Do Headlight Covers Really Reduce Drag Forces?

[stevo_gj]

I did however test head light covers on my old race Zed and they improved top speed by almost 5 mph.........sorry to mention the word test.

 

This post in another thread sparked my interest as I've seen the drag coefficients on hybridZ from their wind tunnel testing. I thought I would see if their figures agreed with you Alan. I started a new thread to avoid taking the old one off topic.

 

If you make the assumption that the drag force is equal between having headlight covers and not having headlight covers at the max speed (which corresponds to the max forwards force your engine produces) then by rearranging the drag equation you can calculate the ratio of V1/V2 = SQRT(Cd2/Cd1)

 

Cd1 = 0.471 (no covers)

Cd2 = 0.467 (with covers)

 

This gives a your ratio of max speeds to be V1:V2  0.995:1 which is not a great improvement! You would hardly notice a difference according to those figures. For reference, to get a 5km/h improvement with that ratio you would need to be going over 1000km/h with respect to the air velocity.

 

I am not saying you didn't experience a higher top speed but was it definitely because of the headlight covers and not perhaps related to a change in wind direction (which would be more likely to give such a big improvement)? Were your results of 5mph higher consistently over several track days?

[luvemfast]

Maybe Alan did a run without them, then fitted them and had another run is very similar conditions.

However, what he may have failed to mention is that he went to the loo and lost a few kilo's 

But, does raise the weight issue with less fuel? 

At Winton earlier this month I couldn't get to 5th gear earlier in the day but as the car lightened with using fuel, I got to 5th and was still holding flat for many seconds.

This would be many Km/h difference, couldn't tell you though as wasn't looking at speedo.

 

[benny]

Here is a bunch of data from windtunnel results.

 

http://forums.hybridz.org/index.php?/topic/55944-windtunnel-test-data/

[620Z]

Top speed on a track also depends how well or fast you approach the straigh from the last corner. There are a lot of variables short of going to the salt flats. But I think if you are really concerned about reducing drag and down forces then a G-Nose will do much more than Headlight Covers. thats why I did mine. 

[dalee]

At Winton earlier this month I couldn't get to 5th gear earlier in the day but as the car lightened with using fuel, I got to 5th and was still holding flat for many seconds.

At least you get fifth, I just get 4th, however that probably has more to do with the Bitsashitty I was rowing at the time.

[stevo_gj]

Weight will have no effect top speed, it will only affect the top speed you can reach if you have a limited time/track to accelerate.

 

The numbers Benji links are the ones I used.

 

Given the results from the wind tunnel in that link I would argue that the Gnose will hardly make any more of a difference than the covers. It seems that the main thing to improve Cd was preventing air entering the engine bay with an air dam.

[260Coupe]

Yep I agree...........my testing was not the most scientific , there may have variations to wind speed , weight, temperature, atmospheric pressure, humidity, speedo variations  etc etc ........(but I do recall the weather being calm with little wind).... I read in some literature some where that the G- nose improved top speed by as much as 8-10mph.........hence I tried my own basic  form of testing with just experimenting with headlight covers.

Whilst my methods may have been "less than scientific" I did find them to be repeatable at the time .

On the subject of a G- nose - For Datsun to actually produce a more streamlined front end ..........there must have been some valid reasoning for this (ie better CD /less drag). Car manufactures rarely spend money and produce aerodynamic add ons if they do not actually work on the race track (ok with exception to cosmetic spoilers and wings as fitted to modern road cars)

Whilst you CD calcs show little measurable change may be this is better demonstrated in practice?

 

Alan

[stevo_gj]

I don't think so Alan, pretty sure if the wind tunnel shows its not an improvement then its not. With unscientific testing I think sometimes it might be a case of people see what they want to see, not trying to offend anyone with that just saying it's inconclusive unless you take a scientific approach.

 

However that is a good point that the Gnose wouldn't exist if there were no benefits.

