|Topic Review (Newest First)|
|05-05-2019 01:28 PM|
Time for a new thread me thinks!
Yeah, I think there is partial truth to your claims but I am not sure you are 100% correct as there is information supporting both. It's difficult to fully believe your claims without evidence.
I do not believe the Giulia's roofline is too high for a spoiler on the trunk lid. The spoiler I speak of is the OEM spoiler nothing else. I am not speaking of anything aftermarket.
"Spoilers are used primarily on sedan-type race cars. They act like barriers to air flow, in order to build up higher air pressure in front of the spoiler. This is useful, because as mentioned previously, a sedan car tends to become "Light" in the rear end as the low pressure area above the trunk lifts the rear end of the car."
"Where most road cars (using your terminology here.. ) get into trouble is the fact that there is a large surface area on top of the car's roof. As the higher pressure air in front of the wind screen travels over the windscreen, it accelerates, causing the pressure to drop. This lower pressure literally lifts on the car's roof as the air passes over it. Worse still, once the air makes it's way to the rear window, the notch created by the window dropping down to the trunk leaves a vacuum, or low pressure space that the air is not able to fill properly. The flow is said to detach and the resulting lower pressure creates lift that then acts upon the surface area of the trunk. This can be seen in old 1950's racing sedans, where the driver would feel the car becoming "light" in the rear when traveling at high speeds. See the diagram below."
"Not to be forgotten, the underside of the car is also responsible for creating lift or down force. If a car's front end is lower than the rear end, then the widening gap between the underside and the road creates a vacuum, or low pressure area, and therefore "suction" that equates to down force. The lower front of the car effectively restricts the air flow under the car. See the diagram below."
"What all these "ideal" attributes stack up to is called the Drag coefficient (Cd). The best road cars today manage a Cd of about 0.28. Formula 1 cars, with their wings and open wheels (a massive drag component) manage a minimum of about 0.75.
If we consider that a flat plate has a Cd of about 1.0, an F1 car really seems inefficient, but what an F1 car lacks in aerodynamic drag efficiency, it makes up for in down force and horsepower."
Anyway, might want to start a new thread and see if you can get some folks in the field to respond!
|05-05-2019 06:26 AM|
Drag coefficient is used for marketing, and it is pretty much a useless number for consumers. Example was the big change in the Chevy Caprice from angular to bulbous body. The drag coefficient improved but I'd bet the drag losses were not reduced at all. GM just increased the frontal area cross section number by making the car body fatter all over. By making it organically rounded it looked "smoother" and more aerodynamic but it wasn't.
For different models of the same car body, however, the difference in coefficient is meaningful in that the lower coefficient version definitely produces less total drag. The efficiency difference however is not indicated by the delta in the coefficient. Only the fact that it is better for the version with the lower number. This results from the formula: higher cross sectional frontal area with no increase in drag will produce a lower coefficient but no savings in fuel economy. In the case of the Giulia it is clear the QV version develops significantly more drag than the normale. The TI will develop more drag than a version with smaller tires. The QV will develop more drag if only due to its wider tires. Various aero appendages will make that worse. I very much doubt the QV version generates any true downforce. Any reduction in lift is likely irrelevant to actual performance except at very high speeds. Even then back to back comparisons between the QV and the TI if fitted with the same power would be needed to establish that reduction in lift compensated for reduction in top speed. Maybe the aero effects just make the QV easier to drive at very high speeds rather than actually improving performance.
Returning to the brake upgrade issue it is critical for safety that you look for a brake upgrade package from a supplier who is competent to design one. Brakes are a system and any changes must be effected as a system even if you upgrade only one part. Buying brake upgrade parts you choose yourself is a very bad idea and potentially dangerous. You can very easily make the brake system worse.
For example, fitting bigger front brakes changes the brake bias. That changes weight transfer which can actually increase stopping distances (before the sceptic experts leap in you should recall that tire grip is related to load but not in linear fashion and, no, ABS cannot compensate for all brake imbalances). Changing just the rotor size without matching the compound to the heat requirements can result in a brake that develops less brake torque than stock because the temperature rise in the pad is insufficient. Counterintuitively, bigger brakes on the same car benefit from softer pads, or, looking at it the other way fitting more heat resistant rotors means your stock pads will likely work better than a set of harder pads. Just changing the pads to more heat resistant pads is the most common and worst amateur error because on the street you can substantially increase stopping distances when the brakes are cold, which they almost always are. If you can touch your wheels after a spirited drive your brakes are fine as they are.
The most egregious effect of fitting unnecessarily large brakes is unsprung weight. Car makers now fit the thinnest brake discs they can get away with for reasons of weight. Many modern cars wear out their discs as fast as their pads.
Holes and slots reduce weight a little but also reduce heat absorption capacity. There are zero proved benefits to holes or slotting in brake discs for road use, btw. This unsprung weight issue also arises if you fit bigger calipers, not just if you increase rotor size. Those four piston calipers you lust after can be heavier than the supposedly inferior sliding calipers your car maker fitted.
Like wheels and tires, brakes should be the smallest and lightest components you can find that perform to requirements. Going larger than you need is purely an aesthetic choice and one that I do not understand, frankly. Brakes become ugly after the first very hard stop.....
|05-04-2019 11:18 PM|
Originally Posted by Alfissimo Int. View Post
|05-04-2019 08:16 PM|
Standard Giulia has a cd: 0.25
Giulia Quad: 0.32
Not sure exactly what that tells us? Extra drag but more downforce.
|05-04-2019 05:21 PM|
Lowering the front ride height only delivers the aerodynamic effect noted on the BMW 1M. This isn't an effect of aerodynamic bodywork. Any road car will decrease front lift relative to rear lift if it is raked downwards towards the front axle, spoilers or no.
