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How to pick out a hubmotor kit for your bike build

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    How to pick out a hubmotor kit for your bike build

    The most important thing when starting a bike build is to figure out what is going to work best for the bike you have and what you need it for. What you need to buy may change depending on how hilly it is in your area, rider weight, the design of the frame, and how fast you want to go.

    Some of the topics we will discuss below are: measuring your dropouts, deciding how much you want to spend, how much power you want, whether you want front or rear, whether you want direct drive or geared, what battery you want this with, whether you need torque arms, pedal assist, and accessories.

    Measuring your dropouts
    this is one of the most essential things to confirm before buying a hubmotor. Most companies selling a hubmotor kit will provide this information. So you need to make sure that what you are buying matches what can be put on the frame.
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    This picture gives you the general idea for what we are going for here. You want to measure from the inside face of the portion of the bike frame where the wheel mounts onto. On most regular bicycles you will get a number like 100 mm on the front and 135 mm on the rear. For more information on measuring dropouts see this link.
    Please note if you happen to have a expensive mountain bike you may have thru-axle, which is a different standard than the sort of dropouts we are discussing here. If your bike doesn't use nuts and bolts at the axle where the wheel sits then it is thru-axle, otherwise you are fine.
    So you now know what motors will fit on your frame, at this point you need to know whether you want front or rear.
    Typically for most people you want rear, please see this link for a discussion on pros versus cons of front versus rear
    of particular note here is that if you have front suspension on the bike it's not desirable to put a hubmotor on the front. Suspension forks were not built to be pulled laterally in that way and it may wear out the seals over time.

    Power, torque, price
    These considerations are somewhat related. First thing you want to consider is whether you will be going up a lot of hills, are overweight, or are towing something. If so you may want to consider a geared hubmotor, this allows the motor to spin faster than it would otherwise and is more efficient at lower speeds as well as provides significantly more torque. The downside of this is that it has more moving parts so you may take a hit on long term reliability. Additionally since geared hubmotor has more moving parts it may make more noise although practically speaking it's not enough to be noticeable in most instances.

    When it comes to power, this is often linked with price, the more power you want the more expensive it is. For the most part high power geared hub motors do not exist above 1000 W, and you might find some as low as 250 W. For most direct drive hubmotors you won't find many that are less than 500 W and some go as high as 10,000 W. The legal limit for power output in the USA is around 750 W in most states, which in either motor is enough to get you to around 28 mph, give or take depending on conditions.

    For more information on the differences between these types of motors check out this link
    Torque arms
    most of the time you want at least one torque arm. This prevents your motor from spinning out of the dropouts, in other words the motor wants to spin the axle with a lot of force and the dropouts want to hold it in place, it is possible to generate so much power that it breaks the dropouts. So what the torque arm does is reinforce the motor axle onto the frame of the bike so that it won't spin out.
    For more information on how this works and its usage please check out this link on torque arms
    while it's generally a good idea to have at least one, often times a kit will only include one and it might be good to have another on the other side if you are using a high power hubmotor. We sell individual torque arms here
    Pedal assist
    for various reasons this is mostly a feature that you find on mid drive kits versus a hubmotor. On the few hubmotor kits that do have pedal assist many users find that it is substandard, that's why it's not a common feature. If you need pedal assist and you consider this to be a dealbreaker you may want to go with a mid drive instead, see here for an example of a mid drive
    see this link for a comparison of mid drives versus hubmotors
    Battery options
    often your battery option will depend on the shape of your frame and how much space you have. For example full suspension bikes often have little to no center triangle and as such it makes very difficult battery placement, some riders choose to keep the battery in their backpack in these situations. If you happen to have enough space in the triangle for a hardcase battery to be mounted this is a good choice since it has some lockable security as well as a protective case. Your other option is soft pack batteries in a triangle bag or a battery box of your own design.

