Revised February 2012
The stock-standard Raleigh Twenty is a perfectly serviceable bicycle, however a number of improvements can be made to it's performance.
Perhaps the best overall upgrade for the Raleigh Twenty is to replace the heavy chromed steel rims with aluminium alloy ones. This will reduce weight, and improve acceleration and braking. Some were supplied with 20" x 1-3/8" wheels on 451mm rims, whilst others were fitted with 20" x 1.75" wheels on 406mm rims. The latter is a common BMX size and a wide selection of rims and tyres are available.
Most bicycles now come with alloy rims (though some cheap kid's bikes still have steel ones), so it should not be too difficult to obtain them used. I picked up a set of Araya alloy rims off a discarded BMX on the side of the street (I also got some knobby tyres which would be great for off-roading). I rebuilt the rear wheel with an Sturmey-Archer SRC3 which also helped solve the problem of the poor rear stock brakes on the R20, so I will always stop, regardless of the road conditions. The wheels are now fitted with Schwalbe Marathon 20" x 1.5" tyres which are surprisingly quick and very durable.
For 451mm rims, the SUN CR-18 are available, but supplies are not always readily available (best to inquire through your usual suppliers). Velocity make their "Glider" rim in 451mm, and is suitable in width to mount tyres anywhere from 1-1/8" to 1-3/8" wide. With the growing interest in recumbent bicycles a growing selection of quality tyres are now available for the 451mm rim format.
The single most important upgrade to make is to improve braking. As supplied, the braking is typical of many English 3-speed bicycles - poor in the dry and non-existent in the wet. A good motivation to ride at a relaxed, steady pace for the most part!
There are several pathways you can take to upgrade the brakes, depending on your particular requirements. Linear-pull or "Vee-Brakes" can be installed by using the "Dimension Linear Pull Brake Mount" - this will work on both the forks and the rear triangle. Another option is the "Tektro U1 U-Brake Adapter Plate" which would allow the use of BMX-style U-brakes.
The rear brake to suit 406mm wheels is a very, very long reach calliper which isn't particularly strong. There are several more brake options with 451mm wheels as various modern medium to long reach brakes fit into the original mountings. If you consult a frame builder you can have mountings attached for cantilever or V brakes.
Forks and Headset
There are several fork options. The stock fork is perfectly serviceable, but can be improved. John S. Allen wrote to me saying that the stock fork can be cut down and re-threaded for a threaded top headset. It is also possible to use a 1" ISO threadless headset and a couple of spacers with the original 26tpi nuts for better performance, but you will need to replace the stem with a standard 1" quill stem in place of the stem clamp.
FireCloud Cycles make a 20" BMX fork with a 280mm long 1" threaded steerer suitable for the Twenty which includes canti-studs for Vee-Brakes.
A better quality set of front forks can be bought as a spare part from EasyRacers.com. The fork off the EZ Sport CX has a steerer the same length as the original "Twenty" forks. You will need a few spacers for 1" thread-less headsets. Update 2015: Sadly this particular part is no longer available.
Chris Harris fitted suspension forks to his "Twenty". "Forks are SR Suntour elastomer damped version. Bought cheaply off Ebay a couple of years back. I believe they were made for Recumbent bikes originally, since they need a long steerer. The Ebay seller gave me the choice of about five different steerer lengths. I chose the longest, and bought one threaded, and one un-threaded set, both with a 1-inch steerer." He also says that with the smaller wheels that suspension forks make a huge difference in absorbing the bumps and shocks that small wheels tend to pick up in the road surface.
Bottom Bracket and Crankset:
This is now covered by a separate article: The Bottom Bracket
Hubs and Gearing:
This is now covered by a separate article: Re-gearing the Raleigh Twenty
The standard mudguards are perfectly serviceable. However, they are often damaged or missing. A number of replacement mudguard intended for other 20"-wheeled folding bicycles are available as spares and can be fitted to the "Twenty". Planet Bike Fenders are also available in 20" wheel-size intended for recumbent bicycles, but they are perfectly suitable for the Twenty. I would recommend the version with the long mud-flaps already attached. SKS also now make a set of 20" mudguards, which are worth looking into.
The original racks were either a Pletscher rack, with its characteristic "rat trap" clip, or another type of rack designed to carry the originally-supplied carry bag. These both make perfectly serviceable carry racks, and particularly lend themselves to having boxes attached. You can re-use the original rack with a large rack-top or trunk bag in a similar manner to the Moulton bicycle.
Modern carry racks are readily installed, using the existing eyelet in the rear drop-out. Normally they are intended for 26" or 27" wheels. This leaves a rather useful space under the rear carry rack for various items, such as a toolkit, or a battery pack to operate one's lights. At least one person has already used it as a place to mount an extra water bottle mounting. Modern carry racks would also provide a useful mounting point for a rear light and would help make the Raleigh Twenty a useful bicycle for short touring. Be careful about rack and pannier choices, as the rear chain-stays on the Twenty are a little shorter than most other bicycles and there may be heel-strike issues.