With all these sexy, sleek, and expensive production bikes hitting the market in the last 6 months (Specialized Turbo, Stromer ST1, Neo Jumper, Currie Eflow) it is tempting for the home builder to just go out and buy one. They are breaking the 20-MPH legal speed limit (between 20-25-MPH) and doing it with torque sensing systems that make them a really sweet hybrid that is super lightweight (for an ebike) and still pedals and feels like a bicycle. Not surprising, some ebike builders are inspired by these bikes, and have decided to build one of their own. (See our mini mountain bike story for some great examples).
Home builds offer the advantage that you are able to build a bike at a more affordable price, while using higher quality components, and are able to break the watt and speed limits that exist in most countries. (read our Ebike legal story)
Home builds are usually a lot sloppier and heavier than a production bike, but there are a few builders who are capable of building an ultra clean home build…Kepler is one of those.
John “Kepler” Wessel of Melbourne Australia is known in the DIY community as the builder and modifier of fast electric bikes. He is known as an owner of a Stealth Bomber that he modified to have a top speed of close to 60-MPH. (read our story on Kepler’s modifed Stealth Machines) He also owns a Stealth Fighter which he modified to be more lightweight Read his Stealth Fighter Review. Kepler is a rare breed of Endless Sphere member who really appreciates not only the power of electric bikes, but also the elegance of keeping them more like bikes…most notably being light-weight and clean looking.
It was this fixation on lightweight bikes that inspired Kepler to develop his own drive system, which is a friction drive using a tiny RC motor. The Kepler drive system was ideal for quickly converting lightweight road bikes and other production pedal bikes into simple and lightweight electric bikes with moderate power and speed levels.
Kepler and Production Bikes
Kepler is a hard man to impress when it comes to electric bikes. He was impressed however when he got a brief test ride on a Neo Jumper (read our review), and it was that experience that spurred the thinking necessary to sell his beloved hot rodded Stealth Bomber (read about that amazing ebike), and look for something more tame and civilized. At first he thought he would buy the $4,000 Neo Jumper, but the price tag and the fact that it was only 250 watts, made him look for other alternatives. In the end, Kepler decided he could use his skills and build a better ebike (this time a tame and very stealthy E-bike) and even save some money.
Kepler made a splash in the DIY community when he suddenly announced he was putting his Bomber on the auction block and was thinking of buying a lightweight under powered Neo Jumper. Trade a 60-MPH Stealth Bomber for a 20-MPH production bike? Endless-sphere die hards were banging their heads against their monitors and throwing up their keyboards and mice.
Kepler decides to build a 29er Carbon electric bike
“Recent experiences with the some nice quality commercial e-bikes got me fired up to look at building a low-powered but high-speed commuter / flatbar road bike that would be hard pressed to even be recognized as an e-bike by the general public using the local bike paths.
As a bit of back ground, after having a quick test of an e-motion Jumper just around a parking lot, it felt so impressive that I seriously considered purchasing one. So the next move was to go visit the suppliers and take the bike for a proper test ride. First impressions were very good with some nice assist. However, the bike was limited to 25-kph assisted as per Australian / Euro regulations. So to ride the bike at a decent speed, you needed to work twice as hard because effectively the motor-assist was doing nothing for you, and you were now trying to push a 40-pound bike at a decent speed.
In addition to e-biking, I have to admit I am a closet Lycra and average about 100-km a week on my carbon road bike. Every time I get on this bike I am always blown away by the efficiency of these modern high performance road bikes. I have no doubt I could average a better speed on my road bike then I could on an Australian regulation compliant e-bike.
So with this in mind, I figured it was time to blend the best of both worlds and build an e-bike that free rides as well as a high spec road bike but at the same time had e-assist capable of a sustained 50-kph under favorable road conditions.