 

Therefore I would say that either the benefits of the Gnose are not Cd related, OR..

 

they only had a Gnose car at stock ride height in the wind tunnel testing - maybe the Gnose doesn't make a noticeable difference until you lower the car.

[HS30-H]

I don't think so Alan, pretty sure if the wind tunnel shows its not an improvement then its not. With unscientific testing I think sometimes it might be a case of people see what they want to see, not trying to offend anyone with that just saying it's inconclusive unless you take a scientific approach.

 

Sorry, but the HybridZ wind tunnel testing was full of holes. In the case of the 'G-nose' tested, it wasn't anything like the proper OEM parts and therefore doesn't say anything about the proper OEM parts. The details inside the radiator duct are quite important, and many of the parts you can't see have an effect. Many so-called 'replica' kits don't actually replicate the details of the original factory parts...... 

 

One of the other things that people seem to overlook is that the Fairlady 240ZG production model was simply used to homologate some of the parts that were used in racing, and that the factory always intended the 'Grande Nose' to be used with an air dam / front spoiler when actually raced. There are mounting holes on the OEM parts for this very purpose, and there were several different evolutions of air dams and spoilers used - with the later evolutions getting lower and wider, and matching with the extra wide Overfenders. And what was going on inside the rad ducts of the works circuit race cars is a salutary lesson in effective air management, too. If anyone doubts that such measures were effective then I'd like to hear their reasons.

 

Sorry, but for me the HybridZ wind tunnel testing seems to have revealed nothing that wasn't already being addressed with these cars 40 years ago.   

 

[Whittie]

Sorry, but for me the HybridZ wind tunnel testing seems to have revealed nothing that wasn't already being addressed with these cars 40 years ago. 

 

Regardless it was still good to see it all put down to paper scientifically. Some of the tests they did, like to non-legit G-Nose, are not very useful but they did deduce that for these cars the best three things you can do to reduce drag and lift (NOTE, Reduce lift does not mean downforce, it literally means reduce lift) is the MSA spoiler, Headlight covers, BRE rear spoiler.

 

You can see these differences when you compare the side-by-side tests of the car in the tunnel with and without the headlight covers. It shows a difference and therefore will make a difference, how much that means on YOUR car is another matter as it wont be identical to the car they tested them on and therefore the results wont be identical either.

[HS30-H]

....but they did deduce that for these cars the best three things you can do to reduce drag and lift (NOTE, Reduce lift does not mean downforce, it literally means reduce lift) is the MSA spoiler, Headlight covers, BRE rear spoiler.

 

Two out of those three were factory option items anyway. What they are calling the "BRE" spoiler was simply a case of BRE copying the factory option rear spoiler ( which was standard equipment on the Fairlady Z432-R at launch in October 1969 ) and the headlamp covers were part of the original design of the S30-series Z.

 

Front spoilers were being addressed by the factory in late 1969, and they were introduced as standard equipment on Export RHD and European LHD models in 1970.

 

Managing the amount of air flowing under the car and through the engine bay is half the battle. The factory was attempting to address this - again in 1969 - with the long FRP undertray of the Fairlady Z432-R model. Factory race cars could be seen with partially-blocked radiator ducts from mid 1970, too. I don't think there's all that much new under the sun when it comes to these cars..... 

[stevo_gj]

That makes much more sense! Thanks for the info

[620Z]

Yeah I read Stevo's comments and didn't want to buy into an argument but given the comments and support of the G-Nose I will. And this might be the first time Alan (260Coupe) and I would agree. But I think the G-nose combined with a good air dam make a big difference to a Zeds handling, down force and aerodynamics. Disregarding the wind tunnel reports logic would tell you that if you extended the bonnet down nearly 8inches to the level of the front bumper bar. Then you are obviously narrowing the wind in take under the bonnet and into the radiator area.

