Are Torque Arms for Rear-Hub Motors necessary?

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Nickelodeon   1 mW

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Are Torque Arms for Rear-Hub Motors necessary?

Post by Nickelodeon » Oct 13 2018 3:47pm

Hi,

I'm looking to put a 1000 watt hub motor on a hardtail mtb aluminium frame.

Since this will be a rear-hub motor is a torque arm really necessary? Logic would suggest that any pressure from the hub spinning would go inwards on the frame and not backwards where it "could" brake off the dropout?

I completely understand the logic of fitting it for a front-hub motor, but for a rear hub???

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wturber   100 kW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by wturber » Oct 13 2018 3:54pm

With 1000 watts and an aluminum rear dropout, I think most people would recommend using at least one steel torque arm. I use two on my aluminum 1000 watt bike.

The motor axle is only about 14mm in diameter. Given that small radius, the amount of torque applied to the dropouts can be quite high. And compared to the hard steel that the axle is made from, the aluminum used for the dropout is soft. So steel torque arms are generally considered a good idea. The little bit of extra cost to buy them or time to build them is well worth the peace of mind and protection from potential damage that they provide.
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Iron Horse 3.0 hardtail - 48V / 1000W / 470rpm generic Chinese DD Hub motor (ebay)
8 x 36v 4.3ah 10s 2P battery packs - 1500W 30A DC Boost Converter delivers 54v and about 1000 watts peak
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flat tire   10 kW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by flat tire » Oct 13 2018 3:58pm

Yes. Plenty of people have opened their rear dropouts with hub motors. It may not happen right away on 1000W but you will stress the dropouts every time you apply power. There is not that much metal there and although you have worked out that the axle won't have optimum leverage on the weakest part, you are tempting fate to not run torque arms.

It really sucks having your motor spin out of the dropouts.

For power levels that are actually fun, forget torque arms: a custom rear arm with huge reinforced slots is necessary.

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spinningmagnets   100 GW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by spinningmagnets » Oct 13 2018 4:20pm

If it was a 250W rear hubmotor and a stout steel frame, I'd say it's OK. Steel would usually start to bend, and possibly give you some warning. Aluminum is more likely to develop a crack, and then suddenly snap off.

You may nor fear a crash at low speed. However, besides the crash there is the repair to the bike and hubmotor to consider. There are pictures of the aftermath available. Spinning the axle will tear out the wires. Three fat phase wires, and five tiny hall sensor wires.

Repairing the wires is not technically difficult, but it is a time-consuming pain in the @$$, which requires disassembly of the motor. Your call, proceed as you wish.

999zip999   100 GW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by 999zip999 » Oct 13 2018 5:04pm

Yes.

ScooterMan101   1 MW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by ScooterMan101 » Oct 14 2018 6:34am

With a heavy 1,000 watt direct drive rear hub motor,
you are better off by just buying a frame that is designed for such a heavy hub.

Vector Frames
EEB Frame
Futur Frame
ION 2.0 Frame

You can find examples of these frames at the ... Items for Sale - New , section here on Endless-Sphere.
My first conversion ...

https://endless-sphere.com/forums/viewt ... 1#p1077497

It's 2018 already, lets get some real , improved e-bike / e-velomobile / e-motorcycle designs .

RLD70   100 W

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by RLD70 » Oct 14 2018 6:38am

I wouldnt use any DD hub without atleast 1 TA attached. Ive had the unfortunate experience of having the hub spinout of the dropouts and ruined the hub.

Nickelodeon   1 mW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by Nickelodeon » Oct 14 2018 7:39am

flat tire wrote:
Oct 13 2018 3:58pm
Yes. Plenty of people have opened their rear dropouts with hub motors. It may not happen right away on 1000W but you will stress the dropouts every time you apply power. There is not that much metal there and although you have worked out that the axle won't have optimum leverage on the weakest part, you are tempting fate to not run torque arms.

It really sucks having your motor spin out of the dropouts.

For power levels that are actually fun, forget torque arms: a custom rear arm with huge reinforced slots is necessary.
Wouldn't a 1000 Watt BBSHD apply the same kind of pressure on the dropouts? Just coming from the chain instead?

