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

General Discussion about electric bicycles.
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

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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.
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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

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

Post by miro13car » Aug 05 2020 12:29pm

riding Townie bike at 75km/h with stock V-brakes is insane and suicidal I must say.
there is simply not enough braking power to stop this thing
at this speed you need motorcycle type of brakes.
Eplus, Bionx

BDamari   100 mW

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

Post by BDamari » Aug 05 2020 1:48pm

HK12K wrote:
Aug 01 2020 1:56pm
No such thing as too much power, just too little bike.
I'd use that as my signature if I were you.

BDamari   100 mW

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

Post by BDamari » Aug 05 2020 1:53pm

miro13car wrote:
Aug 05 2020 12:29pm
riding Townie bike at 75km/h with stock V-brakes is insane and suicidal I must say.
there is simply not enough braking power to stop this thing
at this speed you need motorcycle type of brakes.
I don't mean to go that fast at all! it will probably still happen somehow :?
Anyways I already ordered adapters to convert it to hydraulic discs, and some Maxxis hookworms to get a bigger contact patch.

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

Post by BDamari » Aug 05 2020 1:54pm

dogman dan wrote:
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.
Nice, I thought it was just a popular conversion donor because it looks good.

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

Post by Balmorhea » Aug 05 2020 2:14pm

miro13car wrote:
Aug 05 2020 12:29pm
riding Townie bike at 75km/h with stock V-brakes is insane and suicidal I must say.
there is simply not enough braking power to stop this thing
at this speed you need motorcycle type of brakes.
It would benefit from better pads, and maybe even booster arches to stiffen the brakes up a bit.

Here’s a racing motorcycle that exceeded 175km/h. Note the brake:
Image

They used that kind of linear pull brake for four seasons of international competition, so it seems to have worked.

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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 05 2020 2:30pm

Obviously it was not a braking competition. :mrgreen:
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

Balmorhea   10 kW

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

Post by Balmorhea » Aug 05 2020 3:14pm

MadRhino wrote:
Aug 05 2020 2:30pm
Obviously it was not a braking competition. :mrgreen:
GP racing requires braking as hard as possible, as little of the time as possible. Even more so with a bike that only makes 16 horsepower.

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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 05 2020 4:32pm

Racing courses in those years were very different from modern GP. This bike had re-used brakes from 1920 to save weight and gain some speed advantage over the competition. It did, but the rims were hardly lasting one race.
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

markz   100 GW

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

Post by markz » Aug 05 2020 6:24pm

Too much power is when it gets too jerky to ride, or your always spinning the tire and laying down rubber.

Its ok to go 80kph on a Townie with V-brakes, just do a pass first, on a straight boulevard with plenty of room to spare and check for potholes at 40 or 50 before going 80.
I got up to 60kph once on 72V Lipo, mxus 3kw, it wasnt my cup of tea. Plus I prefer to ride under the radar.

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

Post by j bjork » Aug 06 2020 1:57pm

Balmorhea wrote:
Aug 05 2020 2:14pm
miro13car wrote:
Aug 05 2020 12:29pm
riding Townie bike at 75km/h with stock V-brakes is insane and suicidal I must say.
there is simply not enough braking power to stop this thing
at this speed you need motorcycle type of brakes.
It would benefit from better pads, and maybe even booster arches to stiffen the brakes up a bit.

Here’s a racing motorcycle that exceeded 175km/h. Note the brake:
Image

They used that kind of linear pull brake for four seasons of international competition, so it seems to have worked.
Well, if it is going to be about how much or little brake you can get away with, look at speedway bikes.
No brakes at all, only motor braking when you release the throttle.
So I guess only regen on a 100km/h e-bike is fine?

Balmorhea   10 kW

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

Post by Balmorhea » Aug 06 2020 2:09pm

j bjork wrote:
Aug 06 2020 1:57pm
Well, if it is going to be about how much or little brake you can get away with, look at speedway bikes.
No brakes at all, only motor braking when you release the throttle.
So I guess only regen on a 100km/h e-bike is fine?
On a flat oval speedway track in a racing setting, sure. On a GP track or the street, not at all.

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

Post by Voltron » Aug 06 2020 2:26pm

Basic Townie dropouts so we're all on the same page...
Electra Townie 7 rear dropout.jpg
Electra Townie 7 rear dropout.jpg (266.73 KiB) Viewed 55 times
Electra Townie 7 rear cassette.jpg
Electra Townie 7 rear cassette.jpg (293.85 KiB) Viewed 55 times
Should be easy to mount torque arms, as the dropout isn't too sculpted out. Just fyi, usually you have to file out the dropouts to get a hub motor axle in far enough being a different profile than a regular axle, or it really leverages just the tips of the dropout. Filing it out can lead to it's own cracking problems if not smoothly rounded off with no sharp corners to focus the stress, but usually has to be done.
Edit- just saw that already pointed out 👍
Last edited by Voltron on Aug 06 2020 4:08pm, edited 1 time in total.

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

Post by Voltron » Aug 06 2020 2:51pm

Just for some perspective on basic 30/low 40s (mph) feels on what normally seems like not too bumpy city streets...the vid doesn't really capture how blinding squinting thru ones suddenly watering eyes can be.
One thing about the Townie, it has the foot forward setup, which makes it really hard to lift ones weight off the saddle for sudden potholes. I'd def consider a suspension seatpost... My bike has a very seat-over-the-pedals, jockey like position, with a long travel suspension post, and I've still taken some brutal ass to lower spine shots. The worst is new black asphalt over a pothole.. super hard to see coming

https://youtu.be/gux0etlFN18
(Peak speeds start at about the 1:45 min mark)

I actually feel relaxed after this type of ride, but you really have to be in a state of hyper alertness the whole time.

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