Motorcycles can be a fun and efficient way to get around, but they can also be (quite literally) a pain in the back to ride. Motorcycle-based trikes are nothing new, but modern trikes are significantly more versatile and advanced than older designs. However, all this tech comes at a cost: between £1,950 and £6,500, to be exact. However, some smart shopping and creativity can get you all the fun of any high-end ride at a fraction of the cost.
Trike parts are like any other; the more specialised they are, the more they're going to cost. The trick to keeping cost and complexity to a minimum is using components that lend themselves to universal design, and that means car parts. An automobile rear axle would seem ideal for trike applications, but come with one serious limitation -- cars use a driveshaft, and many bikes don't. The simplest solution is to simply purchase a bike that came with shaft drive, which many did until about 10 years ago. A good 500 cc shaft drive bike will run you between £9,750 and £1,300. To put that into perspective, a trike conversion kit will run you at least £1,950; and that doesn't include the bike. If you already have a chain-drive bike, shaft conversion systems can usually be purchased for under £325.
Choosing an axle
The only limitation on axles is the requirement for an offset differential, similar to those found on the front end of any four wheel drive vehicles. Jeep axles are narrow enough to make for a good trike, and many front axles are fairly light. Hard core off-roaders are always replacing their front axles with stronger units, so your local off-road enthusiast community might be a good place to do some shopping. You'll need to put some hours in to make the axle work. First off, the differential faces the wrong way, so make sure that you get one that can be flipped. Secondly, truck front axles have steering knuckles, so you'll have to either lock the knuckles together with a tie rod, or replace the axle tubes with those designed for a rear-drive axle.
Putting it together
Hard-tail bikes are easy to build; connect the axle tube to the bike frame with a set of ground-parallel frame rails and down-tubes. Hard tails are light, cheap and fun, but can be a little hard on the back. If you're building a soft-tail bike, then you'll need a U-joint or CV-joint equipped, articulated driveshaft and "trailing arm-type" suspension. You can connect the lower control arms (trailing arms) to the frame with either a set of Heim joints or rubber bushings on either end. Support the bike with coil-over struts connected to the seat down-tube. For inspiration, you need only take a look at the trailing arm suspension of a Volkswagen Beetle, Citroen 2CV, Acura RSX or any number of vintage drag-racers.