What’s the weakest link in my electric bike?

I’m hoping someone can show the math for figuring the torque. I believe gearing isn’t a factor and can just go off these ingredients:

180 amps max possible from the esc, 130kv motor, and a no-load speed of 40mph.



I’ve been told a typical 32 spoke mtn bike wheel can handle a lot of torque but how will the chain and aluminum freehub do? I see steel freehub bodies available but will that amount to a stronger internal ratchet and the external interface I’m not worried about?


Would a 3/32” chain be quieter or better in some way than a 1/8”?

What’s the weakest link in my electric bike?

It's usually operator error.

nicobie said:

What’s the weakest link in my electric bike?

It's usually operator error.

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first trying to avoid an operator purchasing/design error. surely someone else has put a lot of amps through a standard mtb freehub and chain but its hard to find in a search.


its a good math problem at least to figure the torque

"My electric bike"? What is your electric bike? If you have a 24 pound ultralight MTB vs. a 45 pound, burly DH rig, what is the weakest link? They will have differences. Is your ebike a factory production rig, a conversion, what?

It'll be the freehub.

Grantmac said:

It'll be the freehub.

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Agreed.

If you use an aluminum freehub body, you need to combine it with a cassette that carries the larger sprockets on a spider with a wide mounting surface. Mounting steel plate sprockets separately on the aluminum spline will cause them to dig into the spline and become difficult to remove. That's a problem apart from the ratchet mechanism (which also works better when it's all steel).



Gearing absolutely is a factor, because the motor output torque is reduced or increased by the gear ratio when it's transmitted to the rear hub.

So apparently we're only focused on the rear wheel and rear drivetrain components? Can't qualify this as to ebikes because I don't think we've seen enough of them as factory bikes...Trek and Spec...or homebrew bikes like with a Bafang for a long enough time come through the bike shop to get a decent feel for specific failure parts.

However, we've had a long exposure to gasoline engine conversions. Not an exact comparison, of course, but it is power to the rear wheel on a normal bicycle with a motor. There is one consistent failure with most all of them...the rear wheel. Some of these are a single rear cog setup and some have a normal multi-cog rear drive. Surprisingly it's the rear wheel that seems to disintegrate at the spokes...breaking and out of tension...instead of any drivetrain components...though I'm sure chain wear and cog wear are accelerated. Still, the rear wheels don't stand up to this use at all.

Now, here's another angle. We don't work on these gas conversions...liability and the funky nature of most of the designs. But we have told many of these owners that a standard single wall rim isn't up to the task of this application. Some are on Walmart level POS bikes with cheesy wheels, so one could chalk it up to that. However, we see this too. Most of these guys are absolute cheap bastids, and a lot of them are DL suspended drunks. We tell everyone of them when they come in to buy a wheel that a cheap, single wall rimmed wheel...even steel...that it won't last. We tell them if they're serious about making the rear wheel last, they need a stronger wheel. Only a handful were willing to try that. We make no guarantees on any wheel that goes on a gas conversion...we note in our store computer.

We've actually had a few buy a near DH level complete MTB wheel and hub setup. Those lasted a very long time, but even those eventually fell apart. Now, I'm pretty sure those wheels weren't properly maintained over that time with good attention to spoke integrity, but at least they did last a good while.

Now...does this apply to the OP's question about the rear wheel and drivetrain? I don't know. But I can't currently recall any drivetrain failures these gas engine guys have had other than chains and rear wheels. Obviously these gas conversion guys are all over the board with single cogs in the rear, cassettes, freewheels, and even some cheap IGH's. It's still the rear wheels that have fallen apart in "most" of these cases before any of the other components. Just throwing this out for food for thought...not sure if precisely applicable at all.

Now let me throw out just my personal experience. I did my BBSHD conversion to my older '08 Santa Cruz Nomad early this year and have been riding hard off road since April...my first ebike of any kind but obviously not my first bike. My rear wheel has an XT 6-bolt disc hub with a solid axle conversion with track axle nuts. The rim is a Mavic EX325-Disc mounted with 32,14ga, DT, SS spokes. This rim is an AM/Enduro level rim and is quite sturdy. I've used 3 of these identical wheels on my long travel, big hit MTB's with absolutely no issues over years of abuse...Moab and other hardcore off road riding...hardly even have to look at them for truing and such. In the short time I've been riding my Nomad/BBSDH with a relatively new wheel...same EX325...I've had to check and tension spokes every 2-3 rides...something I never had to do. I'm not riding any harder and no Moab trips.

Here's my 2 cents. OP seems interested in how power is/can affect the rear wheel and drivetrain. I think we're overlooking how weight plays into this too. Sure, my BBSHD is pumping more torque into my nice, strong rear wheel, but that wheel is also now dealing with a 50 pound bike rather than a 34 pound bike...and then throw my 8 pound battery in my backpack on top of that. It seems that "most" ebikes are heavy. That "AND" the power are both hard on our rear wheels. Throw in using the bike for real off road riding, and you have rear wheel torture.

