MANY old codgers (your columnist included) imagine they would be safer riding a tricycle than trying to balance themselves precariously on two wheels. Certainly, adding an extra wheel to a bicycle allows the vehicle to stand upright on its own while stationary. By contrast, a bike at rest will wobble and fall over if no action is taken by the rider. However, something which is not always appreciated is that, though tricycles may be stable when stationary, unlike bicycles they can be horribly unstable once on the move. All the more so if the rider tries to turn too sharply, and especially so if the turn is attempted while hurtling downhill.



A conventional tricycle’s instability is caused by having a single steering wheel out front, and two driving wheels behind. With such a “delta” configuration, the vehicle’s centre of gravity is not only high but also well behind the steering axis, which passes through the tricycle’s handle bars. In attempting to make a turn, the contraption becomes, in effect, a pendulum—albeit one swinging in a horizontal plane parallel to the road surface rather than vertically. Even a gentle squeeze of the brakes can then be enough to nudge the pendulum bob (the tricycle’s back half plus rider) into sideways motion. If heading downhill at the time, this yawing moment is amplified by gravity. A bruising tip over the top will then ensue as the tricycle topples sideways.



Like a four-wheel vehicle, a tricycle’s rollover resistance depends on the ratio of its half track-width (half the distance between the two side-by-side wheels) to the height of its centre of gravity. The further the two wheels are apart, and the lower the centre of gravity, the greater the trike’s safety margin.



One way of improving a tricycle’s rollover resistance is to reverse the configuration. A “tadpole” arrangement—with two wheels out front and a single one at the back—puts the centre of gravity close to the steering axis. Doing so reduces the tricycle’s radius of gyration (ie, the length of its imaginary pendulum arm) to something close to zero. Presto, the yawing motion is then virtually non-existent.



The tadpole trike, thanks to its geometry, is far less of a handful than its delta sibling. Three-wheelers with a delta layout are natural oversteerers—ie, have a propensity to turn more than the rider intended. By contrast, three-wheelers with a tadpole layout are natural understeerers—ie, tend to turn less than commanded. In extremis, an oversteering vehicle tries to swap ends when attempting to turn too aggressively, while an understeering one likes to slide headlong in the direction it was originally travelling. Another way to make any three-wheeler handle better is to arrange for the front wheel (or wheels) to lean into a bend, as a motor-cycle does. This complicates the design, but can render the vehicle immune to rollover—provided, of course, that the lean angle is sufficient to keep the vehicle’s weight and centrifugal forces in balance. In such circumstances, the distance between the two side-by-side wheels becomes irrelevant. In principle, a tadpole trike fitted with tilting front wheels could then be made almost as skinny as a bicycle.

Such handling know-how can be applied equally to motorised three-wheelers, though the characteristics of the tyres and the compliance of the suspension play a far greater role in determining whether a motorised three-wheeler will slide or spin when pushed to the limit. That said, most drivers find understeer less daunting and easier to cope with than the kind of oversteer that can cause a vehicle to snap into a spin. For that reason, all popular four-wheel vehicles are designed to err on the side of understeer.

Not so sports cars, especially mid- or rear-engined ones, which will often oversteer viciously at the limit, if not fitted with electronic traction aids to prevent that happening. The logic is that a tendency towards oversteer endows the vehicle with sharper handling. Oddly, however, it can also be safer. For instance, if a high-speed crash becomes inevitable, side-swiping a roadside object in a spin is generally less lethal than sliding into it head on. Also, as unnerving as it may seem, a spinning car scrubs off speed far quicker than a sliding one, and comes to rest in a much shorter distance.

As far as three-wheelers are concerned, the vast majority of push-trikes and motor-trikes favour the delta layout, with one wheel out front and two behind, even though they are more unstable and have a greater propensity to oversteer. Anyone who has watched the hilarious BBC “Top Gear” episode featuring a Reliant Robin will testify to the flaws inherent in the delta design.



The sole reason for the delta’s popularity in the past was cost. The mechanism for steering one wheel, rather than two, is a lot simpler and cheaper to build. Also, most motor-trikes are conversions of existing motorcycles, with minor modifications to the rear part of the frame, bodywork and final drive.



The Reliant Robin—which was introduced in Britain at the height of the 1973 oil crisis and remained in production for 30 years thereafter—had its merits, despite its delta layout. It got 70 miles per imperial gallon (4 litres/100km), had a claimed top speed of 85mph, and could be driven on a motorcycle licence, with considerably lower tax and insurance costs. Both the Robin and the even smaller Bond Bug (subsequently acquired by Reliant) were widely popular in the depressed north of the country where, until then, car ownership had been beyond the reach of many.



