Are your tyres slowing you down? There’s a whole new science behind achieving the optimum pressure for your bike tyres.
I’m just back from riding what appears to be the latest craze in cycling - a fat bike. These are so named because they have huge ‘balloon’ tyres that run at pressures as low as 5psi (34kPa) and enable the rider to cross snow, sand and mud with relative ease. When set beside a regular road bike it’s like comparing an Aston Martin with a JCB.
My bike had tyres with a diameter of 4in (100mm), over four times the width of the skinny 23mm tyres on my road bike. Such narrow tyres are, of course, designed for smooth tarmac, and being both narrow and inflated to high pressure (up to 120psi in my case) they allow the bike to go faster. Or do they?
Built for speed
It’s been common knowledge in road cycling for decades that thinner tyres equals a faster bike, but that idea is now turned on its head with more and more riders, both professional and recreational, opting for wider tyres of either 25mm or 28mm diameter.
It’s not just a question of speed, however. Professional teams in long, gruelling races such as the Tour de France are opting for wider tyres since they provide a more comfortable ride (‘comfortable’ being a relative term when you may be riding 200km over the top of the Alps) and cause less rider fatigue. In more extreme races such as the Belgian Classics with their sections of cobbled ‘pavé’, some teams will even use 30mm tyres.
The comfort factor is easy to understand, since a wider tyre soaks up imperfections in the road more effectively, but how can it also be faster?
It’s because if you run a 23mm tyre and a 28mm tyre at the same pressure, they both have the same size ‘contact patch’ on the road. However, the 28mm tyre will tend to flatten across the road surface width-wise, while the narrower 23mm tyre will have a tendency to flatten more elliptically along its length.
This actually creates more rolling resistance since the 23mm tyre has deformed more and created a longer contact patch. The reduced rolling resistance in the 28mm tyre saves energy. Since the cost of changing tyres is one of the cheapest upgrades a rider can make, it’s not surprising that more people in the ‘real’ world are also opting for wider tyres.
A further advantage of wider tyres is that they offer better traction in wet conditions, and ‘pinch flats’ - when the inner tube of the tyre gets nipped between the wheel rim and the tyre - are less common.
The advantages of wider tyres continue. When narrow wheel rims, say 19mm, are fitted with a standard 23mm tyre, the tyre ‘balloons’ slightly at the sides, leading to aerodynamic drag, whereas a 25mm tyre fitted to a 23mm rim has less drag.
In the world of professional cycling these ‘marginal gains’ count for a lot. It can add several seconds time advantage over a long stage race, all other factors being equal. The concept of ‘marginal gains’ was adopted by British Team Sky Cycling’s performance director Dave Brailsford in 2010. At the time, no British cyclist had ever won the Tour de France, but what Brailsford referred to as “the 1 per cent margin for improvement in everything you do” has largely been responsible for three British Tour wins in four years.
Petri Hankiola, president of Wheel Energy, a Finnish company that specialises in researching and testing rolling resistance of bicycle tyres, says: “At the moment, research shows that 24 to 28mm tyres have the lowest rolling resistance. But you cannot say directly what size is best, because the manufacturing materials influence the result and many tyre manufacturers use a variety of materials”.
So how much of the quality of the tyre materials affects the rolling resistance - presumably more expensive tyres will perform better?
“Tyres must be divided into two categories. Low rolling resistance tyres and puncture-protected tyres. Tyres without puncture protection are usually faster than those with puncture protection, but expensive tyres are not necessarily the fastest because the tyre may be an older model that uses materials with higher rolling resistance.”
Feeling the pressure
Wheel Energy’s research indicates that the key to reducing rolling resistance is, however, not minimising how much tread is in contact with the ground but minimising the energy lost to tyre casing deformation, since wider casings exhibit less ‘bulge’ and also have a shorter section of deflected sidewall.
Wheel Energy claims a 25mm tyre will have 5 per cent lower rolling resistance on average than a 23mm tyre. Consequently, tyre manufacturers are incorporating this into their designs by reducing the amount of rubber around the shoulder of the tread and reducing the rubber thickness in the casing. These help minimise the energy lost in casing deformation as the tyre rolls along.
So just how much time does a racing cyclist gain through using a wider tyre? Cyclist magazine recently conducted a series of tests using a set of Continental Grand Sport Race tyres of 23mm, 25mm and 28mm diameter respectively to see which was most efficient. Each tyre was run at 100psi and 85psi from a standing start, with a cyclist freewheeling down a 100m course with a 7 per cent gradient (to ensure that air resistance wasn’t a significant factor).
The tests, while not held in a scientifically controlled environment, indicated that the most efficient tyre was the 25mm with a pressure of 85psi - results that would have surprised cyclists in the past who would have expected the narrowest tyre running at the highest pressure to be the most efficient.
Admittedly the differences between the various diameters and pressures were not huge - the fastest combination of 25mm/85psi covered 100m in 17.97 seconds, while the next fastest were 25mm/100psi at 17.99 seconds and 28mm/100 psi at 18.22 seconds.
However, using the ‘marginal gains’ approach the difference over 100m would potentially add up to a significant time difference between cyclists racing over a long course.
As for the differences in tyre pressure, Hankiola says: “Tyres’ pressure is very important. My own view is that the minimum pressure for 23-28mm with tyres should be 6.0 bars (87psi) and the maximum 9.0 bars (130psi). Wider tyres over 28mm should have a minimum pressure of 4.0 bars (58psi) and a maximum of 7.0 bars (101psi).”
The perfect balance
Tyre manufacturer Vittoria regards it as “impossible to make a general recommendation on inflation pressure for a specific bike and rider” as it depends on too many variables.
The firm lists “frame and wheel material, construction and stiffness of these, weight distribution, road surface, width and material of tyres and tubes, weather and temperature, and not least riding style” as just some of the variables.
Vittoria also suggests an alternative means of finding the best tyre pressure for your bike and riding style to that used by Cyclist magazine.
You should find a short circuit, preferably with a variety of corners, and ride a lap using the tyre pressure its tables suggest.
Then each following lap, lower the pressure by 5psi until you feel the tyres start to ‘wallow’ - this is your lowest pressure point. You then raise the pressure by 5psi each lap until the tyres start to bounce and skip across the road surface - this is your maximum pressure.
Vittoria suggests keeping a ‘tyre pressure log’, which should include factors such as road surface, technicality of the course, weather conditions, type of tyre used and so on.
The firm states that “this log will become a valuable tool for referencing tyre pressures and road conditions in order to optimise your performance on race day”.
Rob Scullion, the director of Cambrian Tyres, which imports Continental tyres into the UK, points out that in real-life conditions things are inevitably more complex. “Continental’s 28mm GP4000S II tyre is faster in the lab than the 23mm and 25mm versions, but in the real world aerodynamics come into play and hinder efficiency.”
He adds that “the 25mm is the perfect all-round package for speed, grip and comfort and [the tyre that] all our race teams choose”.
So there you have it; the happy middle ground seems to lie between 23mm and 28mm - for the professionals at least.