Rant: Stop bothering with "energy/distance at a given s

[CGameProgrammer]

Lots of people here measure efficiency as something like Wh/mile at a given speed in mph, but that's equal to W / mph, which is a far more simple expression, plus it's easier to measure. All you're doing is reporting how much power it takes to maintain a given speed.

 

[Jozzer]

I disagree. Tell us those wh/m! Its the best way to compare and to work out how far packs will get you.

 

[CGameProgrammer]

But it's literally identical to W/mph.

You use 25 Wh/mile at 25 mph, so you are using 625 W to go 25 mph.
You use 32 Wh/mile at 30 mph, so you are using 960 W to go 30 mph.

I think it's much simpler and easier to understand, as well as to measure.

EDIT: Actually perhaps your measurements weren't taken at a constant speed... in that case they're basically worthless for comparison. Someone can take your bike and ride it at 30 mph cruising speed along "a few hills and some stops/starts" and get wildly different results than you did, with the same bike.

 

[Jozzer]

It makes fro an easy way to calculate expected mileage from battery packs. Some have a 36v pack, others 72, some 84. Much easier to work out the WH of the pack to calculate range I find.
Those figures are average on a journey that I tried to keep to those speeds. THey include acceleration etc. I agree that if you wanted power consumption at a certain speed than watts used at that speed would be the way to measure it, but thats not the goal. There is a thread about that somewhere too though! Even that measurement is dependant on too many variables to be duplicated by another rider on a different bike on a different road with different weather conditions. We only want comparisons though.
The other good reason to do it this way is that everybody's drainbrains shows the wh/m figure, with no calculations needed.
Another point is that, on the same bike I had a X'lyte motor, which would draw very similar power @ 30ph, and 25mph, yet when I get back from a trip the wh/mile figure is much lower on the new puma. The puma uses less power to acdellerate it seems. How would you work out how many watts it takes to accellerate to 20mph/30mph etc without a "watt vs time" figure?

 

[Toorbough ULL-Zeveigh]

I agree that watt-hours per mile at speed isn't the bestest way of doing things but unfortunately there's no real good absolute method for comparison. It only serves as a rough thumbnail sketch of relative performance. Alternative forms of measurements have been discussed off & on b4, so come up with something else. You'll find it tough to get people to agree on something because every method has it's shortcomings.
Democracy has spoken & love or lump it were stuck with it.

 

[xyster]

Lots of people here measure efficiency as something like Wh/mile at a given speed in mph, but that's equal to W / mph, which is a far more simple expression, plus it's easier to measure. All you're doing is reporting how much power it takes to maintain a given speed.

Agreed. The units cancel such that there's no difference. I report my energy consumption that way sometimes:

400 Watts at 20mph = 20 Watt-hours per mile.
800 Watts at 26mph = 31 Watt-hours per mile.

Jozzer makes a good point about the different energies required for accelerating. But generally, Watt-hours per distance is reported for cruising energy consumption.

 

[The7]

But it's literally identical to W/mph.

You use 25 Wh/mile at 25 mph, so you are using 625 W to go 25 mph.
You use 32 Wh/mile at 30 mph, so you are using 960 W to go 30 mph.

I think it's much simpler and easier to understand, as well as to measure.

Wh/mile is based on "energy/unit distance" at certain speed.
W/mph should be based on "power/unit speed" at certain speed.

I think that it should be stated as:
25 Wh/mile at 25 mph;
25 W/mph at 25 mph (NOT "625 W to go 25 mph").

 

[xyster]

Wh/mile is based on "energy/unit distance" at certain speed.
W/mph should be based on "power/unit speed" at certain speed.

Which are algebraically and physically equivalent. See pic.

 

[patrick_mahoney]

While I agree with CGame and everyone else who agreed, I don't think that it matters.

I think people should collect data in whichever format they want. If they, for example, want to do Temperature-hours/per hour then I personally don't mind that time is a factor on the nominator and the denominator, I'm just happy people collect data at all. In metric or British units, in whatever format they want... it doesn't matter to me.

 

[dirty_d]

i like the power / velocity way better