A NASCAR Engineer Explains Why This Weekend’s Sonoma Race Is Going To Be So Fast

Sonoma Raceway has gone by many names through the years: Sears Point, Infineon whatever you call it. Welcome to wine country for a beautiful weekend of road course racing. The California road course has been a staple of the NASCAR Cup series schedule since 1989, except for 2020 when the race was canceled due to the Covid-19 pandemic.

From 1989 through 2017 it was one of only two road courses on Cup schedule, the other being Watkins Glen International in New York. The twelve-turn NASCAR layout features 160 ft of elevation change as the track winds its way up and down the hills of California.

The exact number of corners is a bit subject to interpretation and the numbering convention is rather strange. Instead of re-numbering the corners for the NASCAR layout, the infield horseshoe corner numbers are just skipped over with the track going from Turn 4a straight to Turn 7. Why is Turn 3a not just called Turn 4? Why is Turn 4a not just Turn 5? Who knows? It’s a bit confusing but there’s no sense getting lost in the semantics of it all.

The Ups And Downs Of Sonoma

Let’s take a ride onboard with Ty Gibbs during the 2023 Xfinity series race. There are several things you should note from this clip. First, how rough the track surface is. Second, how much time he spends at partial throttle, struggling to get back to full power. And third, how early he is lifting to try and ease his car into the corners.

Starting the lap, drivers will come flying into Turn 1 and quickly begin climbing uphill. As the car compresses into the hillside, drivers will feel the car grip up and it is very easy to get baited into overdriving the entry to this corner and the transition up the hill.

Turn 2 features a blind crest and a direction change back to the right. Cars tend to understeer here as the hill flattens out and falls away from the car, reducing vertical load at the front axle. Turns 2 to 4a is a roller coaster ride with a slight drop from 2 to 3, another rise over the top of 3a and then another steep drop into Turn 4a.

The apex of Turn 3 is in the trough of this up-down section and cars have a tendency to oversteer here as the front axle loads up under max compression. The track then falls away immediately again as they crest another blind hill over the top of 3a and the balance swings towards understeer as the car gets light and tries to fly over the hill. You will see a lot of drivers run wide and get out into the dirt in Turns 2 and 3a.

After Turn 7, drivers will fly downhill through a series of increasing radius ess bends, gradually building their speed up to Turn 10 where they will have to get back on the brakes and decelerate the car. Turn 10 is likely the most crucial corner to nail as it leads into the best overtaking zone on the circuit in Turn 11. On the flip side, there is little runoff room on the outside of the circuit and a mistake in Turn 10 can easily result in a day-ending incident.

The First Year Of Repaving

Prior to the 2024 season, Sonoma Raceway was repaved for the first time in twenty-three years. The old racing surface gave us some of the highest tire degradation of any road course, and this weekend’s race will be drastically different. Traditionally, Sonoma has been a low-grip track where high surface temperatures combined with worn-out asphalt led to drivers spending the majority of the race slipping and sliding around as they tried to preserve their tires. The fresh coat of asphalt has not only smoothed out many of the bumps in the racing surface but added tremendous amounts of grip. In tire testing earlier this year lap times were reportedly four to five seconds faster. Instead of tip-toeing around the circuit, drivers will be wringing the life out of their car each and every lap.

On the old surface, drivers would lift early and float the car into the corner before gently easing back into the throttle. Anything they could do to avoid slipping a tire and creating excessive wear. On a fresh repave, drivers will be charging hard into every braking zone and hammering the throttle on exit of the corners. The difference in driving style, as well as increased speeds, significantly changes the loads that the racecar will be subjected to as well as the rates at which the chassis pitches and rolls around the racetrack. The faster pitch and roll rates necessitate a different damper package and the increased loads will require a stiffer suspension setup. When going to an old, worn-out racetrack teams will typically build the suspension as soft as possible in an effort to add compliance to the car and increase mechanical grip. On a fresh repave where grip is abundant, the suspension can be stiffened to not only deal with the increased loads but to maintain a better aero platform and make the car feel “sharper”, or more responsive, to the driver.

So how do teams figure out how to set up their car for a new track or racing surface? Let’s talk about testing.

The “T” Word

Team testing is heavily restricted in modern NASCAR. The most common type of testing, which occurs multiple times per year, is tire testing. At a Goodyear tire test, one car and driver from each manufacturer is selected to participate and it is the manufacturer’s discretion of who gets to go. For the Sonoma tire test earlier this year, Chevrolet selected Ross Chastain, Ford selected Josh Berry, and Toyota selected four-time Sonoma winner Martin Truex Jr. Teams are typically allotted a few hours at the beginning of the test to dial in their setup on a baseline tire before Goodyear will begin cycling through tire compounds. In an effort to control variables, Goodyear does not want the teams changing their setup between tire compounds so this time allows them to get their car driving somewhat reasonably so that consistent testing can be completed.

Once a tire test has been completed and a final tire compound selected, the next step in the process is for a wheel force test (WFT) to be done. The rules for wheel force testing are slightly different than a standard tire test as you can see below.

