Torque arm security: bolt/eyelet vs hose clamps

Hi there,

First time poster - I've started building my first e bike conversion. It's a 48V 1000W rear hub on a Genesis Equilibrium disc road bike.

I'm nearly done, but the rear fork axle/dropout/torque arm vortex is proving to be challenging. An early test ride led to the realisation that the wheel was nowhere near secure enough, as it span out, gently and without damage thankfully.

I now realise that I need to file to dropout so that the axle fits better and deeper. And that I need a better torque arm, probably on both sides, than the crappy Grin v1 knock off I got.

My question is regarding the torque arm.

If I can find a solution secured via the spare eyelet on my rear forks (or even onto the disc caliper mount?), is that going to be more secure than any hose clamp solution (assuming the bolt is good quality)?

This kind of custom thing is what I'm looking at: https://endless-sphere.com/forums/download/file.php?id=242141

The above actually seems to be pretty similar to the Grin v2: https://ebikes.ca/product-info/grin-products/torque-arms.html

Maybe I could just use that?! But it says it's designed for front forks so I'm not sure if it's a good idea.

Finally, I understand and have read that for optimal resistance you want the direction of motor torque rotation to be pushing against the frame, rather than relying on the hose clamps to pull it back, as they're weaker. So presumably bolts lose some of the benefit of a clamped solution, which places the arm underneath the stay, as the force is transferred to the bolt?

But then again, clamps seem to be fairly unreliable vs a bolt to me, but maybe I'm wrong. I'm not sure how to assess that trade off.

As you can tell, I'm a bit overwhelmed with options and information.

Thanks in advance, I really appreciate any advice.

Seems to me the difference between two clamps and a threaded solution would be difficult to determine without rigorous testing, but Grin could have increased the number of clamps from two to three if their evaluation suggested it was required (they went from one to two because they discovered that one was inadequate). One clamp has worked for my 1000w, 48V rear DD system for six years. If you're concerned, use two torque arms, two clamps each, although neater, probably equally effective control could be achieved with a good bolt/eyelet.

Fyi, because of the different profile of the motor axle compared to a regular wheel, you usually have to file out the dropouts on both sides so the axle fits further into the slot, or it's really hard to keep secured. Unfortunately that usually ruins the frame for going back to a regular wheel later..

Edit---just reread you already figured out.

Have two of these 3 clamp TA V4 from Grin. One on each side. Each side is pulling a different direction on the Axel.
One is used forward torque, the other is pulling opposite to take out the play and support regen which is my main braking.

Axel movement is the problem, any movement. Consider my trike a test 3+ years. Have never had a flat so have not taken it apart or tighten it up. around 6k miles. Don't do the drag racing so max watts 3200w, average below 500w. Regen hammers on it more than forward tq.

Next time will build custom plates. purchased tq arms have to much play. any play with regen will not last.

Hi there,

I'm working on a 48V 1000W rear hub on a Genesis Equilibrium disc road bike, and working through the best torque arm options.

I have narrowed my options to:

Grin v4 on the drive side, with the arm sitting under the chain stay.

or

Custom arm, mounting to fender eyelet, disc brake mount eyelets and with hose clamps (see sketch in images).

For the gear side, I'm looking at a Grin v2 into the eyelet.

See images here of the custom arm sketch, and drive and gear sides: https://postimg.cc/gallery/50Ty0q3

I have a few questions:

- Does anyone have an opinion on what will be stronger? Or strong enough? I want to be as secure as possible.

- The v4s seem to be designed to sit on the seat stay, rather than the chain stay. Is this an issue? I believe people use them on both.

- With the custom arm, a) is it any good? (I have no experience in this!) and b) is there any reason not to use the disc brake eyelets to mount the custom arm as per the sketch? I am working on the principle that the more bolts the better, but maybe I have missed something.

- I don't believe I need a c washer despite the presence of lawyers' lips on my dropout, as the nut I have fits cleanly inside the lips. Am I correct?

Thank you in advance, I realise that's a lot of questions, so I'd appreciate any advice you can offer on any one/all of these questions or anything I may have missed.

