Simanaitis Says

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NO, LET’S DISPEL that myth right off. To a large extent, a tire’s inflation pressure—not its width—determines the area of its contact with the tarmac. However, there are still lots of variables involved in what might be the single most complex component of a modern car, its tire. Added here is a bit of Formula 1 intrigue, with its own DeflateGate.


Given a load L and inflation pressure P, a tire’s contact area is very close to L/P. Akin to an uninflated balloon, a flat tire’s sidewalls support only negligible weight. Note, it doesn’t take much load to flatten an uninflated tire.

Now imagine two tires: a wimpy little 175/75R-13 compared with an aggressive (and wider) 225/50R-15. These tires have about the same overall diameter; the latter is an acceptable Plus-Two upgrade of the former.


At left, a 175/75R-13 tire. At right, its 225/50R-15 Plus-Two counterpart.

Next, let’s analyze the outcome of such a modification. Suppose a 2500-lb car with 50/50 weight distribution has its tires inflated to 30 psi. Before and after the Plus-Two change, each tire supports a load of 625 lb (2500/4). Thus, before and after the wider tire, each contact patch has an area of 20.8 sq. in. (L/P = 625/30).


Comparing compact patches of the wimpy 175/75R-13 with a wider 225/50R-15, both having approximately the same diameter. Image from R&T, February 1986.

The contact patches have the same area—but their shapes are different. The wider tire’s patch is commensurately shorter. And, indeed, in dry handling, this wider tire generates greater cornering force with more predictable behavior.

In the other hand, if weather is inclement, the narrower patch of the 175 tire offers potentially better drainage and improved cornering. What’s more, its longer patch is more stable longitudinally and offers better straight-line grip.


Drag racing tires are a world apart, but they tend to be tall and skinny. Image from Super Chevy.

Ever wonder why top drag racing classes have tall (and relatively narrow) tires? What’s more, tires designed specifically for drag racing are optimized for inflation pressures of around 15 psi. Why so low? L/P again: A small value for P gives a large contact patch.

This brings us to Formula 1—where tires are also extremely sophisticated and regulated—with its own version of DeflateGate. As background, FIA regulations define F1 tire dimensions when measured at an inflation pressure of 1.4 bar (20.3 psi). Pirelli has been the sole F1 tire supplier since 2011.


An array of Pirelli F1 tires, different compounds identified by color.

Back in 2013, Pirelli and the Mercedes team got themselves in hot water over secret testing during that season’s tire development. Punishment for what became known as TyreGate was a handslap: Mercedes was prohibited from one of F1’s permitted testing sessions that year. Pirelli also lost face at the 2013 British Grand Prix when the race devolved into havoc with blown tires.

Fast forward to the 2015 Belgian Grand Prix, where there were two dramatic blowouts, one occurring at a speed approaching 200 mph. These led to a Pirelli recommendation, and a subsequent FIA ruling, that all tires on the F1 starting grid had to be inflated to no less than 19.5 psi.


Sebastian Vittel’s Ferrari after its blowout, Belgian Grand Prix, 2015. Image from The Telegraph.

Then came the 2015 Italian Grand Prix at Monza—and DeflateGate. The Mercedes team finished 1-2, Lewis Hamilton ahead of Nico Rosberg. However, late in the race, with Hamilton’s lead secure, he was given mysterious instructions from his engineers to push, extend the gap and ask no questions.


The Mercedes of Lewis Hamilton, Italian Grand Prix, 2015.

It turned out that, with only 15 minutes to go, organizers advised that the Mercedes pair had started the race at less than 19.5 psi: In a pre-race sampling, Hamilton’s left rear tire was at 19.2, Rosberg’s left rear, at 18.4.


The results after sampling inflation pressures of several cars on the grid. Image from USA Today.

The subsequent kerfuffle was settled without penalty, cloaked in technicalities of grid-measured tires cooling a tad after their pre-race warmers were removed. Yet, it’s also interesting that Hamilton was advised to get some heat—and hence added inflation pressure—into his tires prior to the checkered flag.

As Henry David Thoreau said, “Some circumstantial evidence is very strong, as when you find a trout in the milk.” ds

© Dennis Simanaitis,, 2016

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