The Golden Rule of Trike Design

by Patrick Fenner

My investigation into the "golden rule of tricycle design" (having 66% of the weight on the front axle and 33% on the rear) showed that this was more restrictive than necessary, and that following a set of three design guidelines would better and allow greater innovation. These are:

  1. The centre of gravity should be mounted as close to the two-wheel axle as possible to maximise rollover stability.

  2. The height of the centre of gravity should be less than half the track measurement and less than the distance to the front axle.

  3. If the centre of gravity is in the front half of the vehicle, the vehicle will be stable at all speeds, otherwise further calculation is necessary to determine the speed limit of lateral stability.

The release of the Atomic Duck design lead to a discussion on the 'BentRider Online forums about the ideal weight distribution of a tadpole trike.

Because I wasn't aware that the dynamics of three wheeled vehicles were particularly different to four wheeled vehicles, I asked for more information--this is one of the reasons open source works well, exchanging ideas to improve understanding. The responses suggested a requirement for weight distribution that was very prescriptive. To paraphrase:

The golden rule of trike design:

To ensure stability of a trike with two wheels on the front axle, the centre of gravity (centre of mass) must be a third of the wheelbase back from the front axle. This gives a static vertical wheel load of a third of the weight on each wheel; equal to 66% on the front axle and 33% on the rear.

What also came through was, that there wasn't one clear reason why this rule existed. Suggestions were made about balancing rollover stability with braking performance, but my first article on tricycle stability and its research didn't show anything so singular.

After asking about the source of the golden rule, purplepeopledesign told me about "Three Wheeled Vehicle Dynamics" from Jeffrey C Huston, Brian J Graves and David B Johnson from 1982. It seems likely that this is the original source for the rule, they don't cite any previous paper in their references, it was originally published in 1982 and has been used as a source in an IHPVA journal article.

Specifically, they conclude:

To ensure lateral stability for the situation of constant speed straight line motion, it is recommended that the three wheeled vehicle with two wheels on the front axle be designed so that its mass center is located in the front third of the vehicle…

Huston, Graves and Johnson's work on rollover stability is similar to what I presented in the previous post about tricycle stability, though they continued further. But I did find some confusion in the lateral stability calculations. When I re-ran their calculations, I came up with a different conclusion.

As a result, I propose that the golden rule might be better replaced by three design guidelines:

  1. The centre of gravity should be mounted as close to the two-wheel axle as possible to maximise rollover stability.

  2. The height of the centre of gravity should be less than half the track measurement (and less than the distance to the front axle).

  3. If the centre of gravity is in the front half of the vehicle, the vehicle will be stable at all speeds, otherwise further calculation is necessary to determine the speed limit of lateral stability.

Unfortunately, the paper is not freely available online…which restricts how easy it is to discuss the contents. So I've reviewed the main findings about rollover and lateral stability, and how my results differ from the original authors this technical article: On the Golden Rule of Trike Design

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