Tyre test 2008: Test 5 - Dry braking

Wheels Tyre Test 2008: Tyre Test 5 - Dry braking

Wheels Magazine
June, 2008

WHAT WE MEASURED
Over the past decade, the car industry has done a top-notch job engineering the potential for human error out of the emergency braking process, as well as increasing the performance of brake systems generally.

Take ABS and EBD. These are off-the-shelf technologies offered on just about every new car on the market. ABS allows steering under full emergency brake application, while EBD - electronic brakeforce distribution - means the rear brakes are always contributing as much as possible to the braking process (something that was really never achieved with the old proportioning valve system that EBD made obsolete).

Newer technologies like EBA (emergency brake assist) that are just trickling through to the cars average people can afford, mean drivers who don't hit the brake pedal hard enough will still get a full-tilt emergency stop thanks to a smart computer that can infer an emergency stop is warranted, and pump up the required hydraulic pressure in the brake lines to achieve one.

One of the few variables that remains - after the driver perceives the problem and belts the brakes - is the interaction between the tyre and the road. That's what this test is all about.

HOW WE DID IT
At 85km/h - measured by GPS, not the less accurate speedo - the driver hits the anchors. The Vbox starts recording as the car descends through 80km/h, and thanks to some complex (but these days, mundane) arithmetic derived from a bunch of GPS Navstar satellites owned by the US Department of Defense and orbiting 20,200km aloft, stopping distance can be determined down to about 10cm.

No special skill is required, unless you count not running into the concrete barrier at the far end of the skidpan, where space (or more specifically, length) was at a premium. Something that was more probable in the Mazda, seeing as it took longer to get to 80 kays and, curiously, given its lighter weight, also took slightly longer to pull up than the Commodore. The procedure is simple - 85km/h, hit the brakes hard, wait for the scenery to stop. Also required is an ironclad constitution for the Vbox operator as the experience is a deadset motion sickness-inducer.

We did as many runs as were required to get two mutually consistent ones - meaning within half a metre of one another. Once we got there, those two were averaged to deliver the final result. Three or four runs were generally required to get two that were consistent.

FACT OR FRICTION?
There's no difference between grip and friction. They're the same thing - a measure of the sliding resistance between materials in contact with one another.

The so-called 'coefficient of friction' touted conversationally by the propeller heads is just the ratio of the load acting on anything compared with the sideways shove required to move it. For steel on steel it's about 0.2 - meaning a 20kg shove to get a 100kg steel block sliding on a smooth steel floor.

For rubber tyre on dry concrete it's about 0.9 - meaning 0.9G worth of deceleration before the wheels lock under brakes, or before you slide sideways in cornering. In fact, the best tyre under brakes managed an average of 0.94G for its 80km/h stop, while the worst managed 0.85G. In the wet the equivalent best and worst were 0.90 and 0.76G.

SHELF LIFE
Among the hieroglyphics on a tyre's sidewall is a secret 12-digit manufacturer's alphanumeric code. You don't need to crack it all, but the last four digits can come in mighty handy. If it says '...1606', for example, that means the tyre was manufactured in the 16th week of 2006. For the 27th week of 2007 you'll see it ends in '...2707'. Anything older than three years (less than, say, '...2605') should sound warning bells if it's still on a retailer's shelf.

Consumer empowerment time: ask to see the new tyres you're about to buy - before they're fitted. Check the codes, and don't buy anything older than three years.

☆ WINNER: 195/60R15 - Pirelli P6000
Hats off to the Pirelli P6000 for a great result that saw it beat four of the seven 235/45R17 performance tyres - including, perversely, its stablemate the more performance-oriented Pirelli Dragon, by more than one metre. The ignominy continues: every one of the diminutive 195/60R15s came in ahead of the 235/45R17 Sumitomo HTRZ II that pulled up in seventh spot among the 235/45s. Even more perversely, the 195 Sumitomo pulled out a better stop with the Mazda (almost two metres better) than the 235-section Sumitomo on the Commodore. And the 'bigger, heavier' argument here won't wash - because ultimately the best stopper on the day was the Commodore, by about 0.3 metres.

☆ WINNER: 235/45R17 - Goodyear Eagle F1 Asymmetric
It's getting monotonous, isn't it? The Goodyear Eagle F1 Asymmetric, followed by the Falken FK 452, separated by a poofteenth. This duo has dominated right through the tyre test so far. And not a bad result for the Dunlop SP Sport Maxx, either. Then the gulf to the back markers begins. Nine percent, or almost three metres, was the spread back to the Sumitomo HTRZ II in seventh spot. That's more than half a car length, which could easily be the difference between a near miss and a prang, with the associated insurance claim and weeks off the road. Almost 0.1G separates the Goodyear from the Sumitomo in average deceleration.

Read more on the Wheels tyre test here:

Intro:  Let's roll ...	Test 4: Wet slalom ...
Test 1: Dry cornering ...	Test 6: Wet braking ...
Test 2: Wet slalom ...	Test 7: Hot lap & summary ...
Test 3: Dry slalom ...

To comment on this article click here