I am making a particular choice with regard to the motor I am putting on the bike that is the subject of this series. I’m familiar with the various types – their strengths and weaknesses – but for the sake of the reader who may not be, I’m going to do a quick-and-dirty on each mainstream type. If I’m not covering that motor type (looking at you, friction motors) there’s a good reason that further research on your part will reveal.
There are three types of motors that make it into the mainstream of the ebike world
- direct drive hubs
- internally geared hubs
- mid drives
Direct Drive Hub Motors
DD hubs work with no moving parts and are effectively maintenance free. The axle of the bike is also the axle of the motor itself, which is just a brushless DC motor with magnets running around the outer case of the motor (the rotor), copper windings wrapped around the interior stator etc. Apply electricity and the stator repels (or attracts) the rotor, which results in the rotor spinning, and there is your powered motion.
Relative to the other motor types, DD motors produce much less torque. That means there’s not much oomph behind the motor and you either help it along with a lot of muscle if you want acceleration, or you sit back and wait for it to spool up (in a worst case scenario you are hoping to get to the other side of the intersection before the light turns red again).
To get around that lack of torque, you use a really big motor and a really big, powerful battery. Once you get into the 3kw-5kw and greater range, with a 60v or higher voltage battery, now you are talking about a bike that is accelerates acceptably (or insanely depending on how big you went) and can climb hills easily, even loaded with extra people and groceries.
The drawback to the above is that big motor, with its big metal magnets and large amounts of copper wiring is … big. Heavy. And so is the 2XL battery you needed to get big power out of that motor.
For a serious bike that can carry cargo, passengers etc. up a hill at speed, you are probably talking about a 125 lb bike, with a lot of that weight inside of the back wheel. The battery is likely going to be on your rack as well (not your cargo) reducing your carry capacity. At lower power levels (particularly those that are less illegal than the above noted 3-5kw) you aren’t looking at that kind of weight penalty. But you get low performance as a result. A direct drive motor that is not in the hi-power league will need a long run-up to get to cruising speed. And if you want to climb a hill… well its the worst choice for that job of the motors you can choose from unless, again, you go big.
If you want to research the ins and outs of this type of motor further, you need to also look into ‘torque arms’, why they are needed with this kind of motor, and when.
Geared Hub Motors
Geared hub motors do a good job of providing more torque than DD hubs at similar power levels. They do this by installing a planetary gear reduction inside the motor which connects the stator to the outer casing. The motor spins nice and fast as it likes to. The axle in turn spins the planetary gear. This finally turns the outside casing and the wheel at a slower speed, so you and the bike can go down the road at a comfortable rate of acceleration.
Geared hubs tend to be lighter than their direct drive cousins, which helps with range and acceleration. At sufficient power levels – lower ones than what a direct drive motor needs to do the same job – a geared hub gives some pretty good torque. 80 Nm in the case of the above pictured motor. Mate that to a 35 amp controller and a commonly available, medium-voltage 48v battery and you have a really peppy ebike.
Whats the down side? Those nylon gears will take a lot of abuse (really a lot), but they won’t last forever. Especially if you subject the motor to regular extended hill climbs, or you are subjecting it to a lot of stress… like a full cargo load. If you try to solve this problem with steel gears (Chinese Ali Express specials that may not have been the best re-engineering job) you find out why motor manufacturers use nylon: noise… and metal shavings.
You will also have to open those motors up every few thousand miles and re-grease them, as the grease perishes over time. Lastly, geared hubs really do not exist at the higher power levels (most I have seen are the MAC motors peaking at around 1500w with a special controller to get them up that high). Any higher than that and the gears really can’t handle the power. Unofficially, A Bafang G060 fat motor like in the picture above can handle a 60v battery and 35a controller that delivers 2.2 kw peaks … forever. But I wouldn’t bet my motor’s life on it being able to do long term that under severe loads like cargo duty or living in steep hills.
Final thoughts on hub drives
Both geared and direct hub motors power your bicycle directly through your hub axle. They are the hub in fact. What this means is, your bicycle powertrain is entirely irrelevant to what the motor does. The power to the ground is transmitted directly from your axle. In fact, if you want to have some fun with your hub bike, you can remove the chain. Then ride down the street with your pedal assist turned on, and pedal the bike. It will work just great. Of course you aren’t getting any exercise and likely you don’t want to ride like this, but it illustrates the fact that the traditional bicycle powertrain is not needed.
