Fitting torque arm to rear of carbon frame, can this work?

Manc44

Pedelecer
Jun 21, 2021
126
16
I have a carbon frame with carbon dropouts. There's a recess close to the axle where a torque arm could be fitted as shown in the following image, but is this likely to ruin the dropout if the axle slips?

This would have the torque arm pressing into the carbon. My motor can put out 45Nm.

I am more bothered about my motor cable twisting than I am about the frame, it's a rough old frame but that's why it's a potential donor in the first place :)

I know in this image it's not lined up perfectly but I'd be filing the curve flat, that touches the recess. That recess is at a 90° angle to the dropout and it's 9.5mm deep. It looks like the part that would get pressed on is solid carbon fiber.

44159


Here it is with nothing there:

44160

The torque arm in the first pic needs to go clockwise slightly to be lined up with the axle slot, which is lucky because I can file a flat section into the torque arm to make it fit perfectly, along with filing off the square edge on the torque arm where it's contacting the corner of the recess, that's rounded off.

This is the only thing stopping me using this frame, the fact that it's got a Post Mount rear brake fitting and I can only (properly) fit the torque arm I currently have, to an IS mount.
 
Last edited:

Nealh

Esteemed Pedelecer
Aug 7, 2014
14,540
5,638
58
West Sx RH
Even if you do risk fitting the hub to the drop out there are two aspects to look at ;
1. Any hub axle will require the drop depth to bb modified as you will want all of the flat area of the axle to sit firmly in the drop out. For a 12 x 10 axle approx. 1.5 - 2mm will need to be taken out of the drop out seat.
One doesn't need to file or alter the TA's axle profile as it will already be a perfect fit for the axle, it is the drop out profile that will need to match the TA hole profile.

2. Will the interface of the ali/CF withstand the force/torque applied ?

A torque arm (TA)is a belt and braces fixture to prevent the axle twisting out but one needs full contact of the axle within the drop out. And with CF if thinking of that route 2 x TA's and not one.
 

Manc44

Pedelecer
Jun 21, 2021
126
16
Cheers Nealh.

I just noticed the dropouts aren't metal, they only have a thin sheet over the carbon, presumably to stop QR skewers chewing away at it.

I can use 2 stubby little torque arms the way shown in the first image, because the drive side dropout is the same shape, so I could just use a reversed version of the same type of torque arm there.

I'm doing it anyway lol... I'll just have to check the dropouts every now and then. I'm sure the part where the torque arm would press (if the axle slips) is solid carbon.
 

Nealh

Esteemed Pedelecer
Aug 7, 2014
14,540
5,638
58
West Sx RH
In that case definitely not, a sure recipe for disaster.
A no no;
1. In terms of your safety.
2. You will ruin a perfectly good bike.
3. You really need to have the hubs axle sitting deeper as they are min 12mm and 9mm of a QR, so back to #2.
 

Manc44

Pedelecer
Jun 21, 2021
126
16
How are people like this getting away with it, I can't see a torque arm on this at all and as far as I can tell, that's a carbon swingarm, on a China frame, with 52V/350W motor. He says he's done 15,000Km on it up to now...


I'm asking if the rear triangle is made of carbon and if he's using a torque arm.

Elsewhere some guy worked out an 80KG rider traveling at 22 mph that wants to stop in 33 ft needs to apply 120Nm of braking force at the hub. Surely my 45Nm hub motor pales in comparison to the stresses put on the chainstay by braking?

Another comment I saw that got me thinking was "you can probably slow down quicker than accelerate". Exactly! How can a motor put any more stress on the chainstay or dropouts than braking hard would?

I always see people saying don't use carbon but I never see any actual failures or stories proving it's unsafe.

The frame isn't some thin cyclocross frame, it's a pretty beefy MTB.

Are there any reports of anyone putting a rear hub motor on carbon dropouts and it breaking them?
 
Last edited:

jarob10

Esteemed Pedelecer
Jan 22, 2017
261
73
48
Uk
isn’t the deal breaker here that CF dropouts can’t be filed out to suit the fatter hub axle?
 
  • Agree
Reactions: Nealh

Nealh

Esteemed Pedelecer
Aug 7, 2014
14,540
5,638
58
West Sx RH
Yep one mods CF at their own peril.
 

Manc44

Pedelecer
Jun 21, 2021
126
16
First one is made, took about 6 hours of sawing and filing lol

It's 5mm thick. An identical one needs making for the other dropout.

This wouldn't stop the wheel falling out and it's not anchored to anything, but it should stop the (45Nm) motor axle being able to twist, especially if there's two of them.

That means a maximum of 22.5Nm of force on the chainstay each side, right?

I am pretty sure the frame can take that, when the load is spread the way it is here, over the longest side of the metal at the left, that covers about 35mm. The underside of that longest side (and the top edge) is curved to fit the dropout underneath, so it's not square metal putting pressure on a round corner.

I need to dig out the reciprocating saw because that's the tough part, manually hacksawing through this stuff! The filing afterwards isn't so bad, albeit it takes ages doing that alone.

Now I have got this shape cut out and done, I can just clamp it to the next raw piece of steel and score around it with a hacksaw, so at least all the trial and error of the first piece can be bypassed.

I messed it up because the rear isn't as chunky as it could be, I was obsessed with making it look nice and fit to the dropout, which it does, but that rear bit looks a bit thin. I might start over and make a chunkier one (at the rear) based on this first one, that along with an electric saw... wouldn't take anywhere near as long making two of them as this probably did to make the initial one.

And I have no idea why there's a thin triangle missing at the back of it. I'm sure I did cutout the piece of plastic the right way, sigh. That's where it needs more metal ideally... isn't she a beauty!

44231


The dropout without it...

44234


It needs to look more like this IMO.
Red arrows show pressure points if the axle twisted:

44236


This stuff is crazy prices. You'd think a piece of 5mm steel would be about £3 for a 5cm x 15cm piece... nope, it's more like £8 or £9.

Maybe it's possible to get hold of it scrap, but then it has to be flat and it has to be stainless. In the end it's easier just buying it and hoping you don't mess it up when cutting/shaping it.
 
Last edited:

Nealh

Esteemed Pedelecer
Aug 7, 2014
14,540
5,638
58
West Sx RH
Good work there, a simple solution looks like it could work. One can make them a little over size as in the pencil drawing in your pic above and extend/elongate the 10mm axle slot . I think once in situ (as they are 5mm steel) they can act as the torque arm on both sides if you can securely fix them in place, maybe a pair of 3.5 or 4mm machine screws to bolt each one to the drop out.
 
  • Like
Reactions: Manc44