Re: Winter tyres
Posted: 8 Feb 2017, 4:39pm
Threevok wrote:Compound makes no difference on ice …
If the ice is anything but perfectly smooth, this is not true.
Tyres have two sources of grip: molecular adhesion and hysteresis grip. Molecular adhesion requires direct contact between tyre and road, so is vastly reduced in wet conditions (and ice at British temperatures usually includes a thin layer of water, even if only momentarily under the pressure at the contact patch).
Hysteresis grip comes from the rubber not rebounding with as much energy as was absorbed when it distorted around surface asperities (road roughness). This is the main source of traction in wet or icy conditions (or on diesel spills for that matter), but it depends on the surface having appreciable roughness. Wet manhole covers can be deadly because they are smooth, robbing the tyre of most of its only significant source of traction in the wet. However, even manhole covers usually have moulded features to offer some hysteresis grip, so tyres make a difference even there. That manhole covers don’t have many more, much smaller features is a failure of design.
Traditionally, tyre compounds used carbon black as a filler. This greatly improved hysteresis grip (i.e. wet grip) along with other desirable characteristics. However, increased hysteresis came with increased rolling resistance, which we obviously didn’t want on bicycles.
To alleviate this unhappy compromise, tyre makers have recently (popularly in the last decade or so) replaced carbon black with silica. Silica compounds have the fascinating property of variable hysteresis with stress frequency. They can be made to have low hysteresis at low frequencies (corresponding to rolling deformation) and high hysteresis at high frequencies (corresponding to the tyre sliding over small asperities). This makes possible low rolling resistance and high wet grip in the same tyre.
But it is still not magic. The hardness of the tyre also matters insomuch that optimum hysteresis occurs around the so-called glass transition temperature (at which the rubber changes nature from brittle to flexible). If the compound is used in temperatures far below or above the glass transition temperature, hysteresis grip is dramatically reduced. Therefore you need to choose a tyre for the prevailing temperature of operation.
Winter tyres therefore need to have high hysteresis (preferably only in the high-frequency domain) and a low-temperature compound.
For narrow tyres, three seem outstanding in these regards:
- Michelin Pro4 Grip (only available in 23 mm)
- Vredestein Fortezza Senso Xtreme Weather
- Continental Grand Prix 4-Season
There may be other suitable tyres too, but manufacturers hide their tyre properties beneath reams of useless marketing terms (e.g. that “4-Season” moniker above), so getting good information is hard.