I say this frequently but it applies. Many/most of these discussions about ebikes are all over the board. Do we have any idea what application the OP's "my electric bike" covers? How does it get used? And what is the "typical 32-spoke MTB wheel"...an XC weight weenie racing wheel with 24 spokes or a DH level bombproof anvil? I'm not dissin' the OP. I'm just saying we don't have enough info to answer his question. Then why am I even interested in the issue? I think I'm starting to see the weak link in my application...the rear wheel strength/integrity...for ebike use for off road. And some of this may apply to ebikes in general in some sort of way. We have more power than our puny bodies can produce on the pedals..and often a throttle option...and we have noticeably more weight to deal with. Maybe street applications will just take a little more time to destroy their parts than off road. I'm going with the rear wheel at the spokes and rim as being the "general" weakest link based so far on what I'm seeing. Sure, if you try to run super lightweight hubs and such, you add another weak link to the mix, but right now I'm going with spokes and rim. It's a hard life back there for a rear wheel and a motor.

I’m using a carrier for the cog so the outside of the freehub will be good (it’s an add-on big gear that I drilled for a single speed carrier) I guess ill see about the inside ratchets and wish I’d gotten the steel freehub.


Im only focusing on the parts subject to motor torque. It’s a custom steel mtn bike frame I’m getting made so as to add a motor mount and just some eBay 32 spoke wheels and I guess 3/32” chain. Will use a mid drive using a standard out runner. It’ll just be ridden on road and I’m 150lbs

https://www.ebay.com/itm/26-DT-Swiss-533D-tubeless-ready-6-bolt-disc-bike-wheels-32-hole-red-sealed-hubs-/203554892990?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l6249&mkrid=711-127632-2357-0

Can see the free hub ratchet here:
https://m.aliexpress.com/item/4000378334198.html




But I believe with the math to figure the torque the gearing won’t matter and while I’ll be doing a 12.1:1 ratio, which will increase the torque, the motor kv will have to be very high and therefore low kt and it balances out and I think the only variables are: kv/kt, the no-load top speed, and current. I also thought I was going to end up with huge torque but instead I’ll end up with great efficiency

If I was having a custom frame made, I would choose a modern 12mm bolt through dropouts, axle and hub combo, especially for an e-bike.

Avner.

the bolt thru axle is probably stronger but I don’t think the older style mtn bike axle n quick release is going to have a problem with the torque. The axle will be pulled hard against the front of the dropout with throttle but shouldn’t be a problem..I dont think.

On a ground up build with what you're trying to do I'd use an OSET hub and forget about the freehub. That'll let you run regen as well.

Grantmac said:

On a ground up build with what you're trying to do I'd use an OSET hub and forget about the freehub. That'll let you run regen as well.

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I’ll be using this freewheeling crankset as a jackshaft (I’ll weld on a big pulley) but figure with that hub I still could do regen. I wonder if would be more drag from cogging and the double gear reduction than a hub motor. I’ll use this relatively small 80mm motor, and then a belt, n then chain. I should be able to get a good idea even with the standard hub I have with the bike upside down spinning by hand. I’m guessing drag would be more than the hub motor. Maybe I’ll end up going that way but for now will use the wheel n cog I got. Thanks for the idea.

(Pics I posted in other thread but motor mount in seat tube). Then belt (12mm GT2 8mm pitch) down to cranks n then chain to rear. I think could be the most efficient drive with the high gear ratio of 12.1:1 using belts n chains instead of doing the gearing with any other method.

And then if I don’t do regen I could do higher voltage battery safely ..but at this point of 42mph no-load speed ...if I go higher voltage, maybe it’s worth it raising the no-load speed way beyond what you'd do riding but benefit with more torque? A slight hit in esc efficiency but greater torque?



and will use the Unity double esc wired for a single motor and supposedly possible. A vesc that also has push to go for 300$ and i think will put out 180 amps at 60v.



which brings up lifepo4 and its weird initial voltage drop. how many cells in series you think is a healthy limit for the 60v rated escs? im doing 16s and assuming each cell at 3.6 and 57.6 top voltage but rather go up


and then there's these and they say can charge lifepo4 to 4.1? and there's no data on them anywhere and i bought them and will do a discharge test with my nifty constant load tool. seeing no data anywhere isnt encouraging. see what’s between 3.6 and 3.0v with 3 amp discharge. https://batteryhookup.com/products/full-case-of-120-k2-26650-3800mah-lifepo4-cells

Looking at the size of this motor, “83100”(69x55.5 stator), and knowing it’s a 7:1 ratio to 26” wheel, why am I not getting bone crushing torque with my flipsky 75100 Vesc, set at like 150 amps n 85v?

I’m jumping the gun asking here and still need to get an Android phone to hook up Bluetooth and see what current is actually passing but regardless shouldnt a motor this size with such a gear ratio have no trouble getting flipable torque off the line and maybe even at higher speeds?

The motor claims a max output of 7 newton meters torque. What would be the math to find the possible torque at the wheel when 7:1 gear ratio and what torque would be enough to flip the bike? 7nm x 7=49nm? That’s not happening




I crave more torque and why I made a mid drive but maybe assumed too much.