Electric motor query - power and torque

[newshound]

Ultimately, it's the power which decides how fast you can get up a hill, or how fast you can go on the level (and that will depend on the wind). I *don't* think you should pay too much attention to claimed torque values. I think you need to try them all and decide which one feels most suitable. I'd have thought the other key parameter was range, a good indicator of that would be the amp-hours of the battery (bearing in mind that it also depends on voltage: a 12 volt battery with 10 amp hours contains the same amount of energy (hence the same range) as a 6 volt one with 20 amp hours.

There are of course other factors like electrical losses in the motor, wiring, connectors, etc. and frictional losses in bearings, and rolling resistance. Both bicycle and electric motor technology are fairly mature and the ways of controlling losses are well understood. Although you can reduce losses up to a point by throwing money at them.

[harry]

The cyclist is also putting in power. It's only an assist device. In practice the power output will be limited by the battery charge state.

[newshound]

A continuous 15% gradient is actually pretty steep, in the context of things like wheelchairs and mobility scooters. And 3.6 mph is a good walking pace on a slope.

[Vir Campestris]

3.6MPH up a nearly 1 in 6 hill?

AIUI Tour de France riders put out about 400W. Mere mortals plus 250W from the battery should be able to get close to that - at least for a while - which would be a great help.

Andy

[David]

This made me finally look up the conversion between the old "1 in 4" road signs and the more modern "25%" style.

I had assumed that 1 in 4 was equivalent to 25% and 100% would be straight up. Not so, apparently.

Interesting reading, and one of the times where you can have more than

100% without getting into sporting speak of the "I want all my players to give 110%" variety.

According to the table, 15% is roughly "1 in 6" or around 9 degrees which doesn't seem that steep (at least for a short distance).

Now hunting down a route profiler to check how steep the diddly little hills I ride up really are.

Cheers

Dave R

[The Natural Philosopher]

I have climbed, with ropes, a 45 degree rockface.

which is 100% by their token

Its also IIRC the slope of the great pyramid at Chichen Itza, which I have also climbed, with a rope hand hold. (and a 25kg camera bag) They wont let you do that any more

15 degrees (1:4)(25%) is about as steep as ANY road gets. The infamous porlock hill is that.

[Rob Graham]

Going back to my original enquiry, a bit of googling does a little repair t o my memories of this area of mechanics.

It may well be that my 'ergo' is not correct though and I'm more than happy to be corrected.

If we apply power to something, that is energy per time and we measure that in Joules.

For torque as a rotational force - measured in Newton Metres - for one comp lete rotation the distance is 2 pi x the radius.

Thus 1 Nm = 2 x 3.14 joules / rev.

Also 1 joule = 1 watt-sec.

Now is it a coincidence that for a drive system producing "40 newtons of To rque" with a motor input power of 250W, that 40 x 6.28 ~= 250w ?

I suspect that this is just chance as the inference would be that the 80Nm system would then require 500 W of electrical power which is double the UK allowed power.

Without knowing very much about modern electric motor design, could the sou rce of double the torque be related to much more powerful permanent magnets in the 'up-market' system. Or is it that in some way the electronics for t he less powerful system just 'chips' the power down?

[The Natural Philosopher]

Neither.

Basically what 'powerful magnets' do for you is allow a better power to weight ratio. BY and large on a small DC motor the dominant losses are (resistive) copper losses, which you can avoid by simply having 'more copper'

Or by having more magnetic fields, you can get away with less copper.,

For the same torque.

In terms of constant power, the output powers should be constant. If its double the torque its delivered at half the RPM which may suit a differently geared bike. I don't know how these things are used.

[Dave Plowman (News)]

Torque doesn't include speed of any sort. Only if you know the RPM at which it is being measured at would the power input be of note.

Torque is used to set the tightness of a bolt etc. Very little movement there and at a very low speed. It is simply the product of weight and the distance from the fulcrum. If you use lb.ft. it would be one pound of weight on a 1 ft long spanner to give you 1 ft.lb

There is bound to be some spread of efficiency with motors. However, I'd be surprised if two designed for the same job were so much different. ie, one producing twice the torque at the same output shaft RPM and power input than the other. The low efficiency one would get very hot since there's nowhere else for that power to go.

[harry]

Wrynose pass is 1 in 3 in places. You want to try descending tHAT in a Minor 1000

[PeterC]

Are you sure that it's 1 i 3? I've cycled up it several times and, although steep, is nowhere near as bad (good) as Robin Hood's Bay or Rosedale Chimney.

[PeterC]

It's the Tangent of the angle - gets nasty over 45 deg. Same with drive-side spokes: the tension is proportional to the tan. In the days of early 6-speed blocks and 125mm rear ends I applied much cunning and brute force to gain a few mm on the drive side. With a 531 frame, it was possible to get 5mm preferentially on the drive side by standing on the LHS chain stay near the bridge and heaving. Then simply build the wheel with a longer axle and dis it across to the LHS. Not good with gas-pipe frames as they tended to bend rather too easily.

[Clive George]

It's marked 1:3 on one of the sides I think. It's easier from the west. IMO Hardknott is harder. But Harlech has the steepest - marked as 40%. That was hard.

[bert]

I don't think that is correct The 1 in 4 notation measure the sine of the angle not the tangent. It's a rise of 1 foot for each 4 feet you travel up the gradient.

[The Natural Philosopher]

WTF are you on about?

[T i m]

Really?

Bicycle wheel and frame design / modifications. Derailleur driven cycles generally have an offset 'dish' to them to accommodate the drive sprocket that is on one side.

HTH

Cheers, T i m

[PeterC]

um, now not sure. If the gradient is 1:1 (100%) at 45 deg., that suggests tan. The diagram showed, IIRC, Opp/Adj which I think is Tan.

[The Natural Philosopher]

yes . I always thought it referred to distance along teh map and height above sea level, which is tan.

Then at some point someone else assured me it was sine, and now we are back to tan.

The page linked to seems pretty definite that its tan so I'll go with that for now

[bert]

I always thought that the 1in 4 style notation was in fact sine, i.e. opposite over hypotenuse

[PeterC]

That would be more useful in the real world, as hypotenuse is measurable on the road and opposite can be taken from a map. Both of these are needed to get the adjacent, as that would be difficult to measure without a tunnel.