Take A Wild Guess : Selecting The Best Front Chainring Size For A Mid-drive Ebike On The First Try

Karl Gesslein / May 26, 2017

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

• The diameter of your ebike wheel (smaller wheel = bigger chainring)
• Whether you are using a cassette, single speed or IGH and the size of the rear cog(s)
• What you want the top speed of your ebike to be
• How steep of an incline you need to be able to climb effectively
• Clearance and chain line issues
• Whether it is a commuter or a singletrack machine or both
• How much stress you want to put on the internal reduction gears (larger ring = more stress)

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

What I use

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.

Commuters

• Orange Crush – (52T/34-11T/26″ wheel) Normal 26″ wheel with 11T-34T “megarange” cassette, tried lots of combos for the BBS02 but my favorite is the 52T Bafang steel crap-ass chainring. Average speed with PAS is about 25mph, have to be careful to not start in too high of a gear or I could strip the Nylon primary reduction gear. The bike is pretty much useless on real trails.
• Burning Chrome – (44T/18T/29″ equiv) Cyclone 3000W with a 3″ fat tire stretched onto a 100mm rim with the stock 44T crappy steel Cyclone chain wheel in the front and an 18T Sturmey Archer 3 speed IGH in the rear. If you keep it under 25mph the 18T is a good choice for that wheel size with lots of wheelie power, but I think a 15T cog would be a better choice and would allow you to pedal along at 30+mph. The Cyclone is awesome at 72v but somewhat ho-hum at 52v. Street use only.
• Death Trap 2000 – (46T/30-11T/29″) BBSHD with stock 46T steel chainring and super skinny 700c (29er size) rims and tires and a 8 speed 11T-30T cassette in the rear with limited range. This ebike was an exercise in stupidity. Way too fast and scary even at 30mph on the level, went much faster downhill. If you wanted to hit the high 30’s on the level then just swap out the 46T crap-ass steel chainring with a 52T chainring and make sure your health insurance is paid up.

Singletrack trails / Shallow powder / Groomed snow

• Duh Banana Bike – (42T/22T N380/29″ equiv) BBS02 with a 42T Lekkie Bling ring and a Nuvinci N380 with a 22T cog on the rear with large 4″ fatbike tires on 26″ rims. I still ride this bike several times a week and I love it. One of the cheapest ebikes I’ve ever built it took a lot of effort to get the gearing right, but the 42T/22T combo with the N380 is a winner winner, chicken dinner. The N380 leaks cvt fluid like a sieve and I’m running it till it breaks, but 20 months of abuse and it is still going strong. Good on the road because of the N380’s massive range, can climb almost any incline although the CVT started to slip today on a really steep pitch (25+ degrees).
• Sun Kiss – (32T/22T Nexus3/29″ equiv) BBS02 with a 32T Raceface ring on a BCD 104 adapter with a 22T cog paired to a Nexus 3 speed IGH and 4″ fatbike tires mounted on 26″ rims. Another fatbike I ride all the damn time and take with me on trips cross country on the back of the car. This bike has seen insane amounts of use and abuse and although I have tried many different chainring/cog combinations I always go back to the 32T/22T combo. The Nexus while indestructible does not have the range of the N380 or a regular cassette. This ebike’s top speed is about 20mph so it is useless as a commuter, but it can climb anything and most of the time on singletrack I’m in 2nd gear which is what you want because the pawls lock.
• My Little Bronie – (42T/34-11T/26″) BBSHD with Ludicrous controller 42T Luna ring and a 9-speed cassette with 34-11T range and normal 26″ rims with 2.25″ tires. I don’t care about this bike so it’s the one that gets thrashed on when I go on trips. It sucks pedaling uphill when the battery dies, but this ebike shreds the trails and you can still pedal along at 30mph. This is the perfect ebike for me to ride when I’m on the road and ebiking on the trails where it is forbidden or illegal. The 2.3″ skinny tires combined with a tiny 3.3 lb Mini cube battery under the seat are very discreet.

Deep powder (over 6″ deep)

• Phat Phuk – (30T/36T-11T/29+” equiv) BBSHD with 60 Amp Ludicrous controller and 30T Luna Mighty Mini and 10sp 36-11T cassette and massive 5.05″ tires on 26″ fatrims. I love this bike, it is my favorite by a far margin. For plowing through over 6 inches of snow the 30T mighty mini is amazing. I would go no bigger on this bike.
• KHS 3000 – (36T/36-11T/29″ equiv) Same as above but with a slightly smaller diameter Snowshoe XL on the rear. I tested this bike with the 36T Lekkie HD Bling ring and decided that it was a better match for the smaller tire diameter than the 30T Luna ring. Better top speed than the Phat Phuk but still no commuter.

