large chainrings on mid-drives?

[CrossRoads]

Sorry if this is a dumb question. Noob to mid-drives. Are there any hard limitations to how big of a chainring you can get on a mid-drive?

I have two hub-drive e-bikes and I like going fast and i'm curious about mid-drive but i haven't tried it yet. Seems like the BBSHD comes with 40 - 44T chaingrings. There is an adapter kit for any 130mm BCD chainring, looks like, so it seems like i could get a 52T on there. Why don't they come stock with larger rings? I would think you would want at least a 52T ring on there. Maybe bigger? Is there any actual engineering limit to the chainring size?

 

[m@Robertson]

The hard limitation is if you bog the motor, you can kill it. Or if its a really powerful one, the torque that can't be converted to motion (because you bogged it) turns the chainring or rear cog into a taco shape. The widely agreed-on generic ideal is a 42T ring. But this varies by bike and by the job you are asking of it.

Selecting a chainring on a mid is also about chain alignment, which is a bit of a black art unto itself. Bottom line is to build a mid drive bike, the bar is significantly raised with regard to the details you have to figure out and make good choices on. This is why you see people crying about how their mid drive does mean things to their drivetrain etc. What really happened is the builder didn't do their homework and now they are blaming the equipment.

This explores the chainring topic a bit:

https://talesontwowheels.com/2020/05/03/mongoose-bbshd-update-musical-chainrings/
I have a 52T ring on this bike, but it lives exclusively on flat land and I was looking for straight chainline and optimum cadence, which in this combination of gears is about 3 cogs in. It was never meant to be run on its highest gear of 52x11T and chainline with its short performance-oriented stays would not be acceptable. Oh and also its a 2wd. The front motor takes the initial load off the line to keep the aforementioned 1750w tacos from being served.

There's more details on the whys and wherefores here:

https://talesontwowheels.com/2022/08/01/how-to-build-a-mid-drive-ebike-that-doesnt-break/

 

[Reclyndrider]

I made my own by using the existing 44T with offset and attached a slightly modified 52T chain ring.

 

[Reclyndrider]

There is a guy on YT that builds his own e-bikes on a Cat Trike frame and he uses 80T and plus, chain rings. I looked up the place where he gets his massive chain rings and they are really expensive.
I got the idea of shorter cranks watching his videos.

 

[kmccune]

Hope the bikes and brakes can handle that speed, maybe some of you Guys need an electric motorcycle or moped, OTH "Each to His on" I know Guys that can go really fast and daring, till they cant, one of my Friends also a jump and speed lover almost tore His foot completely off, it was hanging by a tendon, nerve and vein, just a slight amount extra damage and He would have lost that foot.

 

[CrossRoads]

K, well affectively what I ride is a moped, it's just a moped where the human still gets to contribute to the power output of the drive-train at speed, thus increasing efficiency, range, and versatility. I can get 42 miles out of a 25 Ah battery whereas the Onyx moped gets about the same distance from a 40 Ah battery. I get that by 1, pedaling, 2 having a lower cross-sectional area on a recumbent, and 3 keeping speeds to the mid-30's rather than mid 40's (where aero drag scales quadratically with speed).

For both cars and motorcycles, the accident death rate curve sees a sharp exponential rise at around 40 mph, so I hardly see an electric bike that tops out at 38mph as a death wish. More like a mildly risky mid-life crisis alternative that's better for the environment than a ferrari.

... so as i understand it, if you're going for a higher wattage system on a mid-drive, it's optimal to keep your chainring tooth-count low to keep the motor happy and that just means sacrificing the human contribution. Motors like high rpms. Humans don't. I think this sort of answers my question more or less, that a mid-drive is not an optimal platform for a higher speed long range e-bike compared to the tratitional approach of DD hubs.

