Parisians vote to penalise SUVs

[9873210]

As for fuel economy, weight is only a factor whilst accelerating. Once you reach cruising speed fuel burn is mostly a matter of overcoming friction and air resistance, not weight.

Weight is a factor in rolling resistance (e.g. tyre deformation). Rolling resistance is typically about half of air resistance, but still non-negligible.

In theory a longer car can reduce air resistance, since it allows more scope for streamlining.

[ReformedCharacter]

As for fuel economy, weight is only a factor whilst accelerating. Once you reach cruising speed fuel burn is mostly a matter of overcoming friction and air resistance, not weight.

Weight is a factor in rolling resistance (e.g. tyre deformation). Rolling resistance is typically about half of air resistance, but still non-negligible.

In theory a longer car can reduce air resistance, since it allows more scope for streamlining.

You're correct about longer vehicles having better drag but I don't think the rest of your comment about rolling resistance is right:

almost all of the opposing force on the vehicle is air drag. Friction with the ground is very small. The rolling resistance is proportional to the weight of the vehicle and for car tires the coefficient is small, we'll round and say 0.01.

For air drag, it depends on the drag coefficient and surface area of the car. This coefficient is orders of magnitude larger than the rolling resistance. But drag also depends on the speed squared. So doubling your speed causes a force 4 times larger. This is a completely dominant effect once the speed is big enough to be interesting (a few mph)...

which means that once you get over roughly 11 m/s

for speed (roughly 21 mph), the air drag begins to take over as dominant. Obviously this is all approximate, but it should make it pretty clear that once a car is moving, air drag dominates.

I should note that the surface are approximation here is much larger than the real surface area. However, it doesn't change the results by all that much. The point is, at any speed you are likely to encounter outside of a parking lot, the air drag will contribute the majority of the opposition.

https://physics.stackexchange.com/questions/163788/how-much-of-the-energy-of-a-car-is-required-for-overcoming-air-resistance

RC

[9873210]

Weight is a factor in rolling resistance (e.g. tyre deformation). Rolling resistance is typically about half of air resistance, but still non-negligible.

In theory a longer car can reduce air resistance, since it allows more scope for streamlining.

You're correct about longer vehicles having better drag but I don't think the rest of your comment about rolling resistance is right:

almost all of the opposing force on the vehicle is air drag. Friction with the ground is very small. The rolling resistance is proportional to the weight of the vehicle and for car tires the coefficient is small, we'll round and say 0.01.

For air drag, it depends on the drag coefficient and surface area of the car. This coefficient is orders of magnitude larger than the rolling resistance. But drag also depends on the speed squared. So doubling your speed causes a force 4 times larger. This is a completely dominant effect once the speed is big enough to be interesting (a few mph)...

which means that once you get over roughly 11 m/s

for speed (roughly 21 mph), the air drag begins to take over as dominant. Obviously this is all approximate, but it should make it pretty clear that once a car is moving, air drag dominates.

I should note that the surface are approximation here is much larger than the real surface area. However, it doesn't change the results by all that much. The point is, at any speed you are likely to encounter outside of a parking lot, the air drag will contribute the majority of the opposition.

https://physics.stackexchange.com/questions/163788/how-much-of-the-energy-of-a-car-is-required-for-overcoming-air-resistance

RC

I agree with most of the numbers but disagree with the interpretation. He's a bit glib with "a few mph" and "completely dominates".

As your source says at roughly 11m/s (~25mph not 21mph) rolling resistance is roughly equal to air resistance. At 35 mph air resistance is twice rolling resistance. You'd have to be doing about 80 mph for air resistance to be an order of magnitude greater than rolling resistance, at which point it could be said to dominate.

For driving in, say, Paris or London, rolling resistance is non-negligible. For the odd person who commutes from one motorway rest area to another at 2AM perhaps it is.

[swill453]

For driving in, say, Paris or London, rolling resistance is non-negligible. For the odd person who commutes from one motorway rest area to another at 2AM perhaps it is.

Similarly, in stop/start city driving, most of the time you are using fuel you will be accelerating, when the weight of the vehicle has a massive influence on fuel consumption.

Scott.