Michelin Pilot Street Radial Motorcycle Tyres

Brilliant biking inventions…Radial Tyres

Some want the best money can buy: others will swop round old, bald remoulds and not care a jot… but want to know what a great leap forward radial tyres were? Then read on…

Tyres: they’re just black and round, right? Well, yes… but also ‘no.’

For bikes, one of the biggest jumps forward in tyre technology was the introduction of the radial tyre. But what is it and how was it developed?

Well, as is usual, such advances came from racing. Freddie Spencer in fact was the man who is looked at as the one who helped develop the radial tyre for Michelin in the early to mid-1980s.

But first let’s look at the humble ‘cross ply’ tyre. These were all fine, but a cross ply tyre was/is heavy and that means they can really affect a motorcycle’s handling. Look at Suzuki’s race bikes of the 1970s, especially the big-bore RG500s… Legendary spanner man for Barry Sheene himself – Martyn Ogborne – reckoned that front and rear-end chatter came directly from the weight of the tyres themselves: using air-assisted forks did mask some of this, but would never fully eradicate it.

As power levels and speed increased towards the end of the 1970s and into the 1980s the issues grew. And so did the tyres, because at high speed the diameter of cross ply tyres grows due to the huge centrifugal forces exerted on them. Tyre experts don’t like this, as it means the tyres themselves will run too hot and wear prematurely and this affects both handling and stability as well as consistency. What you want is a tyre that works predictably across a wide-range of speeds and conditions. By the early 1980s, the limit with power and speed and chassis demands was being reached thanks to the tyres on the race bikes themselves.

The writing was on the (tyre) wall for the old cross ply by 1983, for it was in this year that Freddie Spencer taking the first radial tyre victory in 1983, with a Honda NSR500 fitted with a rear Michelin radial only: a year later Randy Mamola would take the first win on both front and rear radials. Some years ago, Freddie told Two Wheels Only magazine: “Radials just seemed to give me more feedback and grip. It was a massive leap forward, but it was hard work! If you look at my double year of 1985, not only was I developing both the 250 and 500cc Hondas, we were developing the Michelin tyres. Sometimes we would have something like 200 tyres to test and what we would pick would determine the direction of development. It was a busy time for us.”

Michelin (and Freddie’s) hard work would eventually pay dividends not just on track, but on the road too, because Michelin’s first radial road tyres came out in 1987 with the AM59s, remember those?

So what are the differences between ‘cross ply’ tyres and ‘radial’ tyres?

Well, cross ply literally means that their carcass plies criss-cross each other, making it inherently stiffer. Instead, a radial tyre has plies running radially, parallel to the sidewalls and the beads themselves. These radial plies do not cross each other.

The cross ply tyre offers a more rigid structure compared to a radial, so it doesn’t flex as much. It also has a stiffer sidewall and therefore doesn’t mould itself to the road surface like a radial does, so during use it generates that much more heat in its construction and has a limit to the grip it can provide and it will wear that much quicker. With the more flexible construction of a radial, the tyre in actual contact with the Tarmac wants to move, rather than the whole, tyre itself, so there’s less movement around the wheel rim itself.

Overall then, radials are lighter than cross plies and this basically helps both quicken the steering of a motorcycle and puts less stress through the suspension.

Radial tyre design has been constantly developing since the late 1980s, in both carcass construction and compound, such as the introduction of silica in the rubber to help speed-up warm-up times for tyres as well as developing new tread patterns and ‘sipes.’ In carcass construction, the aim has been to keep the tyre footprint stable and consistent to prevent the compound itself from over-heating – an issue the older cross ply had.

So it was game over for the humble cross ply then? Nah, of course not! Some bikes – like our older classics – work well on cross plies as they don’t generate the speed/heat/stress that (say) a sports bike of the last 20 or so years generates. Also, some bigger, heavier machines – such as tourers – can actually work just as well on the heavier, sturdy cross plies.