Torque arm or not torque arm?

[anotherkiwi]

Aluminium frame with 7 mm thick drop-outs, rear 250 W hub - 705 W peak with battery hot off the charger.

I am tempted to go without a torque arm because the material is thick all around the drop-out.

Opinions?

 

[Fordulike]

Aluminium frame with 7 mm thick drop-outs, rear 250 W hub - 705 W peak with battery hot off the charger.

I am tempted to go without a torque arm because the material is thick all around the drop-out.

Opinions?

Why take the risk?

Torque arm every time on hub motored bikes!

 

[Yamdude]

I've not bothered...... haven't bothered with brake cut out switches either.

 

[anotherkiwi]

Why take the risk?

Torque arm every time on hub motored bikes!

- Because the frame was free,
- because I need to mount a trailer hitch and I don't think the axle is long enough with a torque arm as well,
- because with this much material and a carefully enlarged drop-out I think the risk is minimal (road use only).

There, three good reasons

 

[danielrlee]

I'd never go without one on aluminium dropouts. The failure mode is too unpredictable and when it goes, it's catastrophic.

Steel dropouts I wouldn't bother sub 1kW peaks and even then I might not until nearer 2kW depending on how meaty the dropouts are.

 

[Fordulike]

- Because the frame was free,
- because I need to mount a trailer hitch and I don't think the axle is long enough with a torque arm as well,
- because with this much material and a carefully enlarged drop-out I think the risk is minimal (road use only).

There, three good reasons

I think you just answered your own question lol

 

[Nealh]

You may get a way with it but can you be sure that there is no casting fault or unseen hairline crack in the drop out area. How about trying to make a modified TA and hitch adapter and kill 2 birds with one stone.

 

[anotherkiwi]

The hitch won't fit with or without the torque arm on the other side...

I would have to mill off about 1-2 mm to fit a torque arm which seems kind of counter productive.

Aren't the drop outs milled from flat sheet then welded in?

 

[Hermann R.]

You will need something to handle the torque (at the very least torque washers, but those also will add a few millimeters).
Otherwise, do the test... stick a carriage-bolt of suitable diameter into the dropout (the square part of the bolt), fix with a nut and washer, put a nut and counternut on the other end of it, then get a torque wrench and set it to between 40 and 50 Nm, try to spin the carriage bolt and see what the dropouts do.
If it had a tight fit when inserted and you can actually spin it with that torque, then your dropouts will have widened, and the alu just may have started to break...
As fordulike said: Torque arm on alu dropouts, with motor power above 250W? Always! And better make sure it has a tight fit, so that it doesn't wiggle every time you accelerate.
Maybe the question is not whether the frame was for free, but what it will cost to replace it because of broken dropouts, and/or what a possible accident might cost.
Also, would you ever try to tighten an M12 or M14 nut a thousand times to several tens of Nm with a 7mm thick Aluminum wrench... and expect that wrench to last? That would be the equivalent of what those dropouts would have to handle.

-H-

 

[anotherkiwi]

Problem solved, details in my signature