One of the most common emails I get from builders is people needing help trying to select a front chainring size for their ebike build. The whole process of deciding a chainring size for your first mid drive can be a complex and unintuitive process for the uninitiated. Getting the right chainring size has a lot to do with

In order to select the right chainring for your build on the first try, you will need to have a rough idea of all of these questions before you plunk down the cash for a chainring. This article focuses on the BBS02 & BBSHD drive units, but the same logic is applicable to most other mid drive setups on the market.

I highly recommend springing for an aftermarket chainring for the BBS02 or BBSHD. They have better offset than using a BCD chainring adapter or the stock steel chainring

Before I start explaining all my logic I want to give examples of my favorite chainring size for different applications. After the bike name I list the (Chainring size/Cassette or rear cog size or range/Tire size or equiv size) for quick reference.

Singletrack trails / Shallow powder / Groomed snow

Although I own about 5 other ebikes, these bikes are the ones that I ride most of the time and give you a good idea of what chainring size I’ve settled on for different configurations. Now let’s talk about why I settled on these chainring sizes after years of playing musical chainrings.

The Luna Eclipse 42T is arguably the most attractive of all the aftermarket options. You can easily swap the teeth up to 48T if you want, or just replace them when they wear out (they probably never will though)

Every mid drive system I’ve dealt with always has a fixed reduction ratio between the motor and the pedals. This can vary between 13:1 to a massive 40:1 as in the Tangent Ascent. With a 13:1 reduction that means for every time the crank goes around once, the motor goes around 13 times. The perfect drive ratio between the motor and the wheels would mean that you are pedaling at a comfortable cadence at the top speed of the bike as well as on the lowest gear on a steep incline. Unfortudently for most trail bikes while this is an optimal configuration, it is just not achievable. More often than not you shoot for a comfortable pedaling cadence on the steepest slope (often over 20 degrees) and at top speed, the motor just spins so fast that there is no way that you can keep up. It’s common for mid drives to have a top spinning speed that is faster than a normal human can possibly pedal. Since the range of most IGH, Cassettes and CVT’s are somewhat limited you are stuck making choices about gearing that will affect your torque and top speed.

Here is a sample of ranges for some popular CVT and IGH. Keep in mind that the lower double and triple ranges are for systems with a front derailleur which your mid drive probably won’t have.

If you want a higher top speed then you gear your system higher using a larger front chainring. If you want wheelie popping torque then you need to gear your drivetrain lower so you can crawl up insanely steep inclines without burning out your motor. One way to get around that problem is to get a cassette with a very large range. I tested the aluminum grannied 42T Sunrace chainring but it self-destructed when I pumped 2500 watts through it. There is a steel version of that cassette which I have yet to test, but one of the problems I found with a normal derailleur is problems with clearance on the biggest granny. With a 10 speed X9 derailleur, you have to space the derailleur so far from the cassette for the 42T granny that often the higher gears like the 11T will skip under load because there are not enough teeth engaged. If you buy a derailleur that is specifically designed for cassettes that have more than 36T then this will not be a problem. I like to gear my road bikes so that they don’t wheelie too much in the lowest gear on flat ground which generally gives me a very high top gear. It’s nice to be able to pedal along at 35mph. With my singletrack bikes, I gear them so that I can have a comfortable pedaling cadence up a 20-degree slope in the lowest gear. The deep powder bikes are geared impossibly low with a 36T granny cog and a 30T front ring which means that when I get stuck in foot-deep slush the bike can spin the back wheel quickly with tons of torque to dig myself out.

The newest 1X groupings with the massive granny gears have ranges without a front derailleur that we used to only dream of, the problem is that the cassettes are not that durable

If you gear your commuter too high then you run the risk of creating more stress on nylon primary gear which could fail faster if you use too much power from a standstill. The best way to test to see if your gear is too high is to shift into the lowest gear and then start moving while pedaling on level ground. If the pedaling cadence is reasonably fast then the motor should not be under undue strain. A bike geared with normal pedal cadence from a standstill on level ground in the lowest gear is going to make a poor trail bike because in order to get moving from a standstill on a steep incline you will need much lower gears. Paved road inclines rarely are more than about 10 degrees but if you are trail riding on normal singletrack it is not uncommon to have to ride up 20+ degree inclines. It takes a lot more torque to climb a 20-degree incline than it does to climb a 10-degree incline. The BBSxx series like to spin fast when they are under heavy loads.

