Long-time readers of this blog will know that we’re huge fans of using data (especially video) to improve driving performance. A recent talk given by Andy Hollis at the SCCA Motorsports Expo reminded us that data comes in many shapes and sizes; and that it often involves zip-ties.
Andy started his presentation with the rhetorical question, “What is Data?” He asserted that data is the digital representation of a car’s performance across a set of constraints, either recorded or perceived. Your “butt-dyno” just isn’t calibrated fast enough for the rate of inputs per second of a car at speed. The advantage of data over “feel” is objectivity.
For example, do you sense that your suspension is bottoming out in a given corner? Put a zip-tie around the strut shaft where it meets the tube and see how far it moves. If it moves as far as the photo below, then it is bottoming out.
Want to know how much body-roll you have? Someone is always taking pictures at autocross or track events, so find a good high-resolution image of your car head-on and measure the angle? A good amount is probably in the two to four degree range. More and you get too much weight transfer to be efficient, too little and you’re losing grip. If the angle looks good, what is the loaded tire doing? Do you have enough negative camber? How about air pressure? This photos shows body-roll of about three degrees, but the loaded front tire is deforming suggesting more negative camber is needed.
Data will show some surprising results, such as all other things being equal, a narrower car will be faster through a slalom (think old vs. new Miata). In Andy’s experience, tire compound is more important than width. And testing requires a different mindset than competition. You need to be consistent and remember your objectives when testing so you can isolate and focus on the aspect that you are testing.
For suspension tuning, he starts on the skid pad. Time is the most important factor, not feel. Use zip ties to see if bottoming out. Look at tire temperatures front/rear and side to side. To get good data run in 3rd gear. He uses a 100 foot radius skidpad. For alignment changes, work on the end of the car that lets go first. For a front heavy car, a thicker front bar can help it push less (again counter intuitive.) Rake is important, but consider dynamic toe effect when changing rake after changing ride height. Once you have steady state on round skid pad, then go to oval skid pad and introduce the pitch variable. If too extreme, go back to round skidpad and adjust. Next add braking/acceleration on the oval skid pad. If the rear steps out, it could be a brake bias issue. If you can’t adjust bias, then go back to suspension. Next he moves on to the slalom and takes acceleration of out if to just work on the transitions. This requires a constant speed through the slalom. For autocross, a little toe out in the front can help (not recommended for the track.) For an underpowered car with over-steer a little toe out in the rear can actually help, but can also make for an interesting ride in a straight line over bumps. The B Spec cars in PWC are running as much as a half an inch toe out [which is nuts.] In the end you may have to compromise, but consider where you spend most of your time on course. If you’re running autocross in a small parking lot with many tight turns, you may make changes that are different if most of your time is spent in long sweepers. The same goes for the track.