How does silica lower a tire's rolling resistance?

July 22, 2009 6:04 PM

Physics/chemistry/tire tread composition filter. Why does adding silica to the rubber compound in a tire tread reduce rolling resistance?

In quest for better fuel economy, I understand that tire manufacturers continue to experiment with the tread compound to make them roll with less resistance while still retaining adequate friction with the road for starting and stopping. I believe recent tires use more silica and less carbon black to reduce rolling resistance.

I'm not a scientist, and I'm having a hard time wrapping my mind around how adding silica to rubber would cause it to roll with less resistance. Any thoughts would be appreciated.

posted by tronec to science & nature (5 comments total)

I would think it would make the rubber stiffer. Less bending means less energy turned into heat in the tires.
posted by Chocolate Pickle at 6:22 PM on July 22

This is probably the best explanation you're going to find.
posted by beagle at 6:50 PM on July 22 [1 favorite]

In general, there's a tradeoff between rolling resistance and grip: the softer and stickier the rubber compound in the tread is, the better it will grip and the worse will be its rolling resistance.

Tread rubber also has reinforcing fillers in it for wear resistance. Before the invention of silica tyres, the fillers were mostly carbon black (because it's cheap and bonds well to the rubber polymers). Using silane, silica can be persuaded to bond equally well.

Silica is harder and tougher than carbon black, so the silica grains exposed at the tread's wear surface are harder to break than carbon black grains. That makes them stick out further for longer, making the wear surface on a silica tyre rougher than that of a carbon-only tyre with similar rubber, and giving it better grip.

Silica's increased grip for any given grade of rubber enables the use of a somewhat harder rubber compound (which would otherwise grip less well), and that's what actually lowers the rolling resistance and improves the wear performance.
posted by flabdablet at 6:57 PM on July 22 [2 favorites]

Rolling friction is caused by deformation of the rolling object. That's why a steel ball bearing will roll a large distance before coming to a stop whereas a football will come to a stop very quickly - the steel bearing resists deformation much better than the football.

Adding silica to rubber increases its hardness and makes it less susceptible to deformation, thereby decreasing its rolling resistance.

Charles Goodyear's vulcanisation process (adding sulphur at heat) accomplished much the same feat.
posted by alby at 3:08 AM on July 23

Actually, according to beagle's excellent linked article, the previous crop of tires with added silica were less durable than ones made with carbon black and had a poorer grip:

At first silica seemed to be the magic elixir tire engineers longed for, but they soon realized it too offered no free ride: its lower rolling resistance came at the expense of lower durability, decreased traction in wet weather...

And low rolling resistance doesn't have that much to do with grip, nor is it necessary for a tire with low rolling resistance to resist deformation (and that's not how the high silica tires are doing it):

As much as 90 percent of a tire’s rolling resistance can be attributed to hysteresis--the dissipation of energy that occurs when the tread, sidewalls, and carcass of a tire are deformed as the tire rolls. The remaining 10 percent of resistance results from aerodynamic drag and the friction between the tire and the ground and between the tire and the rim.

Tires with low rolling resistance are ones which give back the energy which is put into them when they are deformed in the first place, as a good steel spring would, say-- a property known as low hysteresis.

The article is frustratingly silent about what manufacturers have done to increase the durability or traction of silica enhanced tires:

Now, however, Michelin seems to be in the driver’s seat with a closely guarded proprietary silica blend that lowers a tire’s rolling resistance below that of carbon black but doesn’t sacrifice the favorable tread characteristics that carbon black provides and ordinary silica doesn’t.

posted by jamjam at 2:55 PM on July 24