A large 42T+ aluminum granny will not survive long with a high power mid drive

For different mid drives, there are a plethora of aftermarket chainrings out there in a variety of different sizes. The biggest appeal of the 42T Lekkie and 42T Luna chainrings are that they are big enough to completely surround the secondary reduction gear and move the chain line back towards the bike by over 2 centimeters. That might not sound like much, but I assure you, when dealing with adding a mid drive a cm is a lot of offset. The best case scenario is to have the chain-line line up with the middle of the cassette, but that rarely happens. Most of the time it is farther away from the bike and lines up with the higher gears. While this is fine for commuters, it sucks for trail riding machines and deep snow bikes because you spend almost all your time in the lower granny gears. If the chainring comes back too far and hits your chainstay then you can always space it back out using a chainring spacer.

A perfect candidate for grinding away the case. Lift up the drive unit and mark where it hits the chainstay and start grinding away at the case, BD Lurch shown (120mm BBSHD required for this bike)

Smaller chainring adapters allow you to use chainrings as small as 32T (if you grind the adapter down a bit so the chain can seat properly on it) however your chain line will get pushed farther out. You can grind down your bottom bracket on the drive side to move the drive unit over, or you can carve away a notch on the mid drive case where it hits the chainstay. The metal on the case is surprisingly thick and I’ve had good luck taking about a cm of material off the case without breaking through to the primary reduction gear. Use a Dremel grinder and go slow and stop if you see a hole going all the way through. It also helps to line up the drive unit while you are working from time to time to make sure that your grindings are actually making the drive unit sit deeper on the bike. I’m a big fan of the Luna 30T ring which sits about as close as it can to the case (mine actually gets chain wear on the case with that ring in the granny gears, it’s so close). For most applications, the Lekkie Bling Ring 36T BBSHD ring is probably a smarter choice as a 30T ring will be too small for most applications.

Changing drive ratios on IGH

When working with IGH you have the added bonus that you can select a front chainring size and then easily change the drive ratio by swapping out the cog on the IGH. Most SRAM and Shimano 3 speed IGH have the exact same cog and there are sellers on ebay who sell very large steel cogs that will fit the Nexus-3 or the Sturmey Archer 3 speed IGH well. These cogs seem to be custom made and sell for around $10 or so + shipping. Do an ebay search for “Sturmey Archer 24T” (or whatever size cog you need) and you should get several options. Make sure it is for the3-speedd IGH as the 8 speed SA uses a cog with a much larger hole (insert inappropriate comment here).

You can easily swap the rear cog on 3 speed IGH all the way up to a 24T cog

When resizing the front chainring if you want to keep the same drive ratio on your bike you’ll also need to swap out the rear cog. The best way I’ve found to do that is to keep track of the tooth difference count. If you are running a 36T front ring and a18T cog (36T – 18T = 18 tooth difference) and want to bump up to a 42T front ring then you will need to bump up to a 24T cog to have approximately the same drive ratio (42T – 18 (difference from original drivetrain) =24T). Although this system is not perfect it works pretty well and it’s easy for my puny brain to understand. The Nuvinci drive units use standard single speed cassette cogs which I have a huge pile of lying around so it seems like I can always find what I need for my CVT’s.

Chainrings on the market right now

No matter what kind of chainring you opt for with whatever mid drive you have I only recommend going with Narrow\Wide setups. The N\W chainrings have proven to be able to tolerate incredibly out of whack chain lines without dealing with constant derailments. Although there are several brands of N\W chainrings, the ones I have tested the most extensively are the Race Face brand. Although they are aluminum, they wear like steel and I have yet to have a single failure using those chainrings.

If you have to use a BCD 104 chainring, the Race Face brand is second to none, makes me hungry just looking at them

My Very Rough Recommendations (don’t blame me if it doesn’t work for you)

Here is what I recommend for different setups

Commuter for best top speed and limited hill climbing ability (mostly <10 percent grades)

Singletrack and shallow snow with limited top speed and great torque

Deep Powder for maximum torque and <20mph top speed

The 30T mighty mini is the weapon of choice on my serious deep powder bikes, nothing else comes close to the raw turtle-crawling power a 30T chainring

The Cyclone 3000W kit and the 6000W Tangent Ascent kit both ship with industry standard BCD adapters and chainrings to match. The large Cyclone chain wheel is designed more for commuters and in my opinion, that drive unit is too heavy for trail riding (I’ve tried it, it sucked). The Tangent Ascent ships with a much smaller chainring which should work fine for singletrack trail riding. The Ascent has such an insanely high RPM range that the top speed does not really suffer, although it is nigh-impossible to keep up while pedaling as the cadence is just too fast. If you want to use different chainring sizes on either kit you’ll have to swap out the chainring with another steel or Narrow\Wide Raceface chainring. If your chain line is bad then you will need to use a narrow wide chainring (although I can’t find any steel N\W rings out there), but if you have a clean and straight chain line, then any stamped out steel chainring should be fine. I’ve had problems with the stock Cyclone freewheel not working properly and Luna has come out with their own replacement freewheel called the Lunatic which runs a steep $155 (available here) but should hold up better than the cyclone one does. It also looks a hell of a lot better.

