Dunlop and Michelin are castaways on the Island

WHEN you’re talking about GPs and WSB, Michelin and Dunlop are the only names that matter in tyres. The two firms enjoy a monopoly in world championship racing, and no-one uses anything else.

But when you bring the Isle of Man TT into the equation, possibly the toughest testing ground for tyres in the world, the French and British firms aren’t even in the running. As far as the Island is concerned, Pirelli is No1.

Last year, David Jefferies won the F1, Senior and Production races on the Italian brand. So far this year, Michael Rutter has used Pirellis to clean up at the North-West 200. And both Jefferies and Rutter are using them to good effect on the Island right now. According to their V&M team boss Jack Valentine, nothing else will do. Which begs the question: ” Why? ”

According to Mark Hepworth, Pirelli’s man on the Island this year, it’s down to stability. He said: ” If you took the three top racing slicks, the Michelins would have the most grip, the Dunlops would be the most forgiving and the Pirellis would be the most stable. That’s the feedback we get from racers. ”

He believes that stability comes from Pirelli’s unique, patented steel belt construction. He said: ” The construction is more important for road riding than outright grip. If the construction is right the tyre can hold its shape and that’s vital when you’re going fast on the Island. ”

The problem with rival slicks is that they are built purely for short circuit racing. They are as light as possible, they only have to last 50 miles or so and the riders are on full throttle and running at top speed for only a fraction of a lap at a time. No short circuit on earth is like the Isle of Man. Nowhere are race bikes held flat-out for so long on what are nothing more than public roads.

The enormous centrifugal forces combined with the heat generated by running across Tarmac at 190mph make tyres change shape, and this change is more pronounced with the softer compounds used on short circuits.

Subjected to this sort of abuse, the rolling radius of a tyre increases, which can dramatically affect the way a bike handles. Not good news when you’re riding at 190mph with a hedge to your left and a dry stone wall to your right.

If you used a soft Michelin or Dunlop slick under these conditions the sidewalls would flex and the profile get taller and narrower, which has several negative effects:

n It reduces the size of the contact patch, making the bike more unstable at speed and reducing grip under braking

n And a narrow profile makes the bike more sensitive to surface undulations, of which there are more than a few on the Island.

Pirelli’s steel belt construction doesn’t suffer these problems because the stiffer sidewalls retain most of the tyre’s original profile, even under severe stress. The firm even goes as far as saying that their construction reduces the tyre growth almost to zero.

Hepworth said: ” A huge amount of research has gone into this, and while there’s still a small change with our tyres when the going gets hot, it’s so small it’s almost nothing. ” There’s also a more indirect pitfall which this aspect of the Pirellis can overcome, and that’s the effect that a tyre’s changing shape can have on a TT bike’s finely-tuned suspension.

Hepworth says: ” Imagine if you’ve put all that effort into getting the right steering angle and ride height and then your tyres change shape and get bigger. It can mess everything up. ”

Of course, all this high-speed performance doesn’t come without cost – and that’s less outright grip. Hepworth admits it’s a typical reaction for a first-time Pirelli user to complain about a lack of grip while praising the stability.

That is backed up by the experience of Vimto Honda rider John McGuinness, who has switched from using Dunlops at the NW200 to Pirellis at the TT in a bid to make his SP-1 more rideable. In Ulster last month he could only run up to fourth gear along the straights before the bike became so unstable he had to back off.

But on the Island last week, in his first time out on the Pirellis, he was able to go flat-out down Sulby Straight, though it took him a few miles to trust the grip.

You could easily argue that Pirelli is only dominant on the Island because Michelin and Dunlop have concentrated all their development work on short circuit slicks, and that if they tried, they could easily make a TT tyre to match, or better that from their rival.

But that doesn’t bother Pirelli, which uses its experiences from the TT to develop road-based rubber. Hepworth said: ” A slick that lasts 220 miles around the Island is a tyre that’s relevant to road riders, our customers. Winning here is worth more to us in terms of advertising than any number of short circuit wins. ”