 

[m@Robertson]

... so as i understand it, if you're going for a higher wattage system on a mid-drive, it's optimal to keep your chainring tooth-count low to keep the motor happy and that just means sacrificing the human contribution. Motors like high rpms. Humans don't. I think this sort of answers my question more or less, that a mid-drive is not an optimal platform for a higher speed long range e-bike compared to the tratitional approach of DD hubs.

So long as you are on reasonably flat ground, then yes. A DD hub is going to be a better choice if you want to go fast on pavement and not concern yourself with pedaling. In that scenario the lesser torque of a DD hub is not really a bad thing, and if you amp up the power (literally) the DD hub overcomes its *relative* lack of torque.

Put enough power into it and the issue of hills goes away too. If we're talking a QSv3, a moped wheel and, say, 72v... you can do whatever you want.

EDIT: If you want to pedal along, as noted above you can get hold of some really big chainrings up front. The biggest I have used on my singlespeed hub bikes is 60T. Beyond that you are looking at some really specialist stuff, but the 60T rings are Taiwan-made and not hellaciously expensive. Holy crap I just looked on Ebay and Stone narrow-wide chainrings - I know they work well as I own one - go up to 70T (!)

 

[kmccune]

K, well affectively what I ride is a moped, it's just a moped where the human still gets to contribute to the power output of the drive-train at speed, thus increasing efficiency, range, and versatility. I can get 42 miles out of a 25 Ah battery whereas the Onyx moped gets about the same distance from a 40 Ah battery. I get that by 1, pedaling, 2 having a lower cross-sectional area on a recumbent, and 3 keeping speeds to the mid-30's rather than mid 40's (where aero drag scales quadratically with speed).

For both cars and motorcycles, the accident death rate curve sees a sharp exponential rise at around 40 mph, so I hardly see an electric bike that tops out at 38mph as a death wish. More like a mildly risky mid-life crisis alternative that's better for the environment than a ferrari.

... so as i understand it, if you're going for a higher wattage system on a mid-drive, it's optimal to keep your chainring tooth-count low to keep the motor happy and that just means sacrificing the human contribution. Motors like high rpms. Humans don't. I think this sort of answers my question more or less, that a mid-drive is not an optimal platform for a higher speed long range e-bike compared to the tratitional approach of DD hubs.

Very salient points, some of us have slow reaction times and we are better off at lower speeds for safety reasons, on the rpm matter , yes! That little motor loves the higher speeds to equalize the field strength and keep the 'eddys' in check, what people do not realize is if the stator gets to move a little torque is almost instant try it on a hub motor and you will see what I mean, the little mid drives get to get that effect almost instantly if there is even the slightest movement of the drive system( a little slack even will do the trick) when things can move freely above stall eveything is happier and as we know the little guy at optimum is more efficient than the big guy who doesnt have time to get things moving. I find it hard to follow Genuises like 'Justin of "Grin tech"( italics and formulae just stop up my gray chowder) graphs are Greek to Me( though I found out I could do a bit of Algebra) the observation and reasoning and of course exchanging of ideas has helped me over the years get a bit of understanding to what is going on remenbering the old adage[ there are old pilots and bold pilots, there are no old and bold pilots] one reason while riding in a friends airplane, no thanks you keep the "con'.

 

[CrossRoads]

EDIT: If you want to pedal along, as noted above you can get hold of some really big chainrings up front. The biggest I have used on my singlespeed hub bikes is 60T. Beyond that you are looking at some really specialist stuff, but the 60T rings are Taiwan-made and not hellaciously expensive. Holy crap I just looked on Ebay and Stone narrow-wide chainrings - I know they work well as I own one - go up to 70T (!)

Indeed, 69T is what I'm using on this bike. Profile pic shows an earlier iteration. The brand is generic "driveline", probably from china, found on amazon, not too pricey, bout a buck a tooth, feels solid enough. It's nested with a 62T, my "climbing" gear, on a two ring crank. The rear hub motor takes a freewheel not cassette, so the smallest cog is 14 teeth, demanding an even larger ring. If i had an 11T cog on a cassette i could get by with a 62T chainring for the big gear.

