How they keep the rubber on the road
An intricate network of mechanisms in the suspension system undergo a wide range of adjustments to keep a race car's grip on the road.
When a team begins setting up a race car for competition on a particular track their concern is how to maintain a consistent and reliable tire patch in contact with the track surface.
The size and type of the track dictates what type of suspension components they will use to accomplish that goal.
A flat surfaced oval track will generally require a stiffer setup than a banked track because the car will experience more lateral load that causes the body/chassis to roll over more in the corners.
On a banked track the car will experience less lateral load and gain more vertical load as it turns into the banking of the corners.
A road course will require a totally different setup from either a flat or banked oval track because the driver will be turning left and right into turns varying in size and degrees.
The family minivan out in the driveway has suspension components similar to a race car but the engineers who designed it anticipated that you would never be pushing it to 100 percent of its capability. They therefore compromised on some of its handling performance to gain a more comfortable ride for you and the family during that long trip to grandma's house.
Race cars are not designed for trips to grandma's house. They are designed for trips to Victory Lane. The in-car cameras that give you a view of what the driver sees are informative and entertaining but they do not give you a feel for the controlled chaos that the driver is experiencing while he is at full speed, or racing speed, on the track. Race cars are extremely noisy inside and experience constant movement caused by variances in the track surface and the high-speed wind that is continually buffeting and bouncing them around. Where you might make a half-dozen steering corrections in a minute to keep the old minivan going straight, a race car driver is constantly adjusting his steering wheel to compensate for the forces acting upon the car.
There is an old racing adage that states: "Low, Left and Light" that you may hear mentioned on TV occasionally. That is referring to controlling the roll center of the chassis to minimize rollover when entering the corners. You do that by moving the roll center as low and as far left as you can and by minimizing or moving the weight that can cause the chassis to want to roll over.
You are attempting to convert lateral load to vertical load whenever possible to help stabilize the car and keep the tire patch in constant contact with the track. NASCAR dictates how much a car must weigh (including driver) and how much of that weight must be allocated to the right side of the car so teams are limited to how much weight they can move around to lower and offset the cars' center of gravity. That places a greater emphasis on how the car is set up to manipulate its roll center.
So how can you manipulate a car's roll center while meeting NASCAR's dictated weight restrictions? First of all, NASCAR's dictated weight limits are based on static weight when the car is sitting on the inspection scale. However, when a car is in motion that static weight becomes dynamic weight and the ballgame changes.
NASCAR has not yet developed a way of measuring a car's dynamic weight while it is at full speed which allows the teams to apply their knowledge of physics and geometry to somewhat fool Mother Nature. This involves raking (lowering the front end) and tilting (lowering the left side) of the car's body/chassis so it sits down in the left front to help lower the roll center and control yaw.
Additionally they will use track or torsion bars along with spring and shock combinations to help transform lateral load into vertical load where possible thus converting some of that right side static weight into left side dynamic weight. Lowering the front also presents more front end surface to the oncoming wind for increased aerodynamic downforce. Aerodynamic downforce creates added vertical weight to the front tires and it is free weight since it does not exist unless the car is at full speed.
Aerodynamic manipulation has become a major factor in setting up today's race cars. Much more so than 10 or 15 years ago. Back then teams did not do extensive wind tunnel testing so they relied more upon their limited knowledge on how to make the cars aerodynamically slick. They would make the bodies as smooth and slick as possible and then wax and polish them to perfection to reduce the friction caused by wind drag. One crew chief from that era used to say: "I want that car so slick and polished that a gnat will bust his ass if he tries to land on it".
Today's race cars, still have aero slick sides on them but the newer style noses split the air away from the sides to create a slight vacuum that reduces wind drag and friction. It also gives today's gnats a better shot at making safer landings. As with anything on a race car, a gain here creates a loss somewhere else. By making today's cars more aero slick for added speed, the engineers have given up some of the driver's handling comfort. That is why you hear statements like "He took the air off of me and got me loose" or, "I've got a bad aero push."
