The first goal of this chapter is to determine how adjustments to the rear wing, chassis ride height and drive train affects the speed and handling of the car. So, we take the Ferrari to Talladega for speed testing.
http://www.formula1-dictionary.net/downforce.html
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The above link will take you to an excellent "primer" on downforce and drag.
The illustration shows a modern formula car, but the Ferrari 488GTE uses most of the same tricks. Notice that downforce is generated by more than just the front and rear wings. The body, floor, underside, and rear diffuser has a lot of effect. And, all of these are affected by the "angle of attitude" or Rake of the car--relative height of the rear being higher than the front.
To determine the downforce produced, we first need to establish the relationship between downforce and changes in ride height. To do this, we make a simple test--check ride height empty of fuel and then full of fuel. The change in ride height can be used to estimate the rate of deflection from added weight, front and rear. This is not a perfect figure, as we do not know exactly the center of gravity of the fuel load, but it will do for our purposes:
For the relatively stiff springs we chose for the test, we determine that the deflection rate is:
Front: 870#/inch. (15.6 kg/mm)
Rear: 750#/inch (13.4 kg/mm)
For the tests at Talladega, we used three different 6th gear ratios:
1.174 192 mph (309 kph)
1.190 189 mph (305 kph)
1.217 185 mph (298 kph)
The car is probably capable of doing well above 190 mph (309 kph) in a draft or when going downhill. I did not test it at Bathurst or Phillips Island, but 182 mph in the first long straight at Lemans was possible without a draft. At Talladega--there is no downhill so it is sheer power vs aerodynamic and mechanical drag.
With a full fuel load, we tested a top speed of 179 mph or 289 kph (using 1.174 top gear).
This was using ZERO rear wing and minimum toe (front and rear) with static ride height of 2.243" Front and 2.760" Rear = 0.517" (13 mm) Rake
Estimated Downforce:
Front: 376 # (171 kg)
Rear: 675 # (307 kg)
Dynamic Rake: ZERO (Front and Rear Ride Height essentially the same.)
We ran the test again with minimum 1.6 gallons of fuel and changed the 6th gear ratio to 1.190. Top speed still 179 mph with engine revs a little higher. (Perhaps marginally faster than with full fuel. Remember that Tallagega is high banked so corner and top speed is not affected much by weight.)
Next test was to see what effect a change in Rake would do to speed and downforce. We dropped the front 2 mm (0.80") to 2.226" and RAISED the rear 2 mm (0.80") to 2.943" resulting in Rake of 0.717" (18.2 mm). This is a 40% increase in static Rake.
Top Speed increased slightly to 180 mph (290 kph).
Estimated Downforce:
Front: 675 # (307 kg)
Rear: 947 # (430 kg)
Dynamic Rake: 0.230" ( 6 mm)
Note: Keep in mind these figures above are for ZERO rear wing! A high portion of the downforce is coming from the underside of the car and all the sophisticated body features. And, lowering the front while increasing rake made a significant difference. There is no doubt the car would have more downforce if we lowered the front more, but there is a limit of how low you can go, as the front dips significantly under braking and the car is pushed down significantly in high speed turns. Any front ride height below 2.226" (56.5 mm) requires a test for ground clearance under braking and in high speed turns. Be especially careful when using the 1029# front springs--watch how the front wing is driven to the ground braking for the hairpin with 1029# front springs and a front ride height of 2.250".
Note: The pics above is a form of "poor man's" telemetry--I change the Z axis in the replay camera to be able to see under the car to see ride heights.
More comment: One might wonder why not use stiffer springs and adjust the front ride height lower. Well---iRacing does not allow you to go below 2.175" on the front. So, stiffer springs (>1257#) are actuall slower since downforce will not push the front low enough. The best speed in our Talladega test was with the weakest front springs (1029#) allowed!!! (The 1029# springs were too weak for Sebring's braking zones--see above) Here is a video of our fastest run.
So, let's now test to see how much of this increased downforce was due to Rake vs lowering the front. Keeping the front at 2.226", we dropped the rear to 2.754" so the Rake was close to our first test with 0.517" (13mm) Rake. (Actually it was 0.528" in this test.) Still running the 1.190 top gear.
Top Speed still 180 mph (290 kph)
Estimated Downforce:
Front: 527 # (240 kg)
Rear: 775 # (352 kg)
Dynamic Rake: 0.040" (1 mm)
Note: Notice that the downforce decreased but the top speed did not increase. As is the case in many cars in iRacing, downforce efficiency or drag increase for an increase in downforce improves with a little Rake.
Next we checked to see what would happen if we raised the whole car 0.30" (7.5 mm) to a higher ride height, keeping the rake the same. The result was very interesting.
Top Speed decreased to 178 mph (287 kph)
Estimated Downforce:
Front: 595 # (270 kg)
Rear: 933 # (424 kg)
Dynamic Rake: ZERO
Note: Notice that raising the car increased downforce slightly, but the efficiency dropped off dramatically--we lost top speed.
Next, the question you were anxious to ask. What happens when I adjust the rear wing?
We set the car with a static ride height of 2.306" Front and 2.862" Rear = 0.556" (14 mm) Rake.
With a rear wing setting of 2, Top Speed dropped 2 mph (3 kph) to 178 mph.