 

[CrossRoads]

I made my own by using the existing 44T with offset and attached a slightly modified 52T chain ring.

Reclyndrider, nice hack. Is the 44T still functional? I'm imagining like the old tour de france days before deraileurs when they would dismount before a big hill and flip the flip-flop hub over for their climbing gear. Is using a derailleur (perhaps a modified one), even an option with mid-drive?

 

[Reclyndrider]

No. The 44T is just to get the backset and be the support for the 52T

 

[m@Robertson]

The brand is generic "driveline", probably from china,

Driveline brand is actually one of the better ones. Several of my hub motor chainrings from 52 to 60T were that brand. They were made in Taiwan, at least at the time. My Stone is a recent addition, used on a mid drive and it was either 46 or 48T. It is a solid disc like the one you have in the pic. The Drivelines all shared a fairly distinct cutout design.

 

[KEITH VASS]

Sorry if this is a dumb question. Noob to mid-drives. Are there any hard limitations to how big of a chainring you can get on a mid-drive?

I have two hub-drive e-bikes and I like going fast and i'm curious about mid-drive but i haven't tried it yet. Seems like the BBSHD comes with 40 - 44T chaingrings. There is an adapter kit for any 130mm BCD chainring, looks like, so it seems like i could get a 52T on there. Why don't they come stock with larger rings? I would think you would want at least a 52T ring on there. Maybe bigger? Is there any actual engineering limit to the chainring size?

I see it as a question of how fast do you want to pedal to? I have been using a 65T chainring on a BBSHD for some years without any issues. The exception being that I can only pedal up to 52 kph and when I reach over 60 on a downhill, I can only ghost pedal. So I am now thinking of getting a larger one. Maybe 70-80T. I spend most of my time in top gear cruising at 40+ kph. Always puzzles me why there is any limitation at all!

 

[kmccune]

I see it as a question of how fast do you want to pedal to? I have been using a 65T chainring on a BBSHD for some years without any issues. The exception being that I can only pedal up to 52 kph and when I reach over 60 on a downhill, I can only ghost pedal. So I am now thinking of getting a larger one. Maybe 70-80T. I spend most of my time in top gear cruising at 40+ kph. Always puzzles me why there is any limitation at all!

air resistance,when your battery quits you will understand why chainrings are only so big.

 

[m@Robertson]

Always puzzles me why there is any limitation at all!

Well, the DIY groups on the interwebs are littered with pics and stories of people who went big, bogged the motor and either destroyed a chainring, or ruined the motor either thru blowing the controller or just plain cooking the copper wires 'til they're black (EDIT: Also there's the infamous snapped chain). With a BBS02 its usually a blown controller. With a more robust BBSHD its usually the drivetrain that gives out.

If you have found a way to keep from breaking things then revel in it

 

[CrossRoads]

Funny that no one replied to my 2 chainring on mid drive thread but replied to this year and a half old thread. Since OPing , I have built up a bbshd bike and it is now quite obvious to me why, at least for the bbshd, smaller is better if you want to hope to climb any hills. I have to watch the temp and stay in a lower gear than my legs prefer to keep the motor performing while climbing steep grades, so a larger ring would be obviously a bad idea.

Having said that, it would be really nice to have a larger ring for speed so I wonder why two chainrings on a mid drive is not where the industry is moving. There's nothing which prevents this in principle, although with the way the bafang bb drives are built, it doesn't seem easy without deep mechanical / machining intervention. Seems more feasible with the cyc.

 

[m@Robertson]

I don't think its particularly feasible with any motor-assisted front chainring simply because of the consequences of shifting under any level of power. Its bad enough when you do that on the back. Doing it on the front would have to be potentially much worse. I tone my BBSHDs waaaaay down insofar as engagement and power ramping are concerned, as well as eliminating the lags. No way would I run a 2-ring system with a derailleur.

