Torque Arm for hydroformed aluminum frame

Am planning to put a 500W Bafang rear geared hub drive motor on this aluminum framed hybrid/ adventure Motobecane 7 speed 29er bike, with a 48v 17.5ah shark battery. I weigh 155lbs. and am not looking for great power or speed; just decent range.

Not sure if I need torque arms, but I'd rather be safe than sorry. Potential interference to hose clamp type arm attachments include non-round chain stays and a kickstand (which I find rather handy) on the left side. I've been looking at slotted two piece arms and considering securing to seat stays if the angles and leverage work out, but its tough to tell without items in hand. Suggestions? Your two cents would be appreciated, as this is my first conversion project.

Your flash blows or the contours, so it's hard to see the critical areas, but, I'd use the Grin TorqArm_V2. You'd need a spacer washer behind it, to get things to line up, and probably modifying the other part that secures to the rack mount eyelet (probably need to drill a hole in the plate since the distance is so short).



https://ebikes.ca/product-info/grin-products/torque-arms.html

Always a torque arm, two is even better.
First off, a hose clamp doesn't care if the tubing is round or square(ish), it conforms.
The way I approach TA installations is to get a pair of genaric types when I place the motor/controller order and try different positions at the first wheel fitting. Bysides using them as they are, I have been known to use just the "snail" part fixed like the pic above, or paired w/ a home made "arm" or even visa versa. It's always good to have extra TA parts and hardware around.
Out of dozens of fitments, only a couple of times was I stumped and both times the Grin (Ebike CA) V4 TA came to the rescue. Once w/out modding and once with. The Grin part is a little pricey, but the alternative was fab'ing my own from scratch.

Just to be ready, it's pretty often that you have to file out the dropouts deeper to account for the different shaped motor axle compared to a regular bike axle, or it won't go in deep enough, and can change the brake rotor position, so sometimes the caliper mount needs shimming, or shaving, or a bigger rotor to get it to clear the motor body etc. If that's needed, obviously that first before fabbing a torque arm.

Also, don't buy the "grin" torque arms you find from random sellers--they are clones, and from various reports here on ES generally don't fit as well as the originals. No way to know the materials and such, either. :/

Yes, go to Grin Technology, and buy their rear Torque arm. Likely it will fit best on the derailleur side. Dont hesitate to grind a few mm off any corner of it that interferes, like perhaps the derailleur mount bolt or whatever.

The other option if you are handy, is to cut your own TA, custom to fit your bike best. Tools needed are a drill, small grinder, or bench grinder, and some small files to enlarge the hole. It may be possible to modify a front torque arm, making your own custom arm to bolt onto it. If you make your own, I'd cut it in one piece, to bolt to those holes in the frame on the brake side.

One torque arm will do.

1.5"x4"x 3/8" steel and an angle grinder would fit well on that bikes chain stay, if you ask me.
Hose clamps, and strips of rubber dinner table mats from dollar store to save the paint job.

or

https://ebikes.ca/shop/electric-bicycle-parts/torque-arms.html
Two is ideal, one will get you by with low power hubs.

Genuine Products
https://em3ev.com/shop/grin-tech-torque-arm/

motomech said:

The way I approach TA installations is to get a pair of genaric types when I place the motor/controller order and try different positions at the first wheel fitting. Bysides using them as they are, I have been known to use just the "snail" part fixed like the pic above, or paired w/ a home made "arm" or even visa versa.

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I like the idea of using the snail part and bolting a home made arm to one of the M5 fittings on the dropout. It seems to me the key is a snug fitting snail. I could fab one if my life depended on it, but it makes more sense to get them already precision cut if available.

Where do you get your "generic types", and are they snug? Thanks for your advice, and also to the fellow who mentioned filling dropout slots deeper.

That makes me wonder if you can possibly leave the slots alone and grind the brake mounts to restore full pad contact, or at least keep slot filing to a minimum, just enough to get radius correct at the top for full contact. First time's an adventure, so my plan is to take my sweet time and go slow with any filing.

I get a kick out of the promotional advertisements that say you can install a kit in under an hour. Maybe if you're a pro who's done it a dozen times and has a shop full of ebike odds and ends...

Where do you get your "generic types", and are they snug?

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I used to order motors from BMS Battery in China and that was the time to stock up on the little stuff I like, such as TA's, left-hand, Half-twist throttles, 9-pin connectors, etc. Their Ta's are good and the stuff is dirt cheap. But I doubt you would want to pay the freight from China for TA's.
I just ordered a pair off Ebay and they were a sloppy fit even though they were stated to be for 12mm axles. I used them anyway because I could easily fit two and it's a low-powered system. Whether or not you want to spend the $$ for Grin TA's, depends on the system power and whether or not you can fit two. You haven't told us much about your build except that it's a 500 W rated Bafang. It could be a 12mm axle SWKX (whatever) motor or it could be a 14mm BPM.