The WFT at Sonoma was done by Shane Van Gisbergen for Chevrolet, David Ragan for Ford, and Jack Hawksworth for Toyota. These WFT events act as a final confirmation of tire selection for Goodyear as well as a chance for manufacturers to gather incredibly detailed information about tire behavior and forces.

A wheel force transducer is a specially designed hub and wheel assembly that measures both forces and moments in all three directions X, Y and Z at the wheel. These devices are incredibly expensive, around $1,000,000 for a full set, and as such the WFT are done exclusively by the manufacturers and not by the teams themselves.

The setup of a wheel force transducer for a road car can be seen here.

And here one can be seen installed on a Gen 6 Cup car:

Each manufacturer is allowed one car with wheel force transducers at the test. #nascar pic.twitter.com/EuEoP3eWPL

— Bob Pockrass (@bobpockrass) January 31, 2017

At the end of the day, the only thing connecting a racecar to the ground is a few square inches of rubber. As such, it is crucially important to understand the forces acting on the tires and its effect on their behavior. For a breakdown of these forces as well as their directions and axes please reference the diagram below.

The main forces and moments measured by a wheel force transducer are as follows:

• Normal Force (Fz): The vertical load on the tire. This can be varied by chassis weight transfer (braking, cornering etc), track banking, downforce and bumps in the racetrack
• Lateral Force (Fy): The cornering force acting perpendicularly to the tire during cornering
• Tractive Force (Fx): The force of acceleration or braking acting parallel to the tire
• Aligning Torque (Mz): The moment about the vertical axis that steers the tire. This moment often acts in response to slip and tire deflection caused by camber but even with zero slip and zero camber there is still a moment created as the tire deflects due to asymmetries in construction and contact patch load variation.
• Rolling Resistance Moment (My): What that must be overcome to roll the tire along the road surface. This can be significantly altered by tire deflection, which itself will be altered by inflation pressure and vertical load
• Overturning moment (Mx): A cambered tire has an asymmetrical pressure footprint. The distance between the center or pressure and the center of contact is known as pneumatic scrub. The pneumatic scrub length acts as the lever arm for the tires overturning moment.

When setting up a racecar, there is a box of parameters and settings within the bounds of which the tires will behave most optimally. The point of a WFT is to define where the edges of this box are. The data system on the car will also collect readings on tire pressures, slip angles, and surface temperatures to fully map out the behavior of the tire. As changes are made to the setup of the racecar, the forces generated at each tire will also change and this will start to draw the lines of the box in which the tire wants to live.

At these tests, teams will often do sweeps of various settings and they can be plotted against lateral force and tire wear to better understand how to set up their racecar. For example, a tire will generate increasing amounts of camber up to a certain point when the increased camber begins taking away contact patch area and the tire starts to produce less grip. However, the location of this point of diminishing returns changes from tire to tire and track to track and is dependent on numerous other settings. Each setup component (camber, toe angle, bumpsteer, slip, pressure and temperature etc) has a sweet spot that will enable the tire to produce the most grip and they all influence one another to a certain degree.

Processing the WFT data will give teams a very good idea as to what the dimensions of their setup box are. Manufacturers will normally send out a brief summary of the tire data to teams in advance of a race weekend. By piecing together all of that information a team may determine that their right front tire needs to race between 43 and 46psi at -4.0* to -4.5* dynamic camber with 3.5* to 4.0* of slip angle.

Once the targets are established for each corner of the car the setup can be developed working backwards from where the tires want to operate. There are dozens of different ways to keep your tire working in this window and finding the best solution is the team’s primary job.

Wheel force tests are no longer done for the Xfinity and Craftsman truck series tires but there is still a lot of valuable data that can be extrapolated from the test results. A track map for the driver simulator can be made by using the suspension travel to figure out bumps, curbs and other track content. The grip level can be determined by looking at the percentage of lap time gain that was seen for the cup cars. By using all of this data to create a lap for the driver simulator it is possible to unload off the truck and have your setup be very close.

Even with all of the pre-weekend information available, teams will still have “real” practice this weekend. During the standard warmup style practice sessions, only pit stop adjustments are allowed. These adjustments include right-side track bar height, rear wedge rounds, and external shock adjusters. With open practice (pre-Covid style) teams are allowed to change anything they want to. Instead of just messing with ride heights and shock clicks teams will be able to make wholesale changes. Teams can change suspension geometry, springs, shocks, ballast location, really just about everything.

These “real” practice sessions are also a valuable opportunity for teams to try and experiment with their setups. On a standard weekend, teams are pretty well married to whatever they showed up to the racetrack with. It can often be a hard sell to try something radically different, when it not working would mean sacrificing an entire race weekend. With full practice, teams can show up with a setup that may not have otherwise left the world of simulation and try it out risk-free.

In addition to giving teams the opportunity to dial in their setups, the extended practice is crucial to determining strategy. At a repaved racetrack fuel mileage is significantly worse, but how much worse is still to be determined. With increased grip from the new asphalt, driver’s on-throttle time will be a much higher percentage of the lap. Without tire falloff, the on-throttle time will remain high for longer into a run. The combination of fuel mileage and tire wear needs to be understood for teams to be able to determine their race strategy and having a full practice will fill in all the pieces of the map that are still missing.