Small Wheels for Adult Cycles

The safety bicycle's influence on wheel size

The key design elements of the familiar diamond frame 'safety bicycle' evolved a century ago. Wheel diameters commonly adopted for adult bicycles have changed little since then, being generally in the order of 26 to 28" (650-700mm). It is easy to see why the Victorians adopted the 28" (700mm) pneumatic tyre. It worked well because it was the biggest that could conveniently be accommodated in the robust, uncomplicated and easy-to-manufacture diamond frame.

For a given tyre construction, cross-section, tyre pressure, load and road surface a bigger diameter wheel rolls more easily than a smaller one. In an unsprung frame the bigger wheel also gives a more comfortable ride. This is because it falls less deeply into small depressions in the road surface and, on hitting bumps, rises and falls more slowly. Quite simply, the 28" (700mm) wheel gave the best balance of comfort and rolling resistance that the diamond frame could accommodate. Anything bigger would have compromised the ease of mounting and dismounting that gave the 'safety bicycle' its name.

Specialised uses for small wheels

Despite the enduring dominance of the larger wheel formats, Sutherland's tyre and rim listings include substantially more small formats than large.1 Many are rare or used mostly for non-cycling purposes, such as wheelchairs, barrows and handcarts. However, established cycling uses include:
children's bicycles, for obvious reasons of scale,
the front wheels of carrier cycles, leaving extra space above for cargo,
portable cycles, to reduce the space needed to stow the machine when folded or separated,
unisex 'one size fits all' family shopper cycles, to allow good luggage carrying capacity over the wheels, and to allow extra height adjustment of saddle and handlebars,
BMX, for fast acceleration and quick steering,
front wheels of pursuit and motor-paced track cycles, to enable closer drafting of the vehicle ahead,
front wheels of time-trial cycles, to facilitate a lower, more aerodynamic riding position,
the front wheels of recumbents, to facilitate convenient location of the cranks and drive train without impeding steering, and for aerodynamic reasons, including easier incorporation of fairings.

Most of these uses are for special purpose machines and capitalise on one or more advantages of the smaller wheel (most often simply its smaller size) in a trade-off for certain disadvantages.

However, over the years some powerful advocates have argued for smaller wheels for regular bicycles. Surprisingly, it is nearly a century since balloon tyres of 12 to 16" diameter (300-400mm) were tried for club cycling in England.2 A Mr. Edmunds who rode a machine with wheels about 12" (300mm) diameter and 5" (125 mm) wide around Birmingham was "a racer of some ability" whose youthful challengers "sometimes received quite a surprise."3

Subsequently the two principal advocates of smaller wheels have been the eminent French cycle tourist Paul de Vivie (in the 1920s) and the distinguished English engineer Alex Moulton (since the early 1960s).

The Velocio approach - medium diameter balloon tyres

Paul de Vivie, alias 'Velocio', was the 'father' of French cycle touring. In the 1920s he advocated balloon tyres of up to 2.25" (57mm) cross-section on 20" (500mm) rims, giving an overall diameter of about 22" (560mm).4 He reached his conclusions during a lifetime in which he cycled the equivalent of 15 times round the world 'all of it as careful experimental touring work with a view to improving machine design and method of riding'.5

As early as 1911 he wrote:

"My own experience has gone no further than to 50 centimetre wheels furnished with 50 millimetre tyres, but I can guarantee that in an experiment extending as far as 15,000 kilometres covered, they will not have the smallest disadvantage from the point of view of their running. It simply seems to me they are more prone to skidding, but this is perhaps due to the fact that their tyres have no tread and that the bicycle is very short."6

Velocio died in 1930 and his obituary in the CTC Gazette7 included a photograph of him with an open-framed small-wheeler. Over the next ten years several British cycle tourists emulated his use of smaller wheels. They included A.C. Davison, Cycling magazine's technical expert, and Medwin Clutterbuck, the CTC Consul for Sussex. Both riders used tyres of about 24" x 1 5/8" (600mm x 40mm). [Clutterbuck used 22" x 1 3/8" rims (560mm x 35mm).] Davison covered some 5,000 miles (8,100 km) on his 'Little Wheels' and declared it "a quite satisfactory bicycle".8

Medwin Clutterbuck had two small-wheeled cycles built by F.W.Evans of London. On the first of these he toured the Alps, Dolomites and Norway, often on poorly paved roads. In England he covered up to 200 miles (320 km) in a day. Half a century later he still considered his second Evans-built small-wheeler "the epitome of what a touring machine should be".9

The idea of a reduction of tyre diameter being matched by a corresponding increase in cross-sectional area certainly has merit. The volume of air and pressure remains the same as in the conventional tyre, while the wider cross-section compensates for (and can even improve on) the otherwise harsher ride of the small wheel.

As for rolling resistance, a reasonable prima facie indicator is the length of the tyre print (under a known weight) divided by the inflated tyre radius.10 For a given tyre pressure and load, the contact patch area is approximately constant, regardless of tyre diameter. (For example, a tyre inflated to 50 psi and carrying a load of 100 lbs has a contact patch with an area of approximately 100/50 square inches, ie. 2 square inches, whatever format the tyre may be.11) However, with the Velocio approach to small wheels the patch is wider but shorter. Thus compensation is obtained for the otherwise higher rolling resistance.

