Beribo Replica Automobiles

A commonly asked question - Are three wheelers stable?

Yes, they are... provided they're designed properly!

Here is a little of the (simplified) theory:

If a car is static it is only producing one force... downward. The static car sits on the road surface quite happily although there is obviously an upward force preventing it from sinking! When in motion though there are many other forces affecting the cars movement.

These include the obvious forward motion and sideways force produced when cornering. It is during cornering however that one particular type of force can have a disastrous effect. This is Centrifugal Force and when combined with a sideways force the resultant joint forces can, if the axis of the force is outside of the wheelbase, cause the car to turn over. To combat the possibility of these forces the car designer must keep the centre of gravity low and the track of the vehicle wide.

Engine location is critical as this is by far one of the heaviest components. The engine should ideally be situated along the centre axis of the car and be as low as possible. In the case of the Reliant Robin, for example, the engine is installed over the single front steering wheel and when cornering can cause a considerable shift in the centre of gravity which when combined with the weight of the driver (with no passenger) can result in the all too common tipping car.

Taking a closer look at BRA's the weight distribution is as near perfect as you can reasonably achieve, bearing in mind that the car does have to look good!

In the case of the CX3 the weight of the engine is placed directly on the centre axis of the car and is in between the two front (steering) wheels. The car is extremely stable, even at high speed. However, drive is fed to the rear wheel and this is where the inexperienced driver can sometimes become 'unstuck'. When cornering sharply or at high speed there is always a tendency for one front wheel lift and lose contact with the ground.

The rear wheel is providing propulsion and if the front wheel continues to lift then the car is theoretically propelled towards the sky... and over she goes! In this type of scenario it is important to rapidly lift off the throttle in order to remove power the the rear wheel until the front wheel once again makes contact with the road. Experienced drivers of CX3's and similar cars are, with the help of these forces, able to almost drive them on two wheels!  This used to be one of my 'party tricks' when I had a CX3!

With our Citroen powered cars things could not be more different. Again, weight distribution is good but the cars have the advantage of being driven by the two front wheels via a differential. In the unlikely event of a front wheel lifting the differential immediately reacts and removes power from the other wheel. This is achieved without any driver intervention. The resultant loss of momentum will result in the raised wheel coming down and, once contact is made with the road, the differential engages drive and the car continues - in many cases without the driver even realising what has happened.

So yes, three wheeled cars are stable. It has been said by many that when you ride in a BRA you could almost be mistaken for thinking it was a four wheeled car. Treat them with respect and you will enjoy many hours of safe motoring.

See you out there, hopefully on three wheels!

David Wiles.

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