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by Matt Wikstrom
August 19, 2014
TECH NEWS BROUGHT TO YOU BY BIKEEXCHANGE
Our intuition tells us that tyres with a patterned tread are less likely to be slippery in wet conditions, but is that really the case? In this article, CTech Editor Matt Wikstrom looks at the factors that influence the grip of road tyres.
A quick survey of road tyres reveals that some companies have a preference for tread-patterns, others eschew them, and the remainder swing both ways. Race-oriented tyres tend to favour a slick tread while training tyres and general-purpose tyres are more likely to feature a tread-pattern.
A tread-pattern, no matter how subtle, promises more grip than a slick tyre since it increases the surface area of the tyre and allows the tyre to deform so as to maintain traction. This notion is certainly true for off-road use, but on the road it is much less relevant.
Back in 1997 Jobst Brandt wrote an article on the grip of road tyres that dismissed the importance of tread altogether. His primary argument was that when a surface is harder than a tyre, then the tread cannot leave an impression, and therefore is unable to improve the tyre’s grip.
Ultimately, the grip of a road tyre depends upon the size and quality of its contact patch. There is no need for the tread to deform when the entire tyre deforms as it comes into contact with the road. That is why tyre pressure, thread count, rubber compounds and rim/tyre width are far more important than the pattern in the tread.
The air pressure used for inflation dictates the behaviour of a tyre. All cyclists are well aware that low pressures make for slow tyres as the wheels sink into the road (and predispose clinchers to pinch-flats). As a consequence, there is a tendency for over-inflation, and while there is evidence that rolling resistance decreases with higher pressures, the tyre is less able to deform, thereby reducing grip on the road.
It is difficult to prescribe the ideal air pressure for tyres. Every tyre is labelled with a range of operating air pressures but shouldn’t be mistaken for directions. The decision on inflating pressure is left in the hands of the rider.
Some believe that that the ideal air pressure allows the tyre to compress 15% under the weight of the rider while Vittoria has developed an app that also considers riding conditions. There is no substitute for experience though and I’d encourage every rider to experiment with different air pressures, remembering that 5-10psi can make a difference to the behaviour of a tyre.
A supple tyre casing assists the grip of a tyre because it readily deforms and by rebounding quickly, rolling resistance is also kept to a minimum. The suppleness of a tyre casing is generally proportional to its thread count (expressed as threads per inch, tpi) since the fibres become finer as the thread count increases. Fine threads are more flexible however a high thread count doesn’t guarantee a supple tyre.
The type of rubber used for the tread has a profound influence on the tyre, regardless of whether it is patterned or not. The ideal rubber compound preserves the casing’s suppleness while being resilient enough to resist wear and punctures. Unfortunately, increased resilience comes at the expense of weight, suppleness and rolling resistance.
There is some science to the formulation of road tyre rubber compounds but it remains one of the arts of tyre manufacturing.
There is a good reason for the recent trend for wider tyres (i.e. 25mm) amongst the pros: wider tyres require lower air pressures and therefore offer more comfort, a larger contact patch and hence more grip, and paradoxically, lower rolling resistance. The only consideration when deciding on their use is to make sure there is enough clearance for the wider tyres in the frame and forks.
The width of an inflated clincher is influenced by the width of the rim (or the volume of the rim bed) such that a tyre is narrower when mounted on a narrow rim (<20mm) when compared to a wider (23mm) rim. As a consequence, wider rims offer many of the benefits of a wider tyre, meaning lower air pressure, more comfort and grip, and potentially lower rolling resistance.
Latex is suppler than butyl rubber, and thus is the material of choice for inner tubes because it reduces the rolling resistance of the tyres. At the same time, there is also the promise of more grip as latex conserves the suppleness of the tyre, though the gains are marginal.
Our notions of grip and traction are rapidly challenged once the rain starts to fall. Every road cyclist fears sliding in the wet and slick tyres appear more susceptible than those with a patterned tread. However, there is little evidence that a patterned tread improves the grip of road tyres in the wet.
A road tyre is already very effective at displacing the water thanks to its round profile so Jobst Brandt has argued that a patterned tread is unnecessary. A broader survey of current thinking amongst tyre manufacturers supports this view, though some see room for marginal gains through a tread-pattern design.
As discussed above, any deformation of a tyre’s tread-pattern is dwarfed by the amount offered by the entire tyre. Thus, the best strategy to improve the grip of road tyres in the rain is to let some air (10-20psi) out of the tyres. Using a wider tyre and/or rim offers a larger contact patch and therefore more grip, regardless of the surface conditions.
Jobst Brandt has warned that a tread-pattern has the potential to increase the rolling resistance of the tyre since it can dampen the tyre so that it spends relatively more time in contact with the road. Indeed, many of the top performers in Al Morrison’s rolling resistance database, are unpatterned, though some have fine stipples or cross-hatching. Overall, slicks appear faster than patterned tyres though there is little data on the effect of adding a pattern to a fast slick.
Recent wind-tunnel testing by Swiss Side found that the tread-pattern of a Continental Grand Prix tyre influenced the aerodynamics of their Hadron wheel design, where the reverse direction performed better than the forward direction.
Engineers attributed this effect to “boundary layer tripping”, whereby the patterned tread helped the air flow over the wheel. The importance of this phenomenon has been considered by others resulting in Reynolds’ Swirl Lip Generator, Bontrager’s Aero Wings, and Mavic’s CX01 Technology, however these approaches concentrated on the sidewalls of the tyre or rim rather than the tread of the tyre.
Perhaps the most important function of a tread-pattern is to improve its aesthetics and appeal in the marketplace. A distinctive pattern allows tyre manufacturers to distinguish their products from those of their competitors, and as such, there is probably more art than science dedicated to the design of road tyre tread-patterns.
The grip of a road tyre depends on a lot more than just the tread-pattern. The quality of the tyre casing and rubber compounds are both important, as is the width of the tyre and rim and the air pressure employed by the rider. There’s a chance that a tread pattern can improve the grip of a road tyre, however it can only offer a marginal gain when compared to the rest of the tyre.
Of all the variables that influence grip, air pressure is the easiest and cheapest to experiment with. There is no ideal air pressure that will serve all riders, tyres, rims, and conditions, but every rider can determine a range that offers a good balance of comfort and grip with performance that can be modified to suit road and weather conditions.
Do you have a particular tyre you swear by? Do you opt for a tyre with greater tread in wet and slippery conditions?