The process of gathering components was then started. The cornerstone of this build is a full carbon 29er frame complete with rigid carbon forks. I decided to go down the path of a 29er because of my height and the fact that it is a mountain bike frame and as such provides a bit of extra strength. Total weight of frame and forks comes to 1.8kg. Nice start. “
RC friction Drive
Kepler is the inventor of the Kepler friction drive. (read our article on friction drives) It was a natural choice that he would go with his own drive system on this new bike. For this bike, many custom pieces were designed in Solid Works and fabricated to hold the drive and controller:
Hiding Components…a home builder challenge
Hiding the battery pack and controller are tricky for the home builder.
Kepler hid his RC controller under the seat:
Like many home builders these days, Kepler went with low cost, high energy density LiPo Hobby King cells. He decided to hide them in a pack behind the seat. This looks super stealthy and neat, but he is only able to hide 44V / 5-Ah for a total of 220 watt hours…not a very large pack, but…it’s easier to hide and it makes the ebike lighter and more like a bicycle:
The nemesis of the ebike builder is where to hide all that wiring. Most ebike builders don’t really worry and let all the wires hang everywhere strapped down with zip ties and duct tape. This is not Kepler’s style, and he was looking for an exemplary fit and finish in his ultra stealth commuter. This is one huge advantage that production bikes have over most home builds. The hollow purpose built frames provide many easy hiding places for wiring, and since they are usually made in China, they have cheap labor to spend hours hiding it all:
A full carbon-fiber frame presented a new set of challenges for Kepler when it came to the hiding wiring especially the bulky battery cables. Most would consider the obvious solution was to just run the wiring inside the frame. However carbon frames are highly stressed and can not be drilled without risking their structural integrity. Subsequently another solution needed to be found.
The solution came in the form of custom carbon fiber cable covers made from thin walled carbon fiber tube. These covers were carefully cut to match the profile of the frame so as to look like part of the original bike.
RC plan aborted!
After completing the bike as a RC friction drive, Kepler made an astonishing “about face”. Even though the bike ran as fast and smooth as expected, there was one element Kepler could not stand. RC drives are notoriously noisy (I know exactly what he is talking about). I abandoned my mid-drive Optibike based on the noise factor alone). On an ebike, for which the intention was to ride completely unnoticed on bike lanes etc, and did an effective job of appearing Stealthy, but it did not seem right to Kepler that the bike gave itself away with a loud “weed whacker” noise coming from the motor. Kepler decided to strip the bike of the RC motor and drive system, and go with a traditional hub motor in the wheel instead.
New Plan, Stealth Hub Motor Build
Kepler decided to go with a compact geared motor but resisted going too small just for the sake of weight savings. A compromise between weight and power was struck with a Bafang SWXH high torque (but low speed) motor selected for the job. At 6.6 lb, this was not the lightest hub motor on the market. However to withstand what Kepler planned to throw at this little motor, some thermal mass was a must have requirement.
This motor is rated at a meager 250 Watts when using 36V, however…Kepler knew from his experience with similar geared hub motors that the motor was capable of much more. How does 75 Volts and 950 Watts sound? Enough to maintain 35-MPH under safe conditions, and climb most hills at speeds that even Tour de France competitors would be proud of.
However, we were now talking about using over one horse power through a set of dropouts not designed for the high twisting loads associated with electric hub motors. To get around this issue, a custom made torque arm was made that transferred the motor’s twisting loads from the chain stay dropouts to the far more robust disc brake mount. Power could now be confidently applied without fear of the back wheel being ripped out of the bike.
New Controller and Mounting Position
Hub motor controllers are not known for their compact size especially when designed for high voltage and high power. To get around this Kepler took a standard low-spec / low-powered compact controller and rebuilt it using 100-volt capable componentry. It was then configured to take advantage of all the latest features available through the latest Cycle Analyst. To continue with the stealth theme, the controller was mounted beneath the bottom bracket with much attention paid to keeping the controller as water proof as possible.
DIY Torque Sensor
Kepler originally decided to set the bike up with a simple RPM sensing pedal assist. However, this method of pedal-assist soon proved too rudimentary for this build and was subsequently replaced with a far more desirable torque-sensing bottom bracket. Again, the V3 Cycle Analyst was used to take full advantage of this method of pedal-assist.
