Is 250w enough power for modest speed up steep hills

Electrically assisted bikes, trikes, etc. that are legal in the UK
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imk123
Posts: 13
Joined: 28 May 2022, 10:48am

Is 250w enough power for modest speed up steep hills

Post by imk123 »

Hello,
I have just acquired a luverly 7 speed Dahon 20" folder, it is about 15 years old and only been ridden a handful of times.
It is in unmarked condition with original Dahon branded tires that still have the molding pips on them.
I intend to use this bike for shopping trips and the odd canal tow path ride.
Problem is where i live (UK North Staffordshire) it is Very Hilly, some hills are so steep that a fit 70s old like myself pauses when walking up.

So my question is, is a 250w mid motor going to have enough power to pull me up hills with modest peddle input?
I have done a bit of research and mid motor seems to be the better decision over rear hub motor.
Also the rear dropouts on this Dahon is 130mm and rear hub motors seem to be minimum 135mm.

I was thinking of getting a Bafang kit, is there a better option?

Many thanks in advance imk
Donbro
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Joined: 19 May 2022, 10:32pm

Re: Is 250w enough power for modest speed up steep hills

Post by Donbro »

I am around same age as you, fitted a mid drive Tongshang 36v 250w to my Raleigh pioneer tour, much better for the hills around here in Hampshire than the front hub I had originally, still not sure about the build quality of the TSDZ2 but early days yet but getting great economy with up to 50 miles on one charge of my 11ah battery.
Nearholmer
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Joined: 26 Mar 2022, 7:13am

Re: Is 250w enough power for modest speed up steep hills

Post by Nearholmer »

Yes.

250W is plenty to pull a bike, a person, and their luggage/shopping up a fairly steep hill at a respectable pace.

What might be an issue with an e-bike that is being asked to deliver 250W over relatively long durations though is battery capacity.

What is the nominal capacity of the battery? And, if the battery is 15yo, even if it’s been looked after in that 15y, don’t expect to get anything like the nominal capacity from it; the chemistry is likely to have degraded noticeably.
imk123
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Joined: 28 May 2022, 10:48am

Re: Is 250w enough power for modest speed up steep hills

Post by imk123 »

Nearholmer sorry but my original post was not clear.

Bike is approx 15 years old and i intend to convert it to ebike so motor/drive/battery is going to be new.
Question is, is 250w enough to pull bike and 70kg me up a steep hill with only modest peddle input as i have arthritis.

Sorry and thanks for the reply imk.
axel_knutt
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Joined: 11 Jan 2007, 12:20pm

Re: Is 250w enough power for modest speed up steep hills

Post by axel_knutt »

Is this any help:
grad temp.png
“I'm not upset that you lied to me, I'm upset that from now on I can't believe you.”
― Friedrich Nietzsche
Lodge
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Joined: 28 Feb 2016, 8:59pm
Location: Staffordshire Moorlands

Re: Is 250w enough power for modest speed up steep hills

Post by Lodge »

I would say that 250 W is plenty.

I'm slightly younger at 65 but also live in the Staffordshire Moorlands and yes, it is hilly round here. I went out for a ride today over to Ilam, up to Thorpe then across to Crowdecote and Earl Sterndale and back towards Leek via Flash. Stunning scenery. All on a 20 inch wheel Tern Vektron folder with Bosch Active Line 250 W motor. In total 50 miles and 5125 feet of ascent in just under 4 hours rolling time, using all of one 500 Wh battery and a quarter of the second.

It's all down to gearing. Make sure that you have a first gear low enough to climb with e.g. 70 rpm cadence at minimum balancing speed, which for me is about 4 mph. This gets me up 20%+ gradients though I will often set the power level to "turbo" to stop my heart rate going through the roof (then back off to "sport" or "tour" for the rest of the time).

Add-on motors like Bafang or Tongshang should deliver very similar performance - they are all 250 W after all. Any differences will likely be in where they deliver peak torque - lower versus higher cadence, for example. But in any case make sure you have plenty of battery capacity as climbing does use a lot of Watt-hours. I've normally a spare battery in a pannier if going any distance!
Bonzo Banana
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Joined: 5 Feb 2017, 11:58am

Re: Is 250w enough power for modest speed up steep hills

Post by Bonzo Banana »

It's not so much about the wattage as different ebikes push the boundary of what is acceptable for a 250W ebike with regards power output. It's more about the Nm output. Bosch mid-drive motors have been shown to peak close to 800W to produce their maximum torque but they can only hold that for a while but that is typically all they need for off-road use. Folding bikes are typically low geared anyway because of their small wheels and so are pretty good hill climbers.

