Hardtops: Extra Weight or Aerodynamic Advantage?

Hardtop? Droptop?

Caymans and Boxsters are often seen at the track with and without hardtops. Some prefer to keep the factory convertible feature where others opt for high-performance carbon fiber fixed roofs. With both seen at track days across the country you have to wonder: how much better is a hardtop compared to leaving the top down? 

More Than You’d Think

According to data obtained by Aniket A. Kulkarni, S.V. Satish and V. Saravanan at the PES Institute of Technology the coefficient of drag on a CFD modeled car increased from 0.75 to 0.97 after the convertible top was lowered. While this is nowhere near the Boxsters coefficient of 0.29 it does have implications on what happens when you start going fast with the top down: turbulence air and drag. Lots of drag. 

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Hardtop Advantages?

As air passes over the windshield of your car it increases in pressure and speed. This fast-moving air then travels over the roof of the car which causes lift. Lift is bad. Manufacturers work hard to minimize this and by dropping the top on a car you essentially disrupt the flow they have worked on sculpting. The air becomes turbulent and doesn’t flow smoothly which is no good for fast lap times or a predictable ride at high speed.

image courtesy of flickr.com 

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Other Gains

Ditching the soft top in favor of a hard top also saves weight which is the #1 killer of lap time. Nothing improves a car’s performance on the stopwatch-like dropping a few pounds and the hard top has you covered in this department as well. In many aftermarket applications, the switch to a hard top can save as much as 60-110lbs depending on how heavy the factory mechanisms are that operate a soft top. Worth the weight? Absolutely. 

image courtesy of motortrend.com

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Wing Control

Rear wing efficiency has a lot to do with how air is flowing before it gets to the wing itself. Trying to make downforce by using turbulent air isn’t the best way to go about things. So, a hard top that smooths out flow will help in creating meaningful use of any well-designed aftermarket parts you choose to slap onto you track weapon.

image courtesy of porsche.com  

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Summary

If your car doubles as a weekend getaway car and a track toy there isn’t really a need to care much about these findings. However, if you are serious about competition or want to best your rivals you might think about getting out your tools this weekend and ordering up a nice fiberglass ($) or carbon fiber ($$$) hard top. Hope this helps you get faster. Thanks for reading.

image courtesy of motonetworks.com

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