The Lexus LS possesses the lowest engine noise levels of any car in its class, while in the rear the level of ‘conversation clarity’ is almost 100% when the car is travelling at 62mph. Many factors contribute to the aura of calm isolation enjoyed by passengers in the Lexus flagship but the striving for perfection has gone much further than many people think. Indeed, the latest innovation is what you might call a reinvention of the wheel.
Lexus engineers already understand the benefits of lightweight wheels, in that reducing a car’s unsprung weight improves maneuverability, acceleration and economy. The engineers also realised that if they could fight road noise at its source — where the vehicle makes contact with the road through the wheels — it would enhance the benefits of all other noise-suppression measures.
Their ingenious solution has managed to harmonise both ideals thanks to a novel alloy wheel design that not only reduces weight but actively suppresses noise.
Road noise is produced by the tyre’s adhesion to the road surface as well as the expansion of air inside the tyre when it is deformed by imperfections. While Lexus has no immediate plans to move into tyre production, it was the management of this audible expansion that particularly intrigued the engineers.
The new wheel they designed to combat noise production features a hollow outer ring just under where the bead of the tyre sits. This cavity is connected via narrow channels to four or five (depending on whether the wheel is 18 or 19″ in diameter) chambers that are themselves connected to the main body of air within the tyre. The hollow ring is therefore pressurised at the same figure as the inside of the tyre.
When air inside the tyre is compressed and released by surface imperfections, vibrations are produced that transmit sound waves from the bead of the tyre into this special cavity built into the wheel rim. But because the ongoing route through the narrow resonating channels to the sub-chambers and into the hollow structure of the tyre is a tortuous one for these frequencies, their energy — or in other words, their audible volume — is all but exhausted.
Heat is a natural by-product of squeezing air through such narrow pathways but the engineers have made sure this is effectively dissipated; first through the heat-sink effect of a route to a cooler environment that gets increasingly easy, and secondly through the heat-conductivity afforded by the wheels’ aluminium construction. Heat also has a handy knack of suppressing air column resonance, which in turn means less noise through less vibration. Double the noise suppression then…
The net result of the efforts of Lexus’s engineers is a new type of noise-reducing alloy wheel for the LS that interacts with the road surface with an almost uncanny silence. But you can be sure that noise suppression is something of a Sisyphean task for the team; they will already be focusing attention on further areas where improvements might be possible.