“I’ve heard that a new high performance tyre has become available in the Brompton’s size. Is this true? And is it worth buying?”

John Wentworth, Buckinghamshire

Yes, the Schwalbe Stelvio is now available in the Brompton size, but before analysing its performance, we should look briefly at the history of small tyre technology.

With the majority of bicycles designed around tyres of 26 inches in diameter, the technology has always tended to follow the larger sizes, with small tyres – generally destined for children’s bicycles – being bottom of the heap. All this changed in the 1960s, when Dr Moulton introduced his ground-breaking 16-inch machines that performed in a broadly similar way to big-wheelers of their day.This performance was due to a number of factors, including full suspension and relatively high pressure Dunlop and Michelin tyres, developed especially for the machine.

By the 1970s, the mass-market 16-inch Moulton had been replaced by the more specialised 17-inch AM model, and the 16-inch format began to fade away. For the next 20 years, the technology stalled, and when the Brompton arrived in the late 1980s, designer Andrew Ritchie was obliged to choose between two rather mediocre tyres – the Raleigh Record and the Schwalbe, more generally known then by its anglicised name: Swallow. These tyres were heavy with poor rolling characteristics, but they were the best available. Of the two brands, the Raleigh was marginally better.

Fortunately, although the Brompton was only selling in small numbers, recumbent HPVs of various kinds were making a big impact in the USA, creating a completely new market for high quality 16- and 20-inch tyres. Quite what happened next is shrouded in mystery, but it seems Vision – one of the largest recumbent manufacturers – approached Taiwanese tyre manufacturer Cheng Shin with a radical new design.

The tyre they agreed to make was the Primo Comet, and when released in 1996, it transformed the market overnight. Most of the rolling resistance of a tyre is caused by flex in the sidewalls, as the nominally round tyre contacts the flat road and distorts.This effect can be minimised in two ways – by producing a tyre to withstand high inflation pressure, thus reducing the degree of distortion, or by making the sidewalls flexible, reducing the work required when flexing takes place. Clearly, these requirements are somewhat contradictory, but the Primo scored in both departments, withstanding inflation pressure of 90psi or more and rolling better than any small tyre by a fair margin. It was also remarkably light, and tough, provided you didn’t try running a dynamo against the flexible sidewall…

For six years, little changed.The very successful 37-349mm and 37-406mm Primos (13/8″ x 16- and 20-inch respectively) were joined by a narrow racing tyre, but this proved too frail for everyday use and demonstrated no marked benefits over its wider cousins.

…Although some users found the tyre ‘skittish’ in certain conditions, the Brompton tyre was a massive hit…

Brompton was so delighted with the performance benefits of the Primo that the company designed its own tyre based on the same carcass technology and built by Cheng Shin. Launched in early 2000, the Brompton tyre was effectively a Primo with a discreet tread and a dynamo track.The tyre was produced in two versions – conventional, and kevlar-belted for added puncture resistance. Although some users found the tyres ‘skittish’ in slippery conditions, the Brompton tyre was another massive hit, offering long life, reasonable puncture resistance and light weight, together with rolling resistance similar to the Primo. Incidentally, the kevlar version rolls less well, and has been known to fail at high pressures, without necessarily demonstrating greater puncture resistance. At about the same time, Schwalbe updated its 1960s vintage tyre, to produce the Marathon, a very smart kevlar-belted tyre with deep tread and a reflective sidewall.

Arrival of the Stelvio

Last year, the small wheel fraternity got excited all over again with the launch of the Schwalbe Stelvio in small sizes.This narrow racing tyre had been around in 700C form for a year or two, but the first 406mm (20-inch) and 355mm (18-inch, primarily suiting the Birdy) tyres were found to roll better than the opposition, by a fair margin. Schwalbe claims that the improved rolling performance comes from the use of flexible silicon rubber in the sidewalls, and a narrower 28mm tyre tread.Tyre width has a small effect on rolling resistance because narrower tyres have less tread to flex, although in this case, the tyre has been beefed up with a kevlar belt, which probably negates much of the advantage.

