Is this a silly theory? Torque arms and rear suspension...

[Manc44]

People with rear hub motors put a torque arm (or two) on their dropouts to help relieve the stress caused by the hub motor, but when you have got rear suspension and the motor applies power, is this not also making your rear triangle move upwards slightly? Think about it, the wheel rotates clockwise (from the drive side), now imagine you had super powers and could grab that wheel and stop it from being powered - won't the rear triangle of the frame put a bit of pressure on the rear shock?

What I am getting at is, on a rear suspension frame with a rear hub motor, isn't the suspension itself acting as a torque arm? The motor does rotate the wheel under load in such a way that I think it might be doing.

Also, if you have a rear motor that puts out say 45Nm, like the 500W Bafang does, but you tightened each wheel nut to 40Nm each, you then have 80Nm of force holding your rear wheel on - so what's the point of a torque arm in that situation when your motor is 35Nm under the force required to loosen the nuts at the dropouts?

All very interesting isn't it.

 

[Woosh]

Of course, crank drive bikes don't need torque arms.
there is another reason for installing torque arms on the motor wheel: to prevent the motor wheel coming off the bike.
The heavy weight of the motor acts a bit like the head of a wood handle hammer. Inertia can make it fly off the handle.
This is the torque arm on the Woosh Gran-Camino:

 

[Manc44]

Whoosh your website is probably the best I have seen for detailing all the stuff I want to know like torque and battery range

 

[vfr400]

The motor transmits torque to the rear subframe mostly by the friction of the axle nuts and partly by the metal to metal contact in the slot in the drop-outs. It's irrelevant what the suspension in the subframe does.

In theory, the friction in the axle nuts would be enough, but the torque is cyclic and in both directions, which could loosen the nuts, leaving insufficient means of resisting torque as many have found out, who decided not to use at least one torque arm.

A500w Bafang motor makes a massive torque at the axle. Only a fool would not fit at least one torque arm, assuming that you're going to run it at the power it's capable of.

Torque arms are insurance against loose nuts. As long as you tighten the nuts properly and have properly installed anti-rotation washers, you don't need them on a rear motor when running up to say 36v and 20 amps or 48v and 15 amps.

Front motors are different. A lot depends on whether you have steel or aluminium/magnesium forks. I'd always use a torque arm if the forks are not steel. Front motors are best avoided anyway.

 

[Manc44]

Cheers vfr400. Yes I keep looking at either a rear hub or a mid-drive, I have ruled out a front hub completely. The only thing putting me off the mid-drive is all the talk about chains derailing etc.

A guy with a tandem and a 73mm bottom bracket shell said his chainline (on his BBS02) came out to 63.5mm! That can't be right surely. It would mean a 61mm chainline on a 68mm bottom bracket shell. Now I can understand it being a few mm out but not 11mm out. It would be pretty shocking to me if Bafang were overlooking something that obvious and creating a chainline so far out, but I have seen the offset on those Lekkie chainrings and it does look like a lot. Really off putting sigh.

 

[Woosh]

standard chainline is 47.5mm.
the chainline on Bafang BBS kit is 50mm for 68mm bottom bracket.
It's OK because you'll use more often the smaller cogs so the chain will be on average working at a good angle.
If you have 73mm bottom bracket, then it will be 50mm + 2.5mm = 52.5mm. It's a bit worse but still OK.

 

[Manc44]

standard chainline is 47.5mm.
the chainline on Bafang BBS kit is 50mm for 68mm bottom bracket.
It's OK because you'll use more often the smaller cogs so the chain will be on average working at a good angle.
If you have 73mm bottom bracket, then it will be 50mm + 2.5mm = 52.5mm. It's a bit worse but still OK.

Cheers Whoosh. My current chainline is set at 50mm because I spaced it that way fitting my crankset last time I was doing it. With a triple + 11s cassette it runs perfectly.