"ARC has released a revolutionary Down Force Meter that mounts sensors between the spoiler and trunk lid so that when you are doing a 150 on the racetrack you can see if you are generating adequate down-force to keep the car stable. The really trick part about this setup is that you can actually log your results in real-time as you parade around the track. This will help you fine tune the car to achieve peak performance and optimal track adhesion. The Down Force Meter is going to priced at around $500

Taken from:
http://www.tune2win.com/index.php/ad...wnforce_meter/

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.