Well, the race is over for another year and the milk has been been spilt on the winner’s chin. I thought it might be nice to share some of the technology that I’ve been able to pick up while at the motor speedway.
Actually it is changing wheels and tires as a unit. You see a lot of tire changes during the race and it sure happens fast. Back in the very early seventies Roger Penske was the guy who brought the “unfair advantage” to every form of racing his teams participated in. One of the innovations that really made a difference was the air jack system. That is where a pressurized hose is connected to a quick disconnect on the car and it pops the car in the air so that the tires can be pulled off and a fresh set can be installed. This reduced pit time tire changes to a matter of seconds rather than dozens and stunned the competition. The next year at Indy everyone used them.
Notice the air gun that’s used. The “nut” that’s used to hold the wheel on is not your usual nut with six sides. Instead it has flutes that the air gun’s wrench can grab hold of. The right side of the car has left hand threads so the gun has to be set to spin clockwise to remove the nuts and the left side of the car’s hubs have right hand threads so the guns will spin the nut counter-clockwise to remove them.
The guns are set to 400 ft/lb of torque to remove and 300 to put on. Here is another shot of a wrench:
Notice the big push pins at the back side of the wrench. That is important. In order to make the tire change as fast as possible the tire man has to do things in an important sequence and this sequence is also dependant on the actions of the other team members on that side of the pit wall.
First the car rolls up and needs to stop exactly where it should. There are marks in the pit for that. The tire changers have the replacement wheel and tire set in a specific spot, too. Just as the car reaches the marks the gun goes to the wheel’s nut and it is loosened. Meanwhile the guy with the air hose plugs it in and the car is immediately jacked up. By this time the wheel is being pulled off the hub and dropped in position and the tire guy has picked up the replacement to push it on the hub. When he spun the nut off the hub he dropped the gun on its side. In doing so the push pin struck the ground and was slid over so that the gun would now rotate in the opposite direction. That way, when the tire guy picked up the wrench he didn’t have to waste any time flipping a switch on the wrench, he just shoved it on the hub and pressed the trigger. The tire changer on the right front side of the car has to be sure the gun and hose gets out of the way of the car so he tosses it toward the pit wall and a team mate pulls it out of the way from the other side of the wall. By that time the air hose has been pulled from the connector on the car and the car slams to the ground.
So it takes four people, one on each wheel, plus someone on the air hose. In a race there is also the guy handling the fuel hose and the guy at the nose of the car telling the driver when it is safe to hit it and take off for the track.
Suspension and Alignment
On our cars we have to deal with a lot of different road conditions. We have to turn left and right, deal with speed bumps and pot holes. We might have to drive in the rain or the snow. For the Indy race it is all left turns and no rain tires.
You may have heard of the Indy track being called an oval. It is, kind of. More like a long straight, a corner and a short straight, another corner, a long straight, a corner, short straight, and the forth corner leading up to that original straight. At two hundred miles per hour it feel more like just two straights with corners on the end. But you only turn left and you don’t use your brakes that much. Not as much as you would on a road course.
As a result of those conditions the brakes don’t have to be quite as huge at Indy as they would be on a road course and the suspensions are set up rather oddly, at least compared to our street cars.
If you look closely at this photo of the rear of this Indy car you should notice that the tires are all slanted the same way. The right tire is tilted at the top to the left and so is the tire on the left. If you measured the diameter of the tires you would also discover that the outside tire (on the right side of the car) is actually larger in diameter than the one on the inside of the car.
On our street car and a car set up for a road racing course we would try to get the tires to be the same diameter, but having a larger diameter tire on the outside helps the car to turn left, which is what is required on an oval track like Indy.
On our street car we would set the tires so they were closer to the inside of the car at the top and wider or away from the car’s center at the bottom of the tire. This is called negative camber. What this does is to provide stability when we drive around a corner. Much like the difference we would find if we stood with both our feet together and someone tried to push us at the shoulder. In that case we would tumble quite easily, but if instead we stood with our legs apart it would make it a lot tougher to push us over and we would be much more stable.
Since the cars make primarily only left turns both the tires are set up to tilt toward the inside of the track at their tops. So the outside tire has negative camber and the inside one has positive camber. This makes for a faster car that is easier to drive left – very fast.
Here is a good shot of a car’s shocks and springs. Notice that they are not up and down nor are they out by the wheels. There are at least a couple of good reasons for doing it that way. One is that it gets these parts out of the air stream. Aerodynamics are critical to speed and unnecessary wind resistance spells “Loser”. It also reduces unsprung weight. That is the weight that is not supported by the suspension’s springs. Usually the wheels and tires are considered “unsprung” weight. Also the brakes, if they are mounted outboard behind the wheels, as well as the suspension arms. Usually half the weight of the shock and spring are counted as “unsprung” weight, but in this design it is not. The spring and shock are actuated through a “rocker arm” (that silver triangle thing) and their weight is part of the sprung weight.
Notice that the engine and transmission are used as mounting points for the suspension. There is no frame to this car. The “tub” where the driver sits is also a “stressed” member. These parts of the car are designed to do double duty and serve more than one purpose.
Take a look at the spring. It is really short. The spring on your car is probably at least ten inches tall and in trucks over a foot tall. These are closer to three inches. That is because the suspension travel on the Indy cars is so limited. The track is quite smooth (although at 200 mph it won’t feel so smooth) and the springs need to resist the down force from the wings on the front and the back of the cars. A lot of suspension travel is not helpful.
The forces generated by the aerodynamic aids on the car (especially the hidden underside) is so great that at over 100 mph the cars could travel inverted if there was an upside-down race track.