The NASCAR Nextel Cup Series...
The NASCAR Nextel Cup Series teams use strong wheels. All designs mustadhere to strict requirements so that the whole racing environment,which includes the drivers, fans, officials, and crews, are less atrisk.
Like so many other parts on our race cars, the wheel has undergoneconsiderable development and improvement over the last 20 or so years.The wheel is under more stress than we might realize and it is the lastpiece of metal between our car's suspension and the racing surface.
The reason we needed to further develop the strength of the circle trackracing wheel is because of the forces that act on it and the way inwhich those forces concentrate on small areas of the wheel. Toillustrate the point, if we have a 2,800-pound car and a 50/50front-to-rear weight percentage distribution, we have 1,400 poundssupported by the two tires at each end of the car. During cornering, thetires resist the lateral forces by gripping the track surface, and eachtire transfers this force to the wheel. The wheel is the firstmechanical part on the car that resists the cornering forces. Here ishow the wheel accomplishes that task.
Defining the Wheel Forces
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 ofincreased thickness where we need the strength. The spun process alsoinsures that the run-out is forced to be correct for a true form as faras roundness and conformity in material thickness.
Even with the left side rules in place allowing a higher than 50 percentleft-side weight, the right-side tires end up supporting around 60percent of the race car's weight because of the weight transfer duringcornering. So, the right-front tire and wheel could end up with (1,400 x0.60 =) 840 pounds of vertical weight.
We can consider that the right-front tire will also resist approximately60 percent of the lateral forces acting at the front of the car. If theg-force at mid-turn was 1.5, a common average for most medium-bankedasphalt tracks and well within range for higher-banked dirt tracks withsome degree of grip, then the right-front tire/wheel combination is nowresisting 1,260 pounds of lateral force. That is a lot for our wheels toresist.
But, in reality, the 1,260 pounds is resisted by only around 6-8 inchesof rim on the inside (towards the inside of the track) of the wheel atany given moment in time. Why? Because the tire contact patch is thefirst part of the car to resist the lateral forces. The force is thentransmitted up to the rim, but only that part of the rim directly abovethe contact patch. The part of the rim at the top of the wheel resistsnothing. In this snapshot of time, there is very little force on therest of the rim on this wheel except that part directly above the tirecontact patch. The wheel is spinning, so all parts of the rim experiencethe force, but only 6-8 inches at a time.
Now imagine if we could mount the wheel horizontally and hang 1,260pounds off the edge of the rim that is attached to it by rollers thatare only 8 inches wide. Now, we spin the wheel with the weight rollersstaying stationary and acting on the entire length of the rim, 8 inchesat a time. If the wheel and rim portion are not strong enough, theconstant weight that is exerted on that small portion of the rim willdistort it and eventually reshape the bead area so that the tire couldeventually slip off the rim. On the racetrack, this would result in asudden 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 thickerthan other non-stressed areas of the wheel. Note that the slots abovethe lug nut in the photo are flanged to add further strength to thecenter section.
In addition to the load at the wheel rim, a torsional, offset load isalso exerted on the wheel center and that causes a good deal of flexthrough the center piece and into the shell of the wheel. This works toweaken the area where the center is welded to the shell.
Knowing this, we now see why wheel companies have worked very hard tomake their wheels stronger and lighter. While it might seem like the useof 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 andlightweight. That is accomplished by knowing where strength is neededand where we can sacrifice some thickness of the metal withoutcompromising the integrity of the structure. Let's study the parts ofthe 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 orby a spun process. If a wheel shell is rolled, the starting piece ofmetal is a round tube and is placed inside of a shaped die. The tube isthen rolled and stretched to conform to the die. This process is fastand economical, but it is inherently hard to maintain a uniformthickness and a true run-out shape due to the way the dies are used andthe wear that can occur over a period of time. The rolled process isfine for trailer wheels and other non-racing applications, and we cansometimes get by using these lower cost wheels for circle track racing,but sources tell us that even though all racing wheels will fatigue andfail at some point in time, the wheels that are produced by the spunprocess might last as much as twice as long.