Coefficient of Drag...OR CD!

[Mikelly]

So I'm reading the newest rags from Car and Drive and Road and Track, looking at the new Z06 and Viper... Reading about the whole aerodynamics thing, power, and all the stats when I get to the new Viper Coupe's CD number and I stop! .39 CD, down .01 from the convertible... .39!

 

So why do you care? Well the Zcar fell into the CD of about .40-.41 range... this is big business... Big news in deed... You see, we who pursue power ruthlessly want to cheat mother natures grasp on all things of solid structure that are in motion....aerodynamics. Why this matters is this: We are now in a new century. 35 years ago Nissan built a cutting edge sports car that looked like it would cheat the wind... All these years later there are some of us (Me included) who have kicked the little Zcar for its horrific aerodynamic numbers... And now, all this time as passed and one of the baddest beasts in American sports car production is producing very similar numbers off the show room floor. I know, I know, It has a V10... This is irelivant if you are reading this thread on this website, as most people here have visions of motor swaps dancing in their heads...

 

So yesterday I was on the phone with Doug Rippie himself, of Doug Rippie Motorsports, and almost as popular as the late great John Lingenfelter. In most GFM circles he always has been. Anyway, I'm trying to find replacement ball joints for my Vette, and GM doesn't sell them... You have to buy the parts they come attached to. So I'm on the phone with Doug and he is telling me how they beat this issue. Afterwards I mention I'm using this vette until my track day car is done and he asks about it and I tell him it is a little beater Datsun 280Z with a 383 stroker/ Gforce tranny and other bits... He keeps asking more info... Then he tells me that that very project is the one he has dreams of building some day. He tells me not to sell that thing ever, as guys throughout the country who do track days always tell of a little Datsun Zcar that owned the track at such and such location, on such and such day! I hung up the phone with a grin on my face. This guy campaignes a C5 in T1, and this coming season will have a C6 Vette to race, and drives one every day, and he is dreaming of... a DATSUN V8 ZCAR!

 

So what does this have to do with the topic? Alot. I mentioned to Doug Rippie that I've got concerns over high speed instability and I qoute the numbers... He tells me to watch what the viper boys do with the new STREET coupe and not the factory race coupe. The race coupe isn't the same car, and has different sheet metal/ fiberglass/ bodywork than the production car, so we Zcar guys need to take ques from how the stock class viper owners make their cars cheat the wind. Bottom line is if they can make those pigs dance in the wind, we can certainly cheat mother nature as well.

 

.40 drag is certainly NOT the svelt and sexy .28 of the new C6 Vette. It isn't a mack truck either. Talking on the phone to Doug Rippie made me rethink a few things while I was re-installing the torque tube, transmission and rear differential on my own vette. The while I was bolting the headers and cat converters back in, I realized how I have funneled way to much money into this corvette, trying to make it into a track car I can live with on the street... The whole purpose oif the Z in the first place! Sometimes it is easy to get distracted.

 

.40 Drag is easy enough to beat... We've got guys on this board who drive their cars on the track at speeds above 140mph and none of them notice the poor handling described by most who drive at stock ride height and stock wheel width. Common sense practices can achieve a greater stability to lower those bumbers to a range within something closer to .35 or maybe lower.

 

We'll likely never know though. I checked into renting time at a windtunnel a few years back when I was building the white V8 Z that JT1 now has. I spent a lot of time on the phone and what I found is that they are BOOKED SOLID by race teams around the coutry. Lots of folks with REAL MONEY book them years in advance. Getting time in one is almost impossible to schedule, and being able to afford it is almost impossible to overcome. They are extremely expensive, and require a second mortgage to afford. So us Zcar guys can only dream and imagine the numbers we pull, as we dream to cheat the wind...

 

Mike

[dr_hunt]

Mike, do you smell business opportunity here???? What about a group of z guys, engineers included, form an LLC, and build one to rent out, and of course special consideration for z projects in the development of some wicked aerodynamic bolt ons for z's.

