“Mid Drives For Dummies”
This article is based on a portion of this post where I discuss the strength and weaknesses of different types of ebike motors. I link that article many times in help discussions, but usually only for the part about how to ride a mid drive without excess drivetrain wear, mechanical failures etc… so I am creating this standalone post on the subject… and stealing liberally from the original.
This post now has a companion: How to Build A Mid Drive Ebike That Doesn't Break.
Mid drive motors on ebikes are very common in the production-line, name-brand-manufacturer ebike world. Its safe to say they dominate the industry for eMTBs. Why is this?
Hub motors power the bike thru the axle, so they are single-speed: their motive power has nothing to do with the chain, chainrings or cogs. Try taking off your chain and then go pedal the bike around. Pedal assist will work just fine. The chain and chainrings are only there for you to slug it out with; the motor couldn’t care less.
Since hub motors are single-speed, that means they are not happy climbing hills… for the same reason your life sucks trying to do the same thing with no gears. The only fix for this is to run thousands of watts thru the hub (we are talking 3-6kw or more, which is approaching e-motorcycle territory).
Unlike hub motors, mid-drives power the bike thru the drivetrain. They use the gears just like you do. This is a good thing for the same reasons its good for you.
Only a fanatic or a penitent rides hills on a single speed bike. So how is it desirable to do that with an electric motor? Spoiler Alert: it’s not. A single speed hub motor is often strong enough to help get you up that hill. But its not happy doing it, and its not good for the motor or (if it has them) the gears inside of it.
If you have only had a hub drive ebike you won’t realize just HOW unhappy until you take your first proper ride up a steep hill on a mid drive ebike. Get it in the right gear from the start and the bike simply doesn’t care that its going up a hill. It scoots right up without breaking a sweat.
The benefit is multiplied when you look at a mid drive’s motor specs. Usually they are more powerful than a hub drive by a wide margin. A typical hub puts out 40-60 Nm of torque, with a few going up to 80 Nm. Production mid drives usually start there as the bottom end. Aftermarket motors commonly put out 120-250 Nm.
Well, if you aren’t familiar with what it means to have X Newton Meters of torque going thru your drivetrain, lets use the more common (but functionally useless) measure of watts:
- That 180Nm motor pictured above has a peak output of 3000 watts.
- A BBSHD or a Bafang Ultra peaks at 1750 watts (peak power on the BBSHD can also be maintained continuously so its REALLY a beast).
- A 48v BBS02 is about a thousand watts.
- your typical street-legal pissant EU motor is rated for 250 watts (pssst… the manufacturers are now all cheating on this. Don’t tell anybody).
- A normal cyclist on an analog bike is capable of putting out roughly 300 watts over the span of a few minutes.
- A professional sprinter/mutant can hold almost 1000 watts, but only for a minute or two (thats not enough to make a slice of toast).
Yeah ‘oh’ is right. Your mid drive is pumping a metric shipload of power thru your drivetrain. That power is likely more than standard bicycle parts were meant to handle. So how do you have a motor this powerful (its not as much of a boost as the math makes it sound like) and not bend, break or snap stuff?
It Ain’t Hard To Do Right…
…but you gotta do it. Here then are the rules of the game when riding a powerful mid drive motor.
The Short Version: Keep the motor spinning.
Now the Long Version:
Keep The Motor Spinning
Here’s a basic tenet that is true of all electric motors: Electrical power goes towards turning the motor and producing forward momentum. If there is resistance – which keeps the motor from free-spinning – then instead of forward rotation, the electrical energy is converted to heat. Mid drives have so much power that they can get really hot, really quick if not allowed to spin up. But they are so powerful, they might not just stop at generating heat.
Lug a powerful mid drive and the torque that is pouring out of it could tear your chain apart – if it can’t rotate it thanks to resistance. Or, you might discover what it means to ‘taco’ your front chainring or rear cog. If your sins are not quite that egregious, and you just lug it gently enough to not tear something apart, then within the span of a single ride it can ‘peanut butter’ the nylon gears inside your motor; bricking that motor and potentially requiring you to carry the bike anywhere you plan for it to go.
Thats very, very bad. So don’t let it happen. Here’s how we do that:
When Coming To A Stoplight, Downshift!
Always. Either that or stay in a gear that is in the middle of your cluster so when you start up again, the motor does not lug itself. You spin up quickly, without any brutality being visited on the drivetrain.
From a standing start, a mid drive will slowly tear into the cassette body, or damage the pawls inside. this will eventually tear the freehub apart and kill the hub. Which means you get to build a new wheel.
If you downshift so the motor doesn’t tear into your drivetrain when you start back up again, you’ll be fine. So remember: downshift before you come to a stop.
Sidebar: This is a gear cluster. Each wheel is a 'cog'. Smaller cogs are higher gears (you go faster). Bigger cogs are lower gears (slower, distributes torque, gentler on the chain). This cluster is a SunRace CSMX8 11-46T: The smallest cog has 11 teeth and the largest has 46.
When You Want to Go Faster, Upshift
When working a mid drive, just like driving a classic sports car, you have to ‘row’ through the gears, both slowing down and speeding up. Wait until your motor is maxed out before you kick it up a gear (up = a smaller cog in the back). Chances are good its going to be smarter to stay one gear down from what you would have used without a motor (down = a bigger cog in the back). Why is that? Your bike will spin up to the same top speed on its next-highest gear (the next-bigger cog in back) as it will the highest one: But it will get there faster if you let it use the bigger/lower cog next to it. Mid drives are like that, especially when going fast on the street. Here again we are going back to not lugging the motor, and letting the mid drive spin faster than you ever would. Unlike you who has to do the pedaling, they like it that way.
