Hubcentric spacers are machined to fit the hubs precisely on the hub side and the center bore on the wheel side. Sometimes they are machined in one piece, sometimes, like the Ichiba, 2 pieces.

Hubrings
http://images46.fotki.com/v1492/phot...MG_1546-vi.jpg

Type 2
http://images47.fotki.com/v1476/phot...MG_1545-vi.jpg

Type 1 mounted
http://images49.fotki.com/v1511/phot...MG_1910-vi.jpg

So its that separate hub-ring part that makes the differences since it acts as the connecting interface between the wheel hub and the actual spacer ring?

Yes

Mod.. I saw on your DIY you went 15mm on both front and back, any reason you didn't go 20mm on the rear?? Am close to ordering these.. definitely 15mm for the front, but have seen 15 and 20 on the rear mentioned here...just looking for some additional input before dropping the wallet.

Thanks

20's on the back
 

i installed 15mm in front, 20mm in back. the back looks killer

Word of Advice, if your going to run a spacer ALWAYS get a hub centric spacer. Non hub centric are known to move around and are not really safe. Hub Centric Spacer are safe, i have a couple friends that road race with hub centric spacers on and never have problems!

I had a set of non-centric (didn't know that at the time) on a Cobra that I tried in order to put Goodyear 315 DR's on the rear and clears the IRS bolts. I have to say that whenever I cornered the backend of the car never felt "safe" or stable as it always felt like the back wheels were never solidly connected to the wheel housing. I would always feel a very slight wobble in the rear wheels when cornering in the slightest...particularly on highway exit ramps.

I learned later on that with the non-centric spacers that with any hard-launch from a standstill I was risking snapping the studs each time. Fortunately I never had them on the car long enough to have that happen.

Not soon after I switched to a Nitto 315 DR's which are about a 3/4" less wide than the Goodyear ones so I was able to eliminate the spacers all together as they cleared the IRS bolt due to being less wide. It was like night and day in terms of the backend without the non-centric spacers.

I learned from that whole experience to "Just say 'NO' to non-centric spacers".