 

I had to include us engineers, cause we think we know everything and more than anyone else.

[DaleMX]

Great post Mike, that's intresting, I have the same problem with getting off course and keep a project board that I update monthly to keep me on track.

 

I must admit I am shocked by the Viper numbers although it does have a strange front and I don't have a real good understanding of CD but I thought that the high speed problems were caused by the turbulant air flow under the Z and the slope of the rear window. Does smoothing out the air flow under the Z reduce the CD? Would placing a small wing at the top of the hatch to deflect air down the rear hatch window help reduce the drag?

[dr_hunt]

I think the problems with the z from what I've read are;

 

1. Too much air gets underneath the car.

2. Air stays in contact with the roofline all the way to the trunk.

3. Air goes around the radiator through front bulkhead holes and actually lifts the front end of the car, adding insult to problem 1.

 

Now from what I've read, there are a host of other "minor" aerodynamic issues like headlights, wipers, mirrors, etc. but when you add up all the minor things, then perhaps they aren't so minor.

[Guest magnadyne]

Yeah, you look at a Viper, and then look at a Z, and it almost becomes obvious what the problems are.

One HUGE one is how much air can get under the car. The more air under the car means more lift. I have also noticed a lot of guys out there with flairs don't have anything covering the front of the tires. Most flairs just cover the top. This is going to cause major drag also.

Aside from that the Z has a very squared off front end. Hell, the whole car has very sharp lines compared to a Viper.

 

Maybe it has been too many engineering classes, but when I look at a Viper, I can picture the air going over it. The only time the air flow is terminated is if it happens on purpose. Like in the grill. Otherwise, it is smoothly redirected around the car.

With a Z I see a ton of areas that are going to mess up air flow around the car.

 

I can think of a few things that would help high speed stability, but I don't see a lot you can do to lower the CD of the Z.

 

It is an interesting post though. I have never really thought about the aerodynamics of the Z before. There is definitely some room for impreovement.

 

I think lowering it and keeping the air from going under the car is probably the easiest thing you can do and get big results.

[Drax240z]

Mike I think you are missing a few key points. Aerodynamic design isn't all about Cd, not by a long shot. Hell if that was the case how do you explain that a F1 car has a Cd worse than a barn door?

 

The missing part of the comparison is downforce. Yes a Z and a Z06 might have the same drag numbers, but while the Z is making lift at any speed, the Z06 is making downforce. As a general rule, the more downforce you have, the more drag you have. F1 has spent countless dollars trying to create the most downforce possible with the least drag possible, and in the end they end up with Cd's of around 0.8. Whether it's the design of a F1 car or the new Z06, the tradeoff of more downforce is almost always more drag.

 

Anyway, just a little segway there to emphasis a point. I think it's pretty damn cool that some of the high ups are dreaming about having a hybridz!

[Michael]

I would be very, very skeptical of those drag numbers for the Viper. Let me repeat that: very skeptical. It’s true that cars designed for downforce will also generate “induced dragâ€, and that (together with exposed tires) is why F1 cars have apparently high drag coefficients. But few modern passenger cars have Cd above 0.35. And even relatively ordinary sedans are down in the 0.28 range.

 

The Z has an advantage in its small size, and in particular in small cross-sectional area. Recall that drag is drag coefficient times area times dynamic pressure. A small car with high Cd might still have less drag force, at a given speed, than a larger car with a lower Cd. I've seen the 240Z's Cd quoted at around 0.45.

[Mikelly]

Michael, I think you're looking at this backwards... CD numbers being lower are MUCH better... So if the C6 pulls down .28 which is amazing. The fact that a viper coupe hits .39 CD isn't cutting edge. The Datsun Zcar is not pulling down less than .40.

 

Richard, I'm well aware of much more factors in play than just CD. and that the trade off in downforce is drag. We've all been on http://www.mulsannescorner.com and there are ways to achieve better downforce to drag ratio with.

