I have been working hard lately on my old 1989 Pontiac Firebird to get it back on the road. We (my brother and I) recently got it running with a Megasquirt II and I have been tying up loose ends so it can go to the dyno without incident. There has been one issue that has plagued the car for as long as we can remember, and its something that we really wanted to get sorted out before taking the car to be dynoed.
The car has had a horrible vibration at around 80mph for a very long time. We have always thought it was a driveline problem, but after measuring the pinion angle, checking the u-joints and having the transmission checked out to make sure there’s no slop in the rear bushing, we never found anything wrong.
The problem turned out to be the wheel spacers that we use on the front of the car. We use wheel spacers so we can run the later 4th generation f-body wheels which are plentiful, cheap and a good size. Plus, some of them just look really nice on the car. The problem with the spacers we had was that they were not hub centric.
Hub centric means that the hub, or in this case a wheel spacer, has a boss that the wheel can slip over. As you might have guessed, this makes sure that the wheels are centered on the hub. Technically speaking, you don’t have to have wheels/spacers that are hub centric because the tapered lug nuts also center the wheel. But even so, it is still possible to have some misalignment between the hub and the wheel. This was especially true in my situation where I had a non-hub centric spacer that also didn’t have a boss to center the wheel – so I had the potential for 2x the error. When I measured the radial runout of my wheel, I found that it was at least .040″ out. Another issue with relying on the lugs to center the wheel (lug centric) is that if you take a hard hit, like hitting a pothole or a high apron at the track, is that it can slide the wheel/spacer just a little bit off center which causes the vibration I experienced.
If you are going to order spacers for your car, you have to make sure that they are hub centric for both the spacer and the wheel to prevent any vibration. If you already have a spacer that is not hub centric, it may be possible to get rings that you can put between the hub and the wheel spacer to bring them into alignment. These rings are usually sold for aftermarket wheels, but they should work with spacers as well.