Picking the right chainring size for your ride is likely to be one of the biggest decisions you will make for your home-built ebike. I’ve tried hundreds of combinations with every chainring I can get my hands on and the biggest suggestion I have to the average builder is, don’t give up. If you don’t get it right on the first try, you will get it right eventually. Keep a couple of power links laying around and a chain breaker and you can swap a chainring in <10 minutes. After you put on the new chainring don’t forget to check the cassette range to make sure your chain is long enough to go from the granny to the smallest road gear without issues. Over they years I’ve collected 4 BBSHD and 8 BBS02’s and I have a pretty big collection of used chainrings saved up. 42T is a great size and unless you are plowing through very deep powder with an insanely large tired fat bike you can almost always make the 42T chainring work, so if you are in doubt, just go with that size first.

Here’s a video from my homey Josh talking about BBSHD mounting and chain line issues as well as how to select the right chainring size while wearing those oh-so-sexy blue nitrile gloves.

What Tire \ Chainring size and cassette/IGH/CVT combo do you use and how has it worked out for you? Leave a comment below so that others can learn from your successes and failures.

Karl I have got so much good info from your writing thanks for being around

Thanks man, it’s been a crappy week or three. I hear your support.

This is exactly the situation I am in now. I have an Origin 8 fat tire with eRad 350 watt mid drive. 36t/18t. I live in the mountains and climb steep hills. I broke the n360 the bike came with. (Started leaking on my first ridiculously steep climb) Got N380 replacement, but it started leaking on my second ride after going up some steep but short sections of trail. Got another N380 (Gotta love the warranty) and instructed the bike shop to lower the gear ratio. They said they couldn’t go smaller on the front due to the motor casing (They don’t like to mod things that are under warranty) so we went with a 22t rear. Still waiting on parts and service. Should the 36t/22t combo work for these hills without breaking another hub? If it does, I’m reduced to replacing the hub with a cassette and I’d really rather not do that.

You should be good with a 36/22T combo. Adjust your programming ‘START CURRENT’ under PAS and throttle to 1. That will help a great deal. The N380 hates the jerk when it first gets a power kick.

I didn’t get the chance to adjust the start current. Ended up putting a 9 gear cassette on and it’s great. The Nuvinci seemed to add quite a bit of drag. Still have to work hard on steep hills with the wimpy 350 watt motor, but that’s probably for the best.

Regarding full suspension bikes, chain ring size can have a very significant influence on suspension behavior and pedaling characteristics. On such bikes, it is unwise to deviate significantly from what the frame intends. On modern 1x frames this will mean 28 – 34T depending on wheel size, so a 30T may be the only option in many cases. This seems to be completely unknown to the e-bike community. If you care about how your trail bike performs AS A BIKE, anti-squat is important. Too large a chain ring can ruin the anti-squat designed into the frame.

Karl, I’ve just bought, still waiting to receive, a Sun Race 11-42t cassette. Now before you jump on the busted aluminum gear rant, it is an all-steel unit. Look up Sun Race CSMS2. I don’t have Ludicrous power, but I climb off-road trails as steep as 30%. My 11-36t cassette isn’t quite low enough, even though I’m using a 30t front chain ring. I want to have the power to climb even when I’m having trouble keeping my front wheel on the ground.

Sure I’ll bite. 500g for $55 on fleabay seems like a good price per gram. Arf, arf.

I liked my alloy spider 42T till it self-destructed then I hated it. Destroy everything you touch.

Can I have some schooling on gears please, what’s good for 1-top speed, 2-hill climbing and 3) what’s good for quickness. Thx

26″ MTB. BBSHD and 11-34T cassette with leckie bling ring = climbs a tree if you can stay seated. I live on a steep mountain (30 % is easy) and nothing i cant climb up….literaly.