 

Mike

[CruxGNZ]

Figured I would throw this link into the discussion:

Early Z Aerodynamics

 

!M!

[ZHeadV8]

Excellent links guys - this is an area I had not really thought about but one that is fascinating, and the time to look at improvements is at the bare shell stage of a project.

 

THANKS !!

[Mikelly]

The point I was trying to make in this long winded post was that maybe .40 isn't a bad number... It isn't great, but not bad either... especially when compared to the likes of an $80k viper!

 

Good link Matt!

Mike

[dr_hunt]

It looks like we could use some hard and fast numbers relating to downforce and drag at speed. Of course the ultimate factor in determining acceleration and top speed of any given vehicle is HP and TQ, to which, the HP to weight ratio of a v8 z probably exceeds most cars depending on the z in question.

 

I think that is immensely interesting, certainly opened some minds, mine included.

[Guest 240zJake]

We could just do what they do at Scaled composites, they do not have a wind tunnel, the have a pickup truck with mounting points for models or full sized pieces of spaceshipone. So we should only need a camera, some yarn, and someone willing to drive their car fast... Maybe use some sort of linear measuring device so we could see the change in ride height and calculate downforce? ??:

[Hugh]

In a drag racing situation, I realize that lift can cause high speed unstability like through the traps... but how much do you think decreasing flow of air under the car would help out in a straight line?

 

I don't care to add any un-needed downforce... but would like to help it move along straight.

[Z-TARD]

As I recall from a previous thread on this, isn't the CD of a 4x8 sheet of plywood (flat side facing out) something like .46? It would be interesting to have a Z in a wind tunnel with a clear base to sit on, so you could veiw the airflow from under the car as it interacts with the road surface. I'd be willing to bet that the majority of the Z's areodynamic problems are caused by the large airflow into the grill area, exiting under the car as it hits the firewall. If you had a clear lexan hood, you could tie strips of yarn in the engine bay to see how the under hood airflow behaves at speed

 

Mike

[JMortensen]

I did a little checking, and I'm not sure if I'm right, but what I found was .5 for the convertible Viper with no top, .45 for the convertible with the targa cover on, and .35 for a GTS hardtop Viper.

 

I only found one site which was like Joe's Aerodynamics site that had those numbers. It did also say that a new Ford Ranger had a .4 if that puts it into perspective at all. The numbers makes sense to me because the original Viper targa setup was definitely not very aero looking.

 

Here's a better site:

http://www.vipercentral.com/buy/specs/specs98.htm

[ZR8ED]

http://forums.hybridz.org/showthread.php?t=91946&highlight=bellypans

 

http://forums.hybridz.org/showthread.php?t=77038&highlight=bellypans

 

http://forums.hybridz.org/showthread.php?t=72849&highlight=bellypans

 

Lots of discussion on this topic.

 

I am very interested in this as well. Working on the grill opening is my next aero project. I have some ideas that I will test with this summer.

[Michael]

I’m not sure how folks misunderstood my posting, but somehow it happened…

 

(1) of course the lower the Cd, the better

 

(2) Cd of 0.40 was OK in 1970, but today it’s outright horrible.

 

(3) Drag coefficient is only part of the story; cross-sectional area is the other part. The Z is small, so even with a high Cd its overall drag is, well, not as bad as one might think.

 

(4) One source of drag, apart from “bad streamliningâ€, is downforce (or its opposite)

 

(5) Multiple sources confirm that the 240Z’s drag coefficient was around 0.45, give or take.

[Mikelly]

I got the figure for the 2005 RT10 COUPE (They dropped the GTS name), which is the lates version of the Viper platform, and is built off the 2004 convertible platform, direct from CHRYSLER in the latest Car and Driver... Which states that the latest 2005 Convertible is pulling .40CD and the Coupe is expected to pull .39... Just reporting what I read...

 

Mike

[Pyro]

Yes Michael, I have always thought the small frontal area of the 240 had a big part in reducing the total drag. Eventhrough the CD was bad, the small frontal surface reduced the load.