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Archives for : September2015

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.

Is The MTuned Miata Big Rotor Kit Any Good?

In a word, absolutely! I got the kit from Emilio at 949 racing and it was definitely worth it. I ordered the stainless braided brake lines, g60 Corrado rotors, M-Tuned brackets and Carbotech AX6 pads.  Here are all the parts I started with.

The brackets and rotors are fantastic for the street and don’t imbalance the brake system or make the steering heavy. The stock brakes on my g meter app peaked at ~.8 g while the upgraded and bled big rotor kit peaked over 0.9 g which doesn’t seem like much difference. That took at least 3-4 Miata-lengths off the stopping distance from 60.  Here is the install process.

The Carbotech pads, however, aren’t really a street pad. They are by far the most ferocious brake pad I have ever installed in a car. But after 3 rounds of take apart, clean, liberally apply anti-squeal and reassemble, the brakes were still howling. I tried feather light braking all the way up to the Wrath of Khan with no difference in squeal. The stopping power was awesome, but the squeal was relentless. I finally found a Carbotech brake pad review buried deep in a forum and found the clause “…amazing at the track, but on the street they squealed like a stuck pig…” aptly applied to the Carbotech brakes. They are amazing pads, but not quiet enough for the street. I will keep those pads for a track day (hopefully).

So I switched to AutoZone ceramic pads which could still handle my stopping needs. They are dead silent and (compared to pre-brake upgrade) the brakes now grip better and better the harder I punish them with repeated threshold braking. I was worried about the ceramic pad not having enough cold bite to stop a light car, but the bigger rotor (~10″ vs 11″) compensates for that perfectly by increasing the braking torque. Basically I will claim that the NA Miatas need 11″ rotors just to keep up with the MUCH quicker stopping new cars on the road.  Here is my review of the brake kit overall.