The hardtop on the Miata is one of my targets for weight reduction on the Super Touring 6 Miata. During the last round of extreme weight reduction on the racecar, I did some testing to try and determine what effect, aside from an 11-lb weight reduction, running without the rear glass would have.
Background
I am on the fence about this one, even more so after testing. There are two schools of thought on the rear glass, and there is very little data on the subject. You can read the testing over at Occam’s Racer for a primer. Although I agree with the results, his data deals with cars equipped with wings and splitters and shows that removing the rear glass negatively affects downforce.
But what if we are only concerned with mechanical grip? The racecars without wings and splitters. Would removing the rear glass from the Miata improve or impair performance?
The theory is that the hardtop acts as a giant parachute. Since we race without windows, turbulent air enters the cab and presses against it, creating drag. Anecdotally, Spec Miata drivers have been busted in the past, shimming the hardtop to allow a path for this trapped air to exit quickly. And usually, if there is an advantage to be had, these guys will try it.
So that’s what I did but on a large scale. If allowing a small exit for air is considered an unfair advantage in SM, then surely removing the whole bottleneck may offer a benefit. I tested the Miata racecar with and without the rear glass during the last race weekend at Nashville Superspeedway. The results were … interesting.
Removing the Rear Glass
Removing the rear glass is simple, even with the wrong tools. First, there are 6 or 8 nuts around the interior perimeter of the glass that must be removed. Then, I used a heat gun and scraper to slowly work the glass out by heating the adhesive and slowly inserting the scraper between the glass and hardtop frame. Working slowly and patiently, the glass will be released from the hardtop in one piece.
Results: How Did Removing the Rear Glass Affect the Car?
In short, it’s complicated, and I need to do more testing to make a definitive statement. But I can say that it didn’t affect lap times by any measurable significance. Let me explain.
I drove the car Saturday with the rear glass intact. This includes race one right after lunch in hot, humid, and relatively static wind conditions. It just so happens that I ran race three on Sunday in almost identical conditions, but this time without the glass. So, let’s compare laps.
I had roughly 50 laps to pull from across these two races. The data set below is the fastest lap, and every lap with a less than 1 percent variance. This worked out nicely to 13 laps per race.
At first glance, the dataset doesn’t reveal much. I used GPS speed at start/finish as a fixed point of reference and the overall top speed per lap. The top speed average varies by .16 mph in favor of window removal, and the average trap speed at start/finish varies by .03 mph in favor of the window. In short, the difference is completely and statistically insignificant.
Now, I need to investigate the 108 mph and 109 mph top speeds. After reviewing the race footage, I found that these outlier top speeds were due to drafting. However, the draft seems more powerful without the rear glass.
Removing these points barely changed things though. The top speed average is now .04 mph in favor of the glass, and the trap speed is .45 mph in favor of the glass as well. It’s still inconclusive.
Drawing Conclusions and Further Testing
Unfortunately, I will need more track sessions to draw a proper conclusion. Throughout the weekend, lap times did not significantly change with or without the window. I am racing at the National Corvette Museum in a couple of weeks and will repeat this test to hopefully spot a pattern.
I will also run the upcoming event on a new set of tires. The tires I ran in Nashville were on the way out, with the left rear only a few laps from cording at the end of race three. As a result, I felt I worked the car harder on Sunday than on Saturday, even though the times were comparable.
As for a tentative conclusion, I suspect the removal of the rear glass from the Miata hardtop will not significantly change things on racecars without wings. If that is the case, then the rear glass and its 11-lb weight saving are perfect for my purposes. As a side benefit, I remained cooler during race three with the extra airflow in the cockpit.
Hopefully, I will report back in a few weeks with a concrete answer.
