The subject of tyre technology comes up rather frequently on these pages, mainly because their inherently higher rolling resistance tends to put small-wheelers at the cutting edge.
In the mid-1990s, small tyres were at a considerable disadvantage against their bigger cousins in terms of rolling efficiency, but this was much reduced with the arrival of the Primo and Brompton tyres, whose paper-thin sidewalls flexed more easily as the tyre rolled, reducing rolling resistance. Ever since, the boffins have been burning the midnight oil searching for further gains, with the primary work being carried out at Greenspeed, the Australian recumbent manufacturer, and Brompton. Folding bikes need small wheels for reasons of folded size, of course, but recumbent designers are showing an interest in the same tyres, primarily looking for a small frontal area and reduced wind resistance. As we saw in A to B 39, Greenspeed is starting to adopt the 16-inch (more correctly 349mm) tyre on its recumbent trikes for just these reasons, and I am indebted to the company for access to its latest research in reducing the already small rolling penalty inherent with these tyres.
The Theory
Why does a tyre experience rolling resistance? Most of the energy is absorbed around the contact patch, the crucial zone where the doughnut-shaped tyre and inner tube mould themselves briefly to the flat road surface. If tyres were 100% springy, this wouldn’t matter (although the rider would probably fly off on the first bump), but rubber exhibits a useful self-damping characteristic known as ‘hysteresis’, which effectively means that not all the deformation energy is recovered when the tyre resumes its shape. This damping effect turns motion into heat, and the process takes place continuously as a tyre rolls. In really bad cases, the tyre will feel warm after a hard ride. Small tyres offer greater resistance than larger ones, because the more sharply curved tyre has to bend more acutely to become flat, and visa-versa when reassuming the curve.
As every cyclist, motorcyclist, and indeed motorist, should be aware, the easiest way to reduce the size of the contact patch, and thus the rolling resistance, is to put more air pressure in the tyres.This seems to work in two ways – firstly by reducing the circumference of the contact patch – the ‘battle front’ of rubber doing the work – and secondly by reducing the angle through which the rubber has to flex when it hits the road. Watch an old chap ride past on a Raleigh Shopper with half-inflated tyres, and you will see all the negative factors at work: a small diameter tyre, large contact patch and extreme angles of flex. One is sometimes tempted to offer a few pump-fulls of air.
There is, however, a limit to the improvement that can be made through air pressure alone, particularly on a bicycle without suspension. Pneumatic rubber tyres have been so very successful because they absorb lumps, bumps and vibrations from the road surface. Inflate a tyre really hard and it begins to act like a rigid disc, which would only be good news if the road were as flat and smooth as a mirror. In practice, roads are more or less corrugated, and a solid tyre will pass these surface irregularities to the vehicle and rider. This is not only uncomfortable, but wasteful, because energy is thrown away as the bike vibrates – effectively lifting and dropping the mass of the bike and rider.
Wider Tyres
Greenspeed 40mm tyres on the Brompton - note the slick tread and very tight clearance around the rear tyre
Tyre shape seems to be worth looking at. Conventional orthodoxy has it that a narrow high-pressure tyre rolls better than a wider low-pressure example.We need only compare the performance of the original Moulton, with its narrow high- pressure tyres, and the frightful Raleigh RSW, equipped with wide low- pressure tyres. Narrow tyres do have advantages – low weight primarily, plus reduced frontal area (and thus wind resistance) – but do they really roll better than wide tyres?
Whatever theoretical disadvantages wide tyres might have, it seems that when we compare like with like (the Raleigh tyres were not only low-pressure, but had heavy, stiff sidewalls) they actually perform rather well. Regular readers may recall my slight disappointment with the narrow high-pressure Schwalbe Stelvio, launched in the 349mm size in early 2003.This 28mm wide tyre rolled slightly worse than the ‘cooking’ 37mm Brompton tyre and gave an inferior ride. In that case, might a wider tyre not roll even better?
Observing that wide tyres, even cheap ones, sometimes rolled better than narrow tyres, Ian Sims of Greenspeed decided to develop his own. Like the Primo and Brompton, the tyre has thin, supple sidewalls, but with a completely slick tread and a width of 40mm (against 37mm).Weight is 280g, against 200-250g for the 37mm tyres.
Greenspeed was kind enough to supply a pair of these new ‘Scorchers’, which I fitted to a Brompton – easy enough on the front, but a rather complex operation on the back, due to the tight clearances. After a period of running-in, the tyres proved surprisingly fast on my standard roll-down test, beating the Primo and Brompton tyres by a small but identifiable margin. Intriguingly, they were also more comfortable than the narrow tyres, under identical conditions.Why?
