After a lot of research this weekend, I learned quite a bit more about electric bikes and how they work. I particularly concentrated on how the motor in the rear hub operates and how it can provide power without any moving parts. My first goal from this point will be to open the hub shell of an Ultra Motor brand hub motor in the rear wheel of a Stromer. I’ll detail all of it in a well-covered post.
Most of the information I found was on just a couple websites which I somehow had not come across yet. Electric Bike is definitely well organized and provided the most information about a variety of hub motors and their corresponding controllers and diagnostic procedures. Ypedal has an amazing and vast knowledge of the systems and has created tons of custom setups as well as details repair on several Youtube videos that is a complete procedure and well explained.
Most of the motors have a similar layout: An outer shell to protect and encase the motor, then a ring of magnets that are positioned between the shell and coils of copper wire. Other than a couple sensors and wires that exit through the center of the wheel near the axle, that’s about it. Very simple and once familiar with the parts, moderately easy to work on. I chose to first learn about the motor because it seems like this is the only component that I haven’t been able to service other than to simply install a replacement wheel. To have the ability to fix the motors once the manufacturer’s warranty expires will be essential — especially as the number of electric bikes is increasing and they are becoming more common.
Back to the sensors and wires in the motor. Usually there are three small square shaped sensors that are positioned between two of the outer magnets in line with each other. These are called Hall sensors and they measure the electrical current coming out of the motor as the wheel spins. A tapered side of the sensor body is always positioned outward and usually is set with a tiny amount of JB Weld or epoxy. Out of the top of each sensor are three uncovered wire leads that are soldered to colored wires carefully running to the center of the hub and exit from the wheel to the controller and potentially a torque sensor. A torque sensor is generally on higher end electric bikes while cheaper versions simply use a cadence magnet to calculate the input of added power. The torque sensor is nicer because it adds power based on a direct measurement of the deflection of the wheel backward against the rear dropout when you put pressure on the chain and pedals. Most are robust and work accurately, giving the rider a better feel of added power when accelerating.
In some situations, I read that one or more of the Hall sensors can go bad and with the right tools, it is a small project for an afternoon that will avoid a replacement ($500-750) or service elsewhere ($100/hr). Even if you aren’t up to doing the task yourself, you will be better able to diagnose and familiarize issues that occur. For the avid mechanic, it is possible to upgrade your hub motor for minimal cost. Larger wires (if they can fit through the frame and center of the wheel) will boost available power and a better controller or throttle can more accurately distribute the power. Unless you have an extreme desire to mod your electric bike, the magnets and copper wire are difficult and costly to replace. Some motors are spun low with larger gauge copper wire which can provide more power, but less torque on hilly terrain while high spin copper will have better power at a high cadence and will last longer.
In order to remove the motor from its hub shell, a car bearing puller is necessary. It is an extractor style tool with three arms that pull on the shell while a center bolt pushes the axle and opposite hub shell side. It is helpful and recommended that you tap the shell with a mallet during this process to aid in freeing the two halves. Otherwise, direct extraction with the bearing puller could break the hub flange and then you’ll be left with a useless motor and no shell.
Considering the Stromer that was detailed in the review and diagnosis, I believe that the torque sensor was overloaded in the frame without that necessary bolt and the motor overheated and fried one or more of the sensors. This would explain the NO_COMM lack of communication between the LCD controller and the motor.
Missing nut and spacer on axle. note the fine shavings of aluminum.
Note the scoring around the dropout.
It makes a lot of sense once the elements are broken down like that. I also think that one of the wires exiting the rear hub is a faulty wire with a weak spot somewhere near the center of the wheel. The torque sensor itself seems okay. I am going to check current through each of the wires to determine the faulty one and then solder in a new one along with the replacement of the sensors.
I was very happy to find this information and come to realization that these bikes can be fixed from the ground up if necessary. Why wait a week or weeks for new parts to install when the local hardware store and Radio Shack have all the necessary parts for a solution? I am going to continue to do more research into the motor repairs as well as connecting different controllers to the bike for operation. It is exciting to realize that with the right research, I can create incredibly fun bikes that are useful for both fun and transportation. Now, once I can locate a car bearing puller, I’ll open the motor and take photos of everything so you can see it. Stay tuned!
– SNC
secondnaturecycling 