    Often if you have a lot of room to place your batteries and want a lot of range, a softpack will be your best bet as hardcase packs are only built so big, while softpack can more easily be built in different shapes and sizes.

    A good rule of thumb when it comes to batteries is to get as much capacity as you can fit or as much capacity as you can afford. The bigger the battery the less stressed it will be when pulling a lot of continuous power as well as less potential range issues if you ever need to go a farther distance than you previously anticipated. Additionally a larger battery, if treated right as described on the battery documentation, will last a longer number of cycles than a smaller battery.

    Also you will probably need to make a power connection, more details in the installation section
    here for soft packs
    and here for hardcase batteries
    Depending on the kit various accessories might be applicable, however one common accessory is often of particular usefulness and that is a wattmeter.
    See here for an example. This is an affordable way to get a much more accurate idea of power usage on your motor than the default display, assuming your kit has a default display. Regardless of the battery type this can be wired up on the power wiring so it goes from battery> wattmeter> controller. There are many designs with various features and prices to choose from.

    Another potential accessory may be lighting. Since the bike already provides power for the bike and you likely will need to connect the power wiring between battery and controller, it is not much more work to splice in wiring to a lighting system capable of doing the sort of voltage your battery will be putting out.
    Last edited by Sebz; 01-04-2018, 10:33 AM.

    As a daily commuter using a hub motor, I just want to remind people that (1) with a hub motor, changing a flat tire in the rear is considerably more work than with a mid drive, and (2) a larger hub motor will interfere with a large cassette (or freewheel), and thus limit your number of gears. If I were to start my project over again, I would strongly consider a mid drive.

    Also, if it is important to the consumer that the electrical system be hidden (e.g. from LE), a hub motor has a big advantage because one can cover the hub motor with panniers and hide the controller and battery in bicycle luggage. On many occasions, I have had passing police cars take an interest in my bike, and I watched them scan the bike and then move on because there is no motor, battery or controller to be seen. This is why I always pedal as much as I can. It is important to me to avoid a ticket or confiscation. If one were to build a legal bike, then this issue is a moot point.

    Also, I like to overbuild a system (I always say "Send a man to do a boy's job") so that the motor, battery and controller will not be constantly pushing against their limits and thus live a long and easy life. We are talking about heat here. As such, I would recommend purposely choosing a significantly larger motor, higher FET controller, larger BMS and higher discharge cells (and/or higher Ah battery) than needed. Of course, this adds cost and weight, but having failed components is aggravating and expensive. Following safety, I think reliability is the most important goal.

    After overbuilding a system, I like to limit the current a little below whatever the weakest link in the system can handle. I ended up limiting my current to 28-30A on my 72V systems, and the motor, battery and controller are definitely having an easy life, but I can still average 30 mph while propelling a (rider + bike + cargo) total of 325 lbs.
    Last edited by commuter ebikes; 08-20-2017, 07:38 AM.


      yes this is a home built trike. My 4th trike but first with electric go-power


        Click image for larger version

Name:	5E79D154-EED6-4DDB-B65D-4D512044715B.jpeg
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ID:	123078 How do I add a rear electric hub on a vintage three speed Raleigh bicycle?


        • 73Eldo
          73Eldo commented
          Editing a comment
          If you did a rear hub motor that would involve removing the 3 speed hub and replacing it with the hub motor. You could do a mid drive like a BBS02 or HD which replaces the crank and pedals with a motor that has new crank and pedals. That would work with the 3 speed and hand brakes just fine.


        threads are spaced 24 to an inch


          Thank you Eldo for your reply and help!

          If I used the rear hub version that would eliminate any gear changing but with an electric motor why would that be necessary if I choose a good rear hub?

          with the mid drive as you recommended I may come across a threading problem. The standard seems to be 24 tpi but my vintage Raleigh bb is threaded at 26 threads per inch.

          I hope I can find a way around this.


            I ended up using a mid drive Tongsheng 350v motor and it is progressing very well!