Estimated Downforce:
Front: 579 #
Rear: 976# (about 200# increase from ZERO setting)
With a rear wing setting of 4, Top Speed dropped another 1 mph to 177 mph.
Here we dropped the top gear ratio to 1.217.
With a rear wing setting of 5, Top Speed dropped another 1 mph to 176 mph.
Estimated Downforce:
Front: 520 #
Rear: 1076#
Dynamic Rake: Negative 0.270" (7 mm)
Note: One can see that downforce increased the most from ZERO to 2 Rear Wing setting, adding 200# of rear downforce, and at a setting of 5, the downforce increased only another 100#. One should also note that with the higher rear wing setting, static RAKE needs to increase.
Well, what does all this mean?
1. Keep the car as low as possible without it hitting the ground under braking, in high speed corners, or when jumping curbs. (A good place to start is 2.226" to 2.306" Front and test from there.)
2. The car likes a bit of Dynamic Rake. Set rear static ride height higher than the front somewhere in the 0.52" (13 mm) to 0.70" (18 mm) range, depending on spring rates and rear wing setting. Important to note--nothing in life is free--raising the rear to gain Rake and downforce will come at the cost of a higher center of gravity and a bit of tendency for the rear to "roll over" in mid corner.
3. Maximum speed is probably best achieved with the 1.190 Top Gear, 189 mph (305 kph) which gives a gain of 9 mph extra speed when in the draft. (1.174 Top Gear, 192 mph (309 kph) did not however cause much lost speed, so it might be best depending on preference and fuel economy.) Best horsepower is about 6500 RPM--4 green lites, the fourth one blinking.(see video)
4) Changing from ZERO to 2 Rear Wing setting will result in a loss of 1 mph if reaching 178 mph +, BUT that change resulted in an increase of 50# more downforce on the front and 200# more on the rear. One should consider the extra corner exit speed gained from this downforce in the corner entering the longest straight and balance that with the slight loss in speed for a very short part of the long straight. Where maximum speed achieved is less than 170 mph, a setting or 2 should be considered to be minimum. Keep in mind the rear "wing" is not only pushing the car down, it is also "killing" a bit of the lift created by the shape of the body.
5) At speeds below 160 mph top speed (260 kph), a setting of 4 or 5 for the rear wing is probably ideal--choose which based on achieving desired oversteer/understeer balance in critical high speed corners as well as balanced front/rear tire temps/wear/hot pressures.
6) Keep in mind that downforce is proportional to the SQUARE of speed, so downforce increases dramatically at higher speeds---translation: A) You need more wing for tracks having lower top speed; and B) The faster you go thru the corner, the more downforce you have and the faster you CAN go thru the corner.
7) Keep in mind that while downforce increases proportional to the SQUARE of speed, drag also increases with the SQUARE of speed. And, drag is a force, so since F=Mass times Acceleration...an increase in drag will reduce not only the top speed, but also the rate of acceleration. So you are constantly choosing a balance to achieve the best lap time---and beware that another driver may choose a different balance with a very similar lap time, but with different speeds at different places.
Transmission Settings:
The Ferrari has 4 possible settings for the Final Drive. Most commonly used is the 2.455 ratio. The other three being "higher" in number effectively reduce the speed in all gears.
There are three choices for 1st Gear. 2.857 provides 78.9 mph (127 kph) when using the 2.455 Final Drive. The other choices for 1st gear would reduce speed. My limited experience with the car indicates that you want and need first gear, and limiting top speed in 1st gear to less than 78.9 mph would be counterproductive--so run 2.455 Final Drive and 2.857 1st Gear everywhere.
IRacing provides 21 choices for 6th gear. But given that we are going to suggest using only the 2.455 Final Drive, the lowest numerical ratio we would choose is the 1.174 (192 mph). So you only have about 10 to choose from.
My advice is to pick the 6th gear with a top speed that is 105-107% of your desired max speed on the track with no draft and no wind. Best horsepower is about 6500 RPM--4 green lites, the fourth one blinking.
Choosing 2nd-5th is then a matter of splitting the gears to optimize three issues: A) Gearing for best corner exit speed at specific corner/s on the specific track; and B) The principal of greater speed range in the lower gears than the higher ones; and C) You want to maximize the area under the torque curve for maximum acceleration.
The principal in B) above is simple. In lower gears, you accelerate faster, so you increase speed more mph in less time. So the split between 1st and 2nd should be more mph than from 5th to 6th. The splits should become progressively "tighter" as you go higher in gear.
You will sense when this is wrong as the car will feel "off" in some gear changes. Or, you may find yourself shifting at an inconvenient place in a corner. Adjust everything for best lap time.
Example: (all subject to optimizing for corner exit and drafting capacity)
1st: 78.9 mph
2nd: 103.1 mph (24.2 up)
3rd: 124.0 mph (20.9 up)
4th: 142.8 mph (18.8 up)
5th: 159.7 mph (16.9 up)
6th: 173.5 mph (13.8 up)
The principle in C) above is based on the fact that RPM drops when shifting up, so you want to be sure that the engine will pull throughout the range of the next higher gear without bogging down and without acceleration flattening out.
Really good article on the setup of this car. Thank you for taking the time to put it together.
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