How much power is needed for a hillclimbing folder?

[kevinfox]

I am looking to make / convert / buy a 20 inch wheel folding bike for use in the Malvern Hills, by a 70Kg + rucksack wearing adult and would appreciate the advice of those more knowledgable than me. She is about to undergo knee surgery so in the short turn much human powered input is unlikely, and a throttle is needed.
After testing many bikes as Prestigne I had almost discounted 24V on power grounds until I discovered the Cyclone Kits that use the bikes gears. These are available in power ratings from 200W to 1000W and I wonder how much is actually needed as weight of the bike is a consideration. I was thinking 500W may be about right but is that amount of power too much, too little or about right? Certainly it's a lot of amps at 24V, although Ping say they can make a custom battery that will cope. Range needs to be 20 miles min and preferably more than that.
The main task is a 4 mile continuous climb which is 1 in 4 in places and maybe half that for most of it. Speed isn't a great concern as going too fast on 20 inch wheels with pot holes around isn't a good idea.
I welcome all inputs / suggestions / comments.
If there is sufficient demand for this I may turn it into a commercial product.
Thanks
Kevin

 

[frank9755]

Kevin,

To get up a 1 in 4 without significant pedalling input is not likely to be possible with any UK/EU legal motor. An Ezee Quando might be the nearest bet.

Of course, a higher powered motor could do it, but then you are into the territory of scooters rather than electric assist bikes.

 

[kevinfox]

Thanks for your reply, I appreciate the motor may need significantly more peak power than 200 / 250w.
I am interested in knowing what will get the job done electric bikewise instead of taking the easy route of getting a standard moped / scooter.

 

[flecc]

The Cyclone 180 watt is a waste of time, the 360 watt is the best version, and driving through the gears should enable it to handle most hills up to 1 in 8 (12%) or possibly even 1 in 7 (14%) with that 70 kilo + rucksack given very little rider assistance. Progress on the steeper ones would be slow though, down to about 5 mph in a low gear on the steepest.

The 500 watt is a bit of a juice eater as you've already guessed, and chain and sprockets wear could be a bit high. However, if you don't mind a large and expensive battery, it would give plenty of leeway for handling the more difficult hills.

By the way, these power ratings are nominal, the peak powers are much higher, and with driving through the gears you can use peak power whenever you like. These are the Cyclone motor ratings:

Nominal 360 watts, peak power 550 watts

Nominal 500 watts, peak power 700 watts

Nominal 650 watts, peak power 1020 watts

P.S. Here's some example figures of actual nett power required that I've worked out for you, based on a bike weighing 26 kilos with motor kit and battery, rider 70 kilos and 6 kilos assumed for the rucksack, total 102 kilos:

1 in 8 (12%) hill at 5 mph = 300 watts

1 in 8 (12%) hill at 8 mph = 470 watts

1 in 6 (17%) hill at 6 mph = 480 watts

1 in 5 (20%) hill at 6 mph = 560 watts

The peak quoted motor powers from Cyclone may be gross, so to be safe, assume they are and rate them at 80% efficient. That would mean actual nett power peaks of 440 watts for the 360 motor and 560 watts for the 500 motor. That would leave very little for the rider to do with the 360 watt motor on the first three examples, and the steepest example still possible with the moderate rider effort of 120 watts.
.

 

[AndyOfTheSouth]

Very interesting. Thanks for taking the trouble to do this, Flecc. I am sure Kevin finds it useful, but it is also good for the education of (relative) novices such as myself!

 

[kevinfox]

Flecc,
thanks very much for the calculations, very informative! Speaking to someone with one of the 500W motors on his bike it does seem battery life is an issue. The 360W gives me the option of the 24V 15Ah battery which saves a little weight.
Any idea on range impications of the hills?
Regards
Kevin

 

[flecc]

Any idea on range impications of the hills?
Regards
Kevin

Much depends on your riding practices Kevin. I live in a very hilly area but I get the similar ranges on both flat and hilly routes. The reason is that my routes enable me to use my preference for letting a bike have it's head downhills, avoiding use of the brakes. Doing that wastes none of the kinetic energy put in originally to get up the hills, turning it back into performance.

For someone using the brakes downhill to keep speed moderate for whatever reason, the range will suffer according to the steepness of the hills. That range loss I'd estimate could be as much as 30%, though that's an extreme case.
.

 

[kevinfox]

Flecc,
many thanks for the information.
I'll let you know how it goes.
Just out of interest if you increase the weight by 18Kg how much more power is needed to climb the hills?
Regards
Kevin

 

[flecc]

Flecc,
many thanks for the information.
I'll let you know how it goes.
Just out of interest if you increase the weight by 18Kg how much more power is needed to climb the hills?
Regards
Kevin

For weight it's directly pro-rata Kevin, so just divide the power in my examples by the gross 102 kilos and multiply by new 120 kilos weight.

If the weight is added as a trailer the power required is markedly increased above that due to the drag being increased by the added number of tyres.
.

 

[musicbooks]

For weight it's directly pro-rata Kevin, so just divide the power in my examples by the gross 102 kilos and multiply by new 120 kilos weight.

If the weight is added as a trailer the power required is markedly increased above that due to the drag being increased by the added number of tyres.
.

That's great info Flecc.
Very useful. I'm finding that I'm squeezing about 22 miles out of my panasonic battery with about 15 of that on very steep inclines.

bw
musicbooks

 

[kevinfox]

For weight it's directly pro-rata Kevin, so just divide the power in my examples by the gross 102 kilos and multiply by new 120 kilos weight.

If the weight is added as a trailer the power required is markedly increased above that due to the drag being increased by the added number of tyres.
.

Superb stuff, very informative indeed.
Thanks again.

 

[kevinfox]

Update on hill climbing

Thought I'd update things.
My other half had her knee ligament replacement 6 weeks ago and has started using her Quando 1. She is able to do the 7 mile journey between her home and the hydrotherapy pool both ways including climbing the mainly continuous 4 mile uphill drag over the Malvern hills without having to stop.
She is unable to load the bike onto the train due to restrictions on lifting and turning but with this range she can cycle instead.
Very happy with the result, and it's a lot more convienient than the invalid scooter and train that is her other mode of transport.
Thanks everyone for your knowledgable help.