To this day, both British marques have legions of devoted owners, who have learned to live with the handling foibles of their beloved delta three-wheelers. According to a survey made in 2011, Reliant Robin drivers are, statistically, the safest in Britain: further evidence, were it needed, that the safer cars themselves become, the greater the risks their drivers take.



The majority of three-wheel cars, before and since, have been tadpoles, with two wheels out front and a driven one behind. This was the layout chosen by Messerschmitt, in Germany, for the popular three-wheel “bubble cars” it produced in the post-war years when it was forbidden to build aircraft.



In Italy, the egg-shaped Iso Isetta, launched in 1953, had a single door between the two front wheels, which swung out, with the steering-wheel and instrument panel attached, to provide access to a bench-seat for two people. A tiny two-stroke engine drove the single rear wheel (later replaced with a pair of closely spaced wheels to improve lateral stability). The innovative Isetta became the most popular bubble car of all, being built under licence in half a dozen countries during the 1950s and early 1960s. In Germany, BMW alone made over 160,000 Isettas.



By comparison, the most popular three-wheeler of the pre-war years—the open two-seat Morgan V-Twin, built in Britain—adopted a far more conventional tadpole design. The classic model had its exposed motorcycle engine mounted ahead of the radiator and front axle, endowing it with excellent handling for a car of its era. Later models used an enclosed four-cylinder car engine behind the radiator. Thanks to their lightweight construction and novel independent front suspension, Morgan three-wheelers enjoyed notable success at sporting events.



They remained in production until 1952, when Morgan ceased making three-wheelers in favour of four-wheel sports cars. However, in 2011, the firm launched an updated version of its pre-war V-Twin. So popular has this vintage-looking three-wheeler, with its lusty American motorcycle engine, become that it apparently now outsells all other (four-wheel) Morgans in the United States and elsewhere.



There is no doubt that the Morgan’s arrival on American shores in 2012 helped rekindle interest there in three-wheelers generally. As elsewhere, much of this was made possible by a loop-hole in the safety regulations. In most jurisdictions, three-wheelers are classified as motorcycles, and do not have to undergo costly crash testing. Nor must they meet the same emissions requirements as four-wheel vehicles. But not all motorcycle rules apply. In many parts of America, people riding in a three-wheeler do not need to wear a helmet, provided the cabin is enclosed. And as long as they have a car licence, drivers do not need to take a motorcycle test.



Polaris Industries, a Minnesota-based maker of snowmobiles, all-terrain vehicles and motorcycles, has taken full advantage of this loop-hole in the regulations. In 2014, to widespread acclaim, it launched a three-wheeler called Slingshot. Powered by a 2.4-litre engine from General Motors, this tadpole sports car has an open cabin with side-by-side seating for two. Where the retro Morgan three-wheeler wallows in nostalgia, the Slingshot looks to have come straight off a "Star Wars" set.



Starting at around $20,000, the Polaris Slingshot is a bargain compared with the new Morgan, which costs $45,000 and up. Want yet more thrills and spills in an even pricier package? Try the T-Rex from Campagna of Quebec, which lists at $62,000 before extras are added. These are big-boys' toys, not econo-boxes like the Reliant Robin.



Why no re-incarnation of the frugal little Robin, offering e-car economy at a quarter the price? Well, despite tumbling oil prices, one such fuel-sipper is in its final stages of development. Elio Motors of Phoenix, Arizona, plans to get its crowdfunding ball rolling this week, with a request to the federal authorities for permission to issue $25m of equity to the public. The start-up already has more than $30m in promises, from some 8,200 potential investors.



The extra cash is needed to get Elio’s tadpole three-wheeler—offering a fully enclosed cabin, with two seats in tandem, air-conditioning, traction control, three air-bags, anti-lock brakes, a reinforced rollover cage and 50% larger crumple zones than any comparable vehicle—into production at a former General Motors plant in Shreveport, Louisiana. If the company can make it all happen, Americans will be able, late next year, to buy a sprightly commuter car capable of 84 miles per US gallon (2.8 litres/100km) for less than $7,000.



Sad to say, the road to motoring nirvana is strewn with shattered promises and punctured dreams. But your columnist would be a thoroughly happy Elio owner if the dream were to survive the coming year or so of punishing reality. A nimble little three-wheeler would be the perfect antidote to his late-life crisis.