Why half an inch matters

TYRE size is one of the burning issues in 500 GPs this year. If you’ve watched any of the recent rounds on TV you’ll have heard commentators going on about 16.5 and

17-inch rear wheels. So what’s the difference and which is best?

Aussie Garry McCoy was using a 16.5 when he took his Red Bull Yamaha to victory in the opening round in South Africa with a breathtaking display of rear-wheel sliding.

But after the French GP at Le Mans, the argument over what rear tyre size is the one to go for was too close to call. Alex Criville’s success in France made it three wins for the

17-inchers, while McCoy’s win at Welkom and Norick Abe’s in Japan make it two for the 16.5s.

Michelin first introduced its original 17in tyre back in 1984 – at a time when most teams ran 18in wheels. The 16.5s didn’t appear until the early ’90s. 1993 world champion Kevin Schwantz was one of the first converts and became the first rider to win using a 16.5in tyre on his Lucky Strike Suzuki at the 1994 British GP at Donington Park.

The smaller tyre was developed because riders wanted more grip to cope with ever-spiralling 500 power outputs. Tyre technology just couldn’t keep up with engine developments and riders were constantly highsiding. The results were frequently painful as tyres slid when riders powered out of corners and then suddenly snapped back into line – launching their victim over the top of the bike.

Michelin’s initial answer was to increase the tyre-to-Tarmac contact patch with a 19cm wide 17in tyre. One of these was used to good effect by Australian Daryl Beattie to win the 1993 German GP. But this wider tyre was heavier and technicians had to make radical changes to the suspension set-up.

Going for a smaller diameter tyre allowed Michelin to keep the wider contact patch and keep weight at previous levels. The 16.5’s profile is radically different to the 17’s with a triangular shape that puts more rubber on the track when the bike is cranked over on its side.

Michelin’s motorcycle competition chief Nicolas Goubert explains: ” The 16.5s allow increased corner speeds. The larger contact patch also keeps temperatures more consistent, so riders can generally use a softer compound. ”

So if 16.5s are that good why isn’t everyone using them?

Goubert has the answer: ” The 17 offers better driving traction once the rider has lifted the bike back up and opened the throttle. Most riders still prefer this characteristic. ”

And while 16.5s offer more grip in corners, it’s the 17s which turn in better because the tyres are narrower.

” There are still some aspects of the 16.5 on which we need to work, ” Goubert admitted. ” The extra rear-end grip can cause the front tyre to push, resulting in understeer. And the 16.5 can cause front-end chatter. ”

This means that on tight and twisty tracks the 17 tends to be a more popular choice, while on fast, sweeping circuits the 16.5 becomes more common.

Red Bull Yamaha’s McCoy is the only full-time user of the 16.5, which he finds perfect for his wild, sideways style. He told MCN: ” I find the 16.5 has more grip mid-corner, which is where I like to get on the power. The 17 is flatter on the top of the tyre, so there’s not so much rubber on the ground when I’m cranked right over. ”

But current series leader Kenny Roberts disagrees and uses a 17in tyre on his Suzuki RGV 500.

” The 16.5 rubber has better grip in places, ” he said. ” But I still believe the 17 is a faster tyre. ”

So there’s no hard and fast rule. Whether a rider picks a 16.5 or a 17 is down to a combination of their style and the kind of track they are racing on that weekend.