This is the kind of information and guidance that is just not found anywhere else man! I’m jealous of the riders that now don’t have to go through all the trial and error that I did. Excellent article. (again.)!!

Two things: Wolf Tooth does make 104 BCD narrow/wide chainrings in stainless steel. https://www.wolftoothcomponents.com/products/104-bcd-stainless-steel-chainrings

Ever heard of anyone using a derailleur with N/W chainrings? I found a way to stack a 30T Mighty Mini under a 36T PreciAlps (pretty much same as a Lekkie.) I just move the chain from one to the other by hand when I go from street to trail. If I were somehow able to modify the front derailleur to work 20mm outboard from stock (unlikely), do you think I could shift from one N/W to another? Seems like the relative clocking of the two rings would be critical.

The last time I looked about 8 months ago I couldn’t find any steel N/W. Surly was talking about building one, but I could not find one for sale. Glad to see Wolf Tooth is making one. Their components are top notch.

There are several people who use front derailleurs with a BCD adapter. Just don’t shift the front derailleur under load or the chain is almost guaranteed to break apart.

Could you provide me more details on your double chainring setup? I’m looking to do a BBSHD commuter build on a 26×4″ fatty with a 10-speed 11-36t SRAM cassette. Ideally I would like a low/high setup that I can change, either like you manually, or with a derailleur. A nice small (30ish) chainring for pedal only (no motor power) riding, and a bigger one (48ish) for commuting with electric. How’s your chainline with the double?

http://eehouse.org/fixin/formfmu.php

I have used this in the past for getting the magic gear ratio.

The bike had an IGH with no chain tensioner or horizontal drop outs. It ran like a sewing machine.

For those who just don’t care about the price, the Rohloff IGH is as good as it’s hype. I care about the price, but got one anyway. On the Montague folder I have it on, it has worked flawlessly. Rock crawler and high speed road runner all in one! 38 mph top and on the low end I’ll break traction and/or flip it over before running out of torque. It seems to be one of those regrettably expensive things that is, dammit, worth the dough.

How can i get lekkie 36t chainring?

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What are your recommended sources for BIG chainrings? I have a 20″ recumbent trike (commuter), standard cassette and interested in anything bigger than a 52T.

A 52 tooth should be fine, if you want to go bigger you will need to use a custom chainring on a BCD 130 adapter. Be aware that starting off in too high of a gear will stress the nylon gear.

With the 52T, the cassette is mostly in the 11T sprocket. I was looking for a 70-80T chainring to get (mostly) into a 17 or 20 cassette sprocket. I anticipate the need to go to a lower gears when starting off.

http://trisled.com.au/product/80-tooth-chainring-complete/ Never tried it. Thought an 80 tooth chainring sounded like an impossible thing. But here it is.

Karl, Thanks for this info and frankly all of your articles that have helped me understand what bike and technology for my own build. I just got the bike (Motobecane Fantom 27.5 Plus Bike w/Boost from BD) and now I’m getting the BBHSD kit from Luna. I was thinking the Luna 36T Or Lekkie 36T (5.2mm offset). I am most interested in PAS mode for climbing single-track and fire road hills in Laguna Beach, not speed. The bike came with a 1 x 11 SRAM with a 30T. My concerns are chain alignment versus offset, for climbing on the lower gears and the wider chain stay of the Boost type frames. I am looking to maximize battery life (stealth Mighty Mini under seat) and will likely program the PAS 1, 2 & 3 just for climbing / slow cruising modes. Would I be better off with a 30T or 36T based one the above? Any advice on the best way to measure the bike with my calipers to make sure the offset is enough for a BBHSD on this particular build?

You will be happier with the 36T ring.

This is an oddly rare shot of similar to mine, from the non sprocket side, showing the axial drive motor integrated with the crank shaft & frame (good cooling).

It allows the usual 3x front sprockets on mountain bikes.

The result is an over the top ~24 gears, but it does allow it to be both a commuter and a bush basher.

It seems a very well made and specced bike. My only grumble is that the too pretty frame restricts battery size in the ideal location.

https://www.alibaba.com/product-detail/High-quality-mountain-e-bike_60094428724.html?spm=a2700.details.maylikehoz.5.783d7ba0upBzu8

I cant understand why its so obscure. It should be a hit?

