How To Build a Mid Drive Ebike That Doesn’t Break

This article is the follow-up to How To Ride a Mid Drive Ebike Without Breaking It. That article points out how most of the online tantrums about unreliable mid drive ebikes are bad riding, not bad equipment.

Quick Jumps:

• Frame
• Chainring
• Chain
• Derailleur
• Shifter
• Cassette cluster
• Cassette Body
• Rear Hub
• Rear Wheel
• Dumbass!

This article’s main points are all found in different posts here on this site, and together, here and there, they all cover the ground I am re-covering here. However, this subject comes up so often I decided to try and consolidate things into one place.

2024 Update:
Added 8-speed options to the drivetrain lists below, and expanded discussion of 11-speed options..

I am not trying to list every single thing you need to do to build a bike (I don’t mention tightening all the bolts or putting air in the tires, for example). I’m trying to shine a light on the more common mistakes. Don’t make them and you stand a good chance of having a trouble-free bike. Some of this stuff is pricey, so maybe you’ll want to work it in bit by bit as budget permits.

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Remember: a successful DIY mid drive is about both building and riding optimally. Mid drives – particularly the ones made for USA-legal and adventurous, off road DIY builders – up the ante on the required competence of both builder and rider. There is no way around this. If you want idiot-proof and simple do a hub kit. If you want the versatility that comes with a mid drive, though, you need to put in the extra time and effort.

I’ll focus solely on the mechanical bits this time, and break the process down into key component areas. We’ll start with:

Pick a Frame…

To get a mid drive to work properly, you need to pick a frame that can handle it, and this is not a given. Lots of frames are a bad choice. So what are we looking for?

… That Handles The Torque

I can still remember looking down at my very first mid drive build, a 4kw Cyclone, and saw the motor flex when I hit the throttle. With that flex, the whole bottom bracket flexed with it.

Thats a bad thing. Pick a frame of very sturdy construction. You are going to have an electric motor giving one hell of a pull on a chain that is connected to your back hub. That pull can flex the entire bike frame.

Can your typical mountain bike do it? Yes. Can your road bike with Columbus tubing from the 1980’s do it? Ehhhh lets say no on that one. Whats the problem? Designed for light weight and strength keyed to relatively smooth roads and human power, the stays are too spindly. All that power can pretzel the poor, innocent chainstays and seatstays when the power of ten pro riders yank on the chain.

How do you fix that? A lighter-duty BBS02 with the amp output dialed down is one solution I have seen done successfully several times. The lower amps do not yank on the chain hard. Some effort on the builder’s part to change the settings so pedal assist is kind and gentle is also important (and also preserves an authentic cycling experience).

… That Fits The Motor

Modern downtubes on mountain bikes tend to be curved and swoopy, coming into the bottom bracket at an arc that equates to a roughly 3-o’clock position. That arc means an installed external motor like a BBSxx has no choice but to hang straight down. Kiss goodbye your ground clearance. Here’s a picture of my 2018 Guerrilla Gravity Smash:

Take a look at how the down tube is curved, and how, on the right image where the drivetrain has been removed, its clear the only way to put a BBSxx-style motor on a bike like this will result in that motor hanging straight down.

You can see, sitting on the floor in that right picture, a Cyc X1 Pro motor, which has long arms that mount the motor as far forward as possible to avoid this ‘angle of the dangle’ problem.

How did I do in terms of mounting the motor and preserving ground clearance? Well, look above at the final installation. Look how low the chainring is. Draw an imaginary horizontal line parallel to the ground from that chainring… the motor is above that line.

The higher the better, but bottom line here is the motor is above the drivetrain so we haven’t lost a lot of ground clearance on this frame by using it (with the right mounting kit a Tangent Ascent motor will fit inside the triangle, just under the shock). So, this frame that is totally unsuited to one kind of motor can be pretty well suited to another.

Here’s another example frame. This is the type of frame typically recommended for a BBSHD, BBS02 or similar motor:

The down tube of this frame is straight as an arrow and attaches to the bottom bracket at a high angle. This allows you to rotate a dangly motor like a BBSHD up as far as possible so you lose as little ground clearance as possible. How did this shake out once this bike was built?