Thanks for your advice, and also to the fellow who mentioned filling dropout slots deeper.
That makes me wonder if you can possibly leave the slots alone and grind the brake mounts to restore full pad contact, or at least keep slot filing to a minimum, just enough to get radius correct at the top for full contact.

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No, don't touch the brk. mounts
How much futzing is required for a rear hub fitment varies greatly from bike to bike. Although all my donor bikes have been high quality, my hand-built Rocky Mountain was a pain to fit these wide (that's the rub , literaly), geared mini motors. My Taiwan GT's were very easy. Little filing, few spacers and little or no "dishing" to center the rim.
Likewise, the Taiwan made Motobecane (Great bike, did you get yours from Bike Island?) looked easy. I did a frt. mount, but I tried a rear motor to get an idea how it would fit. It dropped right in and the only filing looked to be the usual 'rounding" of the deep end. The key to gd. fitment is using the correct "C" washers(not the ones w/ the tab). In the pic you will see the round boss surrounding the drop-out slot (the inverse is a round recess w/ "lawyer's lips" around the perimeter). Only file the slot to center the C-washer on the round boss/recess.
Depending on how wide the motor/cassette-freewhl is, a flat washer may be required between the end of the cassette and the inside of the drop-out to keep the spline from rubbing(washer w/ tab is ok for this.

I get a kick out of the promotional advertisements that say you can install a kit in under an hour. Maybe if you're a pro who's done it a dozen times and has a shop full of ebike odds and ends...

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No, that's joke. I've maybe done a dozen builds and even a much simplier frt. mount takes me hours. But I customize all my cables to length and make all my controller connections. l like things tidy and stealthy

get a kick out

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You'd also get a kick out of the other exaggerations like distance per charge.

"They" say you can get 100 miles on a charge on a 36V 10Ah battery
yeah maybe downhill half the time
or 1% throttle and 99% sweat

It wouldn't be too bad to shave the adaptor between the frame and the caliper down, instead of the frame end of it...easy to replace if you jack it up. I've had to do it on a few regular frames with factory crooked mounts. But I'd still be worried about the axle be getting deep enough in the so the torque isn't right on the tips of the dropout. I've been surprised at how much the torque can spike just hitting a pothole (spun axle and torn motor wires) or landing off a curb with the motor still spinning up hard (sheared keystock in the clutch plate so no output to the wheel after).

Voltron said:

It wouldn't be too bad to shave the adaptor between the frame and the caliper down, instead of the frame end of it...easy to replace if you jack it up.

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Depends on the laod the system sees during braking--on my heavy SB Cruiser trike, I ground the adapter a bit to get the caliper and pads to sit on teh rotor flat (cheap fork legs I used have mounting tabs welded on crooked), and while the brake bites harder this way, if I brake hard it flexes the mount enough to cause various effects that didn't happen before I did that (though it is possible it's just the cheap fork leg twisting or the mount bending--we'll find out when I rebuild the fork using some very thick pedicab dropouts. Tabs/fork legs ain't gonna bend or twist then!).

motomech said:

Whether or not you want to spend the $$ for Grin TA's, depends on the system power and whether or not you can fit two. You haven't told us much about your build except that it's a 500 W rated Bafang. It could be a 12mm axle SWKX (whatever) motor or it could be a 14mm BPM.

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Great info from all for a newbie, thanks. I'd tell you more about the build if I knew more. Waiting until end of month to order kit for personal budgeting reasons. You likely answered one of my main questions, is it 12 or 14mm; pretty sure it's the SW.... motor, so 12mm I guess. If that means less filing it's music to my ears. As for the Grin torque arms, I don't mind shelling out for them, if I can't find a better value solution. The only V2 ones I can find are from the UK, and they don't say Grin but look to be the same.

I got the bike from bikesdirect because I couldn't find a suitable one after watching bike island for a few months. It's called an Elite Adventure x21, and I bought it because all the better ones were sold out, I want to work on it this spring, and it looks like the same frame they use for their ebikes (minus modifications to fit Shimano mid drive) so I figured it must be a reasonably strong frame. I had hoped to put a mid- drive kit on it, but the chainstays are much too wide. 68mm BB and takes 2+" tires, so I should have known; live and learn... It's got 38x700c tires and 180mm rotors and will fit a 48v 17.5 Ah downtube battery. Tourney 7 speed shifters and RD, Sunrace 11-34 cassette and 28/38/48 triple w Sunrace FD and lockout front suspension.

Cheapest bike I own, but my road and gravel bikes are too good to butcher.

Chuckles1 said:

As for the Grin torque arms, I don't mind shelling out for them, if I can't find a better value solution. The only V2 ones I can find are from the UK, and they don't say Grin but look to be the same.

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It's not a value if it doesn't fit tight. Cheap knock offs will always look the same, since that's how knock offs are supposed to look. If you're going to cut corners, go with one instead of two, since you aren't running regen. You can file it to match the flat contours of your dropout so it seats flush, the the other part can line up with the upper rack/fender mount.

Put the savings toward a good, not cheap, suspension seatpost (thudbuster, suntour ncx, etc.)