Tyre test 2008: Test 1 - Dry cornering

Wheels Tyre Test 2008: Tyre Test 1 - Dry cornering

Wheels Magazine
June, 2008

WHAT WE MEASURED
Outright cornering potential is probably the most important thing we measure across both days of the Wheels tyre test. People who like driving get off on cornering because there's such a delicate balancing act between speed and grip near the limit. Those who are bad (read: twitchy) passengers hate cornering for the same reason. This fine line is very easy to step over, because small changes in speed make big changes to cornering loads.

Here are some convenient rules of thumb about playing the cornering game: add 10 percent to your speed; you'll get 21 percent more cornering load (sideways shove, in other words). Add 20 percent to speed, get 44 percent more lateral load. Add 30 percent, get almost 70 percent more.

The challenge with testing cornering capability lies in keeping everything constant - the vehicle's path, for example, and the track surface - except, of course, the cornering speed, and then keeping the car on the limit for long enough to take meaningful measurements.

HOW WE DID IT
Take one smooth concrete skidpan and mark out two concentric circles with witches hats, one 40 metres in diameter and the other 48 metres. This might seem like rather a loose fit, but it isn't if you're the one driving. The theory is the car drives around the centre of the lane, on a 22-metre radius. (This is, of course, an assumption, but with the same car and the same driver doing umpteen laps, it's a safe bet and a consistent treatment across the field.)

Cornering load is a function of only radius and speed. For benchmarking, a car doing 53km/h around our impromptu roundabout is cornering at one G. The theoretical lap time for that is 9.4 seconds.

The remaining variable - the track surface - is controlled because the cones mean we always use the same portion of track.

Lap time is measured electronically using a cross-track infrared beam with a receiver suction-cupped to the car. It records lap times down to one one-hundredth of a second - that's around 0.1 percent of the duration of each lap.

McKay's brief? Go out there and keep lapping until you're confident you have it nailed. With six or eight laps in the can, we extracted the best one.

GETTING BUFF
It's a bad idea to thrash brand-new tyres right out of the blocks. New tyres are coated in residual mould-release compound, which assists their extraction from the moulds used to form them in the manufacturing plant. In this situation, being slippery is an asset; on the road it's a liability, however. As are the 'vent pips', the rubber stubble-like projections commonly found on new tyres.

Best to buff them all off by driving sedately for 30km or so. We didn't have the luxury of time allowing 14 different Miss Daisy-style drives in the country, so we settled instead for six highly abrasive have-a-fang laps on the skid circuit instead. You can be confident our results reflect the actual grip offered by the tread compound, not the relative slipperiness of each company's mould-release compounds.

☆ WINNERS: 195/60R15 - Pirelli P6000/Goodyear Excellence
The 195 field was very tightly packed here, with just 1.6 percent (in cornering speed) separating the Pirelli and Goodyear in equal first from the Accelera in last place. Equate that to cornering load and the difference is greater, at a smidge over three percent, but still not a real big deal.

Conclusion? They're all pretty equal in this event - and because the circles are the same for both tyre sizes, there's a fair indication of the benefits you get when you run a performance tyre - about five percent more cornering load. Call it a safety margin.

☆ WINNER: 235/45R17 - Goodyear Eagle F1 Asymmetric
Goodyear's Eagle F1 Asymmetric took the chequered flag here, with the field spread over just 0.31 seconds - call it three percent. That doesn't sound like much, but it's worth remembering that lap time here relates directly to cornering speed. This is so because speed is distance divided by lap time, and the distance - our circle's circumference - is fixed. There's a relationship between speed and cornering load, too. And our measured three percent variation in cornering speed across the field equates to a six percent difference in cornering load - a good enough reason to buy the Goodyear over the Bridgestone if outright cornering capability is a high priority.

Intro:  Let's roll ...	Test 5: Dry braking ...
Test 2: Wet cornering ...	Test 6: Wet braking ...
Test 3: Dry slalom ...	Test 7: Hot lap & summary ...
Test 4: Wet slalom ...

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