It came out pretty good. Bearing in mind this bike used a smaller-than-usual 40T chainring, the motor is roughly at the same height above the ground as the chainring. Maybe just a bit below. This is as good of a fit as you can expect on a bike for this kind of motor.

So… the lesson here is to think through your motor choice if you already have a frame to work with. Or the reverse if you have a motor on the shelf that needs a frame.

… That offers Good Chain / Crankarm Alignment

This is a tough one to nail down in advance. On chain alignment, you can come close during frame selection but you’ll never know for sure until you actually fit a motor into the frame, along with an assembled rear wheel so you can drape the chain and figure out how it lays.

As to crankarm alignment… thats one you will have to work out once you have a bike on a stand during the build. The key is to remember that your desired final result is to align the pedals directly underneath you… not the crankarms. Focusing on the pedals gives you a couple of extra options over and above finding an offset pair of arms to even things up. I have used uneven-width pedal spacers for some big changes, and different washer counts on one side or the other (from zero up to two) to move the pedals an additional 1.5mm to 3mm in either direction.

uh oh… lots of extra space on the drive side. Can’t use standard crankarms on this bike

I covered these issues in excruciating detail in How to Build an Ebike From Scratch: Tinkering. So I’m going to link you to that page and stop, rather than going through it all over again.

Buy The Right Drivetrain Parts

Many builds fall on their face because the builder left cheap parts on the bike rather than replacing with strong ones.

Chainring

Here is another topic already done to death elsewhere. The options laid out here result in significantly different chain alignments. This is a pretty good general resource on the subject:

Musical Chainrings

Generally speaking for BBSxx style motors with a large secondary housing located where the chainring ordinarily resides, you want a chainring with an inward offset that helps with chain alignment.

You also don’t want to overdo the size of the ring. Generally, “smaller is better” thanks to the rules that go along with riding a mid drive ebike (See below). You don’t want to pick a big ring unless you know exactly what you are doing with regard to gear selection and chainline. 42T is typically considered ideal unless you are riding singletrack. You go as small as you can get away with in that case.

Do a little shopping and you’ll find quality offset chainrings from the usual big name players aren’t cheap. Especially post-COVID. Here’s a new one I found recently on Amazon that offers about 15mm of offset and is well constructed. The offset looks to be more than that simply because the alloy used in construction is so thick.

I bought this ring and it appears a solid alternative – with a great tooth profile – for about half the cost of the other high end rings.

Chain

A beefed up chain is often overlooked and just as often results in an Epic Fail. You can’t run a powerful mid drive and expect to use the cheapo chain you already have on your bike. Whatever you do, don’t use a cosmetic (painted) pretty chain, or one of those pricey skeletonized weight-weenie chains. Instead, spring the bucks and buy a proper strong chain.

Chain strength tests show narrower chains associated with higher-speed-count drivetrains last as long or longer than wider chains for lower-speed-count drivetrains. Do a little digging on the interwebs and the general consensus seems to be that quality matters. If you buy a good quality chain it is less likely to suffer from imperfections that cause problems like a snap from, say, a pin that is not seated quite right.

I run 11-speed systems with a KMC e11 chain – the KMC ‘e’ line is specifically designed to take a mid drive’s punishment. These 11s chains are brutally expensive (today’s price is over US$47), but this is part of the cost of admission if you want to run 11s and a high powered motor that doesn’t snap chains (or wear them out really fast). 11-speed is a wonderful thing to have on an ebike – particularly on a bike you pedal instead of throttling – but the cost of durable 11s drivetrain parts is a deterrent.

2024 11s chain test in-progress
I recently purchased a Sunrace CSMX8 cassette in 11-51T (all cogs are steel except for the 51T big one). While my current chain is not worn out, its best practice to change a chain when you change the cassette. As such I am using this as an opportunity to try out the new-ish Shimano Linkglide CN-LG500 chain whose cost is US$22-28 depending on what day it is. The BFD longtail needs two of them to make a roughly 210-link chain. We’ll see if this gives us the first economical 11s mid drive chain.

The story gets a lot better if you are using an 8, 9 or 10-speed setup. The SRAM EX1 ebike chain is used for the SRAM 8-speed EX1 mid drive-stressed tool-steel cassette, derailleur and shifter. Here’s the thing though: That chain has an MSRP of US$28, and is usually sold for about US$25. The link above to Amazon has it on sale right now for US$18.95.