What I have seen is people doing 2-ring systems where they get off the bike and shift the front ring by hand. Hopefully after physically turning the motor off to avoid getting the new nickname of 'stubby'.

I'm also not sure there is much market for it. As a heavier rider, I have found a flat upper limit at around 33-34 mph. Scuttlebutt on the interwebs confirms this is about it as the HD motors have rpm limits, and 1750w on a 52v pack only goes so far. Bantamweight riders are needed to get up to a (claimed!) 40 mph or so. I have done 36 steady cruise with a strong tailwind and that 52T ring about 3 cogs in (and pedaling). BBSHDs need higher voltage and custom controllers to go faster (and they do) without crazy drivetrains.

Now... this is the one place where a Cyc motor can be considered to have an advantage as their rpm limits are sky-high. But owning an X1 Pro... I chickened out at 4kw and 40 mph with clearly more room to go on the acceleration curve. Hearing that 10,000 rpm-capable motor spinning that hard so close to my ankle (and spinning a chain that fast as well) was not comforting. On the minus side for the Cyc is the fact that when you pump those motors up to that level of output, the Photon overheats and is expressly recommended against for doing that. The X1 Pro starts wearing things out hard and fast. BBSHD with say a High Voltage Kits BAC controller can add back in some reliability but you're on a bit of a knife edge there.

So... in conclusion I'd say if you want big speed and reliability you go big hub motor, big amps and big volts. Stealth Bomber territory. 96v, 100a. Light motorcycle style build. You really are pushing bicycle equipment too far at higher speeds. Rims and brakes in particular.

 

[CrossRoads]

Fair points. It does seem a little silly though, that we have to choose between optimums. Can you imagine a car salesman who started out with, "do you want to be able to go highway speeds or climb hills with cargo, can't have both."

I have a DD hub bike that does 40 mph and it's awesome. That same bike climbs like s**t when the bags are loaded on an 11% grade. So i have a long-tail cargo bike with a mid-drive to take cargo into the hills. I also have a long-jon cargo bike which i use for handyman work, taking tools around town in the flats, currently set up with a rear hub motor so i can use the regen in the stop-and-go. Three bikes, three optimal purposes. I'd rather have one bike that does everything.

The average cyclist intuitively understands the problem of shifting under load. You don't change chainrings while standing and cranking on the pedals. You also don't slam on the gas while the clutch is disengaged on a truck or motorcycle. I don't see right away why this wisdom couldn't translate to a motor on a chain-driven drivetrain. Do we really need to always develop products expecting the pilot to be stupid? I think if i were to do a double chainring bike it would be throttle only, no PAS, then i always have deliberate control of when power is applied. I would power up to speed, then shift gears turning the cranks with my feet delicately, just as i would on a normal bike, then throttle on the new ring.

On the speed / power questions. I recal a post of yours claiming that putting 1500 to 2000 watts in a chain drive was no big deal. A reasonably aerodynamic bike with minimal cargo can do 30 - 35 mph with 1500 watts. I cruise at 40 mph on the flat with 1500 watts on my very slick recumbent and I need a 69T chaingring to pedal at a comfortable human cadence in order to do so.

If the cyc can output 4000 to 5000 watts in bursts, then 2000 watts steady-state should be no big deal for the drivetrain or the motor.

 

[m@Robertson]

Three bikes, three optimal purposes. I'd rather have one bike that does everything.

uhhh... I feel your pain

The average cyclist intuitively understands the problem of shifting under load. You don't change chainrings while standing and cranking on the pedals. You also don't slam on the gas while the clutch is disengaged on a truck or motorcycle. I don't see right away why this wisdom couldn't translate to a motor on a chain-driven drivetrain. Do we really need to always develop products expecting the pilot to be stupid?

