By using two valves, a bleeder...
By using two valves, a bleeder and a standard tire valve, this team caninflate or deflate the gas in the tire without removing the bleedervalve. The bleeder is sensitive to handling and may not performaccurately if it is constantly removed and reinstalled.
The spun process is done a little differently. The original tube ofmetal is larger than the die and placed outside of it. It is then spuninto the die and tightly against it. Since the die is machined perfectlyand the shell is forced to conform to that shape, the result is a muchbetter, almost perfect run-out with control of critical thicknesses inareas where we would see the most flexing and stress.
Spun wheels are more expensive due to the extra time in the process, theexpense of the CNC machinery, and the quality control that isincorporated.
In choosing our wheels, we can sacrifice a little weight and still havestrength by buying a heavier wheel at a lower cost if money is an issue.But if the budget allows, we should opt for the lighter, truer, andstronger spun wheel for all of the right reasons. In no event should weuse a light wheel that may not have sufficient strength to endure thelateral forces that may be encountered. Time will tell, because if yourwheel is not strong enough, it will eventually give up.
Center Section
The center section is stamped and then spun to create the shape that isneeded. The critical areas that need to be stronger (as in thickermetal) are where the wheel stud holes are located and where the edge ofthe center piece is welded to the shell. The metal between the studs andthe shell mostly spreads out the forces and can be of less thickness.
Modifications have been incorporated into the design of the centersection in order to accommodate the handling of the wheel, especially tohelp speed up the pit stops during a race. Reshaping the holes in thecenter section can help a crew member grab and hold onto the wheel as itis placed on the hub. Developing a deeper dish shape moves the holescloser to the outside of the rim so the tire handler does not have toreach as far into the shell to grab the holes in the center section aswell as providing for better brake rotor cooling. The back side of thecenter piece where the holes are located can also be cambered to allowthe studs to "find" the holes easier, and that lets the tire changerlocate the wheel on the studs quicker.
A standard lug nut is shown...
A standard lug nut is shown on the left next to a spring mounted lugnut. Note the flange on the back side of the spring nut. This helpslocate and center the socket more square to the nut reducing the chanceof the nut turning sideways and sticking in the socket.
Another reason the wheel stud hole areas need to be made thicker forsome of the touring series teams is because they make pit stops duringevery race and those teams glue the lug nuts onto the wheel right beforethe race starts. After the race, the glue must be removed from theholes. Most teams use a metal wire wheel. Using this type of tool does agreat job of cleaning the wheel holes and removing paint and glue, butthe process also removes a small amount of metal. If the team uses thewire wheel long enough, that area of the wheel will become thinner andweaker. A small crack that originates here can grow all the way to theshell and cause a total failure of the wheel.
Shell Strength
The two areas of the shell that need added thickness for strength arethe rim area located next to the bead and the middle where the centerpiece is welded to the shell. As in the design of the center piece, thepart of the shell that lies between the rim and the middle where thecenter is welded can be thinner than other highly stressed areas. In thespun process, this area is shaped so that some of the metal is forcedinto areas of high stress for added strength and that is one of theadvantages of this process.
Again, we need to imagine where the forces are located and strengthenthose areas as a priority. Racing wheels must maintain strength at bothrims because the wheel might be used on either the left or the rightside of the car. Mostly the "inside" rim, meaning the rim closest to theinside of the racetrack, is loaded from the lateral forces that occurwhile cornering. It would be a good idea to inspect those areas fordeformities after each race. At least one racer has noted that thefailure of the rim was noticeable once they knew what to look for.
The most common failure for racing wheels is in the area of the center,either where the center section is welded to the shell, or where thewheel studs come through the wheel. That is why most higher qualitywheels add strength to both of those areas.
Back Spacing
Back spacing refers to the distance from the outside edge of the rim tothe back of the center piece where it meets the hub face. This spacingcontrols both the track width and the lateral location of the chassis.If we need more or less left side weight, we can use different offsetwheels on each side to accomplish those goals. We cannot widen the carbeyond the rule specifications, but we can take the track width to themaximum by using lower back spacing.