It gets better still: The EX1 system is 8-speed, but the chain is sized for a 10-speed system. That means you can use this chain on 8-, 9- and 10-speed drivetrains. Since 9s is often considered the sweet spot for mid drives, this chain can be considered almost everyone’s inexpensive default. To sweeten the deal, the chain comes with 144 links. So it will fit everything but a longtail without having to buy and section two chains together.

Rear Derailleur

This one is quasi-optional. A quality derailleur is always a good thing. Especially if your alternative is something like a cheapie Tourney or similar.

11 Speed

I use a SRAM GX 2.1 long cage rear derailleur, and the SRAM NX . This is not meant for a 1x drivetrain but if you just use a narrow-wide front chainring, you’re fine. At over US$100 a pop this is not a low cost option, and one more reason why you need to know you want more gears and more finely-diced cadence options to bother with 11-speed. I use this trigger shifter. SRAM does not make an 11s single shift trigger shifter.

10 Speed

I haven’t done a 10-speed mid drive until recently when I changed my Bullitt hill climber from a Box Components Extra Wide 9-speed drivetrain (discussed below) to a Microshift Advent X. At present the derailleur costs around US$69, and its companion single-click shifter runs about US$31.

Despite the compatibility being labelled “Advent X only” you can mix in any 10s cassette that fits within the range.

The Advent X is only available in one (medium) cage length, but that “medium” cage is a great match for the inexpensive steel Advent X rear cassette cluster, which has a whopping 48T big cog. The Advent X is distinctive from the other derailleurs in this list because of its larger pulleys and that at-least-theoretically shorter cage. It uses the same excellent clutch that the 9-speed Advent version has. Like the Advent, it is inexpensive and workmanlike in its finish.

I use it to shift a lot on a bike that goes up and down hills and flat ground, and it shifts smoothly and surely. Especially when paired with an 11-speed chain. Good to know: 10s and 11s chains have the same internal dimensions. The 11s chain is just a hair narrower on the outside. So an 11s chain on a 10s drivetrain has a little more elbow room. When I fitted an 11s chain to an Advent X drivetrain the resulting silent performance a) made me realize how much chain noise we put up with on a bike and b) was so quiet its like I am running with a belt.

9 Speed

I have found two I consider to be stars:

Box Components Prime 9
I have two bikes with these: The Apostate and 2Fat, my 2wd adventure bike that got the Box upgrade when I made it into a sand crawler. My preferred version is the Box Two Prime 9 Extra Wide, coupled to their Box One single shifter, which is tailored for ebike use. The Extra Wide part refers to extra wide range on the rear cluster, which in English means Big. You can use their 12-50T ebike-stressed cluster if you really want to go big (the 12T minimum is also better for a mid drive’s survival as it helps keep you from bogging the motor or cracking the cog). Combined, shifter-plus-derailleur is about a US$185 solution. I like it better as a premium option because… it shifts spot-on, was super easy to initially adjust and runs silky smooth.

Microshift Advent
I like the Long Cage version, which runs US$60 and is meant for 2x systems but works fine on 1x. The Pro shifter is single-shift like the Box, and costs a whopping US$29. This setup is significantly cheaper than the Box. It works just a little less slick, and the fit/finish is more… workmanlike… but its nothing to complain about given the price range its in.

I have to say I like the adjustable clutch on the Advent better than anything else I have ever used.

8 Speed

I am going to ignore 8 speed freewheel systems because, frankly, they are junk. The cheapest of the cheap and you can expect them to perform cheaply. Freewheels in the first world were a dead technology in the 21st Century (outside of childrens’ discount store bikes) until corner-cutting ebikes from the Far East re-introduced this inferior drivetrain to adults. Freewheels evolved to cassettes for good reasons, and the only stuff available now to support freewheel technology in modern times is bottom-of-the-barrel.

For lighter duty and lower power, a Shimano Acera long cage RD-M3020 is capable of wrapping 45T of chain, and costs only about US$25. Pair it with a Shimano SL-M315-8R 8 speed shifter (not a single shifter) for about another US$20.

Take a step up the food chain and you’ll find the Box Four 8-speed component group. The entire drivetrain including shifter, derailleur, mid-drive chain and a steel-cog cluster can be had for around US$135.

