from a hoverboard. At the time, hoverboards were incredibly cheap, as the hoverboard fad just came to an end, it cost 25$ for a battery pack with genuine LG lithium cells.

I eventually CAD designed and printed battery slots, so the bike could accept 2 batteries instead of just 1. I also CADed a holder for the switch that doubles as a way of hiding all the ugly wires and unused connectors. Overall, it looked pretty clean, and there wasn’t a spaghetti-nest of wires between the legs as I rode around.

Overall, this was a fun project. If anything it, put me in a situation where I had to work quickly with what I had. In the end, I had a pretty exciting little vehicle. It drove around at 23 miles per hour, had a range of 4 miles before the BMS (battery management system) kicked in and cut off current. The range was doubled after I added the second battery. Here’s a quick clip of my friend doing a little drive by. The steering geometry is a little more sensitive than an actual bicycle, and looking back, I probably should have taken the time to learn about head tube angle and rake angle, but these skills would be learned and applied in my full-size minibike build.