Your pedals and chain now exist almost solely to provide you, the rider, with exercise. You put as much effort into pedaling as you want, and that can be just mild pressure or pushing hard the way you would riding an analog bike. It is now the motor thru the axle that is doing the real transportation work. This fact is not lost on ebike manufacturers, and this greatly reduced duty cycle means hub-based ebikes tend to have cranks, chainrings and rear clusters that would not survive long if given the hard life an old-school analog bicycle receives.
Geared hub motors need torque arms just like direct drive hub motors, albeit not so much at the lowest power levels (i.e. 250w and 350w).
Lastly, both types of hub drives have this significant benefit: they don’t require any special knowledge or care to use. You can jump on the bike as a complete newbie and start riding. So long as you aren’t riding some kind of hot rod that is hard to control, you already know everything you need to ride happily down the road and not cause any issues. That makes hub motors of one sort or another preferred for most casual riders who are not challenged by their terrain. That isn’t true of your more powerful mid drives, but I’ll get to that below.
Mid Drive Motors
‘Mid drives’ are known as such because the motor sits at the middle of the bicycle. Typically replacing the bottom bracket. As much as hub drives dominate the lower-cost, DIY and upgrade ebike markets, mid drives dominate the big-name commercial-manufacture market. In particular, and most telling as to the benefits of the mid drive, E-MTB’s are exclusively mid drives, and for good reason.
Mid drives work on an entirely different principle than hub drives. Hubs, as we noted above, power your bike thru the axle, and your drivetrain is just along for the ride. In terms of assistance, the hub drive bike is a 1-speed, and this is part of the reason hub drives don’t do so well in hills or fast acceleration, unless you start getting into big power.
But a mid drive works just like you do: It pours on the power thru the bicycle chain. That means if you hit a hill, you can downshift into a lower gear, keep the chain spinning fast and get up the hill more easily.
Gee thats great. And since much of the world has legal limitations to 250w of final drive power, you can’t really put enough power into the system to break anything. An average person in good shape can put between 50 and 150 watts of power into their drivetrain during a ride. Thats what an old-school analog bicycle drivetrain expects to put up with. 250w isn’t a whole lot more than that (a trained cyclist can pour on roughly 400w or so… and sprint briefly up to around 1500w… which is still not really enough to make a slice of toast).
About that 250w limit… First of all, here in the U.S. that number is typically “less than 750 watts” according to our national manufacturing/consumer safety standard. Many of the individual U.S. states have vehicle codes that separately define what is an ebike vs. a moped vs. a motor vehicle.
Beyond that, in the last year or so we’ve started to see rebellion from major E-MTB manufacturers against the almost-a-joke 250w EU limits. What we are seeing is the complete disappearance of any mention of wattage output. Instead all you see references the Newton Meter (Nm = torque) output of the motor. Unstated is the *ahem* potential for the motor putting out more than 250w.
Really, torque output is what matters in terms of figuring out how much assist you are getting, and mid drives just pour on the torque. A Bafang BBSHD, the de facto volume-sales king of the American DIY market, puts out 160 Nm continuous torque when it is fully utilized. That is about double the momentary peak of what the big geared hub pictured above is capable of (and that geared hub is among the biggest of its genre). Direct drive hubs are in the 40-60Nm range unless you go all Mad Max on the power levels.
So … easy choice everyone needs a mid drive! Well, not so fast. With great power comes great
responsibility repair bills if you don’t use your head. That means build it right and learn how to ride it.
Remember the wattage output a normal human is capable of? The level that a quality drivetrain is expected to be able to handle? Well, the above referenced BBSHD in off-road mode, pouring out those 160 Nm, is feeding about 1500-1700 watts to the drivetrain. Continuously.
You want to figure out how much wattage is going to your motor? The formula is Volts * Amps = Watts. So a 52v battery running at its nominal 52v rating, multiplied by a BBSHD running at 30 amps is… 52*30=1560 watts. At a full charge that battery is 58.8v, so 58.8*30=1764 watts. Continuous output. Yes, really.
So, when we build a DIY mid drive bike, we first want to buy parts that are meant to take this kind of punishment. They are out there on the market but frankly a lot of DIY builders, riders and even most ebike sellers are ignorant of this. You want:
A good narrow/wide front chainring
Made of 7075 alloy most likely, but if you can get a steel ring, do it (Wolf Tooth is the only one I know of and they only come in 30T and 32T sizes). This style of ring typically has wider, taller teeth that eliminate chain dropping issues. In particular rings made specifically for mid drives are sold by Luna Cycle, who manufactures their own here in the USA, and Lekkie, a New Zealand company with a stellar reputation who sells thru ebike vendors everywhere. The latter two names are focused primarily on the BBS02 and BBSHD markets although Luna does make rings that will work on other platforms.