However, this compensation depends on superior lightweight tyre carcass construction. This is difficult to achieve because, the larger the cross-section, the stronger the carcass must be to hold a given pressure. For economy of manufacture, the strength of wide section tyres often comes from thicker, heavier and less flexible materials, and results in a higher rolling resistance13.

Velocio therefore advocated canvas-backed, thin, flexible carcasses produced by Edwardian English tyre manufacturers. In 1911 he bemoaned the fact that, for fear of warranty claims, such tyres were not made in France.

Apart from the warranty issues, stronger materials that enable thinner, lighter and more flexible construction cost more and there may be little demand. It is significant that Medwin Clutterbuck abandoned his small-wheelers after World War 2 because it was no longer possible to have his tyres custom made by the Constrictor company.

The Moulton approach - small diameter high pressure tyres and suspension

Alex Moulton (b.1920) is an engineer with a background in steam power, aeronautical engineering, automotive suspension and rubber technology. His great-grandfather introduced Goodyear's rubber vulcanising process to the UK and the Moulton family has been involved with the material ever since. Alex Moulton's rubber and fluid-interlinked suspension systems have been used on millions of cars, from the Mini to the MGF. Yet he is probably best known for his small-wheeled bicycle concept, to which he has devoted much of the last 40 years. The story is recounted in two books by the author.14

Moulton noted the lack of development of the diamond-frame bicycle since the 1890s. He decided "to take the evolution of that most remarkable device beyond its classical form ... to produce a bicycle which was more pleasing to have and to use."

He argued that, apart from those for use on soft ground:
wheel sizes for virtually all vehicles have decreased as design has evolved,
this reduction of the encumbrance of large wheels is always sensible.
Moulton's background in rubber technology enabled him to demonstrate that 16" x 1 3/8" (400mm x 35mm) tyres inflated to 50 psi (3.4 atm) could match the rolling resistance of the then standard lower pressure roadster tyres.15 It is important to note that Moulton did not use standard juvenile tyres but small diameter versions of the Dunlop Sprite, a good touring tyre of the time. Most series-produced Moulton bicycles of the 1960s and 1970s used tyres of this type.

To eliminate the rough ride given by a high pressure small diameter tyre, Moulton added suspension. This not only gave a generally smoother ride than a conventional bicycle, but also reduced momentum losses. Recent tests of the Primo high pressure 16" x 1 3/8" tyre indicated that on a real road a fully sprung 1960s Moulton rolled significantly better than a semi-sprung Brompton using the same tyres.16 A notable feature of the Moulton bicycle has been its sporting success. Moultons have performed well in time trials, solo and four man pursuit, criterium, ultra-marathon, triathlon, Audax and HPV races. The flying 200 metres normal riding position solo record has been held by a Moulton-based HPV for more than a decade.

Early racing Moultons, such as that used to break the Cardiff-London record in 1962, used sew-ups of 18" (450mm) nominal size, with an actual diameter nearer 16" (400mm). In 1964 Moulton, again working with Dunlop, produced a clincher replacement. This matched the performance of the sew-up but was considerably more robust. To enable riders to switch between sew-ups and clinchers, rims for the latter had the same brake radius, hence the birth of the unique Moulton 17" x 1 1/4" format (430 x 31mm, ISO 32-369).

This tyre, made since 1983 by Wolber [from September 2000 by Bridgestone], is noted for its high performance. Tests have shown it to have a rolling resistance on smooth surfaces that matches a high quality 27" (700mm) clincher touring tyre or a cotton road sew-up.17An even faster slick version is also available. This was evolved as part of the General Motors Sunraycer solar-powered car project. The slicks were used in 1990 on Miles Kingsbury's fully-faired recumbent Bean. This broke the world HPV record for the greatest distance covered in an hour by a solo rider.

Conclusion

Both the Velocio approach and that of Alex Moulton demonstrate the potential that smaller wheels offer mainstream cyclists and cycle designers. They need not merely be a fallback for special purposes, such as folding bicycles.

For a given tyre construction, cross-section, inflation pressure, load, road surface and unsprung vehicle mass, a bigger wheel will roll more easily. However, this parity of conditions is not necessarily met. Furthermore, it can often be influenced in favour of the smaller wheel.

Small wheels are stronger, lighter, stiffer, more compact, have lower wind resistance and offer more design options. They facilitate faster acceleration and more responsive handling. Used in conjunction with suspension and supple high pressure tyres, they can be particularly effective.

As Velocio put it:

"That universal agreement has fixed on 70 centimetres as the proper size for wheels does not in any way prove that this diameter is best; it simply proves that cyclists follow each other like sheep.... Make no mistake, uniformity is leading us directly towards boredom and towards routine, whilst diversity, even though it distracts us, holds our attention, our interest and the spirit of enquiry always on the watch. To change is not always to perfect, and I know that better than any others newly come to cyclo-technology. But to stand still, to sink into a rut, that is the worst of things for industries and for men."

About the Author
This article was written in 1997 at the request of the US journal Cycling Science. It discusses the use of small wheels in cycles for adults. The term "small wheels" here means those with a nominal diameter less than 26" (660mm). They can be as small as 6" (150mm) or as large as 24" (600mm), but today are generally in the range 16-20" (400-500mm). All dimensions quoted are approximate.