Jump to the most money you can spend on an 8-speed (and turn back the clock a few years to when this was the only option on the market) and you will find the SRAM EX1 drivetrain designed specifically for mid drive powered emtb’s. This includes the EX1 long cage derailleur that can wrap 37 teeth, and the EX1 single-shift trigger shifter. I bought into this drivetrain myself back when it was first introduced, but its 2024 now and EX1 is no longer the only game in town. Still, if you just have to have 8-speed this is the top quality solution.

Shifter

I have mentioned but not explained why single-click/single-shift is needed for a mid-drive so here goes: Shifting under power is a Very Bad Thing. If you shift a bunch of gears at once, as is easily possible with a standard trigger shifter, its Way More Bad. If you are trying to snap your chain as quickly as possible (or taco a chainring or cog), thats how you do it. Single-shift shifters only allow one gear shift per trigger pull, so you can’t screw that part up. So either learn to carefully shift one gear at a time (certainly you can do that, as I do on my older bikes) or buy something that eliminates the issue.

Generally speaking, you should buy shifters matched to your chosen derailleur by its manufacturer, so I included specific shifter discussion along with the derailleurs above.

This looks like a good time to link to the discussion of whether or not you want to install a gear sensor. Spoiler alert: I don’t use them and I still stay safe.

Rear Cluster

Put simply, you want durability. Forget about light weight. You want all-steel cogs, preferably pinned together into a monolithic block. That block distributes the force across the cassette body/freehub underneath so you don’t dig a trench into the poor thing when you hit it with a shipload of watts.

11-speed

The Sunrace CSMS7 is an all-steel 11-42T cluster, pinned together. I haven’t found anything else that is all-steel in an 11-speed. These clusters are often tough to find in stock and there’s no telling how long the link above will remain valid (I can also find them on Ebay and via Google in the EU).

The CSMS7. Steel spiders, steel cogs and permanently pinned together. Wunderbar.

A much more widely available alternative is the Sunrace CSMX8 (and CSMX80). This version is not entirely pinned together – it uses two separate, beefy alloy-spidered clusters plus a couple of individual small cogs – but the cogs are steel (some wider range ones use alloy on the inner or two innermost cogs). You can find these in a tooth range from 42 to as high as 51T. Sunrace’s web site says the CSMX8 has its two biggest cogs made of alloy, but the for-sale links show only the biggest one is alloy now. This proved to be correct when I purchased an 11-51T CSMX8 in March of 2024.

Worth mentioning: Some of the Box Components clusters are Sunrace with Box labeling. I have both Sunrace and Box 2 and they appear identical. I’ve seen product reviews say the same thing. Sunrace clusters are quality products.

10-speed

The Microshift Advent X 11-48T cluster with hardened steel cogs is the only one I’ve used. It seems perfect for the high power mid drive application. a 48T big cog is great for steep climbs. Its all steel. It is pinned together all over the place to distribute the punishment across the entire cassette body. Its fairly cheap at around US$47.

9-speed

For bikes that don’t require a lot of range, you can’t beat the Shimano HG400-9. They are found in a range of tooth counts – my favorite is the coated 12-36T. They are dirt cheap and functional. All steel except the smallest cogs, and pinned together. Nearly indestructible and easy on the cassette body.

The Microshift Advent 11-46T Hardened Steel cluster is my absolute favorite. A nice wide range up top. No spiders (each cog is 1-piece). Pins are generously used to hold it together and prevent any torque tacos.

And its cheap with a US$35.99 price at the time of this writing. Unbeatable for a 9-speed mid drive build.

8-speed

There are not too many mid-drive-sound options here in part because 8-speed cassettes are not especially sought-after for a quality bike. However you can still find quality options.

The Box Components Box 4 cassette is available in 11-42T and ebike-friendly 12-42T. It uses steel-stamped 1-piece cogs with no spiders and will run you just over US$50.

Go old school and use a Shimano HG50 cassette. It is pinned together, has steel cogs and is very inexpensive at around US$30. Its only drawback is a limited tooth range as Shimano doesn’t have a wide-ranging 8-speed derailleur to support a big cog.