Typically fitting mid-drive motors is more involved but if you find someone has done the same bike as yours with a motor you are considering and it was plain sailing fair enough. You also have to think about the state of the frame, if you are going to be delivering lets say 4x the power at the crank that is clearly a lot of extra stress on the bottom bracket area. Might be worth checking to see where your model of Dahon typically cracks on the frame.

I had a quick look, if you follow this link and then choose images seems like failures are almost exclusively at the hinge. In fact the bottom bracket area actually looks quite overbuilt on many folding bikes so maybe mid-drive is the ideal motor for such a bike. Fatigue is caused by flexing and it doesn't look like the bottom bracket area can flex much at all. So could be a very good choice. I'm more of a hub motor fan myself because of the all-round cost savings mainly but mid-drive does seem a good option here.

https://www.google.com/search?q=dahon+frame+crack
richtea99
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Re: Is 250w enough power for modest speed up steep hills

Post by richtea99 »

Lodge wrote: 28 May 2022, 6:42pm It's all down to gearing. Make sure that you have a first gear low enough to climb with e.g. 70 rpm cadence at minimum balancing speed,
You have to be careful not to generalise:
- if the sensor is on the crank/pedals (i.e. a mid-drive ebike) you're quite right, it's all down to gearing
- if the sensor is on the hub/wheel (i.e. some hub-drive ebikes) then it's down to the speed of the bike, not gearing

An example of the second type are ebikemotion-based bikes (Orbea, Ribble, etc), where the power drops off at low speeds (6mph or lower, say) because the sensor isn't getting enough swipes of the magnets. It doesn't drop off completely, but its definitely beneficial to keep the speed up a little, which means hills steeper than around 1-in-8 can be more challenging on those ebikes than a mid-drive one.

This is one reason why mid-drive ebikes can be preferable for very hilly areas - because the power is less likely to drop off at low speeds.
However, I think the OP is thinking of mid-drive systems, so not a problem.
imk123
Posts: 13
Joined: 28 May 2022, 10:48am

Re: Is 250w enough power for modest speed up steep hills

Post by imk123 »

I wonder if it is time to get the Good Law Project involved https://goodlawproject.org/
As looking into maximum power limit of 250w issue, it seems to me that this legislation was written for bikers in Holland by the flat earth society With no consideration for bikers outside the UK's golden triangle (London, Oxford, Cambridge)
I live in a VERY hilly area with 10% and 15% gradients around every corner and I don't think 250w is enough for up hills minimum safe riding speed in traffic. Considering that mobility scooters are speed restricted but not power restricted.
PH
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Re: Is 250w enough power for modest speed up steep hills

Post by PH »

imk123 wrote: 29 May 2022, 4:42pm As looking into maximum power limit of 250w issue, it seems to me that this legislation was written for bikers in Holland by the flat earth society With no consideration for bikers outside the UK's golden triangle (London, Oxford, Cambridge)
Here's the views of one of those who wrote it:
viewtopic.php?p=1576831#p1576831

I have a 250w Bosch motored Trek (The 85Nm Preformance CX motor), it has no problem getting my 95kg, plus the 20kg+ bike, plus up to 15kg of cargo up any hill, considerably faster than I've ever been able to pedal and with minimum effort should I so choose. That might not be much help to the OP, as others have already said, not all motors or bikes are the same.
imk123
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Joined: 28 May 2022, 10:48am

Re: Is 250w enough power for modest speed up steep hills

Post by imk123 »

PH wrote: 29 May 2022, 6:27pm
imk123 wrote: 29 May 2022, 4:42pm As looking into maximum power limit of 250w issue, it seems to me that this legislation was written for bikers in Holland by the flat earth society With no consideration for bikers outside the UK's golden triangle (London, Oxford, Cambridge)
Here's the views of on of those who wrote it:
viewtopic.php?p=1576831#p1576831

I have a 250w Bosch motored Trek (The 85Nm Preformance CX motor), it has no problem getting my 95kg, plus the 20kg+ bike, plus up to 15kg of cargo up any hill, considerably faster than I've ever been able to pedal and with minimum effort should I so choose. That might not be much help to the OP, as others have already said, not all motors or bikes are the same.
It would be interesting to see a power meter (Volts x Amps) on your Bosch motor as it seems to contradict the graph above.
Problem I am having is I have not ridden a 250w ebike up the 10/15% gradients that approach my home.
imk123
Posts: 13
Joined: 28 May 2022, 10:48am

Re: Is 250w enough power for modest speed up steep hills

Post by imk123 »

PH I just had an interesting thought re your Bosch 250w motor, is the 250w a continuous or peak rating?
As maybe the peak power is much more than 250w but VxA averaged out over a hour to get around the 250w limit.
And people like Bafang are being honest and saying that there 500w rigs are a continuous rating.

