Compounding: The Problem

by Simon Hargreaves

Ever wondered why your rear tyre wears down in the middle but not at the sides? It’s safer that way...

At first glance there’s nothing in common between touring on a Triumph Tiger 800 XC and a 250bhp MotoGP prototype. But there is.

I recently had a 3000-mile ride on a Tiger. Its Bridgestone Battlewings were 4000 miles-old at the start and, as you’d expect, the rear squared-off as the ride went on. 7000 miles isn’t a bad total, and the bike maintained its steering and stability. But eventually it shimmied a bit over white-lines as it rolled across its squared-off centre. And with plenty of tread left on the edges, it seemed a waste of rubber to chuck it away.

So why don’t manufacturers make rear tyres harder in the middle where they wear the most? The answer is, of course, they do – although only one makes a dual compound tyre for adventure bikes which, given they’re used almost exclusively for touring, seems a bit of an oversight.

A tyre compound is a complex compromise that can be simplified into three opposites of wet grip, dry grip and durability. Each is governed by the ratio of silica, polymers and carbon black content respectively (among 20 or so other constituents). Durability can increase by adding more carbon, but that loses wet grip because there’s less silica. Or a high polymer content gives better dry grip but has less carbon and lower durability.

You can’t have both (although you can refine the composition of the constituents, or the manufacturing process, to improve them all). And this is where a dual compound tyre comes in. With a crown roughly twice the ‘hardness’ of the shoulders, it minimises squaring-off.

But you can’t have it too hard. Apart from manufacturing issues, a crown wearing faster than the edges is a lot safer than the other way round – if a hard-wearing centre ended up proud of the shoulders, imagine how dangerous it would feel! You could say squaring-off is, in terms of handling, actually a safety feature.

Like road tyres, MotoGP slicks are a grip/longevity compromise. Racers want lots of grip, but they also have to finish. Preferably, tyres last until just after they cross the line – and that’s not a racing cliché, it’s a genuine balancing act, in the same way bikes sometimes run out of fuel before the flag.

In the days of 500cc two-strokes, a GP slick suffered from short, sudden bursts of wheelspin which, like a burnout, caused localised heat and visible wear on the outside of the tyre. Changing the tyres’ construction solved the issue. But modern, traction-controlled MotoGP slicks spin into, through and out of corners, generating high internal temperatures that wreck the inside of the tyre. Like an over-cooked cake, it might look fine on the outside but on the inside it might be goosed.

So last weekend at Philip Island when Bridgestone found the grippy new surface and high track temperatures overheated their tyres you can’t, whatever the finger-pointing, fault them for demanding a shorter race with pit stops. It was safety first.

A bit like the squared-off, single compound rear tyre on the back of the Tiger.