Here is an electric bike made from the frame of a Puch Maxi moped. The builder is a young man named Bernd, who lives in Korneuburg (near Vienna), in the beautiful country of Austria. Although a moped frame is an unconventional choice, this frame style allowed Bernd (endless-sphere member puchwudi) to hide the significant battery pack inside the frame.
In Europe, mopeds are very common. A moped is differentiated from a scooter in several ways, the most obvious being that they have pedals. Moped laws vary by country, but…generally speaking, the legal definition of a moped is that it must have fully functional pedals, the engine has less than 2 horsepower (1500 watts), in a displacement that cannot exceed 50cc (3.1 cu in), and its top speed is limited to 30 miles per hour (48-km/h). According to California law, moped registration is only $16 for life, which makes them very affordable to use.
We recently began paying attention to mopeds when we noticed a new trend this year in using moped rims and tires for hot rod hubmotors on electric bicycles. Here is a pic of a Puch Maxi (German, pronounced Pook) in full 50cc stock form.
Although the frame is “only” stamped sheetmetal, the thickness and quality of the steel means that this frame is actually quite strong. Plus, European moped history has had a great deal of time to discover what works and what doesn’t.
In the pic above, this main frame version is actually shown after several attempts at a build-up. Here, it has had a steel pipe welded-in to form a top-tube, which strengthens the frame in a well-known way used by moped hot-rodders. The hollow down-tube section is normally used as the fuel tank, and will now house batteries.
Bernd first tried a battery pack made from large cylindrical Headway cells, but he was disappointed with the range of the pack. He then decided to use a smaller cell, like the common 18650 format, and also increase the internal volume of the frame by adding custom steel sideplates to the main engine-mount section.
Bernd wanted a large and powerful hubmotor, and although the motor itself would not be very stealthy, by doing that he hoped to retain the maximum amount of frame space for an internal battery pack, compared to mounting a non-hub motor in the frame. The hub he chose is from the Quan Shun electric scooter company. QS also makes hubmotors in several sizes even larger than this, but…this is the largest hubbie that we recommend for an electric bike, since it easily avoids getting hot when fed 7000W, and yet it is still small enough to use a one-cross pattern when laced to a 16-inch moped rim.
He needed to lengthen the wheelbase a little bit, and that would also allow him to convert from two side shock absorbers to a single centrally-located unit from Forsa. He put together the wheels early in the build process, to help him calculate the wheelbase and swingarm shape.
Don’t laugh because he used wood to determine the swingarm length! I have also done rapid prototyping with wood, and doing that is cheap, easy, and fast.
This picture below shows how the motor is significantly offset to one side when the short spokes are centrally laced. Larger diameter rims could have the spokes laced in a way to allow a certain amount of “dishing”, to move the rim to one side or the other, but…Bernd wanted to keep centrally-laced spokes and a very small-diameter rim.
He chose to add some opposing offset to the left side of the swingarm. In the pic below, the bike is upside down.
For braking force, the rear wheel doesn’t need a very large diameter disc to apply adequate braking (when braking, most of any bikes’ weight shifts to the front brake). A new trend I am seeing more often these days is a large-diameter rear disc being used, so that the caliper can be moved farther away from the axle, since hubmotors are fatter near the axle, and narrower near the motor edge.
The desirable moped rims do not allow for rim brakes, and most large hubmotors are chosen when very high power and high speeds are desired, so…rim brakes would not be an effective choice in that application.
Bernd test rode this ebike at several stages in the development, and at this point he noticed the rear single-speed freewheel had no problems tracking the chain, but the chainring at the bottom-bracket kept dropping the chain. Bicycle chains are able to shift external gears with a derailleur because they are designed to tolerate a certain amount of sideways offset in the chainline. Although the custom chainguides he fabricated and welded-in are an unconventional solution, he is satisfied with how quietly and reliably the chain tracks now.
This headlight fairing may seem like a minor feature, but I am very impressed by it. These pictures show how easy it is for any garage-builder to make some pretty impressive-looking panels. You may not need (or even want) a similar headlight fairing, but the techniques shown here can be used to make very professional panels that custom-fit any part of your ebike.
Stiff styrofoam can be cut by a razor-knife (or an electric hot-wire) and shaped by a coarse file quite easily. Sheets can be glued together to make the mold as thick as you need. Bernd made two different shapes because he didn’t know what shape he would like the most until they were done, and he wanted to get the messy part for both of them done all at once.
Once the final shapes are cut and filed, it is important to coat the shapes with some type of wax, so the epoxy doesn’t stick to the styrofoam after it’s dried.
