Earlier today, I installed a pair of new OEM 9.5" x 18J wheels combined with 275 35 ZR 18 Michelin Pilot Sports up front and 5mm H & R spacers and lug bolts.
When leaving the tire shop, I immediately noticed an unbearable front end shimmy only in the 40 - 60 mph range. I returned to the tire shop thinking that it was a problem with wheel balance. After re-balancing the front wheels, wheel balance turned out to not be an issue). We even rotated the rear wheels/tires to the front with no reduction in the front shimmy.
I started thinking that maybe the cause is either excessive front wheel bearing runout, or abnormal wheel runout. I've since eliminated these as the possible culprits. I also thought about changing the front alignment by increasing the front toe-in about 1/16" (so as to offset the natural tendency of the heavier and wider wheel/tire assembly to pull outward).
I think that I've settled on the real culprit! When looking at the wheel hub, the hubcentric "register" protrudes (visual estimation) about 12mm outwards. But with the 5 mm spacer, the effective amount of register which goes through the wheel center hole to locate the wheel, diminishes to about 7mm.
The problem seems to be that the 74mm diameter of the wheel center hole is not entirely consistent the whole way through! It seems that about 5mm worth of this hole (starting from the inside surface of the wheel center where the wheel mounting pad contacts the hub) has a non-flat conical contour, seemingly to make the wheel easier to slide on and off the hub register.
The remaining 7mm of register (i.e. the spacer thickness takes up 5mm worth of the register) does not fit snugly in the wheel center hole due the mostly conical shape of the hole at the point where this 7mm of hub register makes contact. Therefore, the wheel is only really located by the five lug bolts, which makes for a wobbly situation.
What can I do? I think that the smallest spacer thickness available which has a built-in register is 15mm (due to the fact that such a spacer with integral register has to clear the entire length of the OEM register which is built into the hub itself).
Should I try to have the registers on the front hubs machined shorter (or even removed entirely) so that I can have custom 5mm spacers made with with integral registers? I refuse to run the 15mm hubcentric H & R spacers because (a) they will change the offset too much that was designed into the wheels, and (b) the tires will end up rubbing against the outside bodywork.
Is there any way to fabricate a small piece of metal/aluminum that I can firmly (if not permanently) insert into the inside of the wheel center hole so as create a uniform 74mm diameter that will eliminate the offending conical contour? I think that if I can do this, then even the remaining 7mm worth of hub register will be sufficient to locate the wheel in a stable, non-wobbly and non-shimmying fashion.
DZeckhausen was correct and absolutely spot-on when he warned of the shimmy problems associated with 5mm non-hub centric spacers! However, there must be a sensible solution to rectify this problem.