Tests of the 406mm tyre, conducted by MIRA on behalf of Inspired Cycle Engineering, demonstrated a significant advantage over the Continental Grand Prix and Primo Comet tyres. However, results from one tyre size do not necessarily stand up for another, and the Primo Comet chosen was 1.75″ in diameter, rather than the sportier 37mm. I decided to gather my own data.

Field Testing

Most rolling resistance tests are carried out in aircraft hangers and other large under- cover spaces, but there’s a lot to be said for trials on ‘real’ surfaces, although field tests must be set up and observed with great care, as wind speed and direction, and air temperature, can have a marked effect on performance. For some years, my tests have been carried out over a 630 metre stretch of ‘B’ category road with what can only be described as a ‘mixed’ road surface. Bicycles are timed over this downhill stretch from a standing start, and – provided weather conditions are similar – it is possible to compare results taken years apart.

In January, wind-free days are rare, although I eventually found the conditions I wanted late one evening.Temperature fluctuated between 3.7 and 6 degrees C, which is on the low side, but the important thing is that all the tyres were tested back to back in near identical conditions. Inner tubes were the standard Raleigh 37-349mm for the Marathon and Brompton tyres, but the smaller Schwalbe tube (claimed to suit any tyre between 32 – 47mm diameter) for the Stelvio. Results are averaged from four or five runs and proved reasonably consistent, although the Marathon took a few minutes to warm up.

The Results

First out was the Schwalbe Marathon.Tyre pressures were set at 85psi rear and 65psi front – pressures that give good results on mixed road surfaces for a cyclist of my build.The Marathon achieved an average roll-down speed of 13.2mph, which is broadly what one might expect from a fairly frumpy kevlar-belted tyre at those temperatures.

Then the new Stelvio tyres were fitted to the bike and inflated to the same pressures. Results were much better, at 13.9mph, which is good for 16-inch, but hardly state of the art. Repeating the runs with inflation pressures of 100psi rear and 80psi front brought a slight improvement to 14mph.

At this point, we know from experience that higher pressures can be counter- productive, as road vibration can actually begin to reduce the rolling speed, an effect that I first observed with the Primo tyre back in Folder 17.This, I should add, is true for the Brompton and others, but fully suspended small-wheeled bikes, such as the Birdy and Moulton, can give very different results.

However, access to the Pantour suspension hub (see page 21), gave a unique opportunity to try the tyres at the maximum pressure of 120psi on a fully suspended Brompton.This produced a more satisfactory roll-down speed of 14.4mph, which is right at the top end of 16-inch performance, particularly in such cold weather.

Remarkable? Well, yes and no.The final test runs (at the lowest temperature, incidentally) were of the standard Brompton tyre, inflated to a comfortable 85psi/65psi. Once again, under the same conditions, the rolling speed was 14.4mph!

Conclusion

One should be cautious interpreting results of this kind.Yes, it’s true that on paper, £20 worth of tyres performed just as well as a £170 suspension/tyre/tube package, but roll-down tests do not give a complete picture.What the figures don’t reveal is that the Stelvio/suspension set up was undoubtably faster uphill, making better use of the power input from the cyclist. It’s probably true too, that the new tyre would perform better on smooth surfaces, so one would expect the Stelvio to shine under racing conditions.

But for day-to-day use, the Brompton tyre still seems to represent the best compromise between comfort, price and performance. Despite descending our test hill at race-tyre speed, it proved extremely comfortable – probably the best of all the combinations tried. Handling was predictable too.The Stelvio was fine at low pressure, but with 120psi front and rear, it proved hard to control, especially on rough tarmac.

Once again I have filled my allotted space, but I hope to return to this matter soon (hopefully in the next issue) with a more detailed review of 16-inch tyres.

The 349mm, 355mm and 406mm Schwalbe Stelvio tyres are available from specialist dealers or by mail order from Westcountry Recumbents mail sales@wrhpv.com Prices start at £13.50 each