An ebike-specific chain
The interwebs are filled with complainers crying about how their chain snapped. When you ask how many of them re-used their $6 stock chain, or who just bought a ‘nicer’ bicycle chain, the numbers pretty much climb up around 100% of chain failures (chain alignment will be dealt with below). E-bike specific chains in various widths for various speeds are sold by KMC (my favorite is the X9e/E9 9-speed) in 136 link lengths. SRAM’s EX1 ebike group has its own 144 link chain. Lastly there are the Connex chains by Wipperman. The common factor in why people don’t use them (besides not knowing they exist) is price. These are US$35+ chains (you can buy smart and get them for less if you know where to shop). But… They. Don’t. Break.
Lastly, you can find a specialty vendor and buy a specific length of chain all in one piece, so you don’t have to section two chains together (if you do that, it creates a potential weakness at the joining point). This is by far the most expensive option. I bought a 7-foot length of 9-speed chain from Luna Cycle and it ran me about $60. But my Mongoose Envoy, with the long-cage Deore derailleur I added, needed 152 links to be set up right (I keep a 144-link SRAM EX1 as a hot spare and it will work fine in a pinch).
A steel cassette cluster
You have two choices for this, generally. First is the SRAM EX1 cluster that is an 8-speed, has a range of 11-40, is made of tool steel and meant to be used with the EX1 shifter which will only shift one gear at a time. The rub is the cluster alone runs about US$385. Its worth every penny (I have one on my E-MTB, so it had better be), but that cost is insane. How about spending US$15-25 instead? Just buy any cheap Shimano rear cluster. In particular the HG-200, the HG-400 or the HG-50. All in 8 or 9-speed. They use steel, not alloy, cogs, and most importantly the entire cluster is welded together into a single unit so the punishment dealt to the cassette body is distributed across its entire width. These Shimano clusters are an excellent example of something that is awful for an analog bicycle and highly preferred on an ebike, where durability is vastly more important than light weight.
A steel cassette body
Here again, what sucks for a bicycle is great for an ebike. A steel body will last. An alloy one won’t. Take a look below. On the left is an alloy DT Swiss cassette body with about 1600 miles on it. It comes from a DT350 hub, which is at or near the top of the line as bicycle component brands go. The cassette cluster I used was a welded steel Shimano, so those notches you see still tore into it despite the gentler damage the welded cluster does. I almost exclusively used the 11T small gear on this bike and on the far right you can see that section is torn into further than the rest (the last cog is free floating so no help from the welded together body on that one… we’ll come back to this and discuss further below).
On the right side is a steel version of that DT350 cassette body. Unlike the alloy version, it is much heavier – and I expect it to last forever. Worth noting: DT Swiss has now released a “Hybrid” version of the 350 hub specifically meant for ebikes. It includes the steel cassette body out of the gate as just one of its durability improvements.
Get a ‘star ratchet’ rear hub
There aren’t many of them. DT Swiss, Chris King and Hope are the only big names that sell freehubs with this sort of splined engagement instead of the traditional 3- or 4 pawls. A splined engagement provides dramatically better contact with the hub from the cassette. See that nearly-ruined cassette body above? the stock 18-tooth star ratchet wheels inside went right back into the bike with the new steel cassette body… they were still perfect. Since DT’s patent on their system ran out, other makers have begun to use it and you can now find star ratchet replacements and complete hub systems on Ali Express, EBay etc. Myself, I still buy DT Swiss 350’s. But you can save hundreds with the Chinese hubs.
Learn how to ride it
I mentioned this briefly above. With a couple of narrow exceptions (don’t mash the throttle going up a long hill) you already know how to ride a bike that has a hub drive. thing is, no matter how seasoned and smart you think you are, chances are excellent you are clueless on how to ride a mid drive.
Here’s the short version: Keep the motor spinning.
Now the longer one:
Keep the motor spinning
Lug it and the torque that is pouring out of the motor will focus on tearing your chain apart, or taco’ing your chainring or rear cog, not to mention generating enormous heat (remember the nylon gears in a geared hub motor? Guess what? Mid drives use nylon gears inside too). Even a BBSHD set to off-road power levels is not strong enough to tear up your cogs or chainrings. But it can snap a chain that you are mistreating.
When coming up to a stop light, downshift.
Always. Either that or stay in a gear that is in the middle of your cluster so that when you start up again, the motor can spin up quickly without any brutality being visited on the chain.
