[Soupdragon]

As I do every few weeks, I drove my LEAF down to Birmingham and back this weekend (21st October). Now this was the day that storm Brian visited the UK.

Now usually I get to Tibshelf with more that 20% in the tank, plenty to move on to Trowel if Tibshelf is busy, or bust. This time I was down to 7%, and the car was giving me its "find a charger" alerts. This is as low as I care to get, and it wiped out my plan B. Fortunately no problem at Tibshelf. (Charge after 45mins doesn't seem to be significantly different regardless of what you start at).

My driving style was pretty much as normal. ECO mode on, cruise control at the speed limit.

The only suspect here is high winds. And that's a bit worrying. because that could happen any time, especially this late in the year. I don't know which way the wind was blowing, and I didn't feel the affects on steering but presumably high cross-winds can affect the car's aerodynamics.

Anyone else out on Saturday and notice loss of range?

 

[Alan Gee]

I was driving North on Sunday morning Bournemouth to Southport and my calculated 100% range* varied from 72 to 75 miles as the day went on. It felt like mostly a stiff crosswind. It meant a definite 4 stop rather than the summertime 3 stop. I'm assuming this is more due to temperature and having the whole family on board than the wind.

* I used 95% to do 68.5 miles from home to Chievely services which means 100%=72 miles. I then use this number for forward planning to know what percentage I need for following stops.

 

[Ticklestick]

Yes and Sunday.

54 miles took 75% out of my battery on the M6. I'm very very glad I stayed at the rapid a bit longer in anticipation of a headwind!

Normally I'd do that distance on about 60% max, I am fairly good at getting about 4.5m/kWh on the motorway.

Edit: Actually maybe it was a crosswind. I didn't *notice* anything other than the battery usage, so who knows what it was.

 

[DBMandrake]

The only suspect here is high winds. And that's a bit worrying. because that could happen any time, especially this late in the year. I don't know which way the wind was blowing, and I didn't feel the affects on steering but presumably high cross-winds can affect the car's aerodynamics.

If you drive into a strong headwind at motorway speeds you'll take a massive hit in range. I've had this happen to me on my Ion where my range went down alarmingly fast in one direction (into the wind) but I made it back in the other direction with a tail wind.

The biggest single source of drag on a car above about 40mph is wind resistance. By the time you get to 60-70mph it is several times higher than any other form of drag and is increasing with the square of velocity.

If you drive at 60mph into a 40mph headwind you have the same aerodynamic drag as if you were driving at 100mph!

So yes, headwinds and tail winds will make a massive difference to range, and can be unpredictable as you might not be aware of which way they are blowing and how strong.

I'm not sure how much a crosswind would affect things - probably not a lot, but it may depend on the overall aerodynamics of the car.

If you have no choice but to drive into a headwind and battery is short drop your speed so that the sum of your speed and the wind is not excessive.

 

[leafmonster]

You can tell the wind speed and direction from looking at wind turbines along the route.

If you can see the back of a turbine that is spinning fast, that is bad for range.
If you see the front of a wind turbine, thats good!

 

[mgboyes]

The biggest single source of drag on a car above about 40mph is wind resistance. By the time you get to 60-70mph it is several times higher than any other form of drag and is increasing with the square of velocity.

If you drive at 60mph into a 40mph headwind you have the same aerodynamic drag as if you were driving at 100mph!

It's even worse than that.

Driving at 60mph into a 40mph headwind you have the same aerodynamic drag as if you were driving at 100mph, but unlike if you were actually driving at 100mph you don't reach your destination any quicker.

Power required goes with the cube of air speed, but driving faster means a shorter journey time so these effects counteract each other and the resulting energy usage over a journey is proportional to the square of speed. But a headwind increases the air speed without any corresponding reduction in travel time which means the effect is proportional the cube of speed.

So driving at 60mph into a 40mph headwind has the same energy efficiency as driving at 129mph in still air

 

[DBMandrake]

Wow, I hadn't considered that - that's nasty...