Here you can see the “control panel” of Kepler’s Super Commuter. He is using a Cycle Analyst v3 which is the most sophisticated electronic dash available for electric bikes, and will tell you within a few hundred yards of when your battery is going to die. The Cycle Analyst measures exactly to a tenth of an amp how many amp-hours (Ah) have been used from the battery. It will also calculate your watt hours used per mile and many other useful stats.
Also, I notice that Kepler chose for his hand grips the same comfortable grips used on the Stromer ST1.
Although the bike is a pure pedal assist with no traditional throttle fitted, there is still plenty of power delivery control available at the handlebars. To the left, a button can be seen next to the shifter. This can be pressed any time to override the pedal-assist and provide boost for taking off from a stand still, or making a quick overtaking move on an unsuspecting Lycra.
On the right there is a 3-position switch (Red) that sets the level of assist. Actual pedaling effort is measured through the Cycle Analyst via the torque sensing bottom bracket. The level of assist is then scaled against the amount of human effort applied. Switch Position-1 requires strong pedaling effort to get the level of assist up. Positions-2 and -3 make it much easier on the rider with the added bonus of position -3 having fully variable assist via a rotary knob, within easy reach between the 3-position switch and gear shifter.
Other neat features include the ability to see the actual human Watts produced by the rider plus cadence, so you know exactly how much effort you are putting in. An accurate summary of the rider’s wattage input over the length of the ride is also displayed together with power consumed, so you can see exactly now much you contributed to the ride.
Worth noting also that the Cycle Analyst can run up to 3 different fully programmable profiles. In Kepler’s case he has programmed an “off road” 950W profile, an Aust/Euro compliant 250W / 25-kph pedal-assist profile, and a USA 750W / 20-MPH compliant profile. Although the bike tends to stay in the “off road” profile most of the time, a fully compliant profile is only a few button pushes away (in our Getting Away with Riding an illegal bike story we mention a turbo button such as this).
All ebike builders know that tires can make a huge difference on the way an electric bike rides, as well as its efficiency and top speed. Kepler originally fitted 700 x 28c Gator Skins to the bike, to keep rolling resistance to a minimum. However, he soon found that the combination of a rigid carbon-fiber frame and skinny 100-psi sports tires was simply too unforgiving on a commuter that spent much of it’s time on bike paths. To help improve the overall ride, a set of 700 x 38c Hybrid tires were fitted. This made the bike far more compliant and comfortable to ride with only a minor sacrifice in overall rolling resistance. An added bonus was that puncture resistance was much improved making the bike ideal for road, bike path, and packed trail riding conditions (flat tires on an ebike are more of an issue than on a pedal bike, read why).
Kepler wanted this bike to be the “whole package” with as little compromise as possible. This braking setup was no exception. A set of Magura MT2 hydraulic disc brakes were selected for the job complete with floating discs back and front (same brakes as used on Specialized Turbo). These brakes are designed for heavy downhill duties, and as such provide incredible stopping power when used on such a light bike.
However, Kepler admits to perhaps getting a little too carried away with the brake spec and found the 203mm front rotor just too much for the bike, with even one-finger application being capable of throwing him over the handle bars. To rectify this, the 203mm front rotor was replaced with a more conservative 180mm rotor. This calmed the stopping power down nicely, and at the same time improved overall brake modulation proving that…bigger is not always better.
Home Built vs Production Ebike
The Specialized Turbo has a 250 watt motor, the Neo Jumper has a 250W (or 350 watt motor depending on what country you buy it in). The Stromer ST1 and the Currie Eflows have 500 watt motors. Keplers tiny motor burns up to 950 Watts, making his bike considerably faster than any of the previous bikes. Also Kepler was able to use higher-end components than those bikes including a carbon fiber frame and high quality hydraulic disc brakes. Total weight of the Super Commuter (25 pounds / 11.4 kg) is considerably lighter than all of those production ebikes.
Lightweight Build Accomplished!
