Tyre test 2008: Test 2 - Wet cornering

WHAT WE MEASURED
In the wet, a tyre's workload doubles. It has to do two things, not just one. See, in the dry, all tyres do is grip the road. In the wet they have to continue to grip the road, usually at a slightly lower level, and at the same time pump as much water as possible away. Most people don't appreciate it, but tread is really an endless linear pumping system.

It's pretty obvious that water lubricates the road. This reduces grip, accounting for much of the added skidding, sliding and pranging that goes on when it rains. The reduction in grip when it's raining - notionally about 10 percent - isn't as bad as it gets. When it rains really heavily, or when the tread face is too worn, the water on the road exceeds the capacity of the tread to pump it away. Instead of the tread gripping wet bitumen, it grips a thin layer of water above the bitumen. It's called aquaplaning, and the grip level might as well be zero. Commonest cause? Driving on excessively worn tyres. It's a scary, potentially deadly experience.

This test is designed to measure the tyres' relative abilities to continue to grip while cornering in a consistently wet environment.

HOW WE DID IT
The biggest problem with measuring wet cornering, or wet anything else, lies in defining 'wet', a condition which spans the gulf between 'damp' and 'flooded'.

In our case, 'wet' meant with the irrigation on Eastern Creek's skid pan set to 'high' and generally about 3mm of water flowing across the circle. Unfortunately, the concrete surface is not dead flat, and features a higher (and therefore drier) sector. The best we could manage, therefore, was consistently inconsistent wetness around the circle - meaning the same conditions for each tyre, but not the same degree of damp at all points on the circle.

This made things interesting for McKay as he negotiated his 50 or 60 laps of the wet skid pan, but what it means for you is, because the geometry of the circle (and all the other testing methodology) remained unchanged between the wet and the dry circle, you can compare directly the wet and dry results.

The only thing we're not completely clear on is the relationship between wet concrete, which we tested, and wet bitumen, which is far more common in the real world.

TOTAL CONTROL?
Keeping everything constant is what we strive for when tyre testing. But how do you know if you're achieving it? How can you be certain the driver's not getting quicker (by learning), or the track's not getting swept a little cleaner with each lap (and therefore becoming grippier)? Or the test car's brakes aren't going off and its performance going downhill?

The answer is a control tyre. It's a tyre that doesn't form part of the scoring process (here we used the tyres the car was supplied with by the manufacturer). You fit it and run it throughout the testing process to make sure it keeps delivering the same numbers - if it does, you've got control. If not, you can either compensate for a trend in the numbers mathematically (it's called 'linear regression' - yawn) or repeat the tests and hope you get control the next time around.

A control tyre is a check and balance system that tells you if all the stuff you hope is controlled is actually behaving itself. Or not.

DRIVE VERSUS DISTRACTION
Debate raged about whether to run ESP on or off for the tests. This wasn't a problem for the non-ESP-equipped Mazda 3. But the VE has it standard across the range and ultimately, we decided that for real-world relevance we'd do the tests with ESP on. Last year, we found that a top-notch driver can be marginally faster with ESP off, but the fact is just about all VEs will be driven with ESP on.

We did let McKay hold the cars in second gear for the cornering and slalom tests, and manually change gear in the hot lap test. These latter concessions were more a means of ensuring driving consistency, rather than introducing the possible variable of auto downshifts. And it affords more throttle-off forward weight transfer, which is useful in both the skid circuit and slalom tests.

? WINNER: 235/45R17 - Goodyear Eagle F1 Asymmetric
What's astounding here isn't that the Goodyear won both wet and dry cornering tests; rather, how little pace it lost on the limit in the wet - just three-hundredths of a second. (The Falken did equally well there, too, albeit at a slightly lower threshold in both tests.)

The field spread out significantly, too, with the Bridgestone wooden-spooning the wet and dry tests, but losing a somewhat more significant half a second in pace when we opened the faucet.

If you want to quantify what happens to cornering capacity in the wet, on concrete, the average lap time for the field in the dry was 10.22 seconds, growing to 10.41 in the wet - that's two percent slower, or four percent less cornering grip.

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