I can tell you that at 250klm my car hugs the road far better than if it didn't have one. Refer pics. Steve compare the pictures of the front of my car to your avatar picture. There is a massive difference in air dam clearance which must translate to improved performance. 

[stevo_gj]

Time for the auszcar to do some wind tunnel testing! heh

 

Yeah I agree I think the wind tunnel results aren't reliable for G-noses.

 

On a side note: It's hard to read their data because their naming conventions and test data isn't logical, but it seems like the vortex generators had absolutely NO effect on Cd.

[620Z]

Yeah I read those test results years ago before I went down that road. I wasn't convinced they were accurate. Like we have said there are way to many variables. Only way to work out what works or not would be to conduct our own tests. But some how I can't see that happenning. Although I would be first in line if someone could make it happen.

[Whittie]

it seems like the vortex generators had absolutely NO effect on Cd.

 

I looked into buying these for my car about a year back. The reason they don't work, in my opinion, is that the Zed is fairly streemlined in the rear and the 'bump' and 'lip' of the gap between the hatch and roof and then the weather stripping probably do pretty much all the turbulence generation required.

 

For vortex generators to make a difference you really need a steeper gradient change. Think of the Evo 8, the first vortex gen road car, and they have a steep transition from roof to rear window. Creating turbulence before this helps the air to stick to the car, reducing breakaway air and eddie generation. This is what drag is, creating eddies of air in low pressure pockets. By putting energy into the air before the sharp edge (Vortex generators. Energy comes in the form of drag) the turbulent air follows the line of the car. The purpose is that the drag from the vortex generators is LESS than the drag of the eddy currents.

 

It would appear that due to the fact that Zed's don't have such a sharp transition at the rear that this equation doesn't work out.

 

It would be interesting to get data on lift/downforce for a zed with a rear lip spoiler with and without vortex gens. One might think that if the air better followed the rear window it would increase the amount of air reaching the rear spoiler. Did they do a test like this in the tunnel? Did the vortex tested car have a rear spoiler of any sort? A front lip would be good to because that would encourage more air over the top of the car and more to the rear wing.

 

Either way, I concluded that for $80 for 10 or so plastic tabs I could spend my money elsewhere and get better results, such as headlight covers.

[MaygZ]

Slightly of topic, but on the same path.

 

The difference between top speeds of a VY SS and a VY SS with the Police light bar across the top is 15km/h.  I think that is quite a significant difference.  I'm not aerodynamitist (I love making up words), but I wouldn't be surprised if the h/light covers added up to 5km/h or thereabouts.

my 2 cents

 

Maygs

[Zedman240®]

I think anything that removes a flat surface from out of the airstream will have a positive result on the CD figure (lowering that is). Two headlights that are almost flat would have some resistance in the air.

[Riceburner]

My brother was telling me about the E-type jags, long story short, they were testing and trying to prove their claim of 150mph top speed before they started selling the cars. The fastest they could go was 145mph until someone suggested removing the front licence plate.... It worked, they were sold with stick on licence plates and 150mph top speed.

 

I agree any flatness removed from the car would have to help

[sco_aus]

Some interesting points here.  I would think if you are after top speeds, the money I have seen spent on things like aerodynamic mirrors and and head light covers etc (on many different cars), could probably be better spent else where. 

 

Craig: No doubt you have much better down force at high speeds, but do you think it has effected your top speed?

[dazzed]

throw a bucket out the window at a 100 k and see what happens , do you think it would instantly fall to the ground???

[sco_aus]

Bit lighter than a zed though..

[MaygZ]

"Weight will have no effect top speed"  I don't think that is quite correct.

 

An increase in weight will increase friction resistance on unsprung components (tyres, bearings etc), thus requiring more energy for a given speed.  But you are right -  F=ma so an increase in weight will decrease the acceleration.

[stevo_gj]

You are correct but arguably orders of magnitude of difference between the friction resistance and the wind resistance makes it negligible.