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by flat tire » Oct 14 2018 4:47pm

No. The hub motor applies a twisting force that tries to spread the dropouts. Just because the part on the back is at the top of the dropout, doesn't mean there isn't a twisting force stressing the metal. You know about lb-ft? Your motor probably makes in the ballpark of 20. Leverage is proportional to distance; the edge of the axle is probably 1/4" away from the center. So you are multiplying that 20lbft by 12*4. That's 960 lbs hammering on your wimpy little dropout every time you go full throttle. That's a significant portion of the loadbearing capacity of the dropout. More than it can take without eventual consequences. So the problem is fatigue and then breakage at some point later, but probably not right away.

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wturber   100 kW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by wturber » Oct 14 2018 6:12pm

Nickelodeon wrote:
Oct 14 2018 7:39am

Wouldn't a 1000 Watt BBSHD apply the same kind of pressure on the dropouts? Just coming from the chain instead?
No. The force is applied from the chain and to the rear cluster. The cluster transfers the force to the hub, spokes and rim. That assembly spins on bearings that ride on the axle. You can't transfer much torque through those bearings considering that the bearings are specifically designed to allow the hub to spin freely. The whole idea is to convert the torque from the chainring to torqe at the rear wheel. It would be counter-productive to be applying that torque to the dropout.
"Commuter - DC Booster"
Iron Horse 3.0 hardtail - 48V / 1000W / 470rpm generic Chinese DD Hub motor (ebay)
8 x 36v 4.3ah 10s 2P battery packs - 1500W 30A DC Boost Converter delivers 54v and about 1000 watts peak
53T/42T Sakae Road cranks - 30mph+ on flats
viewtopic.php?f=2&t=90369

markz   100 GW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by markz » Oct 14 2018 6:19pm

^^^ Yeaaaa wut they say'ed 8)

People have used wrenches and zip ties.
Definitely dont want to be ecruising and your wires get twisted Not Gud

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spinningmagnets   100 GW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by spinningmagnets » Oct 14 2018 7:10pm

I'm not sure I could explain the engineering reason a mid drive doesn't affect the drop-outs the same way. However, I have been hitting 1500W on my mid drive and the aluminum drop-outs don't have a torque-arm, and show no signs of cracks developing.

Steel drop-outs are stronger than aluminum, and I have seen 1,000W hubmotors ruin steel dropouts when they didn't have a torque-arm.

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Chalo   100 GW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by Chalo » Oct 14 2018 7:35pm

spinningmagnets wrote:
Oct 14 2018 7:10pm
I'm not sure I could explain the engineering reason a mid drive doesn't affect the drop-outs the same way.
Motor reaction torque is always transferred to the motor mounts. On a mid drive, that's the bottom bracket or some other part of the bike frame. On a hub motor, it's the axle ends.
Last edited by Chalo on Oct 14 2018 10:32pm, edited 1 time in total.
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EbikeAus   100 W

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by EbikeAus » Oct 14 2018 8:16pm

For the possibility of "what if" at 50 kmh I installed 2 torque arms with the same setup you have
Choose LiFe 8)

May all your batteries be fully charged and perfectly balanced :P

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Chalo   100 GW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by Chalo » Oct 14 2018 10:39pm

EbikeAus wrote:
Oct 14 2018 8:16pm
For the possibility of "what if" at 50 kmh I installed 2 torque arms with the same setup you have
Axles that are hollowed out for cables on one end can't transmit nearly as much torque through that end as they can through the solid end. So a good thick, tightly fitted, well anchored torque arm on the solid end of the axle is not significantly less secure than having one on each end, though having two is more fault tolerant.
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markz   100 GW

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Re: Are Torque Arms for Rear-Hub Motors necessary?

Post by markz » Oct 14 2018 11:05pm

^^^ Yeaaaa wut they Chalo say'ed 8)

I lost a steel plate but I had plenty more steel plate to go 'round.
Handsome design!
New Torque Arms.jpg
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