I made a small plate from 1/4" x 1" steel bar from Home Depot, and drilled holes to align with the brake caliper mounting holes, and used longer screws to attach it. I went with that so I could drill and tap a hole to align with the Grin torque arm, to avoid using clamps. It also allow removal without having to readjust the brakes. I matched the screw with the same diameter as Grin uses to it's a very tight fit with no movement. It works well.
https://endless-sphere.com/forums/viewtopic.php?f=6&t=97945#p1451659

On the other side I have a cheapo torque arm, and although the fit sucks, I have it clamped in a way to preload it against the direction that regen would spin the axle. Haven't had any issues or detect anything that would indicate any movement.

E-HP said:

I made a small plate from 1/4" x 1" steel bar from Home Depot, and drilled holes to align with the brake caliper mounting holes, and used longer screws to attach it. I went with that so I could drill and tap a hole to align with the Grin torque arm, to avoid using clamps. It also allow removal without having to readjust the brakes. I matched the screw with the same diameter as Grin uses to it's a very tight fit with no movement. It works well.
https://endless-sphere.com/forums/viewtopic.php?f=6&t=97945#p1451659

On the other side I have a cheapo torque arm, and although the fit sucks, I have it clamped in a way to preload it against the direction that regen would spin the axle. Haven't had any issues or detect anything that would indicate any movement.

Click to expand...

Thanks. That looks good. No risks as far as you're aware going into the brake calipers? I saw someone else said avoid them, but didn't explain why.

I'm not sure if it would work for my setup as I have lawyers' lips that would obstruct the Grin torque arm part: https://postimg.cc/njBwCRBG

Maybe I could use a c washer or spacer to solve that.

MaximilianMM said:

Maybe I could use a c washer or spacer to solve that.

Click to expand...

That's generally how it works. I use an extra washer to get mine to line up.

MaximilianMM said:

I'm not sure if it would work for my setup as I have lawyers' lips that would obstruct the Grin torque arm part: https://postimg.cc/njBwCRBG

Maybe I could use a c washer or spacer to solve that.

Click to expand...

I prefer to use an angle grinder and a fine cut file to solve that.

From what I can see, your main problem is not the torque arm itself, but the larger than usual lawyer lips on that frame.

The torque arm is not too weak, its just not fitting right on that funny frame.

If the axle is long enough, stack some spacer washers under the side without the wire. C washers under the other side, if the nut won't fit in the space.

Grind down the lips some if you must, and as you mentioned, deepen the axle notch, but not more than 2 mm.

One way to make a two piece torque arm much stiffer, is to set it to fit your bike, then remove it and put a small tack weld connecting the two pieces. This makes it harder for the nut connecting the two pieces to slip. Then the hose clamp is good enough. However, some hose clamps these days really suck and are not reliable. Yours may be obviously flimsy. Try to find a hardware store that stocks better quality ones. Tractor supply in the USA is the first place I'd look.

It is a possibility to make your own torque arm, custom fitted to work with the eyelet. But I think you will be fine, with 1000w type motor and controller, to just make what you have fit proper. You would want better, or at least two TA's, for a more powerful motor.

Chalo said:

MaximilianMM said:

I'm not sure if it would work for my setup as I have lawyers' lips that would obstruct the Grin torque arm part: https://postimg.cc/njBwCRBG

Maybe I could use a c washer or spacer to solve that.

Click to expand...

I prefer to use an angle grinder and a fine cut file to solve that.

Click to expand...

I didn't look at your mystery link, and I took "lawyer lips" at face value. That's not what you have.

Lawyer lips are protrusions on fork tips that keep the front wheel from departing if the QR skewer or axle nuts are left loose. They are one kind of "positive front wheel retention device" as required by the Consumer Product Safety Commission. When they're in the way, I remove them, because they don't have a practical function.

What you have are hooded rear dropouts, which are shaped that way to allow a good surface for the tube ends to attach to. They have nothing at all to do with lawyers. You can't remove them, so my suggestion above doesn't apply.