Torque plate solution?

[cemelski]

Hey all,

I recently installed and ebike kit on a new (decent quality) aluminium bike, partially by a bike shop, partially by myself (crystalyte G40 rear hub motor with 15amp/36V).

I insisted on torque arms, cause i don't want to crash, not now and not over x time when metal fatigue occurs (i'm planning to use this bike for many ten thousands of km's on high support levels). The guy at the bike shop installed these "torque arms" as in this picture: https://www.dropbox.com/s/rfxwsbhfvx64kzn/torque arm.jpg?dl=0 I'm not quite happy with this solution cause in my opinion, it does NOT take the torque away from the dropouts since the screw is attached to the dropout itself... My first question is: are my worries justified or is this completely safe?

An other solution is to make a custom torque plate as displayed in this picture https://www.dropbox.com/s/mgu6euw7hgtf9xk/torque plate solution.jpg?dl=0 The red border displays the plate position and the plate will be screwed into the 2 holes above the kickstand. What points should i consider here? (is this a safer solution? is there a minimum or recommended thickness of this torque plate? How tight should the plate be screwed into the 2 holes (8Nm?) How crucial are the correct dimensions for the hole of the axle?...

Thanks for the advice!

 

[Voltron]

it looks ok, esp for the power levels that kit will produce. The whole dropout casting isn't going to shear off from the force or anything.

 

[dogman dan]

It's not ok,, it's perfect. I would not change a thing.

I can understand making the torque plate just for the fun of it though.

 

[alfantastic]

Yeah, as the others have said, that's a near perfect solution. When I had my MAC 10T motor running at 48v 30A, I had the same torque arm as in your picture, but I had an extra pivoted extension, just so I could attach it to the same place you have. This wasn't ideal, as the pivoted point added a potential weakness.

Yours ain't going anywhere, so stop worrying and enjoy the ride.

 

[MrDude_1]

That small torque arm is just fine. Nice and clean looking too.

If you think about the axle of your motor, its round with a flatspot on each side, making a double D. It is slipped into the U of the dropout.
Your wheel acts like a big lever turning one way and the axle is trying to turn the other way.. this puts tons of stress on the corners of the flatspots of the axle, trying to twist it. That twist can spread the D.
By simply adding that small washer-arm, you are closing the U, so it is much stronger and adding a lever so less force is needed to stop rotation. You're also increasing the thickness of the metal holding it, so there is more contact area.
For your power levels, theres no need for anything larger, you will never break this. Just keep the wheel nuts tight.

Assuming thick enough metal, a torque plate like you pictured would work for a 70,000watt motorcycle. You're at 540 peak watts right now.

 

[MrDude_1]

btw, dont feel bad about wanting to overkill it.
You should see these monster wires I use to wire all my packs with. I know I can just use 10ga wire, but I still have 6ga battery packs just in case I ever want to constantly pull 100s of amps for whatever reason. :lol:

 

[cemelski]

Thanx for the responses.

So my assumption that these 'torque arms' transfer the torque to the weakest point in stead of the to a point on the frame is not correct?

Btw: if i drive at top speed (20mph) does the motor apply a lot of torque then, or is this minimal?

Btw2: if i ride a regular bike (without motor), is there any torque then on the rear axle? If i accelerate as hard with pedal power instead of motor power, is the amount of torque on the dropouts the same or is this due to the u shape of the motor axle? I don't really get this...

 

[Voltron]

There is no twisting of the axle from regular pedaling... its just sitting there giving the wheel something to spin on. But with hub motor, the axle is whats holding the center of the motor still while the outside spins around it, concentrating a lot of twisting force onto the mouth of the dropout. The dropout only has to spread a tiny bit and the axle will try to rotate.. the whole dropout piece is actually really strong... It just wasn't meant to resist concentrated twisting in that one tiny spot.
With even the hardest pedaling, the wheel is mostly just trying to shift a little to one side... But if it helps to visualize it, imagine putting a rod shaped like that axle into a drill held by a buddy, and then holding it tight enough with your hand that it makes the drill yank out of his hand instead of spinning. That's what the torque arm is helping with.

And your maximum force on there occurs during acceleration from a stop... esp if it's bumpy. Steady speed cruising doesn't put a lot of load there.

 

[dogman dan]

Don't blame you for worrying, don't want to ruin a great bike.

But your existing TA is plenty good.

Simple explanation is that when the motor tries to turn forward, the axle tries just as hard to turn backwards. So you need the frame, the pressure of the nut, and ideally a torque arm helping the first two to resist the desire of the axle to twist backwards.

The forces involved can be big, if the hill is steep, or the weight of the bike is heavy. But your torque arm is plenty good to resist a lot more than your motor can put out. I have the exact same thing on one of my bikes, with a much bigger, much more powerful motor. It's been fine, for quite a few years now.

Pedaling while you start up is a good thing. Pedaling till you are going 10 mph even better. But not to save the frame, that is taken care of. The pedaling will lessen the strain on your battery, and extend range.

personally,, I tend to favor gas it till I'm going 10 mph, then start pedaling easy. My battery is strong, and I'm weak. so I do it that way. Before I got sick,, I pedaled first.