Effective Defog and Defrost Strategies for Racecars…


And why I need to implement them now. In past seasons, races in the rain were rare, with only a few to suffer through. But lately, it seems like every race weekend has weather looming over it. Half the races this season have been in what I like to call dynamic conditions. And with these dynamic conditions comes fog. Making what used to be a side note a big priority. Namely, developing a valid defog strategy for races in the rain to maximize vision and results.

What causes fog in a racecar?

In order to develop a proper defog strategy, we must first understand what causes fog in the first place and how we can control these variables while racing.

I won’t get too scientific, but the fog on the windshield is condensation that forms when there is a difference in the temperature and humidity between the air and the windshield surface. This leads to condensation forming on the glass surfaces. So when the moist air inside the car comes into contact with the cold windshield, the moisture in the air condenses into tiny water droplets on the surface of the windshield, causing it to fog up. This is similar to how water droplets form on the outside of a cold glass of water on a hot day.

A few variables play into the buildup of condensation inside the car. We have airflow, temperature, humidity, and surface tension. We will look at how to use those variables to dial in the defog strategy while racing shortly.

Example of a bad defog / anti-fog strategy

But first, check out what a bad defog strategy looks like. The pics below highlight the defog strategy I have run for years: none.

Clear windshield at the start of the rain session at Talladega Gran Prix in the Spec Miata.
Not too bad at first. Wait till you see the windshield after a few laps.
Windshield foggy in the spec miata after a few laps at Talladega Gran Prix
The same section of track after a few laps. (You can see the tire wall out the left window) There is actually a car in front of me at this point. And yes, I pulled off the track on this lap.
Talladega Gran Prix Raceway: TGPR

On a side note, these pics were taken at Talladega Gran Prix Raceway in Munford, AL. TGPR is a very small, slightly nuanced ‘racetrack.’ I put quotations around it because it would serve better as a kart track. Regardless, they frequently hold open lap days for cars and motorcycles. This was my first time there and definitely was not ideal driving conditions.

Effective Strategies to Deal with Fog on the Windshield

Every seasoned racer has a story of driving a racecar with a fogged windshield. Usually involving the squeegee-on-a-stick story or, in my case, during the later sessions at TGPR, a hoodie in the lap. These stories involve manually wiping the inside of the windshield on the straights to remove as much condensation as possible before the next turn. This is easier said than done while strapped into the racecar, with the driver typically only able to clear a little porthole of vision.

This method is reserved for the unprepared. And in the pursuit of more speed and more victories, it is time to jump into those four variables I listed above to dial in an effective defog strategy moving forward.

1. Airflow

Traditionally, the factory defrosting and air conditioning modes remove fog and condensation from the inside of the car. However, most serious racecars do not retain factory air control systems. (This post also assumes the heater core has been removed That doesn’t mean that airflow won’t help, though. In fact, with the touring-style racing that I tend to do, air tends to be relatively stagnant around the area between the dashboard and windshield, making ideal conditions for condensation during a rain session.

Two methods can be employed to generate airflow over this area. Method 1 involves static ducting using NACA-style ducts in the side windows. The outlet of the duct, when routed with a short length of air duct hose, funnels outside air over the inside of the windshield.

Method 2 involves using a marine-style bilge blower motor. The blower will provide forced air and uses an air duct hose to route air over the windshield. Reference pics below.

A NACA duct installed in the side window and a short length of flexible air duct hose can be used to blow air across the windshield while the car is at speed.
A NACA duct installed in the side window and a short length of flexible air duct hose can be used to blow air across the windshield while the car is at speed. (Photo Credit: NACA duct style vent window – Advanced Autosports (advanced-autosports.com) )
Neoprene air duct hose such as this from Chillout Systems can be used on the outlet of the NACA duct.
Neoprene air duct hose like the above from Chillout Systems can be used on the outlet of the NACA duct to direct airflow.
A marine style bilge pump can be used to direct airflow as a forced air system.
A marine-style bilge pump can be used to direct airflow as a forced air system.
2. Temperature

Warming the windshield directly is one of the most effective ways to prevent fog during a race. If you retain the heater core in the car, you can use the bilge pump method to pull heat from the core and onto the windshield with ducting.

The bilge pump in this configuration is pulling air through the heater core and blowing the warmed air directly onto the windshield.
The bilge pump in this configuration pulls air through the heater core and blows the warmed air directly onto the windshield. (You can read more about this setup at Bimmerforums.com)

Alternately, heat strips, like the ones found on the rear windshield of cars, work well. They directly heat the windshield, with the drawback being the disruption to forward vision.

Defrost strips can be applied directly to the windshield at the expense of forward vision.
Defrost strips can be applied directly to the windshield at the slight expense of forward vision. (This example is from a pretty exciting race posted on YouTube)
3. Humidity

Humidity is extremely hard to modify in a racecar as the car is most likely without side windows, factory defrost, and air conditioning. Conditions inside the car are very close to outside conditions, and the defogging efforts are best directed toward the windshield.

4. Surface Tension

This brings us to surface tension and the altering thereof. Rain-X Anti-Fog is one such product that falls in this category:

Rain-X Anti-Fog works by creating a thin invisible film on the glass surface, which helps minimize condensation and moisture buildup. This film reduces the surface energy required for water droplets to spread out and form fog, effectively altering the surface tension.

Unlike regular Rain-X, which changes the surface tension of the outer glass to cause water to collect, bead up, and roll off the windshield easier, Rain-X Anti-Fog alters the surface to reduce the moisture in the air’s ability to condense and form fog when it contacts the glass.

This all sounds great in theory, but I have had very little luck using Anti-Fog while racing. There are alternate products that seem to work much better. (Insert link)

My Solution to Defogging the Racecar

I will post my solution to my foggy windshield woes after a bit of testing in a follow-up post. I intend to rely heavily on improving the airflow over the inside of the windshield and altering the surface tension. If the results aren’t satisfactory, I will move to a forced air, heated system as I do not like the disruption in vision the defrost strips create.

By Chris Simmons

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