Comments from other sites about basjoos' Aerocivic

[roflwaffle]

I wouldn't even go as far as to say uphill, unless the driver never goes downhill. Crr>W. There, I said it.

[trebuchet03]

Originally Posted by roflwaffle

I wouldn't even go as far as to say uphill, unless the driver never goes downhill. Crr>W. There, I said it.

Only of you coast uphill Otherwise, wasted power :/

Crr does indeed go up with weight... But we're talking of a scalar on the order of hundredths (Crr) that is used in linear a linear equation with respect to a constant (weight) versus a scalar in the tenths (cD) which is used in a quadratic equation with respect to a variable (velocity).

So lets say the car weights 2500lb and cRR is ~.01 (partially arbitrary). That gives us a RR of 25 pounds. This is constant. So, we can linearly reduce that value until we have no wieght in the car whatsoever (not possible - but just for discussion's sake).

So lets say the car originally had a cD of .3 and an area around 25 square feet. That's a cDA of 7.75ft.^2....


That means - given a cD of .3 versus .17.... A difference of 25 pounds is had at 53mph. Of course, we can't remove all of the car from the car... So lets take 500lb off. The even breakpoint would then be at ~24mph (5lbs).

Lets say he goes crazy - and pulls out 900 pounds of car off the car.... He'd then have to go ~32mph to get an equivalent difference in gains (9lbs)


Now, I made some assumptions on cRR, A and original cD. I know they're somewhere - I just used handy numbers from the coast down instructable and an estimated weight of 2500 lbs. I'll eventually come across the more correct numbers and throw it into a spreadsheet

I can also post the converse if you'd like (what reduction in cD is require to break even at some given velocity)...


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To reiterate - removing weight is good (and shouldn't be overlooked)... But reducing aero losses has the potential for greater gains in drag reduction. The biggest (and cheapest), of course, is just driving slower

And finally - an example case... Basjoos' car

[roflwaffle]

If ya go uphill, ya gotta go downhill. Anyway, bad, bad trebuchet03, Crrn it isn't linear. And I didn't mean that Crr goes up with weight, I mean it's easier to address than weight. What's going to be easier, slapping a set of RE92s for a halving of Crr or gutting 1000lbs out of a 2000lb car? If ya do both then it's a matter of diminishing returns and like ya said, ya might as well address aero.

[trebuchet03]

^^ F_rr=cRR*Normal

Seems rather linear to me

I agree - easier to address... And less gains to be had while cruising

[roflwaffle]

Page two under road load parameters.

[trebuchet03]

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...

[roflwaffle]

But I was talking about Crr not cRR! I also think that Taylor series of a function is common compared to googtuple, but I guess that's just another Crr cRR.

P.S. This reminds me of that SAE study where the authors estimated flaps on the back of trailers would save the trucking industry something like three quarters of a billion bucks per year. Not to mention the reduction in externalities like pollution...

[Kuma]

I think a lot of the reason for the ricers to complain about basjoos fine work is simply because it's "rough" and unconventional. It definitely shows what aero can do for fuel economy, but it also doesn't have the spit and polish look of a professional engineer with a team of highly trained machinists at his beck and call.

It's homebrew in it's appearance and has function (great function) but the form seems lacking.

I'm not bashing basjoos at all, in fact this has inspired me to look at what I can do with my Jetta to break above my 25MPG handicap.

[newtonsfirstlaw]

Originally Posted by Kuma

It definitely shows what aero can do for fuel economy, but it also doesn't have the spit and polish look of a professional engineer with a team of highly trained machinists at his beck and call.

I'm not sure what basjoos' qualifications are, but I wouldn't be surprised if he is an engineer. I'm an engineer, and a lot (well, all) of my mods are pretty rough and homebrew in appearance.

The difference is the equipment, the machinists, the professional paint facilities and painters, etc.

Given time, a whole industry will spring up around this, just as there has around the performance industry. Well, acceleration + appearance, as that was all the ricer crowd were about.

[Kuma]

Originally Posted by newtonsfirstlaw

I'm not sure what basjoos' qualifications are, but I wouldn't be surprised if he is an engineer. I'm an engineer, and a lot (well, all) of my mods are pretty rough and homebrew in appearance.

The difference is the equipment, the machinists, the professional paint facilities and painters, etc.

Given time, a whole industry will spring up around this, just as there has around the performance industry. Well, acceleration + appearance, as that was all the ricer crowd were about.

My name's Kuma, and I 100% approve the above message.

I myself would not be surprised if basjoos is an engineer, I was more aiming my comments at the lack of "professional machinist" quality of the mods. Again not knocking basjoos he's done some incredible work, but the ricer community is fixated on that spit and polish look.

To be honest I'll be surprised if my own efforts look half as nice as basjoos, I drive a sedan so I know that my boat tail won't have quite as aerodynamically correct of a shape, but I'll do what testing and solidifying of my designs as I can.