vis wrote:... I don't get this point: do you mean less traction?
...
Other point: so, if I well understood, you say that (in wet condition) anti water-spray tarmac (porous) has less rolling resistance than normal tarmac, because it remains dryer. I also think that in dry conditions is the opposite.
Yes, when I said friction is half, I'm referring to the coefficient of friction (f or mu, where I live). As you surely know, the coefficient of friction, multiplied by the weight of the car gives you the
amount of traction you can develop. A low coefficient of friction is what makes a car slip in the rain.
That is why you have a lower MU under rain.
Rolling resistance is the resistance to advance (Cr where I live). The coefficient of rolling resistance, when multiplied by the weight of vehicle, gives you the
force, working against you moving, that is, the force that finally stops the car if you're coasting: part of it is developed on the wheel bearings and drivetrain, part from the flexure of the tyre and part of it on the interface between tyre and tarmac. This last part (microslipping) is what makes you
having a higher CR under rain.
"The measured rolling resistance arises from hysteresis losses in the sidewall and treadband material, which experiences a deformation cycle every tyre revolution. There is an additional small loss (approximately 10% of the total) due to micro-slip in the contact between the tyre and the test surface."
...
"During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%; air drag is about 5% and tire rolling resistance is about 15%."
...
"Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%."
-- Laboratory Measurement of Rolling Resistance in Truck Tyres under Dynamic Vertical Load, Popov et al. --
What influences more the rolling resistance on
highways (and is under your control) is the proper inflation of tyres and air drag, so, if you wish to save gas,
check the tyre pressure and don't speed. As drag increases to the square of speed, a speed limit saves gas (a lot!), besides lives. That is one of the reasons for the trend of lower allowed speeds on highways in Europe.
On cities, inertia and drivetrain become important:
try to follow the traffic if you wish to save gas, "guess" or learn the green cycles of traffic lights and coast in neutral as much as you can: you will be surprised at how high the inertia of your car is if you try it. If you follow "blindly" the movement of the car ahead in a traffic jam, you can increase gas expenditure by 20%! Not to mention pollutants...
You can save easily 10% of gas on average, if you drive smartly.
About porous tarmacs having greater rolling resistance, you're right, but the answer is not simple (sorry for extending, it's Holy Week!
):
What influences a lot, more than "microtexture" (ondulations under 2 mm, or what diferentiates a porous from a regular pavement), is the "shortwave uneveness", that is, ondulations in the road in the range from half a meter to several meters (by losses in the suspension) and "megatexture", that is, ondulations from several centimeters to half a meter. "Macrotexture", that is, ondulations from a few millimeters to a few centimeters correlate poorly with rolling resistance. "Microtexture" has even less influence on rolling resistance. That is,
it is very important to mantain properly the road to save gas. (tell that to Al Gore!
)
Correlation between shortwave uneveness and megatexture with fuel consumption. Source: Sandberg, Ulf. Road Macro- and Megatexture Influence on Fuel Consumption.
Correlation between macro and microtexture with fuel consumption (it has a worst coefficient of correlation than the previous image) Source: Descornet, Guy. Road-Surface Influence on Tire Rolling Resistance.
American FHWA also concluded that a rough (that is uneven on long waves) can give you 20-30% higher fuel consumption. (Source: Federal Highway Administration Perspectives on Fuel Consumption and Air Contaminant Emission Rates by Highway Vehicles. USDOT.)
In the last graph,
IF you design a porous pavement (and thus, the macro and microtexture are used for something useful, which is less mist at high speeds) and the texture is not simply the result of a poor mix design, you contrarrest a little the efect on rolling resistance when it rains. Besides, you normally wish to design a "two layer" porous pavement (more even on the upper layer and more porous in the lower layer) so the holes that drain the water won't clog so quickly with dirt and stop draining the water, which in turn gives you less rolling resistance.
Summary: rain diminishes friction and increases rolling resistance. A porous pavement and proper maintenance of a road dimishes fuel consumption. Check inflation of tyres, coast as much as possible in city traffic and don't overspeed on highways to save fuel.
And finally: when you try to break the trap speed record, do it in dry weather! (which is the question posed by vis).
