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:

The BBSHD: Musical Chainrings

It seems inevitable. When I build a bike, I go through front chainrings trying to get the gearing just to my liking. My Mongoose Envoy build has pretty much set the world record for tweaks in thi…

talesontwowheels.com

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:

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

Mid drives up the ante on the required competence of the builder. Read this so your bike lasts without things breaking or wearing out early.

talesontwowheels.com

 

[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.