The Subaru numbers result from the ridiculous wing which clearly doesn't work properly at any speed. At least you'd be leaving the road understeering safely into the trees.
At 200 km/hr aerodynamic stability becomes a very important safety consideration but you don't see downforce , that particular BMW excepted. That car demostrates pretty scary handling at any speeds, I would definitely avoid very high speed cornering given the published numbers, exactly opposite to what you require for safe handling on the road.
|05-04-2019 05:02 PM|
I think it can be said that front spoilers and rear wings dont influence downforce much at 80 kph, which is the speed from where aerodynamic aid is starting to work. So not much to expect in everyday traffic. However in countries where there is no speed limit like in Germany, where you can go over 200kph, the spoilers and wings can make some difference. Then also in racing at higher speeds.
The size of the difference is hard to say without comparing a car body without the aerodynamic aids with the same car with the aids. A help comes form a German Magazine Sport Auto which in their Super Test on Nürburgring also measures the downforces on the test cars in windtunnel and lists up these values at 200kph. Its interesting to see what values we can expect from sport and sporty cars for the market.
Audi RS 3 Sportback......F/R...21kg up/1 kg down
Opel Astra OPC.....................7Kg up/ 1 kg up
BMW M135i..........................7kg up/ 13kg up
BMW 1 M Coupe...................5 kg down/ 37kg up
VW Golf R............................17kg up/ 29kg up
Subaru WRX STI...................64 kg up/ 14 kg down
Mercedes A45 AMG...............43kg up/ 15 kg up
Ferrari 458 Italia..................32kg up / 31 kg down
This should give an idea of size of aerodynamic forces on cars at 200 kph. At normal road speeds it will be much lower of course.
One could also notice the Subaru cannot be so much fun with 64 kg lift at the front at 200kph, while the BMWM1 Coupe actually generates a small downforce and should be quite stable at 200kph in comparison.
Have not seen figures for the Alfa Giulia Q, which would be interesting.
|05-04-2019 04:53 PM|
Originally Posted by davbert View Post
A trunk lid lip spoiler stalls the air passing over the trunk lid raising the boundary layer effect higher above the car body. SAAB achieved the same benefit by simply filling in the same area above the trunk lid with a hatchback in their so called Kombi coup. The total effect reduces drag. Station wagon and full hatchback bodystyles rarely show higher drag coefficients than sedan versions for this reason: drag goes up markedly when the boundary layer separates. This's why Porsche chopped their long tails off the Le Mans cars.
The modest change in lift over the rear of the car is merely an artifact of the primary objective of drag reduction. A "rear wing " type trunk spoiler just isn't. It cannot operate as a wing because the slot is too small. At useful speeds the slot simply stops airflow under the "wing". Which is why most often you see just the lip spoiler. Technically the ducktail spoiler on older 911 functions in the same manner as latter wing types. The wing is just more aesthetically pleasing. Except for those big gap wings such as Subaru put on their STI which in addition to blocking the rear view mirror and cresting a lot of turbulence in the wake of the car basically just look dumb and set fuel for no reason. For mature drivers you can get them taken off on delivery.
There are a number of articles in the public domain that conclusively show large wheels and extremely low profile tires do not deliver performance gains on road cars, there is a point of reversal of gain. This results from the suspension travel requirements for road cars. The same suspension travel that prevents any downforce from being generated. Too dangerous.
Just btw, the QV Giulia will have a higher drag coefficient because the frontal area is the same but total drag higher because of wider wheels and tires, among other effects. Those rear diffusers aren't and don't. They are 100% cosmetic.
|05-04-2019 03:24 PM|
Whether this is 100% correct or not it was a fun read:
If you really want to go hardcore into it, you can read this dissertation which again I am not sure is 100% correct either.
|05-04-2019 03:13 PM|
Originally Posted by Michael Smith View Post
Larger wheel and ultralow profile tire do have significant performance gains but not without compromises that are well known
As for aftermarket I can’t speak for them all
|05-04-2019 02:19 PM|
Anyone have Drag (cd) for the Giulia ti?
Alfa only shows Drag Coefficient (cd) for Quad at 0.32
I find it interesting they only show that model.
I did find the Giulia AE (diesel coming next year to EU) which is lowered by 5mm compared to veloce models has a Drag Coefficient of 0.23.
So the question is, does the base models have better Drag than the Q with all the useless spoilers, diffusers, large wheels with wide tires, active front splitter and overall height lower than other models?? This could be telling...
|05-04-2019 12:10 PM|
Originally Posted by Max Pershyn View Post
|05-04-2019 12:01 PM|
Originally Posted by davbert View Post
Road car aerodynamic appendages are either purely cosmetic, such as rear diffusers for example, or aids to fuel economy, such as trunk lid lip spoilers and front air dam types. Downforce? Not really.
Large wheels, ultra low profile tires and about 90% of aftermarket brakes are aesthetic, subjective additions to car ownership appreciation. Very little if any actual performance improvement is available.
That's why I remind the aftermarket enthusiast crowd that tires brake the car, brakes only brake the tires.
|05-04-2019 11:46 AM|
Originally Posted by Michael Smith View Post
|05-04-2019 11:38 AM|
Originally Posted by Max Pershyn View Post
|05-04-2019 10:21 AM|
|davbert||That looks like the lifting body that Steve Austin rode down on to a cloud of dust? :-)|
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