REVISED February 2012
The Raleigh Twenty was variously supplied in 3 forms:
Single-Speed - a basic coaster-brake hub.
2-speed - NZ versions appear mostly supplied with the Sachs Duomatic 2-speed kick-back hub
3-speed - Mostly 3-speed Sturmey-Archer AW hubs, but some were fitted with Shimano hubs
Today, there is a bewildering variety of internally-geared hubs on the market. Sometimes it's hard to chose which one would be most suitable. If you don't want to make modifications to your frame, then go for a hub with a narrower over-locknut-dimension of less than 120mm. Anything wider will require having the rear triangle of the frame spread apart. In researching for this page, I found that there were more than a dozen options out there for people who wanted internally-geared-hubs.
So, why an internally-geared hub? There are several benefits. Less wearing parts means reduce long-term maintenance costs because there are less part to replace. There is reduced maintenance requirements, and less parts to clean. It is almost impossible to throw the chain off the sprockets. You can shift gears whilst stationary - this is the big selling point, especially for people who have to ride in stop-start traffic a lot.
Rohloff 14-speed (526% gear range)
Perhaps the ultimate in epicyclic hubs available today - also the singularly most expensive. I would not leave a Rohloff-equipped bicycle outside for long. Only available drilled for 32-spokes.
Shimano 11-speed (409% gear range)
The 409% gear range will make it possible to gear a bicycle either as a tourer or a city bicycle. It is possible to get a very good spread. It is about double the price of Shimano's 8-speed hubs. It seems that Shimano used gear ratios which best suit 20" wheels, as a 44t chainring and 18t sprocket produce an excellent touring range of gears of approximately 23 to 94 gear inches.
SRAM iMotion 9-speed (340% gear range)
A similar spread of gears as most 18-speed MTBs. An excellent choice for utility trips, but relatively expensive compared to the two 8-speed hubs available. 135mm over-locknut-dimension means that you will need to have the rear triangle spread to suit.
Sturmey-Archer X-RF8(W) 8-speed (325% gear range)
Unique in that gear 1 is direct drive and all other gears are step-ups. It's more difficult to get the super-low gears for touring/load carrying. Your gear range would be 36 to 118 gear inches, unless you replaced the crank-set with a smaller-diameter chain-wheel. The X-RF8 comes in two versions, one of which has an over-lock-nut dimension of 120mm. However, it has an extra (redundant) locknut which when removed reduces the OLD to 115mm. Sheldon Brown used one of these hubs on his Raleigh Twenty. He took advantage of the 6 close-spaced gears for most riding with the extra low gears as the hill-climbing "bail out" gear and the top gear for fast descents since he was against coasting downhill.
Shimano Nexus 8 8-speed (307% gear range)
Perhaps the ideal hub for a Raleigh Twenty upgrade. Overall, the price of this hub makes it an attractive upgrade. You can fit either a 16 or 18-tooth sprocket. The difference is basically whether you want an extra top gear for flat areas or an extra bottom gear for hill-climbing.
SRAM S7 7-speed (303% gear range)
Available in freewheel, coaster-brake and drum-brake versions, this is another good choice for the Raleigh Twenty. The Over-Locknut-Dimension of this range varies from of 132-135mm, which means that you will need to have the rear triangle spread to suit. This hub no longer appears on SRAM's website and seems to have been discontinued.
Sturmey-Archer S-R5(W) 5-speed (256% gear range)
A good choice if you find that your traditional 3-speed hub isn't quite good enough for your needs. However, pricing is not reflective of where it sits in Sturmey-Archers range. It is a little over-priced and for a similar outlay one could obtain an 8-speed hub.
SRAM P5 5-speed (251% gear range)
This hub was available in bother freewheel and coaster versions but no longer appears on SRAM's website and appears to have been discontinued
SRAM T3 and iMotion3 3-speed (186% gear range)
A slightly-wider-ranged 3-speed hub. SRAM offers several types with different braking options.
Shimano 3-speed (184% gear range)
Slightly wider gear range than Sturmey-Archer, and has a very good modern coaster-brake option.
Sturmey-Archer 3-speed (177% gear range)
The original and the best. AW hubs will go on just about forever as long as you regularly oil them. (one small squirt of oil every month seems to be plenty, but refer to current service manuals to be certain). Sturmey-Archer also make a more modern line of 3-speed hubs with alloy shells, better seals and have a variety of options such as trigger, lever, barcon, and grip-shifters. Sturmey-Archer now offer various models which include coaster brake or drum-brake options. There are several different over-locknut dimensions, depending on which particular model you use.
Sturmey-Archer S2 and S2C 2-speed (138% gear range)
A modern version of the Fitchel and Sachs 2-speed Duomatic hub. This one shifts by a small kick-back, so requires no other control equipment. It comes in both freewheel and coaster-braked versions with the same kick-back shifting. The S2C is spaced at 116mm and the S2 is at 120mm.
SRAM Automatic 2-speed (137% gear range)
Not to be outdone by Sturmey-Archer, SRAM has released the 2-speed AUTOMATIC hub. This hub will automaticallt shift from low to high gear at a pre-set speed, utilising the centifugal forces of the spinning hub. Available both as coaster and freewheel versions. The hub is spaced at 120mm, but also comes supplied with spacers for 130mm.
An advantage of either of these 2-speed hubs is that it is possible to only need a single cable from the front brake - clean lines, no shifter or rear brake cable - the hub shifts for you and you pedal backwards for the brake!