Help with torque arm install on rear hub build

[stephere]

Hi all, in the process of doing my first ebike conversion. I'm using the Grin RH212 kit on an old-ish mountain bike I had laying around. I was hoping to use the V6 or V7 torque arm from Grin, but they are both pretty thick relative to my remaining axle length. The limited flat surface around my dropouts also complicates things. I've attached three pictures that show my axle layout on both sides.

If it's not clear from the pictures my axle layout is as follows (based on the hardware / install manual provided with the RH212).

Starting on chain side, Img01.png:

1. Axle nut
2. Axle nut washer
3. Tabbed washer
4. Dropout
5. Washer with flats
6. 3mm space washer
7. Motor
8. Washer with flats
9. Dropout
10. Tabbed tasher
11. Axle nut washer
12. Axle nut

Right now my plan is to try installing the V6/V7 torque arm on the cable side. I'll be able to recover some room by removing the tabbed washer on the cable side (10) but I doubt that will be enough. Would I be okay removing the flat washers as well (5 & 8)? Open to any other advice / recommendations as well

Thanks!

 

[docw009]

If you drop back to a 6 speed or 7 speed freewheel/cassette, instead of the current 8 speed, you might get enough space on the chain side for a torque arm. Depends on whether the spacers under those gears can be changed, to allow the arm to go on the inside. On the outside, you would be squeezing the frame.

 

[E-HP]

It doesn’t look like you need both washers inside the dropout to provide chain clearance on the small cog, so I’d get rid of one of them. I’d eliminate both washers outside the dropout and replace them with the Grin torque arm.

 

[Chalo]

Wherever you use a torque arm, it can replace all the washers on that side.

 

[Epithemeus]

I would not do the cable side. Hollowing out the axle for the cable significantly weakens it. I think I remember reading somewhere the hollowed axle side is half the strength versus non hollowed. Don't have the time to dig for the thread unfortunately.

 

[stephere]

If you drop back to a 6 speed or 7 speed freewheel/cassette, instead of the current 8 speed, you might get enough space on the chain side for a torque arm. Depends on whether the spacers under those gears can be changed, to allow the arm to go on the inside. On the outside, you would be squeezing the frame.

Thanks, will definitely consider dropping to a 6 or 7 speed cassette if I can't make anything else work.

It doesn’t look like you need both washers inside the dropout to provide chain clearance on the small cog, so I’d get rid of one of them. I’d eliminate both washers outside the dropout and replace them with the Grin torque arm.

Thanks for confirmation on removing washers. I think I'll need to keep one washer on the outside to get enough clearance from the frame for the Grin Torque arm to sit flush.

Wherever you use a torque arm, it can replace all the washers on that side.

Thanks!

I would not do the cable side. Hollowing out the axle for the cable significantly weakens it. I think I remember reading somewhere the hollowed axle side is half the strength versus non hollowed. Don't have the time to dig for the thread unfortunately.

Yep I've heard this as well, but I thought it was more applicable for hub motors where the cable comes out the end of the axle (see 03.png for where mine comes out)

 

[amberwolf]

If the axle is modified for the cable, then wherever it is modified it is almost certainly significantly weaker there than the unmodified axle. That weakness can affect immediately-surrounding areas as well.

IIRC, I broke an MXUS 450x motor's axle at the diagonal hole exit point for the cable (the hole starts inboard of the cover bearing, and ends outboard of it at the shoulder immediately inboard of the threaded / flatted axle segment, so while the hole itself is entirely within the thicker shoulder, it still affects the strength of the axle at the shoulder/axle interface, and sufficient torque there can still cause it to fail).

Personally I'd prefer torque arms on both axles, as close to the motor shoulder as possible (so inboard of the dropouts, if hte dropouts themselves are not the axle clamp).

 

[stephere]

If the axle is modified for the cable, then wherever it is modified it is almost certainly significantly weaker there than the unmodified axle. That weakness can affect immediately-surrounding areas as well.

IIRC, I broke an MXUS 450x motor's axle at the diagonal hole exit point for the cable (the hole starts inboard of the cover bearing, and ends outboard of it at the shoulder immediately inboard of the threaded / flatted axle segment, so while the hole itself is entirely within the thicker shoulder, it still affects the strength of the axle at the shoulder/axle interface, and sufficient torque there can still cause it to fail).

Personally I'd prefer torque arms on both axles, as close to the motor shoulder as possible (so inboard of the dropouts, if hte dropouts themselves are not the axle clamp).

Thanks for the advice. I think with that in mind I will try to switch to a 6/7 speed casette so I can fit one of the thicker Grin V6/V7 torque arms inside the dropout on the non-cable side

 

[E-HP]

Thanks for the advice. I think with that in mind I will try to switch to a 6/7 speed casette so I can fit one of the thicker Grin V6/V7 torque arms inside the dropout on the non-cable side

That MXUS has a 45mm stator. Your RH212 may be peppy, but it has nowhere near enough torque to snap your axle, even on its best day. Protect the weakest component, which is your dropout.
If you’re not using regen, then one arm is fine, but two is always better.

 

[motomech]

The first thing I would do is toss those cast C-washers and forget the one-piece TA. I've had cast POS's crack into pieces and make a real mess. Then you have plenty of room to stack flat washers out to a point where a two piece TA will line up to a chain stay arm.
I've used cheap 2-piece TA's, but usually, the slot is too big for a 12 mm axel (althought they can filed out for a 14). Once I got a big enough selection of pieces of different styles, I could combine different shapes of holders and arms to get what I needed. If all else failed, I use the expensive Grin TA (V4?) TA which seems to work everywhere I've tried.
Flat, steel C-washers (w/ the tab) are handy to go inside the chain stay where more space is sometimes needed to keep the cassette/free whl. from rubbing.
Depending on the system power, I sometimes use two TA's, one on each side.
You can buy cheap TA 's and mounting hardware on Amazon

 

[999zip999]

I always use norlock washers on each side. Special.

 

[stephere]

Update just to close the loop - I ended up installing the Grin V6 arm on the non-cable side (inside dropout) - I removed the flat washer that was inside the dropout and then used a less thick tabbed washer on the outside. It was still pretty tight & had to spread my frame a bit more but seems to all be squared away now.

Thanks everyone for the advice!

 

[999zip999]

Torque arms are a must. And not to thin as it could cut into the axle.