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Silencing Wiper Wind Noise

A simple aerodynamic mod that can reduce cabin wind noise

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Seeing airflow over the wipers
  • Trialling a an aerodynamic deflector
  • Making a deflector
  • Testing its effectiveness
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This article was first published in 2005.

These days, most cars are aerodynamically pretty quiet. Whistles and rustles from the A-pillars and exterior mirrors are subdued – the result of many hours of development work in aero-acoustic wind tunnels. However, the windscreen wipers often escape the aerodynamic analysis and so contribute more than their fair share of wind noise.

When not being used, the best position for wipers is below the trailing edge of the bonnet – that way, they’re completely shielded from the airflow and so are quiet and don’t contribute to aero drag. However, taking this design approach requires a separate ‘parking’ position for the wipers, which adds cost to the mechanism. The result is that many cars use a design that leaves their wipers exposed to the airflow all the time.

Click for larger image

In some cases, the shape of the plenum area ahead of the windscreen, or the trailing edge of the bonnet, is used to direct air over the wipers, leaving them in what’s called a separation bubble. In other words, the airflow unsticks from the surface of the car just enough to pass over the wipers, re-attaching on the windscreen a little above the wiper line. The result is much reduced aero noise from the wipers.

Hmm, but what if your car (a) doesn’t run wipers with a separate below-the-bonnet park position, and (b) doesn’t appear to do anything fancy to direct airflow over the wipers? Chances are, you’ll hear whistles and rustles as the airflow collides with the aerodynamically ugly assemblies. (These noise are often most apparent on freeways, where the turbulence created by other cars can cause local wind gusts way above the average airflow speed.)

So are you stuck with the noise? Not at all – it’s easy, cheap and straightforward to make a small deflector that directs air over the wipers. The result is a quieter car.

Testing

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In this case, the car is a ’99 Toyota Prius. Despite having an excellent drag coefficient of 0.29, little attention seems to have been paid to the wipers. They don’t park beneath the level of the bonnet and as shown here, if anything, the bonnet flattens out before the wipers. At speed, aero noise can be clearly heard from the wipers.

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To see what the air was doing around this part of the car, small tufts of wool were temporarily stuck to the car with masking tape. The car was driven at 60 km/h and these pics taken. As can be clearly seen, there is attached flow up the bonnet and then onto the windscreen. (Attached flow is shown by the tufts lying flat along the body and pointing in the one direction – ie not whirling around.)

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Looking closer, it can be seen that reattachment occurs very low on the windscreen – even directly behind the wipers, the airflow is reattached at or below the black line on the glass. In other words, the full strength of the airflow is impacting the wiper assembly.

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This photo shows very clearly how the wipers are exposed to the air stream.

Prototyping

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It’s dead easy to make a prototype deflector out of thin plastic and duct tape. That’s just what has been done here, with an old Penfolds Wines plastic sign cut with sharp shears and bent into the required shape. The trial deflector was left on the car for a week while several hundred kilometres of freeway driving was undertaken.

Two effects were noticeable: wind noise from the windscreen wipers could no longer be heard, and the cabin ventilation system tended to breathe hotter air. The latter point is important: most cars pick up their cabin ventilation air from directly in from of the windscreen and so any change in the aerodynamic pressure at this point can affect ventilation.

Building the Real One

The final deflector was constructed from 4mm ABS sheet. This plastic is tough but can be bent (if heated first, the bend is retained), cut and filed. ABS is much more resilient than acrylic, for example. The sheet was bought from a plastics wholesaler.

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Using tape, the plastic was marked out to the approximate shape.

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Putting it temporarily into position allowed the shape to be assessed. It’s easiest to install if the deflector projects out from under the trailing edge of the bonnet. Look out for wiper clearance – both with the bonnet closed and open.

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To angle the deflector upwards, it needed to be heated and bent along its length. This was achieved a small section at a time, using a heat gun and few pieces of particle board to create a clamp and a lever. It’s tricky to get the bend even all the way along, but ABS can be re-heated and re-bent as often as you like. Practice on a scrap bit first!

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The deflector is held in place with double-sided tape. As can be seen in this view, unless you knew what you were looking for, you wouldn’t even see it.

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This side view shows the angle of the deflector.

Results

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Wool tuft testing was undertaken after the deflector had been fitted. As can be seen here, the flow reattachment is higher up the windscreen – the area below the black line is now within the separation bubble. And therefore, so are the wipers! However, the separation bubble is still very small - which is important for low drag.

Sure, the wool-tuft pics show the changed airflow behaviour, but what about the practical outcomes? In short, the deflector works very well. Aero noise from the wipers is now non-existent, while the final version of the deflector (which is a bit shorter than the white plastic prototype) doesn’t appear to change the behaviour of the cabin ventilation system.

After the installation of the deflector, highway fuel consumption has remained the same – indicative of the change in drag being very small or non-existent.

Conclusion

It’s no wild mod that will knock 2 seconds off your quarter mile, but as something that’s easy to trial and not much harder to do, if you have aero noisy wipers, making a small deflector is worth exploring.

Interested in do-it-yourself car aerodynamics? You’re sure then to be interested in the Amateur Car Aerodynamics Sourcebook, available now.

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