Break the bank and go with the OG of purpose-built mid drive cassettes: The SRAM EX1 HG899 cassette is an 11-48T cluster made of case-hardened tool steel. Its cogs are engineered to better handle shifts under load. Its price tag is also in the ballpark of US$350… and thats on the low side of internet pricing. In the more modern era of e-mtb’s its hard to come up with a case to justify spending this kind of money on a cassette cluster. I put a full EX1 drivetrain on my Guerilla Gravity Smash, but that was back in late 2018 when the bicycle industry still thought of steel cassettes as something only for a low-end analog bicycle.

Rear Cassette Body (Freehub)

Whatever it is, it needs to be steel. If not, with an unrestricted DIY mid drive you’ll dig into it like its made of cheese. This is a big deal. I’m only devoting a couple of sentences to it, but thats because there’s just not that much to say. If you want to avoid tearing up this rather expensive and possibly difficult to replace part, use one made of steel. Period.

Left: Alloy at 1300 miles (and an AWD bike so it had an easy life). Right: Steel replacement. A year later I checked the steelie and it wasn’t even scratched.

Rear Hub

With all the talk above of strong chains, steel cogs and steel clusters, you can see where this is going I bet. With all that durable hardware we are reinforcing the drivetrain further and further down the line until we find the next failure point. After we get to the surface of the cassette, the next thing that breaks are the pawls inside the freehub. Whats a pawl? Here are a couple of pictures where they are visible:

The little things sticking out on the end are ‘pawls’: ratcheting couplers that hold the hub firmly while you and your motor apply force to the rear thru the chain. If they give out, the chain ‘freewheels forward’ and you lose the ability to apply force to the rear wheel to make it go. Mid drives putting out big power put big stress on these poor little pawls, and they can die as a result.

Its not hard to see why. All torque is transmitted through those pawls. What you need is more points of engagement. You can find 4, 5 and even 6-pawl freehubs. You can also buy quality rear hubs that can take this level of abuse (I hear Salsa makes some, as do Sun Ringle). You can also change the game and go to a different kind of mechanism that is effectively indestructible to a mid drive…

Hooray for a ratchet engagement mechanism

Bike Radar did a good piece on freehub construction so I’ll just link to that article here and let you read the details, and the comparison of pawl vs. ratchet. So far as I can tell across several wheelsets and several thousand miles, the DT 350 hubs with standard 18T and 24T ‘Hybrid’ ratchet mechanisms are effectively indestructible.

Rear Wheel

So we’re walking the points of failure back still. ‘Under’ the inside of the cassette mechanism is the hub, spokes and the rim itself.

This is not something anyone wants to hear, but if you want a trouble-free mid drive you put on a quality wheel built for it, by someone who knows what they are doing. This is not cheap, hence the reason nobody wants to hear it. And you may be able to do without. Heck… a lot of Mongoose Dolomite conversions are out there and running just fine. But if you expect to pour on the miles and keep doing so for years, trouble-free. Well, your money will be well-spent on a beefy wheel.

Lets take a typical 26″ example. I really like the SunRingle MTX39. It comes in 32- and 36-hole versions. At US$60-90 each they are not cheap, but also not crazy-expensive, either. Its ridiculously strong, but isn’t light weight.

My MTX39 on the rear of my Bullitt. I also use them on my Envoy.

If I want to blow off the budget and go for strong, lightweight (and tubeless) a DT Swiss FR-560 is a winner. I have them on 26″ and 29″ wheels, and they are also available in 27.5. But at about US$150 a pop you have to want em pretty bad.

Next, what about the spokes? Choose good, strong ones. Sapim Strongs are an excellent choice. So are DT Swiss Champions, or DT Competitions. If you are made of money, DT Alpines are pretty awesome, too. These spokes as a body are not the great big 12-gauge spokes you see on some imported Asian rims. the European manufacturers substitute quality materials and smarter engineering and don’t need that massive construction, but they still provide superior strength.

And lets not forget the nipples! Spoke nipples that is. Once again, choose strong ones, not light ones. That means no alloy. Use the brass ones.

Don’t Ride Like a Dumbass

Build it as smart as you want, but if you do bad things you will get bad results.

How To Ride a Mid Drive Ebike Without Breaking It

So last but not least, follow that link. And don’t be a …