Just a thought, comments welcome.
BTW friend in Switzerland is big into ebike's and looking at there legislation ebike are skill/age limited not power, I guess they have hills :-)
Carlton green
Posts: 3645
Joined: 22 Jun 2019, 12:27pm

Re: Is 250w enough power for modest speed up steep hills

Post by Carlton green »

richtea99 wrote: 29 May 2022, 4:28pm
Lodge wrote: 28 May 2022, 6:42pm It's all down to gearing. Make sure that you have a first gear low enough to climb with e.g. 70 rpm cadence at minimum balancing speed,
You have to be careful not to generalise:
- if the sensor is on the crank/pedals (i.e. a mid-drive ebike) you're quite right, it's all down to gearing
- if the sensor is on the hub/wheel (i.e. some hub-drive ebikes) then it's down to the speed of the bike, not gearing

An example of the second type are ebikemotion-based bikes (Orbea, Ribble, etc), where the power drops off at low speeds (6mph or lower, say) because the sensor isn't getting enough swipes of the magnets. It doesn't drop off completely, but its definitely beneficial to keep the speed up a little, which means hills steeper than around 1-in-8 can be more challenging on those ebikes than a mid-drive one.

This is one reason why mid-drive ebikes can be preferable for very hilly areas - because the power is less likely to drop off at low speeds.
However, I think the OP is thinking of mid-drive systems, so not a problem.
H’mm. Surely it’s down to the gearing for whatever drive you use? A motor powered rear hub with gearing inside of it will produce torque that’s a function of the electrical power in. My own preference would be for a low powered hub motor coupled to a speed reduction gearbox that pushes out high torque at relatively low road speeds. I don’t need to be able to do 15 mph up hills but say 5 mph would be both handy and allow a less powerful motor to be successfully used. The same is pretty much true of a mid-drive, I’d want to use the gears to get up the hill using the lowest practical motor size, a geared down motor seems more likely to do that.
Don’t fret, it’s OK to: ride a simple old bike; ride slowly, walk, rest and admire the view; ride off-road; ride in your raincoat; ride by yourself; ride in the dark; and ride one hundred yards or one hundred miles. Your bike and your choices to suit you.
imk123
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Joined: 28 May 2022, 10:48am

Re: Is 250w enough power for modest speed up steep hills

Post by imk123 »

Hello Carlton Green

To best of my knowledge torque is a function of power and speed or the ability to change speed thus acceleration from n to n plus whatever.
And watts is a measurement power and therefore the amount of power/watts is shown in the graph
as a function of power/watts to raise a given mass against the force of gravity.

So no matter how low you gear your transmission you can never achieve a given vertical speed if you don't have enough power/watts.
That said i could not agree with you more that 15 mph up hill is not necessary and for me 5 mph would be enough for me not to wobble off under the wheels of a buss. But if you look at the graph a modest 10% gradient (like most of the hills around my home) need 250w for 5 mph plus peddle power. Problem is that these hill are not of a constant slop and often hit 20% gradient and now you need 500watts to maintain 5 mph or a lot more peddle power than an arthritic 74 years simply don't have anymore. :-(

This is why i am wondering about the Bosch 250w system is it really 250w maximum or as one thread indicated more like 800watts?
Jdsk
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Joined: 5 Mar 2019, 5:42pm

Re: Is 250w enough power for modest speed up steep hills

Post by Jdsk »

imk123 wrote: 29 May 2022, 8:54pmTo best of my knowledge torque is a function of power and speed or the ability to change speed thus acceleration from n to n plus whatever.
Torque, power and angular velocity are an inseparable triplet. Given the values for any two the third is uniquely determined.
imk123 wrote: 29 May 2022, 8:54pm And watts is a measurement power and therefore the amount of power/watts is shown in the graph
as a function of power/watts to raise a given mass against the force of gravity.
Power is a property that can be measured in watts.

Any relation between mass, gravity and power requires a time component, for example in a velocity. The relation between mass, gravity and energy doesn't require this.
imk123 wrote: 29 May 2022, 8:54pmSo no matter how low you gear your transmission you can never achieve a given vertical speed if you don't have enough power/watts.
If by speed you mean linear velocity and if at a given slope then Yes.

Jonathan
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