It has also made the drivers more susceptible to manipulation by other competitors who can move air off their spoilers or disrupt their air flow in other ways that will make the car extremely difficult to control. That is one reason you will see drivers magnanimously move over and let a faster competitor go. Their generosity is based on a basic survival instinct. You don't have to touch another driver's car with yours to wreck him anymore. Just disrupt his aerodynamics and see if he can hang on to it.
How well he can hang on to it will depend upon how his team has set up his car. What caster and camber they have set up in the front end along with toe and how much Ackerman is designed into the geometry. Camber is when you lean the tire (Note: when I say tire I am including the wheel) into or away from the frame. Caster is the amount of reach or gain a tire will experience when you turn the steering wheel.
Toe means one or the other front tire is turned in slightly -- usually 1/8 or 3/16 of an inch -- so the front tires work very slightly against each other and the car will go in a straight line if you let go of the wheel. Otherwise it would wander back and forth when you tried to drive in a sraight line. On street cars you toe the right wheel in slightly to work to compensate for the crown in the road and to work in concert with the positive caster setting. But in race cars you toe the left tire out to help it to turn into the corner more than the right tire.
Ackerman is the amount of toe out that you can gain on the inside tire in a turn. The outside tire travels a longer distance than the inside tire when the steering wheel is turned. If the wheels turned in parallel then one or the other would be dragged across the track surface because they would not be moving in the same arc.
Caster, camber, toe and Ackerman are what is used to adjust the tires so they will travel freely without one binding or scrubbing. On an oval track you would run negative camber and positive caster on the right front and positive camber and negative caster on the left front. Plus you would toe the front end out (versus toe-in on the family minivan) and have positive Ackerman to help the left front tire turn into the corner more than the right front. This is done by altering the lengths of the upper and lower control arms (think upper and lower A frames on the minivan) and the steering arms -- within NASCAR rules limitations of course.
When teams set up the front end geometry they have to take into consideration that as the car rotates into the turn and the springs compress, the geometry will change. That is known as "bump steer." You want the bump steer to aid in turning the car versus hindering it. Keep in mind that all of these geometric calculations relate to maintaining a consistent tire patch to the track surface and the slip angle of the specific tires being used at that track.
The law of "equal and opposite" comes into play during these equations. You cannot gain in one area without giving up something in another area. Everything that is done on a race car is some form of compromise. For example: if some negative camber in the right front is good then more should be better. Yes and no.
The size of the track and the banking, etc. will dictate how much camber you can run before it becomes a negative in the setup. Negative camber in the right front helps to keep the tire tread flat against the track surface going through the corners (tire patch) but it will also lift the outside of the tire off the track surface when the car is going straight and can cause excessive inside tread surface wear to occur. "He blew a right front and hit the wall". One reason could be that the camber setting was not correct for that track. Another might be that the team was manipulating air pressures to compensate for missing the correct camber setting or, a combination of both.
The teams will work to balance the car's weight distribution between the front and rear during the setup process to achieve "neutral" which simply means the car does not push (understeer) or is loose (oversteer) when going into a corner. They have to coordinate the rear end track bar and control arm settings so that they work in concert with what they are attempting to do with the front end.
All of this is done at the shop before going to the track. They will make adjustments during practice sessions prior to qualifying and the actual race. They are limited in what they can adjust on the car once the race starts. They may start out with "spring rubbers" inserted between the coils of the springs as part of their setup. They can remove some of these spring rubbers during a pit stop to adjust the vertical downforce on a particular tire to either loosen or tighten the car. They can jack in or remove vertical weight (take out wedge) with the screw jacks you see them insert through the holes in the rear windows. They can also adjust the track bar in the same manner.
The simplest adjustments are with air pressures. And, being a former crew chief, there are times the crew chief would like to make an adjustment to the driver by using a three-pound sledge hammer. However, that is frowned upon by NASCAR and the team sponsors, etc.
Most adjustments made during a race are relegated to the rear end of the car because raising the hood to access the front jack screws, etc. costs too many valuable seconds on a pit stop.
Bill Borden is a former championship winning crew chief who operated David Pearson's Racing School for many years.
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