Custom 190mm NB Power 3kw Kit from Overseas Warehouse for Dolomite - Torque Arm Guidance

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Hello E-S community-

I'm new to forums altogether, which means that I'm at risk for posting on a topic that's already been covered or posting in the incorrect area. I'm still learning etiquette and jargon as well, so patience is appreciated.

I think I'm in the right area because my question pertains to the structural integrity of a Mongoose Dolomite steel bike frame with an axle that most folks don't seem to order unless through the manufacturer. A lot of folks seem to put this kit on bikes that fit 170mm/175mm dropouts, so I don't see too much support for torque arms on the Dolomite if that makes sense.

I worked with Anne at NB Power in China over Whatsapp to order a custom 190mm axle with the 72v 3kw motor (rear) kit w/ 80amp sabvoton controller. She threw in a 20ah (**w/ 80amp BMS**) battery for another $630 and sent me a $1488 total invoice, which includes 3 to 5-day express shipping for $185 extra. I dig that. She's assembling right now and swapping out the 170mm/175mm default axle most folks see on Amazon, on track to ship out come Monday. I'm a happy guy so far.

Here's my question:
1.)
I have read of spinouts with the supplied (cheap-looking) torque arm(s?) for this 72v 3kw nb power kit. I don't know if they supply one or two torque arms for this kit, but I wouldn't feel comfortable without two given the industrial-grade torque this produces. Do you think those spinouts were due to poor installation/operator error or do you think those spinouts with supplied torque arms were due to a materials failure given this is a ridiculous amount of power?

2.) I have seen a few beefier-looking torque arms on Amazon, most notably, a torque arm made by Grin. Figuring out which and how many torque arms I need is a difficult determination due to the motor kit not yet being in my possession, so I thought I would leverage any experience other nights have with this 3kw kit on a similar fat tire bike. Do you think ONE really good torque arm by a company like Grin would be sufficient in conjunction with one of the supplied torque arms? I'm all about over-engineering when it comes to safety, but would TWO Grin torque arms be overkill for this kit? I will NOT be using Regen, since it sounds like that puts an incredible amount of stress on the whole assembly and frame. For simplicity, I opted for NO PAS as well.

3.) I also plan on supplementing my torque arms with punch angle supported by metal hose clamps and married to chassis with liquid metal weld like one would use for preparing mufflers. This might be overkill, but I do not want structural worries while going 40mph+.

4.)
I don't see any additional mounting holes on the frame dropouts for support hardware used by some torque arms. Should I avoid torque arms that require I drill and tap a second hole in this drop-out area? These steel drop-outs seem like they're close to 5 or 6mm thick, so almost a quarter inch thick - pretty beefy compared to non-fat bike frames it seems.

5.) With the 190mm axles, will there be around room left for peg-like footrests that come with a rear passenger seat I'm going to add? I'm concerned that the axles will be just long enough for a torque arm and nut to secure it all together, and there won't be enough meat left on the axle if I add footrests. Thoughts?
 

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What size axle thread does the motor have? Should be 16mm or more, but might be 14mm. The smaller the major diameter of the axle, the more torque arm thickness you'll need to resist spinout. Don't get lost in fancy materials; anything stronger or harder than the axle will be wasted, and may well waste your axle.

Beyond thickness and material strength, the closer the flats on the TA fit to the axle, the better. The farther from the axle center you anchor the torque arm, the better. I'd put anchor bolts in the far corners of the dropout windows, using fine pitch fasteners as big as you can manage. That's a garbage quality bike made of garbage quality materials. Cut it whatever breaks you possibly can.
 
Get a pair of ebikes.ca's torque arms - very well machined, thick, and can handle the power. I believe they make a version compatible with 16mm axles too.

If you need to drill the torque arms in, do it. Zero slop is acceptable at the kind of power levels you are about to push.
 
Thank you @neptronix @Chalo . This is good stuff. I’m about to dig into the diameter of the coming axle, which is literally the ONE dimension/measurement that I have not taken into consideration yet. Anne at NB power sent me this picture last night, their Saturday morning as its being built. Very pleased with the communication from NB Power so far.

Regarding the materials @Chalo I completely agree. The bottom bracket rebuild was first indicator of how malleable/poor quality these materials are, which I suppose is better than it being brittle material, lol.