Again thinking of your mid drive ebike as if its an exotic sports car with a manual transmission: In between each gear you need to let off the power, shift and hit the accelerator (the throttle). If you have a gear sensor you will not have to worry (officially) about the ‘let off the power’ part as that will be safely done for you.
Note that above, I am talking exclusively about when you are using the throttle. If you want to pedal the bike thats no problem… just use pedal assist and set your power level to a lower setting. Take care not to overdo the boost, and keep your legs spinning fast via smart gear choices – just like on a regular bicycle – so you never lug the bike with slow pedaling up a steep hill
If you are pedaling slow on flat ground, or downhill, you are not providing resistance to the motor or pressure on the chain. So there is a lot less to worry about there insofar as cadence or lugging the motor is concerned. You do however need to make sure you ALWAYS do the following no matter what the terrain is:
Do Not Shift Under Power
Even if you have a gear sensor. Thats right I said it. Don’t trust the gear sensor unless you are forced to. Lift for just a sec and do your shift.
More Specifics on Mid Drive Shifting here: "Do I Want A Gear Sensor?"
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.
If you treat the gear sensor as a fail-safe rather than taking it for granted, you will be much more likely to avoid disaster. Now, as you become familiar with riding your mid drive and how it behaves, you will naturally figure out how to push it to its limits and minimize that power blip when you shift. You may even get smart enough to do without the blip entirely and just shift full throttle. But for your first few weeks of riding this thing… treat the gear sensor as a backup, not the default.
Here’s a technique you want to learn as part of your education on operating a mid drive: Using your brake lever motor cutoffs as a clutch: Just slightly actuate the levers so the cutoff kicks in, but the pads don’t engage. Lift when the shift is finished. You can stay on the throttle or keep pedaling while doing this so the process is near-seamless.
Many ebike levers have this ability built into them. Magura MT5e levers have a mid-lever hinge that lets you touch the brakes and engage the cutoff without any pressure making it to the caliper.
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 if its a DIY effort.
You really only need three or four gears in the middle of your cluster on a mid-drive-powered ebike. You want them to be the ones that let the motor spin fast. You also want the cogs the bike is happiest in rpm-wise to not be cockeyed, front to back (i.e. bad chain alignment). So regardless of whether you built this bike or you just bought it, when hammering on the power don’t do it when the chain is yawed to an extreme.
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 shift your gears (or when you build the bike in the first place).
If this is a DIY build, 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 on the market. They cost money, but spending that money now means not spending it later after you have walked home.
Just Say No To Your Smallest Cog
Standard advice in DIY build circles is to Stay The Hell Away from your 11T small cog. If you don’t, you will wear it out very quickly – as in only a very few hundred miles. Or you will simply break it. Even if you are smart and pick a steel cassette cluster, the two smallest cogs in that cluster will typically be alloy. And as we all know… steel bends but alloy breaks. Those little cogs are just as likely to crack as they are to wear out superfast.
There’s a second reason to stay off the little cog. If you build your bike right, the best, straightest chain alignment is somewhere in the middle of the cassette. You want that to keep your chain from becoming a chain saw to your front chainring and your rear cogs. Since your teeny little cog is all the way outboard, that is worst case for chain alignment. On some bikes you’ll even start skipping your chain on that little cog thanks to the skewed chain and the high torque. Under serious power that is a recipe for cracking the cog… or maybe even breaking the chain.
And here’s a third reason: When you start riding your new wonderful bike build, you will likely find there is a point of diminishing returns that your littlest cog is well outside of. You can shift into a higher and higher gear but at a certain point… all those cheeseburgers you have eaten over your lifetime impose an upper speed limit. At a certain point the motor will just bog down. If it can’t generate motion, it generates heat instead. And thats bad.
Testing my Stormtrooper’s 30a BBSHD running a 52v battery, I found the following: I built the bike with a cluster whose smallest cog was 12T, knowing the problems with 11T already. I found (with stock programming, before I revised it) I had a 33 mph upper speed limit on that 12T cog. But it took a city block to get up there. If I shifted to my second, 14T cog, my top speed was about 31 mph and I got there within maybe a hundred yards. On my 3rd cog in, top speed was about 28 and I zipped right up to it.
So… no real incentive to use the two smallest cogs.
If you bought your bike manufactured with a mid drive installed from the factory, this part has already been taken care of. If you are building an aftermarket conversion, you will have to buy components that are strong enough to handle the punishment your 1500w+ motor will mete out. Almost 100% of internet whining about mid drive reliability is from builders who fail at this stage.
While a lot of this article is repetition as I stated at the beginning, this is one place where I will just refer you to what I have already written elsewhere. Its only applicable to DIY builders so if thats you, go to this link and scroll down to the Mid Drive Motors section.
Quit Whining… Its Not Really This Bad
I am making this sound like a lot of work. Upshift this and Downshift that. Or perish in flames. Here’s the reality of it: You’ll figure out a happy medium real fast and you won’t need to do much shifting at all. I certainly don’t. Mid Drives live in a much wider gearing range than humans like to so you will naturally need to shift less. You’ll figure this out soon after you begin riding your mid drive ebike.
As a builder the first thing I do is pick a chainring size thats suited to my terrain. Big ones for flat land. Littler ones for steep city streets. Tiny ones the Sierras. Thats part of the magic as well. Most of the rest is you just picking a cog a couple-three steps up from the bottom of the cluster and staying there. Live with a little less top end speed, or say to hell with it and hammer the throttle if you must. Let the motor spin you up so you can whizz past the small children playing ahead of you.
Wrapping it up
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.
And you will have an absolute blast doing it!