The Body Mechanics of Braking Accuracy
Last Updated: 05/15/2014
There has always been a lot of talk about improving braking accuracy in sim-racing. There are many kits and home-brew mods out there and all of them seem to work. In this blog entry, we are going to explore several methods used to increase braking accuracy which modify various characteristics of the brake pedal. So which combination of characteristics is the most optimal?
To answer this question we must fully understand the forces at play. It is often said that ‘muscle memory’ is responsible for accurate brake usage, indicating that our muscles are more consistent at repeating force rather than repeating distance movements. Not only is this statement not true, but the term ‘muscle memory’, which comes from bodybuilding not sim-racing, is actually a myth. Our bodies can replicate distance movements quite accurately. Consider when you walk up a flight of stairs. You are able to lift your foot the same distance at every step without even looking or concentrating on the task. From this example it is clear that our legs are perfectly capable of repeating distance movements while performing other tasks. So what makes it so difficult to move the brake pedal the same distance repeatedly?
There are two reasons for this. First, we don’t have a clear point of reference. This will be clear if we return to the stair example. When we lift our foot off the previous step to move on to the next step we have the previous step as a reference point. However when we are driving we must move our foot off the gas pedal by moving it to the left and then moving down onto the brake pedal. This throws off our mental measurement of distance and prevents us from pressing the brake pedal the accurate amount. Driving without shoes is one method of dealing with this impairment. By driving without shoes we can feel the brake pedal before we press it which re-establishes the reference point and allows us to press the pedal down more consistently. The second reason is the limited travel of the pedal. Let’s refer to the stair example again. If we lift our foot a quarter of an inch higher or lower while climbing the stairs, we can still manage to get to the next step because we have a margin for error in our stride. However, the Logitech G25/G27 brake pedal only has two inches of travel and pressing down a quarter inch more or less results in over 12% error in braking. That is a huge margin of error to have in your brake pedal. To remedy this, we can implement a longer travel on the brake pedal. For instance, by having six inches of pedal travel in our brake we can reduce the error to about 4% when over-pressing by the same quarter inch. While this cannot be accomplished by the Logitech pedals, there are some high-end pedal sets that implement this approach.
So now we have learned that increasing pedal travel reduces the impact that over-pressing the brake pedal has on our driving. However, increasing the travel distance is not a realistic approach. In fact, real race cars actually have very limited pedal travel, which is a result of tight tolerances and high strength in the braking components. In addition, race cars typically do not have power-assisted brakes and the driver must exert significant force to slow the car down.
Let’s take a look at a different approach. Many people seem to believe that installing a stiffer spring on the brake pedal, perhaps even a progressive spring, would make it feel more realistic and improve braking accuracy. This is where the term ‘muscle memory’ is often misused. If we reference the stair example again, we know that our legs are able to lift our bodies consistently to the next step. If our legs were to mistakenly lift with an extra 2 pounds of force, it will still get us safely to the next step. However, the original spring in the Logitech G25/G27 pedal requires only about 20 pounds of force to fully depress, squeezing it with an extra 2 pounds of force results in a 10% error in braking. By installing a stiffer spring that requires perhaps 80 pounds of force, the extra 2 pounds mistakenly applied in our example reduces the error to less than 3%. So, while our bodies are able to repeat force fairly consistently, similarly to how we can repeat movements, we simply can not accomplish it with the accuracy required with a lightweight spring. So why not use both extended travel and increased resistance to improve braking accuracy?
Body mechanics play a large role in how much relative strength our muscles can exert. Extending our legs actually reduces the amount of force we can exert due to the effects of muscle leverage. Think of lifting weights at a gym, the weights are easier to lift when they are closer to your chest and become increasingly difficult to lift as your arms extend. The same effect happens with our legs and this effect would have a negative impact on braking accuracy. If our brake pedal has an extended range of travel, the amount of strength our muscles need to press the pedal changes as the pedal travels down further.
The best results are achieved by limiting the pedal travel to a very small amount and increasing pedal resistance. This way, the knee and ankle are maintained in the same position so the driver can apply force to the pedal more accurately, without any detrimental effects from muscle leverage. Increasing the maximum amount of force that can be applied to the pedal minimizes the error caused by pressing the pedal too hard or too soft. Furthermore, limiting the pedal travel has the added benefit of maintaining the pedal in the same relative position to the throttle pedal, making heel-toe downshifts easy whether you are braking hard while approaching a corner or lightly as you trail brake after the turn-in.
The best way to achieve high resistance with short travel in a brake pedal is with a load cell. Load cells are able to sense a high range of force regardless of the amount of travel in the pedal. There are several load cell upgrades available for the Logitech pedal sets, but many are designed with a significant amount of travel to make them feel like the brakes most people are accustomed to in street cars. In addition to power-assist making brakes feel soft, rubber brake lines and inexpensive cast iron calipers add unwanted flex and mushiness to street car braking systems. True race car brakes have a short and positive feel to them. The brake fluid does not compress, the brake lines barely expand, the calipers hardly flex and the rotors do not squeeze. Additionally, the pedal springs back softly as the brake fluid travels away from the calipers.
It should come as no surprise that the Ricmotech LC27 load cell brake upgrade has been engineered to incorporate the principals explored here which coincidentally makes the pedal feel just like a real race car brake pedal. There is a small amount of travel to simulate the pads moving toward the brake rotors and only the tiniest amount of travel after that to simulate the slight amount of give in the brake system. There is even a small amount of drag when the pedal returns to replicate the feel of the real thing. In case you were wondering, yes, you will have to wear shoes just like real race car drivers do.