In their regular column in Fast Bikes magazine, this month our friends at California Superbike School UK have been taking an in depth look at the science behind traction.
Traction is a bi-product of the chemical’s used in your tyre’s compound plus a number of other factors. These include the construction and shape of the tyre’s carcass which is used to create a contact patch, plus it’s tread depth, temperature and pressure.
If you were to mount a tyre to a wheel, then heat it to it’s optimum operating temperature and apply ever-increasing pressure to it at an angle of 45 degrees, it will eventually slip. The load being placed on the tyre at this exact slipping point represents 101% of the tyre’s static-grip limit.
However, achieving maximum traction while moving is quite different altogether. As your tyre grips, all the combined chemicals, oils and waxes that bind the rubber together will start to wear.
The combination of abrasion and heat essentially ‘cook’ the tyre compound. Track day enthusiasts and racers will no doubt have seen a tyre that has turned a bluish-purple. Technically known as ‘blooming’ it is caused when extreme heat leaches chemicals out of the tyre, forming a residue on the surface. This can usually be worn off within a lap or two.
When these residues appear, a tyre can become dry and slippery. The dry layer needs to be scrubbed away to expose the fresh rubber underneath, and this requires abrasion. This is provided by tyre slippage.
Research by tyre engineers suggests that around 15% longitudinal slippage is enough to clean the tyres during usage to maintain optimum grip and temperature. Slippage isn’t the same as a slide – your bike continues to hold it’s line, it’s just that it helps your tyre to uncover the fresh rubber underneath that dry surface layer.
With no power going through the front wheel, your front tyre relies on three factors to uncover that fresh rubber – slip angle, the friction generated from side-grip and abrasion during braking.
Motorcycle’s tend to go in a straight line unless influenced by an outside force. Turning-in the front wheel creates that influence. It creates abrasion resistance which forces your motorcycle to enter a corner and hold it’s line. At this point your tyres are also slipping sideways to the outside of the corner – that’s known as the slip angle.
If you were able to freeze frame mid-corner and maintain that exact same lean angle and turn-in angle of the front wheel, then jump off the bike and push it, you’d find that your line would be much tighter through the corner than if riding at speed.
Camber force also plays it’s part in the traction story. When leaned over, the outside of the contact patch (closest to the sidewall) is on a tighter radius than the side closest to the centre-line of the tyre. If you imagine a roundabout at a playground, somebody standing on the outside edge travels a greater distance per rotation than someone standing closer to the centre, in the exact same amount of time.
Apply this to the contact patch on your motorcycle tyres, and what it means is that the edge closest to the inside of the corner is turning slower than the opposite edge of the contact patch and is in effect dragging. This creates rubber cleansing abrasion, and also helps the bike to stay on its line.
When taking any corner at a speed which is sufficient enough to keep your motorcycle upright and balanced, your tyres are always slipping, however slightly.
If they didn’t slip, you simply wouldn’t get through corners.
For more great riding tips from California Superbike School UK, check out the very latest copy of Fast Bikes magazine.
Find out more about the ContiSportAttack 3 – as used by the school on their Ducati fleet for both instructors and students.