I have devoted literally thousands of words and multiple articles designed specifically to combat the most frequent and common awfulness found out there in the internet.

https://talesontwowheels.com/2022/08/01/how-to-build-a-mid-drive-ebike-that-doesnt-break/
... and a necessary companion:

https://talesontwowheels.com/2020/10/02/how-do-i-ride-a-mid-drive-ebike-without-breaking-it/
You have to remember that USA cycling was once a niche field populated only by people who knew wtf they were doing. Now its the 21st century and any idiot can buy/build a monster. With legions of other idiots who don't know what they don't know, but freely giving advice anyway. And furthermore, we are faced with an industry that does not try to shield the inexperienced mainstreamer from this lack of knowledge (which is why 60 lb 25 mph ebikes were equipped with cabled brakes until someone was killed and a big lawsuit knocked some sense into the industry).

So when you say "the average cyclist" you are speaking of people I used to share the road with decades ago. But not today.

I think if i were to do a double chainring bike it would be throttle only, no PAS, then i always have deliberate control of when power is applied. I would power up to speed, then shift gears turning the cranks with my feet delicately, just as i would on a normal bike, then throttle on the new ring.

Blech. Screw that. You feel free to do what makes you happy but I'd hate that (and I tried it for a few weeks on a fresh build when my pedal assist sensor went bad and I had to wait for a replacement). I manage power by selecting a lower PAS. Or changing the motor settings so shifting is safe combined with riding technique (or both). And if I want to gun it then sure I love me some throttle. Pedal assist on a motor set up right is a scalpel, not an axe.

On the speed / power questions. I recal a post of yours claiming that putting 1500 to 2000 watts in a chain drive was no big deal.

It isn't if you ride it right. See above. And also include the low-impact settings I go into for an HD motor. Riding it right, building it right and setting up the motor so it doesn't trash the drivetrain are all essential ingredients. As beneficial as a mid drive is over a hub motor... you have your work cut out for you to make it daily-driver reliable over a long period of time with no excess wear and tear. But if you put the time in at the start, you never have a care for drivetrain problems over the life of the bike.

https://talesontwowheels.com/2023/08/09/bbshd-settings-for-a-pedaling-cyclist-2023-edition/
I recently changed out a chain that was measuring out to be just fine after 4200 miles (the green bike in the pic above). And yes thats Four Thousand Two Hundred. It was an 11s KMC 11e. A cargo bike that had been carrying heavy loads (not the least of which was me) throughout its entire life, albeit on flat ground. I changed the chain ONLY because I put on a new chainring and cluster, and its stupid to do that and NOT change the chain (which I kept in my toolkit as my backup in cease I ever break the chain on the road).

If the cyc can output 4000 to 5000 watts in bursts, then 2000 watts steady-state should be no big deal for the drivetrain or the motor.

Making sure we are talking about the X1 Pro at those power levels. Not the Photon. Cyc has gone on record saying Photon is not meant for that. Its a big deal going into the game. Building it, ensuring its power roll-on is not a problem etc. but if you take care of all of the above THEN it becomes no big deal. But...

You want to see high power that has a broad range of application for both speed and climbing capability? Look to the Biktrix Juggernaut XD. That motor uses a jackshaft and a second kart chain running down the left side, single-speed. 2.3kw. But power delivery is independent of the now-human-only drivetrain which solves a whole lot of problems.

https://biktrix.com/products/juggernaut-xd-duo?variant=41967776366783
But now ... you are pretty much in light motorcycle territory, and a light motorcycle's components (wheels, brakes,frame) is better suited to this kind of power level. An UBCO is probably the closest thing to it present-day, and those get to the same place with dual hub motors.

 

[Nine6Teen]

i converted a road bike to ebike using a bafang mid drive kit, and choose a 52t front chain ring because it's a road bike with skinny tires. so I wanted the ability to go 25 - 30 mph. extreme hills are a challenge though. the bafang 52t chain ring is relatively inexpensive