Home  Cyclingnews TV   News  Tech   Features   Road   MTB   BMX   Cyclo-cross   Track    Photos    Fitness    Letters   Search   Forum  

Recently on Cyclingnews.com

Mt Hood Classic
Photo ©: Swift

Tech feature: Continental talks tyre tech, June 17, 2005

Vectran keeps Conti inflated

We've all heard of Kevlar, but another space-age material has made its debut in bicycle tyres: Vectran. Continental is now using the material in its high-end road-racing tubulars and it's unlikely to stop there. Gerard Knapp reports.

Andrea Tafi (right) and T-Mobile riders rolled out on Continental's Vectran-lined tubulars at Paris-Roubaix.
Photo: © Sirotti
Click for larger image

Of all the major bicycle races, no event has been more affected by simple old punctures than the brutal Paris-Roubaix. You don't have to look back very far - like two months, to be exact - to consider how a race could have unfolded very differently had a strong rider or favourite not suffered a flat or two while pounding over the pave. Like, Fassa Bortolo's Fabian Cancellara, for example.

For this reason, building a whole marketing event aimed at cynical cycling journalists is potentially a risky proposition, but for those teams in Paris Roubaix riding on Continental tyres - T-Mobile, Credit Agricole, Phonak and Saunier Duval - no rider could point to an ill-timed puncture.

According to a report in the British Cycling Weekly, these sponsored teams 'only' suffered two flats each in the 2005 race, compared to the usual average of eight flats per team in Paris-Roubaix.


Tubular tyre construction
Rolling resistance measurements for two of Continental's high-end tyres show the Competition tubular has lower rolling resistance than the Grand Prix 300 clincher
Tyre rolling resistance decreases as tyres become fatter, according to Continental's measurements.
Figures from an SRM power meter show that at 50km/h a narrow tyre requires less power than a fatter one. Rolling resistance may be lower for the fat tyre, but the narrow rubber's superior aerodynamics more than makes up for it

Continental used Paris-Roubaix to introduce a new 'Competition' model tubular: a 25mm tyre with a double layer of Vectran, a hi-tech material that is said to offer significantly better puncture resistance than Kevlar and other materials.

The thin Vectran layer resides between the tyre casing and the outer tread. The German company recognised Vectran's strengths, but producing it so it could be used in lightweight bicycle tyres required over two years R&D, resulting in a secret and patented process.

The first tyres to use this layer were used by the professionals in the 2005 Paris-Roubaix, and the day before by a group of journalists who all survived their cobbles experience (see separate story).

Like many bicycle components (or anything performance-oriented, for that matter) the production of a tyre is a compromise of weight over strength. Yes, tyre companies could make a tyre that would be virtually impervious to punctures, but it would be heavy and slow.

But what do we mean by "slow"? How can a tyre be "slow" - isn't it the rider? Well yes and yes; a tyre casing that would be puncture-proof would either be solid or so thick as to have the responsiveness of a gumboot. A tyre still changes its shape when it rolls down the road - regardless of the inflation - and the energy that is required to make the tyre change its shape is determined by its thickness. Therefore, a thick tyre casing requires more energy than a light casing. More energy absorbed = slower tyre.

The challenge, therefore, is to make a tyre capable of handling high pressure, but still be light and puncture resistant. There are other requirements like longevity and grip, but a racing bicycle tyre's intrinsic shape and weight mean those qualities are almost always sacrificed in the name of performance. We're not talking Moto GP here. Tyres don't go off or get shredded by 180hp engines. (Of course if you can have grip without compromising weight and speed, then that's a bonus, and tyre development in the last 20 years has seen substantial improvements in rubber compounds.)

After wind resistance, a tyre's rolling resistance is the cyclist's next best enemy, and Continental presented some research results that explode some myths about tyres and rolling resistance.

Conti's figures further demonstrate how tyre design and construction - even at the high-end - is still a compromise of conflicting objectives. Continental's numbers support the widespread belief that tubular tyres are faster than clinchers, but most surprising was research that showed how wider tyres have less rolling resistance. For example, a 25mm tubular offers less rolling resistance than 22mm and 19mm tyres (see graph).

But when a bicycle and rider build up velocity, resistance is not as important as aerodynamics, and wind resistance becomes the all-important factor. For this reason, Continental found that - conversely - a thinner tyre requires less energy than a fatter tyre to travel at 50kmh, principally due to aerodynamics (see graph).

For this reason, Continental believes the fastest combination of tyres is to have a 19mm front and 22m (reflected in its Grand Prix Attack/Force combination that use a 22mm front/23mm rear combination - see review). But this combination was proven on the velodrome in Buttgen, Germany. In the real world, there are factors such as comfort, grip and not the least, puncture resistance.

What is Vectran?

Which brings us to improving a tyre's puncture resistance without impacting its performance. Continental clearly believes it has the edge with the use of Vectran, and while it is only available in the 25mm "Competition" tubular, executives hinted at its likely introduction into other tyres in the range.

Vectran is - for the chemists out there - a "wholly aromatic, liquid crystal polymer, derived from polyester" that's five times stronger than steel. As a puncture-prevention layer in a tyre, Vectran is said to have the edge over materials like Kevlar because of its near-zero moisture absorption and fatigue performance. It is also said to have better resistance to folding and buckling - essential if it is going to be used in high-pressure bicycle tyres.

Conti believes the material is also superior as a 'breaker' material because it is more cut resistant (see table) but still light and thin so it can be used in bicycle tyres without adversely affecting the tyre's weight, rolling resistance and overall performance. In short, it is something of a breakthrough that is likely to be featured in more products, but only for the road. Special track and time trial models are unlikely to feature the new material.

Time will tell if it is more than a marketing gimmick, but given Continental's background in bicycle tyres, it is something the company believes will give it the edge for years to come.

See also: All stitched up - Inside Conti's tyre factory

More on 'fat tyres have lower rolling resistance'

Everyone knows skinny tyres are faster, so the notion that fat tyres have lower rolling resistance seems counterintuitive to many cyclists. As we've seen above, at racing speeds, narrow tyres need less energy to maintain a speed, so how does this fit with measurements showing higher rolling resistance?

The first thing to remember is that at 50km/h rolling resistance is only a small portion of the forces acting against a cyclist. Rolling resistance increases roughly proportionally with speed, while air resistance increases as the cube of speed. Double your speed and your tyres' rolling resistance doubles - but the air resistance goes up eight times! Air resistance dominates at racing speeds and narrow tyres are faster.

A fat tyre gets its lower rolling resistance from the shorter perimeter of its contact patch.

To understand why the rolling resistance component is lower for fat tyres, we need to understand where rolling resistance comes from. As your tyre tread and sidewall bend on contact with the road, they absorb energy. When they straighten out, they spring back and return most of that energy - but not all of it. Some of it simply gets turned into heat, and that lost energy is the rolling resistance.

For a given tyre pressure and rider weight, all tyres have about the same area in contact with the ground. If you and your bike weigh 200lb and you have 100 pounds per square inch of pressure in your tyres, you're in contact with the road over two square inches of rubber (apologies to people who think in metric, but Imperial units are so convenient for this stuff!)

A skinny tyre has a long, thin contact patch, while a fat one has a contact patch that's almost round. For a given area, a circle has the smallest circumference, so less of the sidewall bends in the fat tyre case. In addition, the greater cross-section of the fat tyre means it bends less, and the combination of a shorter circumference and shallower bend gives rise to lower rolling resistance.