Checking out ebike.ca’s TAs this evening as well. Should have motor kit in about a week, which means more will be revealed! All in all, sounds like I’m on the right track and excerizing the appropriate level of caution.

Thank you so much!
 

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Get a pair of ebikes.ca's torque arms - very well machined, thick, and can handle the power. I believe they make a version compatible with 16mm axles too.

If you need to drill the torque arms in, do it. Zero slop is acceptable at the kind of power levels you are about to push.
That ebikes.ca website is good stuff. Lots of quality options for for many things including torque arms. Seems like their stuff is made to last. Thanks, again!
 
Ya welcome!
 
Grin, i.e. ebikes.ca, makes several versions of torque arms. They sell several of those on Amazon.
Grin is the best as far as all things hub motor are concerned. Though the price, shipping, and customs might make that grin sag.
 
Do you think those spinouts were due to poor installation/operator error or do you think those spinouts with supplied torque arms were due to a materials failure given this is a ridiculous amount of power?

Both. Those ubiquitous thin, poorly designed, poorly manufactured, poor materials, sloppy fit, cheap TAs are not up to the task, and many examples we see here of them installed show that many folks also don't know how to fit them for maximum effect. Someone with a fine mechanical mind could fit them and make them work (i.e. shim up loose fit with shim stock, repurposed tape measure, soda can, etc.; install one on each side preloaded in opposing directions, etc.) but unless you have the experience and knowledge why risk it?

Do you think ONE really good torque arm by a company like Grin would be sufficient in conjunction with one of the supplied torque arms? I'm all about over-engineering when it comes to safety, but would TWO Grin torque arms be overkill for this kit? I will NOT be using Regen, since it sounds like that puts an incredible amount of stress on the whole assembly and frame. For simplicity, I opted for NO PAS as well.

A high-power kit like yours requires at least one high-quality satisfactorily-installed TA. Two would be better (3kW motor, 80A controller).

3.) I also plan on supplementing my torque arms with punch angle supported by metal hose clamps and married to chassis with liquid metal weld like one would use for preparing mufflers. This might be overkill, but I do not want structural worries while going 40mph+.

"Liquid weld" muffler paste is not the correct material for this application. The best of the Grin TAs are the pinch clamp models; they will work fine without muffler paste or punch angle.
 
Both. Those ubiquitous thin, poorly designed, poorly manufactured, poor materials, sloppy fit, cheap TAs are not up to the task, and many examples we see here of them installed show that many folks also don't know how to fit them for maximum effect. Someone with a fine mechanical mind could fit them and make them work (i.e. shim up loose fit with shim stock, repurposed tape measure, soda can, etc.; install one on each side preloaded in opposing directions, etc.) but unless you have the experience and knowledge why risk it?



A high-power kit like yours requires at least one high-quality satisfactorily-installed TA. Two would be better (3kW motor, 80A controller).



"Liquid weld" muffler paste is not the correct material for this application. The best of the Grin TAs are the pinch clamp models; they will work fine without muffler paste or punch angle.

Boy have I sure read about some catastrophic spinouts that sound preventable with the investment into a proper TA. I’m putting WAY too much forethought into this build to chince out on such a critical piece of hardware. Right now, those Grin TAs are $36-$43 on amazon with overnight shipping. I might just go with two so I can sleep soundly.

Thank for talking some sense into me folks!
 
Finished build/torque arm update:

With a good amount of filing, I was able to get the 14mm axle insert on the Grin Gen 6 torque arm to fit around the thick 14mm axle on the NB Power 3kw kit. Filing the dropouts deeper by several MM is also necessary, so anyone reading this should known that this is essential if your axle does sit all the way in your dropouts.

By drilling and tapping a hole in the rear dropout of the disc brake side of the bike, you’re able to add a second cheaper torque arm that came with the kit. Using only the tear drop shape component bolted into the frame is enough support for torque arm #1, and no this is not overkill for a 3000watt or higher kit.

Thus far, everything seems to be holding very well with no signs of potential spin out. I opted for no regen or PAS in this kit to limit the amount of torque on frame. I heard regen puts insane amounts of torque on frames, and I’m not going long distances, so it seems unnecessary.

Statorade (ferrofluid) is also a must for any ebike motor; I learned this last week after motor was getting pretty hot on even short rides. If you’re using all motor like I am with no PAS or regen, I’d say heat dispersion will become a priority for maximum power and efficiency.
 

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