Today, it seems that the bicycle industry has settled on wheels of about 26 or 27 inches in diameter for all adult bicycles. In recent times things have been muddied with the new 29-inch MTB wheel size, which is really just 700C, traditionally a road-bicycle size, with big balloon tyres.
There are five (really, only four) large-wheel sizes which are more-or-less standard today:
26" - 559mm rim diameter. Traditionally with 2" wide tyres. Has it's origins with cruiser bicycles before moving into the modern MTB era.
650B - 584mm rim diameter. A French size which has regained popularity in recent years.
700C - 622mm rim diameter. A French size which became virtually standard with road and racing bicycles.
27" - 630mm rim diameter. Most of the old 10-speed bicycles were fitted with this size, usually with 1-1/4" tyres.
29" - 62mm rim diameter. Basically 700C designed for modern Mountain Bikes.
The large bicycle wheel size has it's origins in the 19th century when few roads were paved, and those that were paved were usually cobblestone! At the end of the 19th century, the "Penny-Farthing" with it's huge wheels, often up to five feet in diameter were an extreme example of the large-wheel popularity.
However, Paul de Vive, known often as "Velocio," didn't think that the large wheel size was optimal. He was the first to advocate the use of smaller wheels, saying that a rim of 500mm in diameter and 50mm tyres were equally good. Funnily enough, this is almost the same as the "Junior MTB" 24" size, with 507mm rims.
The next major proponent of small wheels was Alex Moulton of the Moulton bicycle fame. He found that 16" wheels with mechanical suspension made for an ideal bicycle wheel size. In fact he used 16" x 1-3/8" wheels, with a rim diameter of only 349mm. This system worked well, but suspension made the bicycle more expensive.
Raleigh tried to compete with the original Moulton bicycle with the RSW-16 (Raleigh Small Wheel) without the suspension they used 16" x 2" balloon tyres on an even smaller 305mm rim diameter. The idea was that the balloon tyres would compensate for the lack of suspension. It had mixed success. Whilst it was certainly much cheaper to manufacture, the performance was extremely poor.
At about the same time, other manufacturers were looking to muscle-in on the small wheel concept. Some notable examples were Dawes with the Kingpin and Royal Enfield with the Revelation. Dawes used the French 440mm rim size, whilst Royal Enfield used the slightly larger 451mm (20" x 1-3/8") on their bicycle. Dawes' bicycle was an excellent machine, but the unusual French wheel size and the fact that they were a premium brand limited their sales. Royal Enfield designed the Revelation in a very short period, and it was a most excellent bicycle, but the company was already in decline. The Revelation was their last new bicycle model before all production ceased in 1966. It is unclear how many were made. Some estimates are that they only manufactured a couple of hundred before the company wound up.
Raleigh were a little late on the scene with the Twenty, but a combination of marketing and manufacturing might saw this become the singularly best-selling small-wheel bicycle. They adopted the 451mm rim size, but for some reason also used the 406mm rim size, which would go on to be a popular BMX size. The Twenty had the advantage of being cheap to make, and came along at the right time to ride the craze for small-wheel bicycles through the late 1960s through to the early 1980s.
Like most Raleighs, it was built with something of a Victorian engineering mindset. Everything was over-engineered. The main tube was thicker than the Dawes Kingpin, and when the folding version came along the hinge was massively over-built. Despite being an inferior design (from an engineering viewpoint) it worked well, and continues to do so to this day. The Twenty wasn't an inferior bicycle by any means though. For the average rider of the time, it was lighter than the usual roadster bicycles which had been more or less the standard for over 50 years. It was trendy and practical, particularly as a shopping bicycle. So many were made that they are still a common-place item.
The Raleigh Twenty really deserves the credit for keeping Raleigh in business through the bicycle slump of the 1970s. In 1977 over 170,000 Twenties were sold in the UK alone! That works out as one being sold every 3 minutes 24 hours a day 7days a week for the entire year! And that was only one year out of the 18-year production run.
The small wheel bicycle faded from popularity as from the 1980s a series of fads or crazes washed through the cycling world. These include the 10-speed, the BMX, and the Mountain Bike just to name a few. Small wheels became associated with children's bicycles and were for a long time regarded as mere toys.
With the increase in urban living, and private motor car transport become less and less convenient as populations increase and roads become more crowded, cycling and the small-wheel bicycle is making a big come-back. Small wheels have several advantages over large ones. A small-wheel bicycle takes up less room on a train when you are mixed-mode commuting. Small wheels keep the size down for folding bicycles, best exemplified today perhaps by the Brompton Bicycle. Even when I take my Twenty on the train, I do not often fold it as it takes up so little room compared to a regular bicycle it doesn't inconvenience other passengers. They are also a God-send for smaller people. My own sister is a petit 5'2" and only just fits on a lady's roadster with 27" wheels.
There are also many technical advantages with small wheels. A lesser rotating mass makes for quicker acceleration. There are no problems with toe over-lap, particularly for bicycles with short top-tube lengths.
There is now also an increasing number of excellent rims and tyres available, partly thanks to "racing BMX" and to recumbent bicycles, which often make use of the smaller 20" wheel sizes. Just from practical experience, it has been found that a 20" wheel with a tyre between 38 and 47mm gives a working combination of light weight, good acceleration, good cushioning and good grip.
The classic Raleigh Twenty design has become popular once more. In fact, Raleigh now manufacture a bicycle almost identical to the original Twenty design. Two other companies have also started manufacturing extremely similar bicycles. The design is able to accommodate the very small and very large rider in one simple frame size.
Many Twenties are still being ridden just as they came out of the factory all those years ago. Many are also heavily modified and customised with all sorts of weight-saving and performance-enhancing features, such as alloy rims, new cranks, lightweight mudguards, drop-handlebars, high-performance hubs, etc.
The Twenty will keep going for many years to come. Perhaps due to a certain fashionability, nostalgia or any other number of reasons. But perhaps best surmised by: "If it works, why change it?"
Small-wheel bicycles present several advantages in bicycle design for shorter riders, particularly for the ladies. The small wheels allows for a shorter top-tube length, without toe-overlap on the front wheel, a common problem for smaller bicycles. It also means that the seat-tube and head tube can be at the same angle of about 72 degrees, rather than a steep angle for the seat-tube and a shallow angle for the head-tube.
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