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Old 12-28-2007, 01:56 AM   #36 (permalink)
Join Date: Dec 2007
Location: Bay Area
Posts: 1,151

The Miata - '01 Mazda MX-5 Miata
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Ahh, I thought you'd bring that up... cRR typically refers to the lower wieght simplification (as in, weights less than 2000kg). Keep in mind that these terms, as measured are an infinite sum with each term having a power of n-1 (one, theoretically could argue it's a "googtuple"

In any case, I can factor that in - but doing so, aero resistance becomes a cubic as this is looking at power.

Overall, we increase (very slightly) the velocity in which the break even point occurs. Check out page 4, figure 2. The dominating point occurs between 34 and 55mph. Make aero better - the locus shifts and those velocities slightly incease. I guess it's worth noting - the CR2 factor is on the order of hundredthousandths - .0000522. Much more significant when one speaks of a vehicle weight in terms of tons (~4+ short tons), rather than pounds (3000lb)

Lets apply their equations to a 1500kg car....

Term1 (crr*N) = 0.0678
Term3(excluding aero) = .0000355

Although, we shouldn't compare these directly as they don't have the same units....

Given the low comparative weights - it's really not incredibly necessary to include the expanded series. Stock coast down tests have come very very close to the factory advertised cD - even when cRR must be calculated...

I still stand that we have more potential gains from aero improvements given our high starting cD. Weight is important too - but when it comes to reducing a 20 pound load over a 60 pound load... I'll take 5% of 50 over 5% of 20

Thanks for the link though - very good read. It's pretty interesting that ~6% of a semi trucks energy is in the rotating bits of the axles...
Cars have not created a new problem. They merely made more urgent the necessity to solve existing ones.
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