Some Experiments
At the same pressure and carrying the same load, the contact patches are of near identical length, but the width varies broadly in proportion to the width of the tyre
It is widely assumed that – for a given tyre pressure, loading and wheel diameter – the area of the contact patch will always be the same, irrespective of tyre width: a long and thin patch on a narrow tyre, and a short fat one on a wide tyre. As the crucial dimension is generally assumed to be the patch circumference, it seemed to make sense to aim for a round contact patch, with the shortest possible circumference. Hence the move towards wider tyres.
Perhaps surprisingly, this turns out not to be case, or at least, not with tyres of conventional construction in the 16- inch size. Comparing the 28mm Schwalbe Stelvio, 37mm Primo Comet and 40mm Greenspeed Scorcher, with the same loading and tyre pressure, I found the length of the tyre contact patch to be a function of tyre diameter, irrespective of tyre width. But the width of the contact patch varied according to the tyre width.Thus, the most free rolling 349mm tyres have the greatest contact patch circumference and tyre/road contact area, and those with the highest rolling resistance have the shortest circumference and smallest contact area.
That the wide tyres should be more comfortable seems easy to explain.The extra width is bound to ‘average out’ the pits and bumps in the road, and we now know that the ‘point pressure’ is less with the broader tyres, presumably allowing the tyre to mould itself around obstacles, rather than deflecting. And as all the tyres share the same aspect (ie, height to width) ratio, the broader tyre is also taller, putting a greater expanse of rubber between the road and rim. All these factors might be expected to iron out bumps, but they don’t really explain the improved rolling performance.
Two possible answers spring to mind. Looking again at the illustration, it’s clear that the narrow tyre comes almost to a point at front and rear, suggesting a fairly acute degree of flexure at the front and rear of the contact patch as the tyre assumes the flat shape then springs back. On the wide tyre, the more gently rounded tyre shoulder suggests that the rubber is bending more easily to assume the flat contact patch. As one observer commented on seeing the illustration, the wider tyre displays a ‘cleaner’ ellipse, and this cleaner shape results in lower hysteresis. Presumably too, the improved shock absorption of the wide tyre reduces vibration, and thus rolling resistance.
Whatever the explanation, it looks as though a new generation of small tyres is on the way. Can we expect to see broad, slick designs on everyday bikes? Another widely held belief is that tread somehow improves grip. Obviously, this is true enough on soft or loose surfaces, but on tarmac, a slick tyre can be expected to grip better, roll better, and shrug off debris, reducing punctures, compared to a similar treaded tyre.Whether slicks gain widespread acceptance with the general public remains to be seen.
Unfortunately, most small-wheeled machines are designed for narrow tyres, and weight is important too, so it’s unlikely that wider (and taller) designs will be adopted, unless a considerable performance advantage can be demonstrated. For recumbents, on the other hand, the only downside seems to be the slightly increased frontal area.
Tubeless Tyres
Experimental tubeless tyre - note the cut inner tube protruding around the tyre. This surplus can be trimmed off when inflated
As we have seen with the Primo (see Folders 17 & 18), another solution is to make the tyre sidewalls more flexible, thus reducing the effort needed to overcome rolling resistance without compromising (probably improving) the shock absorbing characteristics of the tyre. But where do we go next? One long overlooked solution is to eliminate the inner-tube.There’s little point in fitting a tyre with paper-thin sidewalls backed up by a stiff, inflexible inner-tube. In practice, most good quality tubes flex quite well, but Greenspeed rig tests have found a reduction in rolling resistance of around 20% by eliminating the tube, so in theory, it’s well worth doing.
State-of-the-art. Running tubeless, the Greenspeed tyre rolls well - better than most 20-inch and some 26-inch tyres
If you want to experiment yourself, simply slice open an old inner-tube around the circumference, splay the tube out flat and fit it to a wheel, followed by a tyre. Inflate as usual (not easy) and trim off any excess tube. Provided the tyre is in good condition (you may need to add some sealing gunge), this home-made solution should work well enough.
Tubeless bicycle tyres are not a new idea, although most designs have required a special rim and/or tyre profile, none of which have caught on.The advantages include easy puncture repair (a soft pencil of rubber can be inserted into a hole from the outside, without disturbing the wheel or tyre), lower weight and lower rolling resistance. On the negative side, tubeless tyres are probably more prone to puncture, more difficult to inflate when off the rim, and they require some sort of sealing system Black Primo Comet around the spoke holes.
But what of the future? Greenspeed is currently fine- tuning the composition of the Scorcher tyre, a process that will no doubt yield another small performance gain.The first production examples should be available early in 2005.
Further information from Greenspeed