When coming up to a hill, downshift.
Always. Can you guess why? Thats right so you can keep the motor spinning. And ‘coming up to a hill’ does not mean ‘already started up the hill’. Anticipate and shift in advance of the climb.
When you want to go faster, upshift.
But wait until your motor is spinning fast before you do.
When you up- or downshift, NEVER do so under power.
Shifting while pouring huge watts into your chain is an ugly thing. You will recognize your mistake the instant the result hits your ears. It won’t kill the chain outright, but as you hear that chain smash from one cog to another you will know your bike hates you very, very much.
You can invest in a gear sensor that will protect you automagically from this. It installs inline on your shifter cable and, when it senses the tiniest amount of movement, it cuts power for an instant. The result is a safe shift. I have them on one of my three BBSHD-equipped bikes and it works great.
But for the two I don’t, I just stop pedaling/freeze my legs, click-shift and then do a single crankarm rotation to seat the new gear at low power. Result is perfect shifting and only a minor blip in pedaling rhythm. But that is a learned behavior.
Others have perfected the use of the brake levers as a clutch where they only slightly actuate the lever. This triggers the safety cutoff which in turn allows a safe shift. If you are like me and you cheaped out and don’t have safety cutoffs, this won’t work.
Keep chain alignment as straight as you can
Mid drive motors tend to work in a lot wider range than humans do. So you can leave the motor in a gear that would be too low for your cadence and let it spin away like crazy… it actually likes it that way. So, this piece of advice is partly about how you ride the bike (i.e. what gears you let it sit in) but also about how you build it. You really only need three or four gears in the middle of your cluster on a mid-drive-powered bike. You want them to be the ones that let the motor spin fast. You also want the cogs the bike is happiest to not be cockeyed, front to back (i.e. bad chain alignment).
On an analog bike you can get away with a lot, since you are only feeding back 150 watts to it. Feed it 1500 and that sideways-skewed chain will become a saw and chew right through your front chainring and rear cog teeth. Be smart when you build the bike, and learn in your first outing or two whether there are any problem gears you should stay away from. There are all sorts of offset chainrings (and 1mm and 2mm shims) available for a BBS02 and BBSHD… they cost money, but spending that money now means not spending it later after you have walked home.
What happens if you don’t do some or all of these things above to install and use a DIY mid drive bike properly?
Well of course it means you go on the internet and blame the equipment. Its not your fault you used the wrong components. And its not your fault you didn’t know how to ride it. Its the mid drive’s fault.
This is the secret message hiding behind a lot of “don’t buy a mid drive” posts on the interwebs.
… If you build with appropriate components, and ride it smart, even a high powered mid drive will essentially last forever. Yeah sure you will wear out the chain and rear cluster in say three thousand miles, the smallest cog in half that, and the chainrings in 10. But thats peanuts considering how many miles you put on the bike. How much does an 11T cog cost (about $6)?
Wrapping it all up
Whew that was a lot of typing. This article lays out in broad terms the characteristics of each motor type. It doesn’t get into what kind of riding each is good for. Lets finish with that:
Direct drive hubs
- Lower/legal power: maintenance free cruiser bikes for paved streets where speed and acceleration are secondary to bulletproof reliability. Want a bike for Mom? Your kids (who aren’t future BMX pro riders) A direct drive 250w-750w hub motor is a viable candidate.
- High power: Sky is the limit in terms of power: Light electric motorcycles only thinly disguised as bicycles. Can range across a wide variety of cycling genres including cargo, dirt and pavement. But big and heavy. Single speed is usually a bad choice for offroad/singletrack riding but there are exceptions, in particular the famous B52 Stealth Bomber and similar. They have so much power they use brute force to overcome the weight issue. But you’d never pedal one of these things.
- The Swiss Army Knife of motors. Not ideal for anything but good for almost everything. Only runs into trouble under very heavy use, particularly in an area that is all hills.
- Their only drawback is maintenance if heavily used. Semi-annual teardowns to re-grease a high mileage motor is advisable.
- Bad choice for singletrack/offroad.
- Best choice for singletrack/offroad
- Got hills? Mid drive. Strong power delivery without significant weight added to the bike.
- Good for hard-use applications.
- Arguably the most efficient in terms of power consumption.
- Requires the most attention to build detail and demands the most attention and learning from the rider.
- Even with proper use and components, more wear and tear on the drivetrain than any other option.
- A kid who gets his right hand caught inside the chain of a mid drive is going to be named ‘Lefty’ from that day forward.