Z aerodynamics

[RB30-ZED]

Originally posted by 260DET

I'ts going to likely mess up your airflow around the rear something awful though. Lots of drag and any spoiler probably won't work well. Much better to leave it clean and use a spoiler IMHO.

Thats very true but from what ive read its a chioce between smooth air flow, which unfortunatly will cause lift as it races down the rear section of the 240's body or breaking up the air flow coursing drag but stopping lift.

Lift scares me more than drag.

Steven

[Alfadog]

Originally posted by SteveK

Sorry to go off topic for a moment but what on earth are "ricers"

I have seen the term used several times and cannot for the life of me think what it refers too.

Ricers are those who put really tacky things on their cars to make them look fast. Generally they own economy cars. A picture tells a thousand words.

[240znz]

Thanks Steven,

Those links made good reading. My 71 has a front dam fitted and factory rear spoiler. I was thinking of removing the rear spoiler but on reading those pages, I might as well keep it on there.

I have seen another Zed with the same front dam as is on mine. Mine was imported to NZ from Australia and I'm wondering if these dams were fitted in Japan or in Australia.

Here's a early pic

[260DET]

Originally posted by RB30-ZED

Thats very true but from what ive read its a chioce between smooth air flow, which unfortunatly will cause lift as it races down the rear section of the 240's body or breaking up the air flow coursing drag but stopping lift.

Lift scares me more than drag.

Steven

Well mine has a whale tail on it with a small air flap (Gurney lip) and it is rock solid at 200+ kph. :classic:

And are you sure that the hatch surface induces lift anyway? It is not the same as the bonnet surface, there air is being forced to speed up and so lose pressure which induces lift.

[RB30-ZED]

Originally posted by 260DET

And are you sure that the hatch surface induces lift anyway? It is not the same as the bonnet surface, there air is being forced to speed up and so lose pressure which induces lift.

No Richard im not 100% sure, but I believe it does. As you say air pressure drops with the increasing speed of the air, and true as it rises over the front of the car its speed goes up and its pressure goes down , but as it is dragged back down the slopping rear of the Zed it is also accelerating in speed back to were it started, so yes, I think it does.

Here is a quote from the ZCCW Newsletter

Source: James Lux

"The smooth top of the Z, gently falling to the tail, encourages high velocity air to remain attached to the roof surface and that attachment produces lift on the aft section of the body. The figure that comes to mind is 120 pounds of lift on the rear of the car, but I don't recall now at what speed that figure is reached, though I believe it was either 60 or 100 mph. Lift on the aft body relieves load on the springs and the aft body lifts slightly. The MacPherson strut rear suspension acts like the front suspension when the body is raised: camber becomes slightly more positive and toe-in changes slightly compared to the body's static setting. The mismatch of lift between front and rear at speed on the 240 is a major reason why the Z is known as an understeering car at low speeds and an oversteering car at high speeds. Oversteer and understeer characteristics are formed by more than suspension type and settings. An optional rear spoiler was available for the 240 almost from the beginning, though relatively few 240's possessed one. Changing the aft body lift characteristics of the 240 involves either creating downforce, as with the original rear spoiler, or "spoiling" the lift before it begins."

A very interesting topic.

Steven

[260DET]

Yes Steven it is interesting and I have some reservations about the ZCCW information. I'm not convinced that air moving down the back of the hatch would be significantly low pressure. After all it has already been accelerated when passing along the bonnet so it surely is still not accelerating by the time it reaches the hatch.

The air rushing in to fill the space behind the car is somewhat different, it is turbulent too. But anyway, as I said, there is no problem with my car there so..... IMHO it is at the front where gains are there to be made, just consider air flow through the radiator and its exit for one.

[beandip]

According to SCG on page 15 of the Gold Portfolio the forces on the 240 Z at 100mph is 270 lbs of drag including the tires . the LIFT to the body at 100 mph is - 85 lbs on the front and -95 on the rear. Which means positive down force . This is with out any air dam or spoiler. For what it is worth , I have noticed that the front of my '73 feels less light at speed after I added the sheet metal pan that was a factory mounted pannel or farring under the rad and front or the engine. I have read tests since, that have agreed with this , as was reported , helped to eliminate the air from building pressure in this area and helping to draw more of the flow from the engine bay. By helping to eliminate turbulance of the air traveling under the front vallance and crossmember. I have also added rear luvers on the hatch window , but I cannot feel any difference in the handeling in this reguard. :classic: to correct a miss statement the rear suspension is not Mc pherson , but is Chapmen strut type. Mc pherson is on the other end. No flame just information.

[RB30-ZED]

Originally posted by 260DET

I'm not convinced that air moving down the back of the hatch would be significantly low pressure. After all it has already been accelerated when passing along the bonnet so it surely is still not accelerating by the time it reaches the hatch.

But if the air accelerated up over the front to the roof, then it has to move back down just as fast, you see its not going any faster, but its still going just as fast as it did on the way up, and more importantly, its moving faster than the air under the car.

Its the imbalance of air speed under and over that courses lift, ether up or down.

Did that make any sense.

But I do agree that your best to stop it getting under in the first place. When I get time im going to hook a spring loaded resister to the rear suspension and take readings at different speeds with the louver on and off, to work out the suspension hight due to lift.

Steven

[PrOxLaMuS©]

Thanks to this conversation....

I have decided to.. buy BRE Spook front spoiler

Can't decide though.... With brake cooling holes or without?

[beandip]

According to the tests that were done there was a down force of 95 lbs on the hatch at 100mph and 85 lbs on the front end , not lift. The drag to the rear of the car was calculated to be 240lbs at the rear of the car . by adding a spoiler, the small one that was on the early Zs with the addition of the spook , that it moved the vacume bubble back away from the rear of the car about 4 or 5 ft. if I remember right . I had all this data down loaded but I was recently infected and lost the Hard drive and all the information so I cannot back up this info . With the exception of the figures of the down force on the rear hatch and hood. I will keep researching my books for the info. This was brought up also as a way of helping the smell of fumes entering the rear of the car as a side effect. by the way the front suspension is mcpherison and the rear is chapman.

[RB30-ZED]

Originally posted by beandip

According to SCG on page 15 of the Gold Portfolio :

Sounds interesting infomation, but what is SCG ?

Can't decide though.... With brake cooling holes or without?

Id go with the brake ducts, as they look better, but if you dont need them block them of from behind to stop air getting under the car.

Steven

[beandip]

S.C.G. Stands for Sports Car Graphic It was a magazine that did testing like Motor Trend and Road & Track. They published compairisons of the new offerings when they came out and tested everything such as cornering, brakeing , stopping , brake fade milage as well as the aerodynamics. they show 15.5 lbs per HP on a '70 240 Z The book DATSUN Gold Portfolio is a 172 pages of tests and articles about the 240 and 260 Z and a little on the 280.