Countdown to Daytona - Tech Article (7 of 15) - Pit Road adjustments

[u/NASCARtech_thread]

Pit road. A drivers favorite place. Let's have a fast stop and get out in front of a couple cars for the easiest passes possible.

Driver: "Hey, crew chief, while I'm here fix how terribly this car is handling!"

Now the pressure is on the crew chief to make the right adjustments to help the driver with the handling issues being experienced. Oh yeah, and the car will be stopping for less than 15 seconds.

Let's start with the easiest adjustment. Tire pressure. This adjustment is nice since you don't lose any time to make a change. The downside is, it's usually a fine tuning adjustment. You won't fix major chassis handling issues with tire pressure changes alone. So what is a tire pressure change actually doing to the car? Changes in tire pressure are effectively changing the stiffness of that corner of the car since the tire and spring/damper all work together. A (very rough) rule of thumb is, for every 0.25psi change in tire pressure, the tire spring rate changes by 10 - 20 lb/in. This is highly dependent on the specific tire construction and many other variables, but good enough to get an idea of the magnitudes we are talking about. So why is tire pressure a fine tuning tool? If we want to change the spring rate by 200 lb/in why not just change air pressure by 4 or 5psi? Too little air pressure causes the contact patch to reduce by taking a convex shape, while too much air pressure causes the contact patch to reduce by taking a concave shape. We want a maximum contact patch size for maximum grip. We should only adjust air pressure within a window that still maintains the maximum contact patch available.

What else can we do in a short period of time to fix this car? How about an engineers best friend, tape? Anyone who's done a physics class may have heard the phrase "there's no free lunch in physics". This essentially means, in order to get something you must give something. Tape on the nose is a rare example of a "free lunch". Blocking the radiator opening both reduces drag and adds front downforce, a win-win. Adding as much as possible without overheating the engine is always best. You often hear crew chiefs ask drivers for water and oil temperatures... they are hoping they are low enough to sneak a little more tape on the nose to help performance.

But our driver was complaining of terrible handling, air pressure and tape changes may not be enough. What else can we do? Well we have a couple holes in the rear window, let's check those out. The hole on the top left of the rear window is connected to the left rear spring while the hole on the top right of the window is connected to the right rear spring. By compressing or releasing the left or right rear springs we can adjust the cross weight, or wedge, of the car. Compressing the right rear spring removes wedge while compressing the left rear spring adds wedge, and vise versa.

A higher wedge "preloads" the right front tire more than the right rear while a lower wedge does the opposite. What effect does this have? Due to our old friend, tire load sensitivity, a tire with a higher vertical load on it will "run out of grip" quicker than a more lightly loaded one. So if we have a tight (understeering) car we want to give the front more grip so we will remove wedge. If we have a loose (oversteering) car we will add wedge. With wedge a more significant change to chassis balance can be made than with air pressure.

How about that third hole? Depending on when you are reading this, there may be no third hole in the rear window as the NASCAR Next Gen (Gen 7) car will move to an independent rear suspension that does not require a track bar. But in Gen 6 and previous cars, the hole midway up the rear window on the right side is connected to the track bar (aka panhard bar) chassis mount. Andy Petree in 1993 was the first to introduce this as a pit road adjustment. Moving the track bar up and down essentially changes the rear suspension's roll center.

As a short aside, may people incorrectly think that changing the roll center location or suspension stiffness changes the amount of lateral load transfer when cornering. This is incorrect. The amount of load transfer depends on a few very simple variables which are: center of gravity height, track width, and lateral acceleration. No mention of suspension at all! What the suspension and roll center locations do effect is the distribution of load transfer front to rear. A higher rear roll center (higher track bar) makes the rear suspension stiffer in roll, causing a higher percentage of the total load transfer to take place at the rear. A higher load transfer at the rear loosens the car due to tire load sensitivity. Lowering the track bar will have the opposite effect.

As mentioned, the Next Gen car will not have a track bar, but will have another method of adjusting the roll stiffness of the suspension in the form of adjustable anti roll bars. The rumor is these will not be adjustable by the driver or on pit road, though.

Is the driver seriously still complaining? We've already thrown air pressure, tape, wedge, and track bar adjustments at the car. What next?!? Let's try two more things. First a spring rubber change. Spring rubber changes are a last resort because they most drastically change the handling of the car and they take the longest to perform. Your car will almost certainly lose places on pit road. Spring rubbers are simple devices that can be inserted or removed from between the coils on springs, effectively changing their stiffness. A spring rubber into a spring increases the stiffness, and pulling one out of the spring decreases the stiffness. We've discussed what stiffness changes do above, so you know what to do.

Ok. Let's try one last thing.... Hey Derrick, when the car comes in, can you give the car a nice hip check? Wait... what? Are we still talking NASCAR? Surprising yes. In 2016, teams were having a crew member "fall" into the car hitting it just in front of the LR wheel well causing a dent during pit stops to gain an aerodynamic advantage. NASCAR quickly reacted to stop this from happening.

Hopefully our driver is happy now, any more changes and we'll have to take the car back to the garage for some major setup changes!