The NASCAR Winston Cup (soon...
The NASCAR Winston Cup (soon to be the Nextel Cup) Series teams use strong wheels. All designs must adhere to strict requirements so that the whole racing environment, which includes the drivers, fans, officials, and crews, are less at risk.
Like so many other parts on our race cars, the wheel has undergone considerable development and improvement over the last 20 or so years. The wheel is under more stress than we might realize and it is the last piece of metal between our car's suspension and the racing surface.
The reason we needed to further develop the strength of the circle track racing wheel is because of the forces that act on it and the way in which those forces concentrate on small areas of the wheel. To illustrate the point, if we have a 2,800-pound car and a 50/50 front-to-rear weight percentage distribution, we have 1,400 pounds supported by the two tires at each end of the car. During cornering, the tires resist the lateral forces by gripping the track surface, and each tire transfers this force to the wheel. The wheel is the first mechanical part on the car that resists the cornering forces. Here is how the wheel accomplishes that task.
Forces Even with the left side rules in place allowing a higher than 50 percent left-side weight, the right-side tires end up supporting around 60 percent of the race car's weight because of the weight transfer during cornering. So, the right-front tire and wheel could end up with (1,400 x 0.60 =) 840 pounds of vertical weight.
Here we can see the area of...
Here we can see the area of a spun wheel between the middle and the rim. This process promotes a lighter wheel while serving to produce areas of increased thickness where we need the strength. The spun process also insures that the run-out is forced to be correct for a true form as far as roundness and conformity in material thickness.
We can consider that the right-front tire will also resist approximately 60 percent of the lateral forces acting at the front of the car. If the g-force at mid-turn was 1.5, a common average for most medium-banked asphalt tracks and well within range for higher-banked dirt tracks with some degree of grip, then the right-front tire/wheel combination is now resisting 1,260 pounds of lateral force. That is a lot for our wheels to resist.
But, in reality, the 1,260 pounds is resisted by only around 6-8 inches of rim on the inside (towards the inside of the track) of the wheel at any given moment in time. Why? Because the tire contact patch is the first part of the car to resist the lateral forces. The force is then transmitted up to the rim, but only that part of the rim directly above the contact patch. The part of the rim at the top of the wheel resists nothing. In this snapshot of time, there is very little force on the rest of the rim on this wheel except that part directly above the tire contact patch. The wheel is spinning, so all parts of the rim experience the force, but only 6-8 inches at a time.
Now imagine if we could mount the wheel horizontally and hang 1,260 pounds off the edge of the rim that is attached to it by rollers that are only 8 inches wide. Now, we spin the wheel with the weight rollers staying stationary and acting on the entire length of the rim, 8 inches at a time. If the wheel and rim portion are not strong enough, the constant weight that is exerted on that small portion of the rim will distort it and eventually reshape the bead area so that the tire could eventually slip off the rim. On the racetrack, this would result in a sudden loss of tire pressure and a quick trip to the wall.
The center piece where the...
The center piece where the lug nut holes are located is visibly thicker than other non-stressed areas of the wheel. Note that the slots above the lug nut in the photo are flanged to add further strength to the center section.
In addition to the load at the wheel rim, a torsional, offset load is also exerted on the wheel center and that causes a good deal of flex through the center piece and into the shell of the wheel. This works to weaken the area where the center is welded to the shell.
Knowing this, we now see why wheel companies have worked very hard to make their wheels stronger and lighter. While it might seem like the use of stronger and lighter to describe a design sounds like an oxymoron, there is a way to design a wheel so that it is truly stronger and lightweight. That is accomplished by knowing where strength is needed and where we can sacrifice some thickness of the metal without compromising the integrity of the structure. Let's study the parts of the whole wheel so we can see where the strength is needed.
There are two main parts to a wheel, the center section and the shell. The shell, or outer portion, can be formed by either a rolled process or by a spun process. If a wheel shell is rolled, the starting piece of metal is a round tube and is placed inside of a shaped die. The tube is then rolled and stretched to conform to the die. This process is fast and economical, but it is inherently hard to maintain a uniform thickness and a true run-out shape due to the way the dies are used and the wear that can occur over a period of time. The rolled process is fine for trailer wheels and other non-racing applications, and we can sometimes get by using these lower cost wheels for circle track racing, but sources tell us that even though all racing wheels will fatigue and fail at some point in time, the wheels that are produced by the spun process might last as much as twice as long.