Understanding Data Acquisition Part 1: Maintaining maximum acceleration to achieve fast laps

October 14, 2009 | Advanced Tech | Andrew Wojteczko | Comments (2)

This is the first part in a series of Data Acquisition related tech articles aimed at building a better understanding of the tools and info available in your data and how to use them to help improve as a driver.

Most basic to advanced data acquisition systems are equipped with a lateral and longitudinal accelerometer. With this it is possible to create a GG diagram. It is called a GG diagram because it plots lateral acceleration or lateral G against longitudinal acceleration or longitudinal G, where G refers to G force and 1g equals 9.8 m/s2 of acceleration. The GG diagram is constructed using a scatter plot of lateral and longitudinal accelerations recorded during a lap or lapping session depending on your data selection.

To understand how to interpret the GG diagram it is first necessary to have a basic understanding of your tire. Your tire is able to generate either maximum lateral or maximum longitudinal acceleration but not both simultaneously. It is however possible to maintain the combined maximum of lateral and longitudinal acceleration while transitioning from braking to steering input, and from removing steering input to throttle application. With the GG diagram we can monitor how well a driver is achieving maximum combined G forces as well as how consistently.

If a car was able to produce equal acceleration in braking/cornering/acceleration the ideal traction circle would be exactly that, a circle. However this is not completely accurate as cars produce substantially less acceleration on throttle than they are able to in braking. Cornering forces are also slightly higher than braking forces though how much is dependent on the vehicle's static weight distribution (for example, a rear-heavy 911 will more closely match braking and cornering g values than a front-heavy VW Golf). The following approximation can be used:

Consequently, the ideal GG shape ends up being elliptical with a squashed top. Anything that changes the load on the tires will have an effect on the ideal shape. A vertical acceleration caused by track elevation can momentarily increase or decrease the wheel loads and impact the ideal traction circle for that corner. Cars with aerodynamics will see the ideal ellipse expand with speed while production cars in stock form that generate lift will see a reduction. These factors can account for the slight variations seen in the scatter plot. However, if you’re driving a car with 0 aero lift/downforce on a flat track then any inconsistencies are most likely between the seat and the steering wheel.

Image 1. The Traction Circle. The Ideal traction circle representing a vehicle producing a max acceleration of 1.2G at two different points of the braking/cornering transition.

With the following formula it is possible to calculate your total G force for any combined lateral and longitudinal G. It is also possible to see that anywhere along the perimeter of our traction circle will result in a combined G force of 1.2, or the maximum acceleration our sample car can produce at any time.

Combined G = √ longitudinal l G² + Lateral G²
Combined G = √ 0.98² + .69² = √ .57² + 1.05²
Combined G = 1.2

Interpreting the GG Diagram

The key to a fast lap is achieving maximum acceleration at all times. When interpreting a GG diagram such as the one below we can focus on the following:

1. Look for a concentration of points closely following the shape of the ellipse drawn over the GG plot. This represents how often the car was able to keep the tire at maximum capacity. Limited horsepower will almost always keep the upper half of the diagram on the low side, as seen on the GG plots below.
2. Keep in mind that not all points will follow the outer edge of the ellipse, as straight sections leading to brake zones will have no lateral input from max longitudinal + (throttle) to max longitudinal – (braking). To minimize this portion of minimal tire loading, ensure a quick release of the throttle and a quick yet smooth application of brake up to maximum deceleration force.
3. Depending on the software package it may be possible to apply a small filter to smooth out the G spikes that occur when hitting bumps. This can make it easier to interpret. the Motec data is filtered using a value of 5.
4. Check to see if braking is at its best. The car in question is a front engine Lexus IS300 Touring Car with front splitter and rear wing and a relatively narrow 235/40-17 tire. Based on our table of Braking versus Lateral G we can expect 93% of cornering force in braking. Based on a 1.25G max lateral G reading, this results in 1.15G max braking. In the Shannonville example we can see that braking force is in line with this estimation and the driver is therefore using the brakes efficiently.
5. The ideal GG Diagram will vary from track to track depending on banking (as noted in the Watkins Glen diagram) surface texture, track layout (see Mosport with minimal braking) and several other factors. The ability to overlay your data with a faster driver in your car will immediately make it apparent where you can improve. To get the most from the overlay:
   1. Compare peak lateral and longitudinal G to see if you’re achieving maximum cornering and braking power from the tire in steady-state.
   2. Next check the transitions around the perimeter to see if you are achieving maximum combined G.
   3. Finally, check for consistency and concentration of scatter points in these areas.

October 14, 2009 | Advanced Tech | Andrew Wojteczko | Comments (2)