Designing a Stable Three Wheel Car

I. Preface

Solar cars usually come in two configurations: 4 wheeled (2 front/2 rear) or 3 wheeled (2 front/1 rear). As you would expect, the three wheel variant comes with concerns about maintaining stability. This document can be used as a resource for designing a three wheel car and optimizing (particularly the dynamics systems) for improved stability.

II. Introduction

• A crucial vehicle property is the location of the vehicle center of gravity (CG). If it is located properly, the vehicle will be “stable” in terms of:

  1. Resistance to “losing the rear end” in turns and crosswinds.

  2. Ability to travel at high speed without continual steering corrections to counteract weaving.

  3. Resistance to tipping over in turns and in encountering changes in road surfaces if sliding.

  4. Resistance to swapping ends in hard braking due to weight transfer from the rear to front.

• The location of the CG should be a design specification. Components should be arranged to achieve a specified location of the CG, instead of arranging components through an ad hoc process of fitting things where it is convenient or accessible or to solve some interference problem arising from previous ad hoc choices

• The information provided here should give you the ability to state desirable vehicle responses in terms of yaw, tipping and braking weight transfer. There will follow a few inequalities that 1 involve the CG location, front track and wheelbase, allowing these values to be chosen to satisfy the desired responses. A major point is that the wheelbase, track and CG location should be treated as design decisions.

III. Center of Gravity Location and Yaw Response

The yaw response of the vehicle refers its tendency to rotate about a vertical axis through the CG, or “spin”.

Tire Response to Side Load

Directional Stability

A side gust of wind applies a side force to the car which, in general, is not applied at the CG. This causes a yawing moment about the CG causing the car to turn.

The weight distribution between the front and rear wheels greatly influences directional stability.

Main factors:

1. CoG height

2. CoG distance from tip axis

3. mass of car

4. weight distribution front/rear

5. CoG longitudinal location

6. Track width/wheelbase ratio