Tech Talk: Know your wet tyres
Q: What constitutes a proper, full on, wet weather tyre?
A: It’s actually two things. Firstly, it’s the grooves; they’re designed both to evacuate as much water as possible, but dimensioned to ensure the largest, widest possible footprint. Secondly is the compound formulation: this is 100 per cent silica featuring a recipe with extreme low-temperature conformability to Tarmac micro-characteristics, which ultimately gives that incredible amount of grip when conditions aren’t ideal – so they don’t need the same amount of heat as a dry tyre to be effective.
Q: Are they road legal?
A: All Pirelli rain tyres are marked with an NHS symbol which means ‘not for highway service’, so they can’t be legally used on the road. This is because their operating range is too narrow to ensure duration in diverse road conditions – they’re developed to be used in the wet and cold, and without water they would almost instantaneously wear out… especially if the asphalt is dry and hot.
Q: How do they work and create grip?
A: They actually work in two ways. The tyres have a way of conforming to asphalt micro-asperities (which is basically any roughness or uneven part of the tarmac), whilst chemically bonding with the water to ensure that there’s contact. Obviously they wouldn’t work through a river, but through standing water, it’s surprising how much contact is offered with the Tarmac as the tyres disperse the water as well.
Q: Will they work in the dry?
A: Although they will indeed work in the dry, they have an extremely narrow operating window for ideal use. So, during a race when a dry line is beginning to appear on the circuit, wet tyres will begin to decay due to the rubber tearing, but they won’t just stop working. This is why you’ll see riders aiming for puddles off-line when using wet tyres on a dry track.
Q: Are there different types of wets for different tracks?
A: For 2019 there are, as Pirelli have two different types of rain tyre to choose from. First up is the Diablo Rain SCR1, which is designed for use at less abrasive circuits or when the circuit temperature is less than 15°C.
The second is the SCR2, which is suitable for warmer conditions (track conditions above 15°C) and abrasive circuits – so for use at somewhere like Thruxton, for example. SCR2 is the most used solution if the track is beginning to dry out as well, as it’s longevity in tougher conditions is better.
Q: How do they work best?
A: They’re no different to a regular tyre in a way! It’s all about having the right pressure and the right compound according to the operating conditions. If you’ve got this, alongside the correct suspension and bike geometry, you should be all good to go.
Q: What technology do they use?
A: There’s a lot that goes into these tyres! The Diablo Rain models are developed with dedicated carcass specifications and dedicated compounds, to give the ultimate grip available. All of this info and tech has been developed and tested within World Superbikes.
Q: How much water do they disperse?
A: It’s difficult to pinpoint an exact figure as there are many factors, but a purely theoretical calculation gives an estimation of eight to ten litres per second at 200kmph for a rear tyre. Obviously this is just a standard mathematical exercise, and other factors like the actual water layer thickness, Tarmac, etc., will alter this performance. It’s still a lot!
Q: Are they incredibly soft?
A: Yes, incredibly! The rain tyres are among the softest compound you may find for racing use.
Q: Should you use warmers on wets?
A: We get asked this one a lot. Tyre warmers should be needed just to ensure that tyre temperatures are kept close to operating temperature, which is around the 40-60°C mark. That said, rain tyres are capable of featuring an immediate warm-up thanks to the soft compound. For this reason they are often used without tyre warmers. If this is being done though, it has to be considered that on a cold tyre the set up pressure needs to be -0.1 to -0.2 bar lower than what it would be with the ideal operating temperature.
Q: What about correct pressures?
A: As for all tyres, air pressure is crucial, whether in the wet or the dry. It varies with different factors, so always consult your tyre technician beforehand. As a benchmark, the ideal operating pressure is 2.0bar, which is about 29PSI for those of you who are wondering.…
Q: How do you know if a set of wet tyres is trashed?
A: Rain tyres are designed to last one full race. When operating conditions allow it, we may consider using an SCR2 twice, but this doesn’t happen often – it’s better not to if it can be helped. Generally, when rain tyres are at the end of their life cycle, a significant drop of performance is easily perceivable, so just be prepared.