Can you reduce road noise by changing your car’s wheels and tyres?
The ancient Jedi art of cutting road noise by reverse engineering the car’s wheels/tyre package. Or not…
Let’s drill down into the tao of noise, and the attenuation thereof.
It’s very difficult to correlate proposed modifications and the effect this might have on NVH. This is one of the most complex areas of engineering R&D in new vehicles - partly because you can measure frequency spectrums until your head separates from your neck… ... but ultimately you have to correlate those measurements to what some poor bastard in the cabin actually experiences. That’s a real challenge.
In part this is because we don’t hear all noises equally. We’re particularly sensitive to noises at three-to-four kilohertz. That’s about the sound of a baby crying or a siren on an emergency services vehicle. Or a smoke alarm.
These noises are selected for a reason: evolution, and sirens and alarms by people who understand human hearing response. They’re the sounds most likely to pull you out of a cognitive coma and get you responsive quickly.
The upshot of this is that a noise at three kilohertz is likely to be perceived as more annoying inside the car than another noise at 100 hertz or 15 kilohertz - even though all three might be playing at the same sound pressure level.
See also:
There are so many variables when it comes to the generation of noise and vibration inside a car.
Like: you drive from a nice smooth piece of hotmix to a section of coarse chip at freeway speeds, and the subjective noise level triples. It’s a completely different set of NVH inputs that you’re trying to attenuate. Dirt roads are a different challenge again - smooth clay or rough gravel? Big difference.
Wind noise is another bugbear - aerodynamic effects vary with something generally between the square and the cube of speed. It depends on the shape, and other properties, like flow separation, and the positions of wake vortices, etc.
This is proper propeller-head stuff. You need a full-on laboratory and a university education in applied science to grasp it. To me, proposing a bolt-up fix is a bit simplistic.
Wind noise - or at least some of the noise generated aerodynamically - could be eight times louder on the highway, compared with around the city and suburbs.
Then you’ve got the engine and transmission - all sucking, squeezing, banging and blowing, churning, burning, turning and squirting at different frequencies. I’ve watched a lot of movies on these issues, and I can tell you there’s more research to be done there. As carmakers make the bodies of the vehicles more rigid, they transmit more NVH. Attenuation is a real challenge. They have targets to meet. They generally meet them, but everything’s a compromise - performance cars tend to be noisier than luxury cars.
It may well be that moving down from 21s to 19s or 20s with inherently more sidewall height (because you’d want to maintain overall rolling diameter) does actually result in a measurable attenuation of NVH - but it’s just as likely that it will not.
In general I can tell you that asymmetric tyres tend to be quieter than directional tyres, but that’s about it. Do you want quieter in the dry or the wet? On what surface? It all matters.
It’s also likely that you might not perceive much of a difference. The tread pattern of various tyres is a salient factor - but, at $1000 a corner, how many tyres are you prepared to test to derive the quietest one for the roads you’re on?
With wheels and tyres it really depends upon the frequencies that are transmitted, the ones you perceive as a problem, and whether the sidewalls are in a position to attenuate them.
And then there’s this counterintuitively kooky phenomenon that occurs: If you are a proper propeller-headed NVH engineer and you successfully attenuate wind noise, for example, the perception of road noise by the owner is going to goes through the roof (because there’s much less wind noise to cover it up).
Let’s say you are breathtakingly successful: you swap your 21s for 19s and choose the Goldilocks of silent tyres, thus slashing road noise. It’s very likely that wind noise will seem much louder now.
In a nutshell, I’d suggest that reverse-engineering NVH with wheels and tyres, and hoping for a real, meaningful improvement is a fool’s errand. You’ll almost certainly degrade the handling performance of the vehicle, and you might spend thousands and not get much of a subjective result, at least on the perceived NVH front.
This is a great advertisement for test driving the car you think you want on the kind of roads you are actually going to drive on. SUVs are generally more tolerable on a noisy backroad than a conventional car, and vice-versa in the city. It’s important to choose the right tool for the job.
