Wheels and deserts

by Andy Green

A Land Speed Record vehicle is first and foremost a car. It may be travelling at jet-fighter speeds (or, in the case of BLOODHOUND, rather faster!) and using a range of aerospace technology, but it is still a car which (under FIA regulations) requires at least 4 wheels. Hence we need wheels which can support and control the vehicle, and which can also take the huge stresses of spinning at an enormous rate. In BLOODHOUND’s case, the wheels will need to support the 6-7 tonnes of the Car’s weight between them and rotate at up to over 10 000 RPM – which will induce stresses of around 50 000 times the force of gravity (or ‘g’) at the wheel rims.

This sort of load requires extremely strong wheels – Thrust SSC opted for forged Aluminium to cope with around 35 000g, while BLOODHOUND is using Titanium wheels to endure 50 000g at 1000 mph. This now raises a further problem – if we have to use solid wheels, what sort of surfaces can they run on?

Thrust SSC’s engineer Glynne Bowsher with a 140 kg solid Aluminium front wheel fitted to SSC.

There are basically 2 choices of surface, salt and alkali playa (beaches were also used up to the early 1930s, but were too soft and uneven for speeds approaching 300 mph and have not been used since then).

Dry salt lakes such as Bonneville are astonishingly hard – it is impossible to stick a knife blade more than a few millimetres into the surface – and they provide a high degree of grip for rubber tyres (as a rule of thumb, the grip for rubber tyres on salt is about half that of tarmac, with a coefficient of friction of around 0.6, compared with a tarmac figure of over 1.0).

The alternative surface is an alkali surface, which is effectively a very fine dried mud. The drawback with alkali, which is a softer surface than salt, is the higher drag. The big advantage is that the surface has a degree of compliance – in other words a bit of ‘give’ in it – that produces a much smoother, more controlled ride for a very high speed solid wheel.

Before Thrust 2, jet cars used conventional wheels with tyres, albeit tyres designed for very high speeds. With rubber tyres, the hard surface of Bonneville salt flats was ideal – good grip to control the car and low drag to help achieve the maximum speed. As the extreme loads on the wheels forced Thrust 2 to use solid wheels, with no give in either wheel or surface, salt was suddenly an unfriendly surface. Only a few cars have tried to run solid wheels on a hard salt surface. Richard Noble and the Thrust 2 team struggled with solid wheels at Bonneville in 1981, finally getting the car up to 500 mph on the salt. Rain forced the team to find somewhere else in 1982, so they moved to the Black Rock Desert where they found that the solid-wheeled jet car was much more controllable on the softer alkali surface. Subsequently, Rosco McGlashan ran his ‘Aussie Invader’ on the salt surface of Lake Gairdner in Australia with mixed success, finally wrecking the car when the car went off line at over 500 mph and hit a timing light. Craig Breedlove also ran his ‘Sonic Arrow’ on salt, at Bonneville in 1996, but found the car virtually uncontrollable. While there are sound explanations for the problems that all 3 cars encountered (including soft salt, non-optimal wheel designs and steering geometry problems), no-one has successfully run a solid wheeled jet car on salt at high speeds. That’s not to say that it can’t be done, but there is an element of technical risk in trying to do it for the first time.

With the desert search programme continuing, it seems quite likely that BLOODHOUND may end up running on salt. To try to limit the technical risk, we will need to consider the best tread/keel pattern for the wheels to give good lateral grip on the salt. With so little information available, it will be hard to decide what wheel profile we should use. We would ideally like to run a test programme at Bonneville, to try several different wheel profiles on the salt, but time and money are (as ever) against us.