How much power is too much power? (hub motor)

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BDamari   100 mW

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How much power is too much power? (hub motor)

Post by BDamari » Aug 01 2020 2:36am

On one hand, I see converted mid-range hardtail mtbs running 5.5kw peaks and handling it... adequately.
On the other hand, I see people on hardtails (with double torque arms) saying their rear drop outs/chainstays broke after running peaks of over 60A. Not the occasional axle chewing them out with torque, but from the mere unsuspended weight of the direct drive hub motor.
There's another possibility to consider though: with the crappy driving norms in my country, it's very common to see people take the cheapest ebikes with dirt cheap forks and ride off sidewalks at 20mph/35kmh as if they're riding a dirtbike, so driving style might be the reason for this.

I want to run 60v with 50a peaks on my Townie 21d, I've already seen on youtube people running 72v systems (45A controller) on the very same bike and going 45mph/75kmh with stock tires and v brakes, so you can say much worse stuff has been done in the past.
Mine has a Suntour NEX fork which might be simple but it softens the ride really well. Of course, I'm gonna install double torque arms and wider tires, and no jumps, stunts, wheelies or anything!
The question is, will the frame handle it?

Thanks

Balmorhea   10 kW

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Re: How much power is too much power? (hub motor)

Post by Balmorhea » Aug 01 2020 3:15am

Breakage at the dropouts, even if not from a spun axle, is still far more likely to be from torsional forces than from inertial loads due to a heavy wheel. Consider that a bike is designed to tolerate many hundreds of pounds applied directly to the dropouts by rider weight and by chain tension. But it is not designed to propel that load by twisting the wheel against the axle. You have to anchor the reaction torque firmly, and apply it far enough from the axle that it doesn't overstress the dropouts or their adjoining tubes.

If you use well fitted, closed eye torque arms and attach their free ends solidly at least a couple of inches from the axle center, I doubt the Townie frame will give you any trouble.

Gross current is only one factor in producing torque. Motor torque per amp is just as important, along with internal reduction gears if you have them.

BDamari   100 mW

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Re: How much power is too much power? (hub motor)

Post by BDamari » Aug 01 2020 3:43am

Balmorhea wrote:
Aug 01 2020 3:15am
Breakage at the dropouts, even if not from a spun axle, is still far more likely to be from torsional forces than from inertial loads due to a heavy wheel. Consider that a bike is designed to tolerate many hundreds of pounds applied directly to the dropouts by rider weight and by chain tension. But it is not designed to propel that load by twisting the wheel against the axle. You have to anchor the reaction torque firmly, and apply it far enough from the axle that it doesn't overstress the dropouts or their adjoining tubes.

If you use well fitted, closed eye torque arms and attach their free ends solidly at least a couple of inches from the axle center, I doubt the Townie frame will give you any trouble.

Gross current is only one factor in producing torque. Motor torque per amp is just as important, along with internal reduction gears if you have them.
I am gonna fit a torque arm on each side and bolt them as far as I can from the axle, but it's mostly torsional stress I'm worried about.
The direct drive hub motor by itself weighs 6.5kg, with the wheel and tire I presume shy of 10kg...

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MadRhino   100 GW

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Re: How much power is too much power? (hub motor)

Post by MadRhino » Aug 01 2020 4:21am

The faster and the harder you ride, the better the bike need to be. But, I guess some believe in their luck, or just have poor mechanical logic.

75 kmh can be slow or fast, according to the bike’s reliability, handling, riding conditions and obstacles around. But some don’t have that consciousness. Distorted perception does fall under Darwin’s law. One can be lucky sometimes, but no one can be lucky all the time.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

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monster   100 kW

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Re: How much power is too much power? (hub motor)

Post by monster » Aug 01 2020 7:33am

When there is so much power that even the slightest touch of the throttle sends you on your back.

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MadRhino   100 GW

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Re: How much power is too much power? (hub motor)

Post by MadRhino » Aug 01 2020 7:51am

monster wrote:
Aug 01 2020 7:33am
When there is so much power that even the slightest touch of the throttle sends you on your back.
Being sent on your back in acceleration, is a matter of geometry, not power. Proof is, if needed, that a bike can be made to never lift no matter the power, and another one that will only need the rider leaning back to lift.

Proper bike geometry does let the rider choose to lift or drift at will, by shifting his position in the virtual cockpit.

That, is the first thing to know before getting into building powerful bikes.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

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dogman dan   100 GW

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Re: How much power is too much power? (hub motor)

Post by dogman dan » Aug 01 2020 1:10pm

Its the hucking that really breaks shit, IMO. Put a honking 25 pound copper weight on your rear wheel of a hardtail, then fly off curbs, up curbs, or just get air from the humps in the road.

That tends to do er, i think. Your townie 21 d however, has a much more robust rear dropout plate than many bikes built to be a millionth of an ounce lighter on each part. Bolt a stock front torque arm to them, by drilling a hole where its needed in that thick plate. Both sides. Then you will be good to go for 3000w of power with zero problems. File the dropout deeper first though, so your hub motor axle center is same as your original wheels center. 1 mm deeper if your motor is 12mm, 2 mm deeper for the 14 mm axle common with dd motors.

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dogman dan   100 GW

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Re: How much power is too much power? (hub motor)

Post by dogman dan » Aug 01 2020 1:12pm

Its the hucking that really breaks shit, IMO. Put a honking 25 pound copper weight on your rear wheel of a hardtail, then fly off curbs, up curbs, or just get air from the humps in the road.

That tends to do er, i think. Your townie 21 d however, has a much more robust rear dropout plate than many bikes built to be a millionth of an ounce lighter on each part. Bolt a stock front torque arm to them, by drilling a hole where its needed in that thick plate. Both sides. Then you will be good to go for 3000w of power with zero problems. File the dropout deeper first though, so your hub motor axle center is same as your original wheels center. 1 mm deeper if your motor is 12mm, 2 mm deeper for the 14 mm axle common with dd motors.
Last edited by dogman dan on Aug 01 2020 1:14pm, edited 1 time in total.
Reason: double post

markz   100 GW

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Re: How much power is too much power? (hub motor)

Post by markz » Aug 01 2020 1:13pm

How much power is too much power? (hub motor)
When you lose a shoe!
https://youtu.be/NCpzXIPXdAQ?t=8

HK12K   10 kW

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Re: How much power is too much power? (hub motor)

Post by HK12K » Aug 01 2020 1:56pm

No such thing as too much power, just too little bike.

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DogDipstick   10 kW

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Re: How much power is too much power? (hub motor)

Post by DogDipstick » Aug 01 2020 2:51pm

I got 8000w out of a 1000w hub motor. Peaks,.. and about 3.5Kw contin, sealed, filled with Auto transmission fluid a little, and hub sinks, 65Wh/mile all day. 100C thermal limit.
83.1v of Ironhorse XC.. :) :bolt: by Chevy :bolt: :D Broke 10 horsies :twisted: (..about 80% healed..).. :? Anybody.. what equals √3 times the line to neutral voltage? Asking for a friend.. :| (gottenymoem4115thangs?Yall?) :confused: Fabricator @ BSECo. Inc.

john61ct   100 GW

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Re: How much power is too much power? (hub motor)

Post by john61ct » Aug 01 2020 4:01pm

The weight you can stand carting around is the limiting factor

power itself can always be designed around

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