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Except for very old style mag wheels that had straight-shank wheel nuts (a topic for another time! - FORGET THESE), wheel nuts (and bolts on European cars, particularly) are tapered and each taper fits into a tapered steel seat fitted into the wheel, or in the case of a steel wheel the taper is part of the wheel. The tapers serve two purposes, to locate and to increase the friction between the nut and the wheel: tapers make it harder to tighten the wheel nuts, but when tight they are far less likely to come loose. Nuts must always be tightened to the manufacturer's specified torque. KEEP TAPERS CLEAN: NEVER ALLOW OIL TO COME IN CONTACT WITH A TAPER!.
As Lampy says, centric rings serve to locate rather than support. It is the initial locating when the wheel is being fitted to the hub that is critical. A wheel / centric ring / hub with a combined close locational fit will result in the wheel being accurately positioned on the hub BEFORE the nuts are tightened. It is the hub spigot in combination with the centric spigot ring that assures accurate positioning of the wheel, rather than the wheel nuts and the sequence in which the wheel nuts were tightened. When all the nuts are tightened properly, the wheel will be accurately centred with the right amount of clamping force being applied to the wheel and hub. Then it is extremely unlikely that the wheel will come loose even under harsh vibration found in rallying. Note that for heavy vehicles the road wheel must be positioned on the hub by a locating spigot that is part of the hub.
Studs can fail for a variety of reasons:
. they become fatigued through being repeatedly tightened and loosened (wheels removed and replaced); when overtightened ("rattle guns" used by tyre fitters are often not calibrated properly, with the consequence that wheel nuts can be overtightened and studs are subjected to unnecessarily high stresses in the process) - how often do you see a car with a flat tyre parked on the side of the road because the driver cannot loosen the wheel nuts?? Yes, studs and wheel nuts should be replaced, especially if a thread looks worn or if there are bits of metal that might be causing the nut and stud to bind together. If a nut has ever been cross threaded when an attempt is being made to tighten it (especially with a rattle gun), both the stud and nut should be replaced.
. some aftermarket studs are dodgy - here the biggest danger is that manufacturers sometimes use a machine tool to cut the threads - NEVER!! Studs (or bolts) must be made from high tensile alloy steel (at least Grade 10.9 - '10' indicates tensile strength of 1000 Newtons per square millimetre (1000 MPa) and ".9" means that when tested no yield occurs before 90% the tensile strength is reached), and threads MUST be rolled - on new studs, one way to check is to trace along the thread with a fingernail - if the thread does not feel smooth or has a deep sharp bottom to the thread groove, particularly as you get close to the base of the stud, there is a good chance it was cut not rolled - cutting creates a "stress concentration" at the root of the thread, especially near the base of the stud - this is where the stud needs to be strongest. Stress concentrations become the most likely place for fatigue cracks to start. The other consideration with dodgy studs is that they can deform or permanently stretch, thereby failing to ensure the wheel is properly clamped to the hub, and further tightening of the nuts does not help - it just makes things worse.
. there is a school of thought amongst some in the rallying fraternity that to stop studs coming loose (especially when wheels are being fitted in a hurry) a bead of weld material should be applied by an arc welder to keep the stud and flange from separating - NEVER!! be tempted to do this - this will cause the steel in the stud to become brittle and the stud will ultimately break. It is important to buy the right stud with the correct number and sized splines: correct studs will not come loose when fitted to a flange that is undamaged - in the first instance, go for genuine replacement studs, sold as a replacement part by the original vehicle manufacturer - alternatively if you need studs that are non-standard lengths go for the factory competition part if there is such a thing (Nismo, TRD, ... whatever). It is foolhardy to think that it's possible to save money by buying cheap aftermarket studs - this is a case where extreme caution is needed when it comes to the aftermarket the seller's hype !
. a bolted flanged joint, such as a wheel on a car is, relies on ALL the nuts being tightened in the correct sequence (usually not in in the sequence 1-2-3-4- etc, but 1-3-2-4-etc) and torqued down properly - usually in two stages. - if one nut is not tightened properly, this could become the source of ultimate failure. (Interestingly some old Citreon cars had three wheel nuts, and it was critical that all had to be precisely tightened - ultimately Citreon changed to a minimum of four (4) wheel nuts, not because three was not enough, but because it could not guarantee that owners would always correctly tighten all three wheel nuts.) When it comes to wheel nuts: one big one in the centre on a hub-centric spline is good; there is no two; three can work fine but they must all be tight; four is better than three, five is better than four, and so on!
SUMMARY: Using a correctly fitting centric ring that accurately locates the wheel on the hub is a really good idea, especially if using aftermarket wheels. Under harsh conditions, such as rallying, wheel studs should be changed at least once per season, preferably with ones made by the original vehicle manufacturer. If you have to use longer studs for any reason, buy high tensile ones that have rolled threads, NOT cut threads. ... and using spacers ??? Whatever the risks might be, using spacers only make things worse. Personally, for me this would be a case of NEVER. There is no shortage of wheels with the right offset and width: it might just take time to find the right ones.
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Things work best when it matters the least, and ...
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