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Archives for : miata duck tail

Another Aero Curiosity, Rear Bumper Cut

I first spotted this mod on some of the forums frequented by the miata endurance racers.  The discussion centered around the coefficient of drag of the stock miata being ~0.36 which is about the same as that of a ’69 dodge charger, better known as the original General Lee.  For reference, a sphere has a coefficient of drag of about 0.41.  This fact left me really carefully considering what I could do to cut down on drag. One of the methods used by racers seems to be this bumper aero cut where the lower ~1/3 of the bumper gets removed so it can’t trap air and create what is often called a “parachute effect.”  I have studied fluidynamics, so I know a thing or two on this topic.  And the aero cut is definitely better than stock, but only by a margin.  It isn’t my final step in the rear, it needs a full diffuser and a custom exhaust so that air is told where to go instead of being left to find its own path.  But as a first step, this was an easy process.

Why a lexan duck tail spoiler on a Miata?

Because it helps reduce form drag on the car.  It may not seem like a worthwhile upgrade.  After doing a lot of research into the aerodynamic properties of the miata I found some surprising numbers.  Namely the miata has the same coefficient of drag as a ’69 Dodge Charger such as everyone’s favorite hero car the General Lee.  The designers of the miata said they borrowed heavily from the Lotus Elan which came out about 1962. With a drag coefficient (~0.36 compared to a sphere at ~0.4-0.5 vs a droplet shape at ~<0.05) comparable to other 60’s cars, its fair to say the miata design team acheived their goal.  Fast forward to 2015 and most car manufacturers are streamlining the cars and really working on tucking the undercarriage bits out of the air stream.  So many modern cars have very low drag coefficients such as the prius at ~0.26.  That is a big part of the freeway mileage increases we have seen across the board on cars and trucks since even 2005.  Before we get into talking about gas mileage and rules of thumb, check your tire pressure, if its at ~25 psi start by inflating them properly.  I tend to run 30-35 psi in my vehicles depending on the situation.

Now back to business. In the 60’s the guys at Dodge worked in part with folks at NASA and retrofitted the Charger to make the Charger Daytona which was quite possibly the most aerodynamic mass produced car of its day.  The main changes were a nose cone, flattening the rear glass of the canopy and adjusting the rear wing and stabilizers until it balanced the front.  They ultimately managed to gain downforce and high speed stability at the cost of a minor drag increase.  I found simulations of the miata in CFD suggesting that the front end of the stock car produces approximately 200 lbs of lift at 100 mph.  That is why the front end feels so light at speed.  I for one want control and grip at speed.  So the first aero work I did on the car was to add a DIY duck tail spoiler.  I prototyped the shape in cardboard, transferred the pattern to 1/8″ thick lexan and bent aluminum support brackets.  After installing it, I noticed my mileage on the same route, shifting at the same rpm, and all else including air temp being equal, increased about ~10%.  On an all highway run, I saw a ~20% improvement in gas mileage for a high of 35 mpg at mostly 80 mph.  That was with 2 people and a fully loaded trunk.  To be clear, this data is subjective.  Others have also seen improvements in the gas mileage at speed when adding a duck tail to the endurance racing miatas, as they observed running more laps on the same fuel.  This is how I constructed my duck tail.