But surely only the proportion of the relative wind speed that is due to the wind (the 40mph) is cubed, the effect from 60mph is only squared, so not 100mph cubed - or did you allow for that in your 129mph calculation ?

Either way its still really nasty!

 

[mgboyes]

Wow, I hadn't considered that - that's nasty...

But surely only the proportion of the relative wind speed that is due to the wind (the 40mph) is cubed, the effect from 60mph is only squared, so not 100mph cubed - or did you allow for that in your 129mph calculation ?

Either way its still really nasty!

Power is proportional to air speed cubed, journey time is proportional to 1/ground speed, and energy is power times time.

So in made up units when travelling at 60mph in still air the energy needed is 60^3 / 60 = 60^2 = 3600.

In a 40mph headwind it's (60+40)^3 / 60 = 16,667.

And at 129mph in still air it's 129^3 / 129 = 16,650.

 

[Mr_G]

Power is proportional to air speed cubed, journey time is proportional to 1/ground speed, and energy is power times time.

So in made up units when travelling at 60mph in still air the energy needed is 60^3 / 60 = 60^2 = 3600.

In a 40mph headwind it's (60+40)^3 / 60 = 16,667.

And at 129mph in still air it's 129^3 / 129 = 16,650.

@mgboyes Neatly expressed and the point well made. If only more schools taught physics correctly.

The variability in the driving power needed depending on conditions is in my view a good indication that the GOM gives inaccurate predictions is in fact an urban myth. It is very accurate because it is sensitive to recent conditions BUT with the assumption that future conditions will be the same. Most drivers coming from ICEs are unaware of how variable the engine load can be and just how often engines are running at a low percentage of their peak output. The fact that they do is why lean burn can be used in petrol engines. The GOM is more accurate than any petrol tank gauge. The GOM protects you by accurately downgrading predicted range due to recent bad conditions. Relief and range recovery comes when you stop going uphill, driving fast or battling headwinds and rain with the heater working hard. Set the trip on a 100% charge and add the elapsed miles to the predicted miles and you will see results that induce confidence in the instrumentation. At 50% charge I often look to see how close the two readings are. If they are apart there is always a good reason based on the recent driving conditions being above or below average consumption.

 

[BarryP]

I use a rough and ready method to judge wind speed, that of noise. Within safety considerations I use windnoise as a guide as to optimum speed.

 

[DBMandrake]

I use a rough and ready method to judge wind speed, that of noise. Within safety considerations I use windnoise as a guide as to optimum speed.

Should EV's have a relative wind speed indicator to warn if you are driving into a strong headwind ? Shouldn't be too hard to incorporate an air flow meter at the front of the car somewhere...

 

[Alan Gee]

Just look at the trees/flags/weathervanes on churches. This is my normal technique. Any on car device would have to account for the normal airflow due to forward motion.

 

[DBMandrake]

Just look at the trees/flags/weathervanes on churches. This is my normal technique. Any on car device would have to account for the normal airflow due to forward motion.

Which is extremely easy to do, because the car already measures the road speed.

Measure incoming air speed at the front, subtract road speed - the difference equals the proportion of the wind velocity vector which is a head wind. What's hard about that ?

A lot of the time I'm driving on motorways (which is when the headwinds really matter) there are no nearby trees to gauge wind speeds from, and with a tree flapping around randomly its not always easy to figure out which way the wind is actually blowing anyway.

 

[Hitstirrer]

This is a long way from a scientific study but if its even partly true it could suggest that much of the wind drag that we experience is wheel related rather than slippy body issues. They half heartedly claim a 20% improvement by changing wheel hub caps for plain ones.

 

[DBMandrake]

This is a long way from a scientific study but if its even partly true it could suggest that much of the wind drag that we experience is wheel related rather than slippy body issues. They half heartedly claim a 20% improvement by changing wheel hub caps for plain ones.

Sounds about right. There was an article recently (or was it a tweet from Elon ?) that said that removing the (in my opinion ugly) "aero" covers from the Model 3's aero wheels reduced range by something like 10% or a full 30 miles on the long range model...