Brake pressure sensors are a logical addition to any data logging, whether it be for club level competitors or professional teams. The information we gather from them can help us with both driver and car development.
Some of the most useful information to be had concerns driver performance and coaching. Braking is the one aspect of car control that goes against the accepted wisdom of smooth driver inputs. Aggressive braking is the key to both fast lap times and the tactical advantage needed to make passing moves stick. This is especially true of cars that produce a meaningful amount of down force. At high speeds these cars typically have much more grip available than the brakes could ever overcome. This means the driver can brake much harder at higher speeds than he can at lower speeds.
The other facet to braking performance is the release of the brake pedal, once described by Jackie Stewart as one of the hardest things to learn how to do properly in a race car. As the road speed decreases, so does down force and braking ability. Some brake pad compounds will also increase in friction over the period of the braking event. Therefore, we need to be gradually decreasing the brake line pressure as we approach the corner to avoid unnecessary lock ups and to ensure that the car is balanced and can transition smoothly into the cornering phase.
Brake pressure traces are the essential parameter for evaluating how well a driver uses his pedal. Here we are going to take a look at what sort of things we would expect to see from correct brake application and what other information we can derive pertaining to car performance.
To set up a basic braking graph, we take car speed, front brake pressure, throttle position, longitudinal acceleration and ABS activity where applicable. We are using a Time based graph as we are mostly concerned with how quickly these events happen, more than where they happen on the track.
The first thing to look for is how quickly both the driver and car can build braking force. The idea in a modern race car with good aero is to build high line pressure while the car has additional downforce then taper that off as the speed decreases and the braking force increases due to temperature rise in the disc and pads. These instances rarely occur together due to a lag factor as the brake pad and disc build up to peak operating temperature.
Figure 1 has been taken from a Ferrari GT2 car with a pro driver in the seat. The green trace in Fig. 1 shows the brake pressure rising to a peak about 0.21 sec after the throttle is released. This is pretty good although less than 0.2 sec is desirable. The points we are looking for when analysing this are the point at which the throttle is released from full and the point at which max brake pressure is achieved.
Fig 2 shows a slow transition to max pressure by an amateur driver in the same type of car. The interesting part is how maximum braking force is achieved before maximum pressure due to the length of time it takes. It also takes nearly twice as long to reach maximum braking effort overall compared to the faster pressure rise in Fig 1.
Jackie Stewart once said that one of the hardest things to learn as a driver is how to let go of the brake pedal properly. This is as vital as how the brake is applied in the first place as it has a large bearing on how the car turns in. Too much and the outside front is overloaded from the word go, too little and it will struggle to build lateral force. Either way, the car will behave like it doesn’t want to turn in, but for two separate reasons.
Some of the car’s braking performance can also be analysed from this kind of trace. The lag time between maximum line pressure and maximum braking force can be seen with the brake pressure and longitudinal acceleration trace. Fig 1 shows how the maximum braking effort lags slightly as the line pressure has risen faster than the brake temperature. It also shows us when the ABS unit is working. ABS is also of interest to us. If the ABS activity is on the whole length of the braking zone, then the car cannot stop any harder and the ABS has become the limiting factor. It does become an excellent “canary in the coal mine” for suspension compliance under braking and turn in transition especially in reference to inside wheel loading.
Fig 3 is from a V8 Supercar and now shows how a brake pad that reacts slowly from cooler brake temps can cause an over braking effect. The initial brake pressure rise is quite quick but the braking temp lags a little and by the time max pressure is realised, the disc surface temp has only risen by a little over 40degC. to approx 250C. The braking force then continues to rise despite the driver releasing brake pressure to counter the beginnings of a lock up on the front left wheel.