They may be round and black, but there's a lot more to your motorcycle's tyres than meets the eye. Here's all you need to know to choose the best rubber for the job
COMPOUND, OR TREAD IF YOU LIKE
If a tyre were made from pure rubber it would wear quicker than a chocolate condom and would never take the required weight. Instead, the 'rubber' (often a synthetic equivalent) is mixed with carbon black to make it tough and resilient, then baked at high temperatures and mixed with up to 60 chemicals. Varying quantities of these chemicals determine the compound's softness and its optimum operating temperature.
The tyre grips by pushing itself against the surface so the softer the compound, the more it will grip. The softer compound will also be more abraded by the road and wear faster, as well as generate more heat by flexing more.
But regardless of compound, there's a temperature at which tyres operate best. Unless a tyre reaches its optimum temperature the compound won't soften enough to provide the intended grip - hence the use of the chemical Silica in road compounds to ensure a certain level of grip in cold, damp conditions and why it is important to warm up your tyres. And there is also a temperature at which all tyres will overheat: after construction a tyre is cured in an oven at a certain temperature for everything to stick together. If the tyre goes over that same temperature again for too long, it will un-cure - at first it squirms and loses traction as it breaks down chemically, then it delaminates as it breaks down physically. So each tyre is carefully tailored to suit its intended purpose.
Take a track-orientated tyre. The compound is designed for plenty of grip at constant, high temperatures reached by the extreme pace maintained on a track - hard acceleration, hard braking and high corner speeds. And because it is designed to live at high temperatures the race compound will also take longer to reach its peak. Use these tyres on the road and even if you ride like your nuts are on fire you'll be forced to slow down for traffic and Gatsos, and every time you stop the tyres will cool off and take ages to muster half their required temperature again. Keep this up and the tyres will wear fast and shed rubber through cold tearing - so they may look like they're being used hard, but in fact they're disintegrating from misuse.
Then, there are heat cycles. Each time a tyre goes from hot to cold it's re-curing itself to become harder, as chemical oils used in the tread to maintain compound are released (hence the blue colour you sometimes see on a tyre's tread after hard use). While this is minimal on a road tyre, it becomes more extreme on race compounds. Specialised race tyres are designed to go through only one heat cycle before compound deteriorates, while the track-focused Metzeler Racetecs and similar are a lot more sensitive to heat cycles than their road equivalent. Hence tyre warmers not only bring tyres up to their required temperature, they also maintain a constant temperature between races or sessions to minimise heat cycles.
Meanwhile, the road-focused Metzeler Sportecs and the likes are good for all the heat cycles you need for 50-odd Sunday runs.
The carcass gives the tyre its required strength (it's more resilient on a road tyre, and how much the carcass lets the compound flex affects heat generation) and its rounded shape. But because the tyre's contact patch is completely flat the tyre has to compress and distort where it meets the ground. This shape changing means some of the rubber has to slide across the road to achieve the new shape, causing wear, while the constant flexing of compound and carcass at this point generates heat.
The old-style cross-ply tyres used many layers of plies moulded at an angle to give the tyre strength. But the sheer amount of material used made them heavy and generate a lot of heat, so harder compounds had to be used to maintain the right temperature.
But as bikes became faster, lighter and more agile, tyres had to follow suit. Bias-belted tyres appeared as a step on the path to radial heaven, and they're still in use on big heavy bikes where sidewall stiffness is more important. But radials broke new ground thanks to the clever layering of fabric (see cut-out caption). A radial tyre is not only lighter and more responsive, it also runs cooler as the tyre distorts more easily. And running cooler means you can use a softer tread for better grip with no increase in wear.The shape of the carcass's crown radius also dictates the way a tyre handles, which together with the sidewall determines profile.
On a 120/70-17 front tyre, 17 is the diameter of the wheel rim in inches; 120 is the width of the tyre in millimetres, and 70 is the percentage height of the sidewall against the tyre's width - so the sidewall is 84mm tall. The higher the sidewall, the more rounded the tyre, which results in slower steering but good stability. Lowering the sidewall by 10mm to a 120/60-17 is equivalent to dropping the forks through the yokes, and the tyre also acquires a steeper profile - it is more 'triangulated'. The results are quicker steering and more grip when leant over, but anywhere in-between - straight line and transition from upright to lean angle - is less stable. The carcass's crown radius also shapes the profile, hence Metzeler's Racetec has a more pointy shape compared to the more stable Sportec road tyre.
The sidewall also acts as suspension for the tyre and comes in varying degrees of stiffness: a big heavy tourer for instance needs the thick bead filler found in the sidewall of touring tyres for added strength and stability. By reducing the height of the sidewall, the tyre is less capable of absorbing surface irregularities and tends to hop when cranked over, causing the bike to understeer. A race chassis with high quality suspension copes well with a quick-steering 120/60 or a racing slick's more radical profile and flexible sidewall, but fit racing slicks to a road chassis and the bike becomes unsettled. As a result road bikes and Superstock racers generally opt for the more suitable 120/70 front tyre, and Yamaha switched to a 120/70 front profile on its 2005 R6. Last year Michelin launched its Pilot Track Day Slicks with a road profile to suit production bikes.
Rear tyre size also affects performance: a 180 section will steer quicker, while a 190 will last longer by coping better with the power battering. On a superbike the 190 trade-off is only worth it if you're running a WSB-tuned machine at race pace over 30 laps...
A road tyre will kick the race tyre's butt in terms of acceleration, cornering speed, braking, durability and stability over the course of a road journey. Meanwhile, if you're a bit of a racing god the latest sporty offering may not be up to scratch on a circuit. So go on, be honest with yourself about the tyres you need.
RADIAL TYRES - THE MAKE-UP
A Tyre's compound (aka the tread) is moulded onto the carcass. On a radial tyre (below) the carcass is made of two plies with strands usually of steel or aramid (that's Kevlar): the first is a radial ply that runs at 90° to the tyre's rotation (note how it is folded under the steel bead), while the second runs in the direction of the tyre's rotation to minimise expansion at speed. The top two plies are cross-plies (with usually Nylon strands) placed at an angle to add strength. The bead that you'll find on all types of tyres holds the tyre to the wheel rim with a 'rubber' bead filler to strengthen the sidewall. This is why tyres are tough bastards
• Low pressures cause tyres to move around and generate heat, while high pressures will reduce the contact patch and the tyre will struggle to warm up. Go with the manufacturer's recommendations for road pressures, but if you're using your tyres on a racetrack lower them a few psi depending on track temperature and pace. Always check your pressures from cold.
• New tyres need careful scrubbing in for around 200 miles to get rid of the slippery mould release agent used at the end of the production process.
• A slick's uninterrupted compound (or tread) optimises grip, but it's unable to clear standing water and debris. Hence a pattern is moulded into a road tyre's tread. A tread pattern also helps generate heat by flexing ('block movement') and is usually no more than 5mm deep to prevent weave.
• Race tyres are available in different compounds and mix-matching can provide an ideal compromise between grip and longevity. A softer tread is used on the front for better grip while the rear gets a harder compound to cope with the power battering. The front is also sometimes fitted with a flatter, more stable profile while the rear can be more triangular for quick steering. But manufacturers do all the work for you on road tyres so don't mix-match.
• Specialised race TYRES get different compounds on either side of the same tyre, so where a track has a predominance of right-hand corners the compound will be harder on the right but softer on the left. Similarly, Bridgestone's dual-compound road tyres get softer edges for good corner grip with a harder middle to stop tyres squaring off with motorway miles. Michelin has just launched its dual-compound Power Race tyres.
Posted: 05/02/2008 at 15:34
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