It’s got to be a little frustrating that there are less than 90 days left before the start of the 2014 season and the rules package for the car isn’t set. Teams hope for a little more clarity following the open test at Charlotte Motor Speedway Monday.
NASCAR’s put a lot on the table – everything from suspension changes (like static ride height) to aerodynamic modifications to the spoiler and splitter. On the positive side, teams are very happy at how inclusive NASCAR is being with their changes – they’re listening to input from the teams, sharing their thinking processes about the changes and their conclusions from the tests.
One of the overriding themes that keeps coming back is a desire to slow down the cars to improve the racing. Let’s look at how that works.
There are always some people obsessed with numbers, but we all know that some of the fastest tracks – the 1.5-mile ovals – have struggled to produce competitive racing. One of the major problems is the aerodynamics of the cars. Drivers complain that they “can’t pass” because of “aeropush”.
Slower? But This is Racing!
It is great fun to stand near the fence and have to hang on when 43 cars come whizzing by at 200 mph; however, I bet you wouldn’t notice much of a difference if they were “only” going 190 mph. Racing is about relative speed – how fast one car is going relative to the other car. It doesn’t matter if car 1 is going 200 mph and car two is going 199 mph, or if car 1 is going 190 mph and car 2 is going 189 mph. The difference is speed determines who leads — and who wins.
Sure, it takes your breath away when you realize that a stock car going 190 mph travels about the length of a football field a second; however, do you want speed or do you want passing? There is an old saying:
An aircraft is a series of compromises flying in close formation
So is a racecar. NASCAR’s job is to figure out which compromises to make, with the goal being exciting racing. So everyone calling into radio shows saying they want both really fast cars and a lot of passing, just stop now.
Some have complained that slowing the down the cars is just going to make the racing more like that in the Nationwide series. But it isn’t just a question of speed – it’s a question of where that speed comes from and the balance between mechanical grip and aerodynamic grip.
Getting a Grip
A tire’s grip depends on two things: how well the rubber sticks to the road and how much force is pushing the tire into the track. The more grip, the faster you can go.
The first element – how well the tire sticks to the track – is a property of the tire. Teams are prohibited from chemically treating or physically changing the tires. That means the big variable they have to play with is the force pushing the tire into the track.
Imagine I give you a tire and ask you to pull it. Not so hard. Now I have Tony Stewart sit on top of the tire. It’s going to be a little harder to pull now, right? That’s because the extra weight pushing down on the tire gives the tire more grip and makes it harder to move.
A caveat: The weight pushing down on each of the four tires isn’t the same because of the initial weight distribution of the car and load transfer (the shift of the body weight during braking, turning and accelerating). To keep this straightforward, I’m not going to to worry about downforce distribution, just talk in terms of total downforce.
When it comes to downforce, more is better.
The car’s weight pushes the tires into the track. This type of grip is called mechanical grip.
A heavy car has more grip just because it’s pushing down on the tires harder; however, you need more force to accelerate a heavy car. A typical weight for a Gen-6 car is 3480 lbs with driver (3300 lb minimum weight, 180-lb driver). You can change the mechanical grip of each tire by adjusting the suspension and tire parameters such as camber and pressure;however, every car has about the same amount of weight to distribute.
If cars depended only on mechanical grip, they wouldn’t go very fast. If we raced in a vacuum, it would be pretty boring.
Luckily, we have air to help with grip. As a car travels, it encounters billions and billions of air molecules. Those molecules bounce off the car’s surface, exerting forces. Each air molecule exerts a small force, but there are a heck of a lot of air molecules.
The forces have different names depending on the direction they push. If the air molecules are pushing opposite the direction the car is moving, we call that drag. If they are pushing down on the car, we call that force ‘aerodynamic downforce’.
Aerodynamic downforce is quadratic: it changes like the square of the speed. If the car goes twice as fast, it gets four times more aerodynamic downforce. In the graph below, the blue line shows the mechanical grip and the purple line shows how the aerodynamic grip changes with speed. The numbers are typical numbers for a Sprint Cup car in race trim at a 1.5 mile track.
This is one reason driving a racecar is so challenging. At a track like Texas, you might come into a corner doing 200 mph and reach 180 mph in the center of the corner. You’ve lost almost 440 lbs of grip just going through that part of the corner.
Here’s the really important difference between mechanical grip and aerodynamic grip: Being in another car’s wake doesn’t change your mechanical grip, but boy does it change your aerodynamic grip. The aerodynamic downforce is generated by the air flowing over the surface of the car. When you get close to another car, that changes how the air flows over your car and thus changes your grip.
Aerogrip and Speed
When you’re running around Bristol doing 90 mph about 12 percent of your total downforce is due to aerodynamic downforce -the rest is due to mechanical downforce. When you talk about 1.5 mile tracks, where speeds hit 205 mph, now about 40 percent of your total downforce comes from aerodynamics. This gives the car in the lead an incredible advantage because the lead car can harness the aerodynamic downforce and the other cars can’t.
There are a lot of ways to slow down the cars and NASCAR seems open to investigating just about all of them – with the caveats that any solution can’t introduce safety issues or be prohibitively expensive.
- Decreasing engine power can be done quickly using tapered spacers, as is done in the Truck Series. A big advantage to this is that NASCAR can change the spacers relatively quickly and easily. It presents a possible inequality for the teams: Because the engines are designed differently, they react to changes in airflow differently and one manufacturer might be disadvantaged more than others by a tapered spacer change made on short notice. They’re looking at taking 90 hp out of the current 900+hp engines.
- The aerodynamic package can be changed via additional parts (like roof rails, wickers or sharkfins, as we’ve seen in the past) and by changing the spoiler and splitter. The disadvantage is that aerodynamics are especially subject to the law of unintended consequences. The aerodynamics of the car play a huge rule in keeping the car on the ground, so any changes have to be vetted for safety in case of spins and collisions. There’s also the issue that aerodynamics changes (like a wing vs. a spoiler) tend to be very visible and fans really like the look of the Gen-6 car.
Anyone hoping for a simple, fast solution is going to be disappointed. A car is a complex system. Changing one part often changes things you might not have expected to change. NASCAR has to balance their experiments with ensuring the teams have enough time to get parts ready and do their own testing to optimize their cars within the new rules.