Here is an ebike where the builder is using a common hubmotor, but…he is mounting the motor into the frame, and connecting it to the rear wheel by a chain. Of course, most available frames wouldn’t allow for a motor to be mounted there, so…he simply brazed-up a custom frame. There are several benefits to doing this, and if this kind of thing interests you, read on…
There are several things I like about this ebike. As soon as a saw it, this ebike jumped to the top of the waiting list of articles I wanted to write. For several years I have known about a variety of builds that used a hubmotor as a non-hub build. However, this one did a great job of engineering the details, and also provided enough pictures to show future builders how it could be done.
A common hubmotor is almost a commodity item. They are literally mass-produced by the millions in China, and the few ebikers in the USA who buy them can only choose from the selection that the Chinese manufacturers provide to Asia and Europe. However, the selection has improved over the last few years, so that we here in the US finally have a fairly good selection of options. Hubmotors are narrow enough to readily fit inside a bicycle frame, and the most common direct-drive (DD) hubmotor size has a 205mm diameter stator.
Geared hubmotors have a lot of torque for their size, but….you must use them within their rated power levels because they have a poor heat-shedding pathway from the interior of the motor to the outside air. Although…I must admit that if you gave a large geared hubmotor the use of seven derailleur gears (like the way this build does, or maybe 3-speeds in an IGH?), then…it would actually be a viable option that was slightly smaller in diameter than the Crystalyte shown.
The DD hubmotors can take quite a bit more heat than geared hubmotors (in their stock form), and…they also have the option of adding Ferrofluid and hubsinks to increase their ability to shed heat. So, geared hubmotors are a great solution for a mild commuter, but for hot-rodding?…a DD hubmotor is the way to go.
Most bicycles don’t have enough room to mount the battery and also a DD hubmotor inside the frame. But…if you manage to do this, there are several performance benefits. It would be easy to use available sprocket-sizes that would provide a 3:1 ratio between the motor and the wheel, so lets discuss that as the theoretical option.
DD hubmotors (by design) will spin at the RPM’s of the rear wheel. For a common 26-inch bicycle wheel that is spinning at the US national ebike speed limit of 28-MPH?…that is only 362-RPM’s. Spinning that slow means you are leaving a LOT of horsepower on the table. Most industrial motors operate at their most efficient speed, which is near 3,000-RPM’s. Higher RPM’s than that have some power benefits, but it would also have higher frictional losses. If we are using an easily-sourced 3:1 sprocket ratio, then…we can spin the motor three times faster, for about 1,100-RPM’s. It’s not the more ideal 3,000-RPM’s, but…it’s a hell of a lot better than the stock 360-RPMs when it comes to getting the max possible power and the resulting lower amp-heat.
This builder has reported that this ebike accelerates quite well, and also climbs hills well…and that is even while it is currently using only 36V.
Since the rear wheel has a freewheeling freehub, the motor only needs a solid (non-freewheeling) sprocket to drive the rear wheel. As you can see, the “track cog” shown at the bottom left of the pic above is a 17-tooth that is made for a common 3/32 bicycle chain (17T is the smallest that will work with the adapters shown in the same pic). The next stronger size up would be a 1/8th inch BMX chain, which is still an option. However, an 1/8th inch chain would not be able to run through a derailleur, and it would be restricted to driving a single-speed, or a 3-speed IGH.
The technical problem that this build answers is…how to let the motor drive the rear wheel, while also letting the pedals remain stationary? If the motor is driving the pedals at the same time that it is driving the motor, then the pedals might strike the ground when turning in a curve (causing a crash), or…it might also swing around and hit the back of your leg, if your foot slips off the pedal…
Most mid-drives use a freewheeling bottom bracket. There is nothing “wrong” with doing that, however…most freewheeling BB’s are expensive, and they have to try to fit a wide variety of frames. If you use a common “non-freewheeling” BB pedal set, they are readily available and cheap. Using the one that came with the frame you have is guaranteed to work. But…there needs to be a freewheel somewhere between the pedals and the motor, so…Mr Appel has used a clever assortment of available bicycle parts to attach a FW to the motor, for handling the pedal input to the drivetrain.
The odd part in this system is the cylinder with threads on the OD, called a “left-side bottom bracket cup”. An aluminum BB cup would probably work fine, and they can also be found in steel.
If your bike has a common bottom bracket, it will have 1.375 x 24 threads (sometimes advertised as 1.370 x 24 tpi). At the top left (of the pic above), there are two adapters which incorporate the 6-hole disc brake interface, and also have a 1.370 x 24 tpi inside diameter (ID) threads. Those are designed to convert a common threaded wheel-hub to a solid fixed track-cog (commonly called a “fixie”). The 1.370 x 24 tpi threads on common hubs are normally used to install some type of freewheel on the right side.
