Jan 10, 2006 update to the Mazda STS2 Miata project car

Weight Analysis

Another key part of the decision-making process for this endeavor is determining competitiveness of the car. In this installment, I’ll go through some of the analysis that shows why a Miata should be at least as good as a CRX and establish some performance-related goals for our project.

My experience has shown that one good measure of potential autocross handling performance is the weight-to-tire ratio. Given equal weights, the car with more rubber on the ground is faster, and given equal tires, the lighter car is faster. Since the CRX and Miata have similar gearing in 2nd, and since that gearing provides a perfect range of speeds for typical autocross courses, both optimally use the same size tires.

The 205/50-15 size is the de facto standard and will be our initial choice. Since STS rules allow up to a 225 width tire, I tested the wider/taller/heavier 215/45-16 size last summer and found that they were only marginally faster on a pure handling course, yet were consistently several tenths slower on a course with some acceleration zones. This result was likely due to the gearing and weight disadvantage of the bigger tire. That said, in the near future we’ll be testing the 205/40-16 size that Falken makes in their new RT-615 line. It’s shorter and lighter, so it could potentially provide a couple of tenths better performance. The only issue on that tire is that its tread width is not quite as large as the 205/50-15. The only way to know which factor dominates is to test.

With tires being an equal factor between the CRX and Miata, let’s now look at weight. Our ‘92 Miata weighed 2015 in racing trim when it ran in E Stock. Let’s work forwards from there to get an estimate of STS2 weight.

Wheel/tire —————————————————>(22.5 - 30) * 4 = 30 lbs added! Seats (we’ll use stock rails on one seat) 60 - 35 = 25 saved Exhaust (Borla -> straight pipe) ————>16 - 4 = 12
Crank pulley ————————————————> 5 - 1 = 4 Springs (lightweight 2.25”)————————>15 - 9 = 6 Perches (aluminum) ———————————->10.5 - 4 = 6.5 Battery (sealed motorcycle) ——————->21.5 - 5 = 16.5 Misc (bushings, intake, sway bar, etc.) ->9

That’s a total weight loss of 49 lbs which puts us at 1966 for a target. Not bad, and if the 205/40-16 tires work out, they are 5 lbs lighter per tire which comes out to a total weight savings of 16 lbs when you account for the slightly heavier 16” rim. That combo will put us right at 1950.

So, what does an STS2 CRX weigh? Well, a fully prepped STS Civic can be right around 2000 lbs and its Honda sales brochure curb weight is 2161. The curb weight on an 88 CRX Si (the lightest year) is 2017, for a difference of 144. That would put the best fully-prepped CRX at 1856. So the Miata is giving up a full 100 lbs to the CRX! Wow!

But wait! There’s more! The weight distribution of an STS Civic is about 60/40. Its likely to be even worse for a CRX since the leverage of the long tail section is gone and the front end is the same. Let’s say 65/35. In ES and CSP trim, the Miata was around 53/47 so that’s unlikely to change much for STS2. In a pure steady state condition, with equal sized tires all around, the front tires will saturate first and will be the gating factor on lateral acceleration (and thus, speed). With 65% of its weight on the front wheels, the CRX splits 1200 lbs between its front tires. The Miata splits 53% of 1950 or 1033 lbs. Hmmm…advantage Miata!!

In addition to the pure steady state performance, the Miata’s drive wheels are at the rear. As the car accelerates off a turn, the undersaturated rear tires have some additional tractive capacity available for forward bite. Thus, power can be added earlier in the corner exit phase. On the other hand, the poor FWD CRX must wait to accelerate until the turn ends, the steering wheel is unwound and some traction is available to accelerate with. Oh, and did I mention that the Miata can run a factory viscous limited-slip differential? While not perfect, that device will take some of the accelerative forces and transfer them to the underutilized inside rear tire.

So, we now have a car that should corner faster in steady state and accelerate earlier off the turn. Next installment we’ll look at potential power outputs (complete with dyno graphs) to see who’s faster in a straight line. Stay tuned…

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