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)

Top Speed vs Torque

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

Picking a chainring

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

BBS02/BBS01

• Lekkie Bling Ring retails for $120 and comes in 52T, 46T, 42T, 36T available in Kiwiland here or through many US & worldwide distributors. I have extensively tested these chainrings and can say they are very high quality.
• Luna Mighty Mini 30T chainring for $55.95 available here. Tested extensively by me and works well for singletrack or snow.
• Chainring adapter that works with any BCD104 chainring available for $22 from Lunacycle here. Add a RaceFace Narrow Wide chainring for $36 : 34T, 36T or 38T. I have tested 4 different BCD104/BBS02 adapters and they are all pretty much the same.
• Bafang sells a bunch of stamped steel rings from 44T all the way up to 52T. In my opinion, these rings are too heavy and are not narrow/wide so I do not recommend them. For only $15 it’s by far your cheapest option (and it’s not worth every penny).
• There are several other sellers in Europe for the Bafang BBS02 custom chainrings but I have not tested any of them so anything I said about them would be hearsay (people generally seem to like them).

BBSHD

• Lekkie Bling ring for the BBSHD retails for $120 and comes in 46T, 42T and 36T and available here direct to NZ or scroll down for other countries.
• Luna Eclipse Chainring for the BBSHD retails for $105 available here and comes in many different colors. The 42T chainring I have tested extensively and if you want the larger 48T chainring it costs $10 extra. I have a 48T chainring but I have not tested it yet. So many chainrings, so little time.
• Luna Might Mini Chainring retails for $56 and is available here in almost any color you could want. I have tested this chainring more than any other chainring in impossibly deep powder and I can say that I really love it. If all you want is raw power, then pair this chainring with a 60 amp Ludicrous controller and you’ll be a happy camper.
• Several people have chainrings and adapters for sale, the cheapest I could find was Lunacycle selling both for $49 here.  This is a BCD 130 adapter that will fit almost any BCD 130 chainring on the market if you don’t want to use the 42T narrow\wide chainring it comes with. Expect that most BCD 130 adapters for the BBSHD will be pretty much the same and will not have as much offset back towards the bike as the 42T Lekkie Bling Ring or the 42T Luna Eclipse.
• Bafang sells a 46T steel ring stock with the BBSHD that you can buy for $25 here (why you would want to is absolutely beyond me, masochistic much?). My advice? Don’t waste your time, this chainring sucks more than my 20-year-old shop vac.
• As above, there is a handful of European resellers who have custom BBSHD rings that I have never tested.

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)

• For 20-24″ wheels and a normal cassette, I would not go lower than a 52T.
• For 26″ wheels and a normal cassette, I would pick anything 42T-52T leaning toward the 48-52T range for higher top speed.
• For 29″, 26″ fatbike tires or 700c wheels and a normal cassette I would go with 42T (higher torque) – 48T (higher top speed).
• For any IGH system, I would start with a 42T chainring or bigger then fine tune the speed/power range by swapping out the rear cogs which are very inexpensive. You’ll probably end up with a 16T or smaller.
• CVTs are going to really struggle when you gear them very high and I don’t recommend them for commuters (although there certainly are many people who use them as such). If you’re going to overload a CVT your best bet is to keep it spinning fast which means gearing it lower which means lower top speeds which means crappy commuter.

Singletrack and shallow snow with limited top speed and great torque

• For 20-24″ wheels and a normal cassette, the 42T is a good bet.
• For 26″ wheels and a normal cassette I generally use a 34T-36T chainring although the 42T can be made to work if you’re not running fatty tires.
• For 26″ fat tires, 27+, 29er or 29er + tires I would go with a 30T-34T depending on the tire diameter. Bigger tire diameter means go with the smaller chainring.
• For any IGH system, I would start with a 42T chainring or smaller then adjust the speed/power range by swapping out the rear cogs, you might have to get a super large cog from ebay but using a 42T chainring will give you the cleanest chain line which is super important with an IGH. I really like the CS-S500 cog ($13 on ebay) which comes in 18T/20T sizes (the 20T is a safer bet).
• For CVTs you can also use the 42T chainring and experiment with cogs sizes, for a 26″ tire you’ll probably settle on the 22T like I did. Be aware that the power limit for keeping your CVT under warranty is only a scant 250W.

Deep Powder for maximum torque and <20mph top speed

• 20-24″ tires – Forget it, to go through deep snow you need the biggest, fattest tire you can get.
• 26″ fatbike tires or 29+ size and a normal cassette should be a 30-36T chainring depending on the diameter of the tire. Bigger tire (like the Snowshoe 2XL means a smaller chainring).
• For any IGH system, I would start with a 36T chainring or smaller than adjust the speed/power range by swapping out the rear cogs which are very inexpensive, you will have a hard time getting ANY IGH other than 3 speed IGHs to survive at over 2500W of power for any real length of time.
• Forget about CVT’s, the only one that will work in deep powder is the N171 and it is just too heavy to consider at almost 9lbs.

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.

Ride On

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.

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