Bernd applied three layers of fiberglass, which ended up with a 4mm thick wall (probably more than necessary). The first two layers were loose fibers, and the last layer was a cloth.
If you coated the mold cores with wax at the beginning, the styrofoam will be fairly easy to remove at this point.
Once the fairing was cut away from the plank, he used a Dremel to cut slots near the edges so velcro straps could hold it onto the suspension fork stanchions.
One of the features of the “Sondors” fatbike that really appealed to a large portion of their polled customers was the triangle-shaped hard case in the center of the frame. It not only held the battery pack, it also hid the controller and the wiring clutter that normally makes DIY ebikes look less professional. Of course, there are so many frames with hundreds of minor variations, that acquiring a perfectly-fitting set of panels is only possible if you make them yourself.
You can do it, so I want to start seeing more custom hard panels on those home-built ebikes!
The Battery Pack
Bernd started with a single pack made from the very large Headway cylinders. He liked the safe chemistry and the high current capability, but…he found the range disappointing. Their large size (38mm diameter, 140mm long), limited how many could be fit inside the frame space.
The black plastic retainers in the pic above were designed by Bernd on a computer screen, and then he ordered them to be made from a 3D printing service. They were surprisingly cheap and arrived quickly in the mail.
By switching to a smaller cell, Bernd was able to cram sub-packs into more of the hollow frame, so he could get the maximum possible range while still hiding the pack inside. How many? He was able to fit 176 of them!
Once he settled on the 18650-format (18mm in diameter, 65mm long), he determined he could fit 176 cells, which would allow a pack configured for 22 cells in series groups, and 8 cells per parallel group (22S / 8P). Using 22S with these 4.2V max cells results in 92V when fully charged, and is referred to as an 81V nominal pack.
He chose the Samsung 25R cell, which has proven itself over the last year as a reliable and safe choice for high performance. Each cell is capable of 20A, so eight of them in parallel will result in a 160A capability. Their 2500-mAh capacity means that 8P = 20-Ah of range.
If the idea of spot-welding 18650 cells together to make a custom battery appeals to you, here is our story of an ebike builder who did that.
The fat red and black wires carry the heavy current to drive the motor, and the thin wires will allow the Battery Management System (BMS) to balance each series group so their voltages end up exactly the same.
In the pic above, insulative padding is being glued into the inside of the hollow frame to add an extra layer of protection. You can also see “spiral wrap” that was added around the cables to help protect them too.
Time for Paint!
Once Bernd was satisfied the frame and battery packs, he dis-assembled everything to get all the parts off to be painted.
In front of the downtube, he mounted an 18-FET controller. It was an EB318 Xie Chang unit from Lyen, which has a programmable chip that was modeled after the Infineon.
Wiring is one of the biggest reasons many new enthusiasts are willing to pay a lot more to get a turn-key factory ebike. Plus having a warranty. However, if you are willing to do your homework, you can learn everything you need to know to do it right the first time, if you are determined.
There are many different BMS’s you can choose, and each manufacturer has its own set of features. One of the main features they all have in common is the ability to make sure that all the series strings are charged to the exact same voltage. After a bulk charge, there can be some variance in the amount of voltage that each series string ends up at. Charging them to 4.1V per series-string will dramatically extend the batteries life, but most ebikers charge to 4.2V to get slightly more range per charge.
BMS’s often limit the charging amperage, and also can be programmed to limit the output current. Some even have built-in heat protection, and a digital voltage read-out.
Austria has cold winters, and that leads to indoor projects! Here is a pic of the wooden stand he built to hold the frame at chest height, to make it easier to sort out the wiring.
In this pic, it’s hard to believe that he has 81V X 20-Ah = 1620 watt-hours of power hidden inside it! Maximum speed on flat ground is 85-km/h (53-MPH).
The 18-FET controller was adjusted to provide 85A of battery current, and 109A of motor phase current, and Bernd is very pleased with the performance. He is using an average of 81V X 85A = 6900W (9.2 horsepower). When the battery is fully-charged, it’s 92V at 85A = 7800W. The modest horsepower number is deceiving, because…electric hubmotors provide 100% of the available torque at one-RPM, all the way up to the top RPM’s.
Bernd beefed up the 18-FET controller in many ways so it could handle this level of power, but sadly…it finally died from the abuse. As of the publishing of this article, he is building up another controller that will handle his power levels reliably, and still stay reasonably cool while doing that.
If you want to see more of the pictures from the build process, you can view the discussion thread here.
Written by Ron/spinningmagnets, November 2015