The newest buzzword when it comes to tires is “Low Rolling Resistance” (LRR). It seems lately like every tire company in the world has suddenly jumped onto the low rolling resistance bandwagon and is marketing at least one tire that they claim is more fuel-efficient than the rest. But what is “low rolling resistance” really, and how does one choose between the storm of LRR tires coming onto the market? How does one make a meaningful comparison of fuel-efficiency between, say, Bridgestone's Ecopia and Yokohama's Avid Ascend, for example? What do RRF and RRC mean, and why do they make even my head hurt to contemplate them?
Here's the lowdown on low rolling resistance.
What is Rolling Resistance?
Car engines generate energy, much of which is eventually lost somewhere along the line. A great deal of that energy is lost in the engine itself and in the powertrain, but some energy eventually makes it to the tires and is used to move the car. Rolling resistance, then, is a measure of how much of the energy that actually makes it to the tires is then lost both to the friction of the road surface and to the process known as “hysteresis.” Hysteresis is the process by which the tire flexes as weight is placed on it, and then snaps back into shape as it rolls. The energy that comes back to the tire when it snaps back is, due to the laws of physics, always less than the energy that went into deforming the tire in the first place, so that the tire is losing some energy to the process of flexing at every moment it is moving. As much as 30% of the energy that ends up making it to the tires is given up by friction or hysteresis.
Ultimately, all the energy provided by the car' engine comes from the gas tank, and this is why trying to retain that energy is so important – the more energy that goes to moving the car, the better the car's fuel mileage will be. With gas prices spiking all the time and environmental consideration assuming increasing importance, fuel-efficiency is the new name of the game. As it is very difficult to reduce the friction in the engine and powertrain any further, this makes the tires one of the best areas available to try and get back some of that lost energy.
In years past, low rolling resistance tires meant having a tire with a very hard rubber compound and stiff sidewalls to reduce friction and flex. While this approach worked moderately well in reducing friction, it made for tires that ran like rocks and had very little grip. Nowadays, new tire compounding techniques such as silica-based compounds and alternative oils are changing the game yet again. Newer compounds are showing some very good rolling resistance properties, while also maintaining a pleasant ride and much more grip
RRF and RRC
RRF and RRC are the two numbers most often used to evaluate the actual rolling resistance of tires. Rolling Resistance Force is essentially the force in pounds or kilograms required to rotate a tire at 50mph against a large steel drum, while the Rolling Resistance Coefficient is obtained by dividing the RRF by the actual load placed on that particular size of tire. The process of doing so is rather ridiculously complex, and there are a couple of problems involved in using these numbers to compare different brands of tires. While RRF is pretty easy to compare, it does not take into account the size and load of the tires, and while RRC does take these factors into account, this makes it impossible to compare tires of different sizes. This is why tire companies most often market LRR tires by using fuzzy comparisons. Most often you will see a tire company claim that their tire is “20% more fuel-efficient than the competitor's tire”, or “10% less rolling resistance than the previous tire.” I have said before and will say again that these numbers are generally either an average of RRC across the entire line of tires or a best-case scenario for a particular size, which makes clear comparisons difficult if not impossible. In fact, my summer project has been to put several different LRR tires on my car for weeks at a time to get clear comparisons of one single tire size carrying an identical load, to give me a clear idea of the real-world differences between the tires.
Present LRR technology will give the a fuel-efficiency improvement of 1-4 mpg at best. While this does not seem like much, taken cumulatively over the life of the tires, it does begin to add up. There are, however some important issues to remember.
First of all, if you spend any time at all reading online discussions of LRR tires, you will inevitably see someone complain that their new LRR tires give worse fuel mileage than their old standard tires. There is a simple explanation for this – worn tires have much lower rolling resistance than new tires. When you put new tires on in place of old ones, your fuel mileage will always drop, regardless of how low the rolling resistance on the new tires actually is. The only fair comparison is between brand-new tires and other brand-new tires, or between tires worn to the same degree.
Second, when using low rolling resistance tires, there are two related factors that are easily as important to real-world fuel-efficiency as the tires themselves.
- Motor Oil: Using the correct weight of oil in your car will cut down on engine friction, and this accounts for at least as much in fuel-efficiency gains as the tires.
- Tire Pressure: Even slightly underpressured tires will quickly bleed away any fuel savings that one might gain from LRR tires. I find that if you want to get the best fuel savings from your LRR tires, it's best to check the pressure literally every time you fill the tank.
All in all, LRR tires do appear to be an effective and useful new technology, for all that it seems to be in it's infancy right now. With gas prices being what they are, it can often be a good thing to have tires that can save you a bit of fuel while they keep your car rolling.