The Electric Down Hill Bike

In 7th grade, I started visiting the electric vehicle forum, “Endless Sphere” https://endless-sphere.com/forums/ . Here, people share their projects, findings, and most importantly their takes on similar problems. Back in 8th grade, a Frenchman put a hub motor in the middle of a downhill mountain bike frame. At that time, a brushless bike hub motor cost around $300, for a kit that came with all the needed components. This $300 was not something I had, but was an idea I wouldn’t forget. I revisited this idea in Junior year of high school, after hub motors became very cheap, lithium prices were on the decline, my knowledge was now expanded enough to take this project on.

Starting and Planning

I started out by assembling a jig together, just to make sure all horizontal axes, whether it be the forks, bottom bracket spindle, or motor axle, were parallel. The hub motor was tucked as close to the frame as possible. The tires have a 13’’ radius and with a measuring tape, the motor was positioned so that during the front wheel’s full travel, there was adequate clearance. The motor shown is a generic 1000 watt, 48 volt unit, though it is overvolted to 72 volts, with a peak of 80 amps. In the photo to the right, you will notice a reduced boss from the motor, flattening the side of the motor casing, allowing a custom drive mount to be bolted to the center of the axis of the hub-motor case.

 Mounting Brackets

Looking at the very first image under the “Starting and Planning” section, one can see some shiny steel tabs hanging off the bolt holes where some suspension hardware is mounted. A big goal of this project was to non-invasively connect this whole motor unit, while having a strong and seemingly intentional placement. The picture to the right shows only one of two dropouts fully mounted to the frame, and it is clearly seen how compact it is. This created a triangle, using the strongest bolting points on the frame.

The mounting brackets used the hub motor as a jig for tacking all the components in place. Once I was sure that everything was where I needed it to be, I then laid down final weld beads. In hindsight, I would mount a cross brace to keep the dropouts from swaying side to side. It has no triangulated support, allowing it to sway like a parallelogram.

Here’s a quick clip of me testing the bike’s clearance. My entire time riding this bike, I was not limited by the ground-to-hub- motor distance. Overall, the motor location, while a bit bulky and unconventional, was not at all problematic. It certainly resulted in better weight distribution than if it were just in the rear wheel.

Drive Hub

This is one third of a confusing freewheel system. The first freewheel is within the wheel itself like a normal bicycle, the second is inside the crank arms, so when the motor is spinning, the crankarms are not dragged along with it. The last is in the hub motor, so in the unfortunate event I run out of battery, I can pedal with no resistance. The most observant will note that (good job, because I had to spend a half hour drawing clockwise and counter clockwise arrows on a piece of paper) there will need to be right hand thread freewheel, and 2 left hand thread freewheels. If a right-hand thread freewheel was needed for the hub, then hooray, because that’s what a rear wheel hub motor comes with anyways. This unfortunately is not the case, and what actually needs happen, is a custom hub needs to be turned. What you see blow is custom hub, with a left hand 1.125’’ diameter boss that gets threaded with a reversed 1.25 metric pitch (weird, I know, but bicycles are a mix and match of metric and imperial measurements).

Here is a better look at what all the chain lines look like. For the smaller front chain, I used a single speed fixie chain, I hoped this would mitigate stretch of the less serviceable chain. For the rear chain, I was forced to use a narrower 8 speed mountain bike chain simply due to having an 8 speed cassette.

Electronics

I originally got the hub motor from a friend who thought it was broken, so I opened it and swapped the hall sensors out after confirming they were busted via a voltage test. The controller is a 72 volt 230 amp peak Kelly Controller unit that I bought from a different friend. The batteries deserve a paragraph of their own.

What you see to the right is 1/2 of a 160 cell battery pack. More specifically 10s 8p. The cells are LG, salvaged from hoverboard packs with a nominal capacity of 2200mah. The pack is consisted of 2 parts, each part draped on either side of the top tube. Each of these packs are 36 volts, wired in series for 72 volts. Now, you might say, what is the logic behind this, and all I can really defend myself with is that, I bought these packs from a friend. I had bought a bunch of cells to build my own, but I had no spot welder, and in the end, I didn’t think I would use this vehicle to the extent where the weird charging procedure would become annoying. I was wrong, I used this bike a lot. No need to worry about safety; there is a BMS (battery management system) in each battery for charging, and the controller was set for a slightly higher cut-off voltage when fully discharged, 66 volts. Overall, it works, but it’s definitely something that will not be repeated in the future.