As you can see, he attached two of the 6-hole brake disc adapters onto a track cog, and then spun that assembly down tight onto the threaded hub of the Crystalyte motor. This resulted in the ID-threaded adapters protruding out farther than the original OD-threaded sideplate-shell, and it presented ID threads for the BB cup the be spun-on. Then, he spun the BB cup into the the brake adapter, which then provided an OD threaded shell for the single-speed freewheel to be attached to it.
This collection of parts was never designed to be used together, but…they all fit, and they create a great optional drivetrain for ebike builders.
By his own admission, the battery pack this builder put together is mild. He is using six DeWalt 5S / 18V 5.0-Ah packs (since he has a lot of DeWalt cordless tools), in order to make a 2S / 3P system resulting in 36V and 15-Ah. He is planning on upgrading to more power someday, but he had the tools and skills to put this together fairly fast, so…here it is.
A while back, I wrote an article on using a variety of cordless tool batteries for ebikes, so if this interests you, take a look here. Plus, here is an article on a cruiser that also used cordless tool packs.
The copper-colored box at the top left of the pic above is the controller. It looks like a common 12-FET unit, which is appropriate for a motor of this size. However, since gearing allows the motor to spin many times faster than the rear wheel (and also the side-plates have had ventilation cooling holes drilled into them)…an 18-FET controller (which could provide more amps) is not out of the question for a future upgrade.
Building a Custom Frame
The pic below shows a frame jig. It holds all the tubing pieces in perfect alignment while they are tacked into place in a handful of small spots while on they are on the jig. Then, the frame can be removed from the jig and oriented so the builder can get the best possible angle to connect a joint with a solid braze or weld.
There are a couple of mild bends in a few of these tubes, but for the most part the frame is made from straight tubing.
The pic below is some straight-up frame-building porn. If you are using a brazing torch to assemble a frame, the filler material cannot fill-in gaps as well as a common MIG/TIG welder, so…the good prep work is needed to start-off with a good fit, in order to achieve good results. Mr Appel decided to use brazing to connect the tubes instead of welding, since he already had the equipment, and was skilled at doing it this way.
APL’s early versions
How did this all get started? This builder has a wide variety of fun projects he fiddles with, when he can find some time to play. As a result he has to bounce back and forth between his home and his shop, so he pondered a way to zip around a little faster. He restored an old 20-inch wheeled bicycle frame, and attached a small brushed motor he had laying around (Currie?). It proved to be a success, but he immediately wanted more power, and a larger frame that fit him better…
In the pic below, you can see the second version (V2), which used a frame he designed and built from scratch.
In the pic above, you can see how this builder had progressed to a larger and more powerful brushless motor, but kept the motor in the frame instead of simply swapping-in a hubmotor to the rear wheel. The controller is located just in front of the motor, and the battery (along with the batteries’ weight) is the rectangle-shape that is distributed across the entire bottom of the frame.
This is a common Crystalyte hubmotor with a 35mm wide stator (the H35 series). I like how he painted the steel magnet-backing rim red. In this application, he could have cut the steel spoke rims off to reduce the diameter and weight (a tip for those who want to copy this build). The aluminum sideplates have been sanded to remove the black paint, and then polished for a sweet-looking sheen. He has also cut ventilation holes into the sideplates to allow any accumulating heat to radiate out, which will help to keep the motor cooler.
The final version of this cruiser uses a motorcycle seat, and it uses a seat-suspension system that has often been found on vintage motorcycles. The support beam incorporates a pivot at the front, and there is a spring-element at the rear. The V1 pic at the top of this section uses two chromed coil-springs, and the V2 pictured just above uses a single air-shock. Since this is a “hardtail” frame, a possible V4 might use a fatter rear tire to help soak up the road bumps…
The national speed limit for electric bikes in the US is 28-MPH, and if I needed a commuter to get to work, something like this is an ebike I could live with every day. My most often used ebike is a stretch cruiser with a BBSHD mid-drive. It allows me to blend-in with the mild traffic near my home, while providing a comfortable riding posture and a decent amount of power.
This build has the comfortable posture I like, and even if you only need a low top-speed, it is nice to be able to “accelerate briskly” to avoid 3,000-lb cars with texting drivers. Having the motor as a separate element from the wheel means that you can climb some serious hills without any fear that the motor will overheat, because the motor can be spinning several times faster than the rear wheel.
Cruisers are a style that is usually very polarizing. Either you love them or hate them, and there is typically no middle ground. The message of this build isn’t that you should consider building a cruiser, but that…you should consider building a custom frame that YOU like, something that you can be passionate about, and something that is fun for you.
Have fun, and ride safe my friends.
Written by Ron/spinningmagnets, August 2018