Thrust SSC - Engineering

About Face
The Reason for Rear-Wheel Steering

The driver, any driver, always has the steering wheel in front of him or her - right? Such astounding good sense leads to the logical conclusion that the steering wheel will therefore be connected to movable front wheels for the purposes of keeping the vehicle on the straight and narrow. Conventional wisdom therefore has it that the steered wheels on any vehicle other than dump-trucks or similar slow moving industrial devices need to be those at the front in order to excercise proper control.

Designers of land speed record-breakers are not usually constrained by what has gone before, so it should come as no suprise to find that a car as radical as Thrust SSC should stand conventional wisdom on it's head and opt for rear wheel steering, a first on any car targeted at the Land Speed Record. The world's fastest dump-truck? - hardly.

Like most other elements of it's radical design, Thrust SSC's rear steer layout emanates from a couple of basic principles that have guided the overall design philosophy from it's earliest days. Firstly, the car has to be inherently safe, which at the sort of speeds envisaged can be summarised in one word - stability. The adoption of the stable dart principle dictates that the heaviest parts of the vehicle, the engines, need to be installed towards the front of the chassis. Each needs it's own specially designed intake which at full power will be forcing several tons of air per minute through the engines.

While air intake design is crucial to the correct functioning of the engines, they also compromise the second major principle of the design, that of minimising cross-sectional area in order to reduce drag. A logical place to install the front wheels is at the outer edges of the engine pods thus providing stability by giving the widest possible track and leaving the intakes free from interruption. However, if you make those wheels steerable it increase the cross-sectional area, and therefore the drag, by an unacceptable level. Answer? - fix the wheels and think of some other method of steering.

Fixed wheels and a movable rudder mounted under the nose of his jet-powered Sprit of America, the first jet car to take the LSR, was an idea first tried by American Craig Breedlove back in the early sixties, but it proved to be unworkable at 300mph so there was no reason to suppose that it would work at 750mph. Recently, it has been suggested that computer controlled vectored thrust, essentially a movable tailpipe to direct the thrust from the engine could be used, but that idea is still a long way from reality.

The long narrow chassis section of Thrust SSC that runs between the engines suggested either a tandem rear wheel or offset rear wheel installation in order to minimise drag and to stay clear of the extremely hostile environment generated by the engines' exhaust. Tandem rear wheels, albeit fixed not steered, were another idea tried out in the sixties, this time by Bill and Bob Summers in a streamliner they built to attack the wheeldriven LSR. Although not fast enough to take the record it proved a point and the lessons learned were applied to their next car, the Goldenrod, which did succeeed in taking the wheeldriven LSR. It was a record that stood for many years until taken by another American, Al Teague, who used offset steered front wheels in order to reduce the width of his streamliner.

Rear-Steer Mini In a flash of lateral thinking one day, Glynn Bowsher asked why Thrust SSC shouldn't use steerable offset rear wheels. Why not indeed? After more discussion and detailed calculations it became clear that this could be the answer to the problem, but theory is one thing and practical application another. In order to remove any lingering doubts, including those in the mind of the man who would have to rely on this novel method of direction, it was decided that a test car would be built to prove the theory. And so was born a very strange looking Mini.

While the engine and most of the drive train remain standard, the front wheels have been locked up and an extended tubular chassis grafted on at the rear to carry two offset steered wheels in the format to be used on the record-breaker. Although it may look like the automotive equivalent of the Frankenstein experiments using a shopping car and a dragster as the unwilling donors, it does actually work. Lengthy runs at the Motor Industry Research Association (MIRA) test track not only proved the point to those remaining sceptical, but also gave Richard Noble valuable experience driving a rear wheel steer car. In fact, after a while he pronounced it more stable to drive that a standard Mini.

Back in the sixties, Craig Breedlove's failed attempts to use fixed wheels and a rudder were solved by making the single front wheel steerable through only one degree. With a proven rear wheel steering concept and a chassis installation that will allow six degrees of steering movement, Thrust SSC should prove every bit as controllable as it's illustrious predecessors. I wouldn't mind a go in that Mini myself!

About this site

Supersonic Race Update Mach 1 Club ThrustSSC Shop
Sponsored by This site best viewed with Microsoft Internet Explorer 3
Digital msie artewisdom

© SSC Programme Ltd, 1997