ARC Downforce Meter

September 11, 2009 | Advanced Tech | Andrew Wojteczko | Comments (4)

MSRP

$525

Pros

• Ability to analyze effectiveness of multiple wings and settings
• Easy installation in many vehicles
• Can be output to data logger
• Multiple functions including downforce, lateral G, longitudinal G, and vehicle speed

Cons

• No English documentation makes first use difficult
• Downforce expressed in Newtons, no option for lb force
• Requires a scaling factor to achieve accurate downforce values

With the use of load cells, also known as strain gauges, the ARC downforce meter is able to measure the force generated by your rear wing. This makes it possible to evaluate multiple wing settings as well as multiple wing shapes without having to spend mega bucks to rent a windtunnel. Installation requires positioning the pair of load cells beneath your trunk’s bump rubbers. These are typically threaded to set the height that the trunk lid will rest at when closed. These bump rubbers transfer the majority of the vertical load from the trunk to the chassis and consequently are the ideal place to locate the ARC load cells. Simply remove the adhesive backing and place one load cell on each side of the chassis, where the trunk bump rubber will contact the load cell. Next, locate the ARC Downforce Meter display unit in the passenger compartment where you can easily read the display. Finally, connect the supplied harness to switched 12-volt power and ground and you are ready to power up the unit.

Calibration can be performed as follows. With the trunk open, power up the meter. The unit automatically calibrates to 0 Newtons every time it is started. Now adjust the trunk bump rubbers full short and close the trunk, the meter should still be showing 0. Open the trunk and start lengthening the bump rubbers equally until the meter reads 100 Newton with the trunk closed. Now simply reset power and you should see zero on the unit. This way there is no delay in response from the unit as force is built from your wing. Next apply a known weight to the wing and record the value on the display. We simply used our body weight fully supported by the rear wing to achieve our scaling value. With our application we found that 145 lb force was equal to 110 Newtons or 11 on the Downforce Meter (Newtons*10). For reference, 100 lb force equals 444.8 Newtons, so it is clear that the trunk mounts, striker, and weatherstripping are all distributing load and skewing the value seen at the bump rubbers. The correction for this is to apply a scaling factor. Knowing that 145lb force = 11 on the display, 145/10 = 14.5.

Now, simply multiply the ARC downforce Meter output by 14.5 to get your actual downforce in lb force. In our case our setup registered 17 on the Meter at 118 mph. 17x14.5 = 246.5 lbs.

Here we are only using one known weight to perform our scaling. To account for non linearity in the deflection of the trunk supports/surrounds you can use multiple known weights close to your target downforce. With this information you can plot a curve in a program such as Microsoft Excel, and then following a run, overlay your display output on the curve to get an even more accurate reading.

Aero Tech

Wing efficiency is measured using lift/drag ratio. The higher the lift/drag ratio, the more efficient your wing is working. With the downforce meter we have evaluated half of the equation. But now how do we evaluate the drag? With limitless budgets simply head over to your local wind tunnel. However, there are affordable ways to evaluate drag. One such way is called coast down testing. To perform this test, find a safe, long, smooth, flat stretch of road such as an unused airport runway or local drag strip. Start with the rear wing removed, with a friend in the passenger seat and a stopwatch, accelerate to 105 mph, then place the car in neutral and record the longitudinal G and downforce values starting at 100 mph and then at 10 mph increments until 60 mph. For best accuracy, carry out the test several times and then average your results. Now install the rear wing and incrementally increase angle of attack from 0 degrees and repeat the test.

Note: downforce and drag increase exponentially relative to vehicle speed, so the drag force and deceleration will be greater at 100mph than at 70mph, not constant.

Now to calculate the drag force at each recorded speed use the following formula. Let’s use 100 mph and an accelerometer value of -0.1g. We will assume a vehicle mass of 2800 lbs.

Force = Mass x Acceleration
Force = 2800 lbs x -0.1
Force = -280 lbs

Now with the wing at max angle of attack and the same speed of 100 mph lets assume an accelerometer value of -0.13g. We can now find the additional drag caused by the addition of the wing at max angle of attack.

Force = Mass x Acceleration
Force = 2800 lbs x -0.13
Force = -364 lbs
364 - 280 = 84 lbs

This shows that the drag penalty at maximum angle of attack = 84 lb force. To find the angle at which optimal efficiency is reached simply divide the lift over the drag ratio. The setting that yields the highest L/D ratio is the most efficient setting. However, it is important to note that vehicle balance can have a greater impact on lap time, and if the optimal efficiency setting is producing too much downforce to maintain aero balance and resulting in excessive understeer, running a lesser angle of attack may prove to be faster, or ideally find a way to increase front downforce with a larger splitter/vented hood/canards etc. On the other hand, if the car is oversteering in high-speed corners, running an angle of attack greater than maximum efficiency may improve lap times. But first, consider adding some rear aerodynamic aids such as a rear diffuser or wing gurney flap, since both will increase downforce while helping to keep drag minimized.

Recap: Coast Down Test

1. Find a flat, safe stretch of tarmac to perform the test on
2. Remove rear wing and accelerate to 105mph
3. Put car in neutral and allow to coast
4. Have passenger record downforce and longitudinal accelerometer readouts starting at 100mph in 10mph increments
5. Repeat test 3 times and average to improve accuracy
6. Install rear wing and repeat steps 2 thru 5 for each rear wing setting
7. Use above formulas to calculate downforce and drag values and determine ideal setting by finding maximum lift/drag ratio

September 11, 2009 | Advanced Tech | Andrew Wojteczko | Comments (4)