classic33
Leg End Member
That's not for you or me to decide though.
20 mph speed limit.
- Thread starter gavroche
- Start date
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Alex321
Veteran
- Location
- South Wales
I never suggested it might be.
albion
Guru
- Location
- South Tyneside
I was reading about the 'eco marathon'.
Seems the eco driving record is 15,200 mpg. Average speed was 18.6 mph. So, was that 56mph thing a lie?
Via wiki, https://web.archive.org/web/2006072...s.com/content_pages/record.asp?recordid=43581
Seems the eco driving record is 15,200 mpg. Average speed was 18.6 mph. So, was that 56mph thing a lie?
Via wiki, https://web.archive.org/web/2006072...s.com/content_pages/record.asp?recordid=43581
Alex321
Veteran
- Location
- South Wales
No. The majority of cars were designed to be most efficient at around that speed. The record breaking car was a special design, very different to the average family car.
Though I'm not sure that modern cars are significantly more efficient at that sort of speed than they are at lower constant speeds. They will still be a lot more fuel efficient at a constant 56 than in stop-start traffic, but won't be much less efficient at a constant 30 than at a constant 56.
the snail
Guru
- Location
- Chippenham
It's debatable whether a car will be any more efficient at 30 vs. 20 though. IMO you are more likely to keep a steady speed at 20, and more likely to be braking and accelerating at 30 which will ruin efficiency. Also you will be generating more particulates at 30.
albion
Guru
- Location
- South Tyneside
No looks at wattage at differing cycling speeds. It is only aerodynamics that stop cars having the same hirrendous exponential efficiency loss as the bicycle. However the increasing loss is still there I am sure.
rogerzilla
Legendary Member
56mph being most efficient has been debunked many times. It was just one of the benchmarks used in an old UK fuel economy test.
Most cars do their best mpg at around 30-40mph. Slow enough that air resistance is reasonably low, but fast enough for good engine efficiency. Petrol engines have quite a peaky specific fuel consumption, and need to be developing a reasonable amount of power to convert more fuel into work.
roubaixtuesday
self serving virtue signaller
Cars have exactly the same physics; resistance goes as the square of speed; power as the cube (power = resistance x speed). Neither is exponential.
The difference for ICE cars is that engine efficiency also varies with power and gearing, but the speed power relationship at the wheel is the same as for cycling.
albion
Guru
- Location
- South Tyneside
Mathematically, drag created by air resistance increases as the velocity squared. However, the power or energy expenditure to overcome resistance during cycling increases as the velocity cubed. Thus, as velocity increases, an exponentially greater level of power must be produced in order to attain that speed.
From google
From google
roubaixtuesday
self serving virtue signaller
All uphill
Still rolling along
- Location
- Somerset
You might want to check the definition of "exponential".
Cambridge University dictionary seems to think that x cubed is an exponential function.
albion
Guru
- Location
- South Tyneside
Seems we have to realise that the US speed limit was set at 55mph during one of the oil crises.
So, it is not 56mph that is most efficient, it is just that over 55mph, mpg gets far far worse.
So, it is not 56mph that is most efficient, it is just that over 55mph, mpg gets far far worse.
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Alex321
Veteran
- Location
- South Wales
Yes it is.
Exponential just means that the rate of increase keeps increasing.
So even a square function is exponential, never mind a cube function.
Dredging from dark recesses of memory...
Power function x^3
Exponential function 3^x
Edit: And a source
https://socratic.org/questions/why-is-x-3-not-a-exponential-function
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Fab Foodie
hanging-on in quiet desperation ...
- Location
- Kirton, Devon.
Is there any feedback from Welshielands about how this 20mph speed limit is going?
Has the country ground to a standstill?
Have billions been received into the Cymru coffers as a result of all these fines?
Are there riots on the streets?
Has the country ground to a standstill?
Have billions been received into the Cymru coffers as a result of all these fines?
Are there riots on the streets?
