Shopping: Real Estate |  Costumes  |  Guitars
This Issue Archived Articles Blog About Us Contact Us

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
Email a friend     Print article
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.


Click for larger image

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.

Click for larger image

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.)

Click for larger image

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.

Click for larger image

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


Click for larger image

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.

Click for larger image

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

Click for larger image

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.

Click for larger image

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!

Click for larger image

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.

Click for larger image

This side view shows the angle of the deflector.


Click for larger image

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.


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.

Did you enjoy this article?

Please consider supporting AutoSpeed with a small contribution. More Info...

Share this Article: 

More of our most popular articles.
A custom PowerChip remap - now she comes alive!

DIY Tech Features - 8 March, 2011

Powering-Up the 1.9 litre TDI, Part 4

Useful parts for nothing

DIY Tech Features - 6 October, 2009

Getting the Good Parts from Scanners!

Some of the best wind tunnel pics you'll ever see

Technical Features - 4 July, 2007

Aero Testing, Part 4

Step by step of keeping drill bits sharp

DIY Tech Features - 20 August, 2008

Sharpening Drill Bits

Developing a new Human Powered Vehicle suspension system

DIY Tech Features - 13 July, 2010

Chalky, Part 8

So what's detonation and how can it be detected?

DIY Tech Features - 2 November, 1999

DIY Detonation Detection Part 1

How Ford in the US is developing safety systems - it's very weird!

Special Features - 29 September, 2009

Water-Blasting Cannons and Shopping Trolleys...

What you need to know about arc welding

DIY Tech Features - 20 June, 2007

Beginners' Guide to Welding, Part 1

This is what happens when you put a current Merc diesel into a 20 year old body!

Special Features - 12 January, 2010

Mercedes Makeover

Squirt your intercooler spray for 5, 10 or 20 seconds - all at the press of a single button!

DIY Tech Features - 2 September, 2008

Intercooler Spray Squirter

Copyright © 1996-2020 Web Publications Pty Limited. All Rights ReservedRSS|Privacy policy|Advertise
Consulting Services: Magento Experts|Technologies : Magento Extensions|ReadytoShip