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wats their purpose and are dey needed

I assume you're referring to wheel center rings. They are used to make up the clearance between the hub on the spindle and the wheel center area. This way a wheel with a larger hub center can be used on a vehicle with a smaller hub diameter. If you dont use them, the wheel studs have to bear the weight of the car rather than the hub. This usually results in broken studs, at the very least.

They help to quickly install the rim concentric to the hub axis to reduce vibration. Other than that, they don't do anything. You can run without them (by nature the bolt pattern of the hub will center the wheel), but they are recommended as they do generally make life a tad easier. The hub centric rings do not carry any load, and not using them will not result in broken studs The loading on the hub/sutds is the same with or without them.

I have to disagree. Take an aftermarket wheel and with the car in the air, put the wheel onto the hub. While keeping the wheel against the hub see how much play there is in all directions. Most aftermarket wheels do not have a very close fitment on the studs. Most are downright sloppy. Now place the centering rings on the wheels and try again. Should be no play.

While the lug nuts keep the wheel pinned to the hub, it's the ability for that movement that takes it's toll on the studs over time. I have seen 1st hand the damage that can result from not using the centering rings.

I was always under the impression that the wheels on our cars, and many others out there, were hubcentric. Which means they are centered on the hub, and simply held in place by the studs. Once the centering is off, some or all of the studs will bear the load, causing failure. I've seen it happen on several cars and this reasoning was the explanation every time. I dont just make this stuff up.

Exactly.

The hub centric does NOT take the load.

Why doesn't it?

Magic! [poof of smoke]

Frictinal force between the wheel/brake/hub takes the load. The studs/nuts provide the normal force via tension. If not accelerating (putting a rotational force on the hub/wheel via the enigne or brake) would shear off the studs.

Why use your own words when you can steal someone elses:

The centering ring is important to precisely align the wheel to the hub while the wheel bolts (or nuts) are being tightened. But once tightened, it's the static friction between the hub and wheel that is taking the load. The fastener has no significant shear force, just the clamping tension.

It is true that the hub is bearing the load, but not at the machined center where it protrudes into the wheel. There is no way that portion of the hub could bear the load unless there was zero clearance between the hub and the wheel.

The hub is bearing the load, not at its center protrusion, but rather at the wheel bearing locations.

Similar to what others have stated, once the wheel is "clamped" to the hub, the hub and wheel become one assembly. This is no different than a two-piece wheel being bolted together. The load isn't taken at the bolts clamping the two halves together as the wheel is essentially one unit at this point.

In short it comes down to the clamping load exerted by the bolts as 03blackrt stated.

And seriously, my hubcentric rings (which I forgot to put on this summer) are plastic, they couldn't support me let alone a fucking neon.

Any time I've ever seen lug failure it's due to improper torque, or failue to re-torque after the given interval. Once the studs loosen off THEN they take all the vehicle load, which is why they fail.

Interesting. So what you are saying is the longitudinal directional load (force of the hub trying to rotate before the wheel does) is taken by the sheer clamping force provided the lugnuts. Does this mean that the lug studs don't take any load at all other than the torque during tightening them?

And by applying this idea, the centering ring does nothing besides make the wheel easier to mount?

*Edit* Based on info from Tirerack, this seems to be the case...

Tirerack Website wrote:The centerbore of a wheel is the machined opening on the back of the wheel that centers the wheel properly on the hub of a vehicle. This hole is machined to exactly match the hub so the wheels are precisely positioned as the lug hardware is torqued down. Keeping the wheel precisely centered on the hub when it is mounted will minimize the chance of a vibration. Some wheels are vehicle model specific and will come from the factory with a bore machined to match that vehicle. Some wheels are designed to fit multiple vehicle models and will use a centering ring system to reduce the bore size to match the hubs of different vehicles. These rings keep the wheel precisely positioned as the lug hardware is torqued down.

So the stud breakage I have seen is because the wheel was mounted improperly?

*Edit Edit*

This is what happens when it takes me 20 minutes to finish a post. My questions are answered

oh so i got multiple answers and i was just curious cus i got a new tire on a rim and da dude put da rim without the ring. so should i put da ring on?

For $10 more they are very good insurance and worth every $$ IMO. I recommend them on every car I put custom wheels on and will always run them on my cars. $10 isnt anything if you know your wheel is centered correctly.

Well that all makes sense to me

I suppose the fact that the lugs are tapered should have given that one away too. They are only loaded when out of torque spec. Once tightened down, they become one with the hub and the bearings take the full load. Sorry for the arguement, it was quite a misconception from other conversations.

I work on semi truck/trailers and there are both hub centric (unilug) and stud centric (budd) designs on the road. The unilug style is the newer of the two, and probably makes up for about 75% of the wheel styles I see. The hub is tapered to a machined fit inside the wheel centerbore, and the studs simply hold the wheel in place. Applying this design and the fact that the unilug design is newer and more common, you can see why I might have been under the wrong assumption.