Kepler’s Super Commuter came in at 25 pounds…not the lightest electric bike ever (see our story on the lightest) but definitely lighter than any production electric bike on the market now.
A lot lighter than Kepler’s Stealth Fighter (60 pounds! 27.2 kg), which he modified to be lighter than stock (read story about modifying the beast). It should be noted that the Kepler’s Stealth fighter is a totally different class of ebike. It has a real full suspension…It weighs 60 pounds but has a 40-MPH top speed and a range of 35 miles. Notice he needs to pick up that bike with 2 hands where as the Super Commuter he can pick up with one:
For an ebike to have ultimate performance (speed and range) you inevitably have to add weight to the bike. Kepler tries to make his electric bikes as light as possible while achieving the performance he wants. Obviously, riding the Super Commuter is a totally different kind of riding experience than riding his Stealth Fighter. Having both bikes in his garage he can choose based on the type of ride he is in the mood for.
According to Kepler: On 12S LiPo (44V nominal) and 350W, the bike will reach around 21-MPH (35-kph) on flat ground when unassisted. A bit of pedaling and still maintaining 21-MPH will see the motor-current drop off to around 200W on flat ground. 15-MPH was easy to maintain on gradients that normally see me drop down 10-MPH unassisted.
On 18S (72V nominal), speeds of up to 35-MPH could be maintained on flat ground with around 650W required for this sort of speed. With some pedaling effort this drops down to around 500W. Not great for maximizing range but nice to have when you want it. Its worth noting that both 12S and 18S setups will provide all the power this bike needs. However Kepler has now settled on 18S as the default battery configuration to take advantage of the higher cruise speeds associated with the higher voltage.
Estimating the range of an ebike can be a difficult task with most manufacturers quoting unrealistic figures based on an ideal set of conditions. On Kepler’s bike, having the ability to measure human input as well as electrical input makes for a far more realistic range estimate. Kepler reports that he likes to contribute an average of around 150W, and can track this average input in real time when riding.
On the low assist setting, the Cycle Analyst is programmed to set a power-to-assist ratio of 2:1. So in other words 150W human input will see the motor assist providing 300W. With 300-WH of battery on board (as pictured above), we now know that the bike will be able to run for an hour at this pace. With 450W total pushing the bike along, Kepler can confirm the bike will hold 25-MPH on flat ground. So based on this data, the realistic range of this bike is around 25 miles at this pace.
Kepler, being an admitted closet lycra rider himself, is allowed to tease the lycra guys a bit. Here is a riding story of his:
“I latched on a nice quick Lycra this morning. He was sitting on 35 to 40-kph most of the way on bike paths and was quick enough for me. Next thing this dude on a noisy clunker of an ebike with what looked to be a Cyclone mid-drive setup puts the moves on us and manages to get past us both on a really tight section of path. I hear the Lycra in front cursing him a bit, #?!* bloody electric bikes…I was having a bit of a laugh under my breath as we came to a stop at a set of lights. I pulled up next to him for a bit of a chat. He starts telling me about how pissed off he was that the ebike overtook him and how it was cheating, usual stuff. I just agreed with him and baited him a bit. During the conversation this guy had no idea I was on an ebike also. So we took off again as best lycra buddies, at which point I switched to 750W mode and silently sped off to a 45kph cruise speed. That was the last I saw of him. Mission accomplished “
What did it cost to build?
Kepler spent $3,000 total on this bike. Of course he needed to spend many hours of skilled labor putting it all together, which are not factored in this cost. The project took 2 months in total to complete, with around 5 hours a week spent working on the bike. The benefit of home building is you can get a much better deal than if you spend the money on a retail bike, and get something that is “illegally” fast.
Compare the Super Commuter to the cost of a production bike which is not as fast, not as light, and certainly not carbon-fiber:
Specialized Turbo 45-lbs $6,000
Stromer ST1 Platinum: 63-lbs $4,000
Easy Motion Neo Jumper 50-lbs $4,000
Currie Eflow 52-lbs $4,000