There was a comment somewhere about no steel rings in narrow/wide….these rings are heavy but the whole spider and narrow wide (steel) ring was about $15 when I looked for long enough. Have a quick look at this: https://www.aliexpress.com/item/BAFANG-48V-1000W-BBSHD-or-BBS03-Mid-Drive-Motor-Cycling-Crankset-42T-Chain-Wheel-Electric-Bicycle/32824042807.html?spm=2114.search0104.3.195.yO0Ql5&ws_ab_test=searchweb0_0,searchweb201602_2_10152_10065_10151_10068_10344_10345_10547_10342_10343_10340_10341_10548_10541_10307_10060_10155_10154_10056_10055_10539_10537_10536_10059_10534_10533_100031_10103_10102_5670015_10142_10107_5660015_10562_10084_10083_10561_10175_10312_10313_10314_5650015_10550_10073_10551_10552_10553_10554_10557-10553,searchweb201603_1,ppcSwitch_5&btsid=8b4a6f43-ed38-4b83-8ce3-ece5bb818a0f&algo_expid=bcd1914c-719f-4dfa-9b35-752629d2e876-24&algo_pvid=bcd1914c-719f-4dfa-9b35-752629d2e876 but I wasn’t super impressed with it’s smoothness of operation, TBH. Thanks for your info, BTW…it’s been really helpful.

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Thank heavens I finally found this article too, now I understand why Karl recommending the 42T as my starter CR. It is a very informative write up. Thank you Karl. Looks like 42T it is then, but I will give the stock SA 18T cog a try first before ordering the Shimano CS-S500 cog. Thanks for that tip too… Rod

I clicked on Death Trap. You’re hilarious I love it man!

I clicked on Death Trap. LOL you’re crazy dude!

BTW I’m putting probably a 44t on my electric ute. I’ve been climbing hills pretty well in Seattle on 56T to 32T But I think i’m going to wear out my motor.

Where are you in Seattle?? I’m taking my fat tire out today with all this new snow. Im up in Fairwood in Renton

great write up mate, excellent info, you have saved me weeks if not months…thank you

New 40T Chainring Precialps for Bafang BBS01 or BBS02

Hi Karl – I can’t thank you enough for the intel man!! Soo good. Really appreciate you getting n00bs like me all acquainted.

With your help I just built out my old single speed Surly Karate Monkey with a BBSHD ludicrous kit and am now puzzling over whether to install an 8-speed or a 3-speed IGH. I live in the San Francisco Bay area and want the bike for all purpose trail and road cruising/commuting/fun – I don’t plan on using it for shredding single track but will inevitably find myself on steep single track.

Thanks again and happy holidays!

I have had food luck with 3 speed IGH, never done an 8 speed, but lots of people on the internet have had problems with 8 speed IGH and powerful mi drives. You might want to go with a Rohloff if you want lots of gears and for it to stand up to abuse.

Excellent. Thanks for the feedback. I bought the 3-speed nexus but have been waffling. I’ll lace it up and give it a go.

Hi Karl, great stuff, had a good read through. I’d like a bit of advice if possible please? I’m just fitting a BBS02 750w to my first proper build (I did a quickie Voilamart rear hub on a non-suspension just to see if I liked it – and I did…). So now I’m on with a full suspension MTB for trail/occasional road work. I’ve got an 11/42 SRAM at the back on 26″ wheels, and the Bafang comes with a 44 chainwheel. Where I normally intend to ride has some longish hills to get up there, I don’t want to overheat anything running the motor hard getting up there (I’m 66, I don’t have much pedaling ability – hence the electrics, and I’ve disabled the PAS so I can modulate the throttle myself), but equally I don’t want to have it screaming at low mph on the flat. Given the 42 on the back, can I get away with the 44 up front do you think, or should I be looking at maybe around 36/38?

Many thanks in advance, much appreciated. 😉

I would try it with the 44T and see how it feels. If it’s to fast on the level you can get a 38T or 40T but you will probably be fine with the 44T. A 44T-42T produces insane torque, hard to imagine that you will need more than that for trail riding unless you are really hardcore.

That’s great info, thank you. I’ve built plenty of enduro and trials motorcycles from scratch, but this e-bike thing is all new to me. I’ll stick with the 44 to start with then, if it’s okay I’ll probably modify it to bring the chain line more inboard, I have room to the chainstay. Cheers and Thank You.

Have you converted a 2021 Salsa Mukluk Deore 11 speed before? It has 100mm BB and runs 26×4.6 tires. The chain stay is tight already to the 30tooth single front chain ring. Concerned about chain line, loosing gears, should I use a 120mm or 100mm mid drive system. I have mechanical experience but this is the first time building a bike like this. Hoping you may have a build list and pass on your experience undertaking this project. Thanks

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