Low Rolling Resistance Tires: are they a good deal?

March 19, 2009 -- Filed under My impact by Louis-Alain Richard

We’ve already had a quick look at the BlueMotion label in this blog, but seeing as they’re a major trend nowadays, let’s examine low rolling resistance tire more closely, and let’s call them LRRT for short (to give my fingers a break).

Tire Resistance

Every time a tire turns, its tread will flatten out to hug the road surface under the weight of the vehicle. As it distorts, the tire heats up and gives off waste energy. This rolling resistance (RR) will be greater or less depending on several factors.

- Speed rating : to ensure stability at high-speed, the tire is reinforced along the tread layer. The more the tread layer is reinforced, the greater the RR.
- Diameter: greater diameter means less rolling resistance in the order of 1% for each additional centimetre.
- Tread design: a 50% increase in tread depth increase RR by 12%. At the end of the tire’s useful life, RR is reduced by 25% compared to a new tire.
- Tire pressure: underinflation increases its distortion and causes it to heat up more: RR is thus increased.
- Type of road surface: the rougher the surface, the greater the RR. The difference can be as much as 40% more.
- Outside temperature: for every increment of 10° C (between 10 and 40° C), RR decreases by 6%.

Are you still with me?
In an ideal world

In an ideal world we would drive on large diameter narrow wheels with tires that are almost completely worn down and inflated until rock-hard. But in real life, this isn’t always practical, or safe… So engineers have designed LRRTs that minimize rolling resistance without losing out in other areas.

For example, bias-ply tires (that haven’t been used since the 70s) were twice as energy-consuming as radials. More recently, silica additives in the rubber have permitted a further decrease of 40%, which means that modern tires have only 30% of the rolling resistance of tires from 40 years ago. It is estimated that there’s room for a further improvement of about 25% that could be accomplished in 10 years.

How was greater efficiency achieved?

To further improve their performance, tires were designed with lighter materials to reduce distortion and heating. The greater the tire’s weight, the greater the distortion, and the more waste heat is produced.
The next step was to change the composition of the rubber by adding recently developed silica components. The silica replaces almost all of the traditional carbon black. A small quantity of carbon has been retained, simply to give the rubber its familiar colour. A beige tire would just look funny, don’t you think?

New manufacturing techniques have been developed as well in order to eliminate certain materials and simplify tire assembly. All this research seems to have given results: a Swiss study has shown that Michelin’s most recent tire, the Energy Saver, delivers a 12% improvement over its closest rival, a figure that goes up to 25% when it’s compared to the worst tire of the same size. And despite its low resistance, the Energy Saver offers close to the same traction as the best in the category in tests on wet pavement (braking from 80 to 20 km/h).
What are the benefits for me?

All these numbers are very interesting, but what do they really mean? With the same Energy Saver from Michelin on a VW Golf, you can hope to increase fuel economy by 0.2L/100 km according to the manufacturer. If you calculate 20,000 km in a year at $1 a litre, that means a saving of $40 a year. That may not seem like much, but these benefits are free, as the LRRT tires cost the same as regular tires.

What can you get in Canada in 2009?

There’s not much available in Canada as yet, but here are a few links to Michelin and Nokian, and a link to Goodyear in the United States.
Consumer Reports

According to the American magazine Consumer Reports, the tires with the best RR rating among those tested are the following:

Michelin Radial X, Harmony and Energy MXV4 Plus;
Continental ContiPremier Contact;
and ContiExtreme Contact General Exclaim UHP.

The worst, if we confine ourselves to the RR rating are:

Michelin Pilot: XGT H4, Exalto and Sport A/S
Pirelli: P-Zero Nero, P3000 and P6 Four Seasons
BF Goodrich: Traction T/A and g-Force T/A
Bridgestone Potenza G009
Yokohama Avid V4s
Goodyear Eagle F1
Firestone Firehawk Wide Oval

So do you think these new technologies are going to turn the situation around? Let’s talk.

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