QuestionI bought a used 99 miata, which develops a vibration at about 60mph becoming stronger when going faster (80mph +). When I take it out of gear at that speed, the vibration remains. I had all wheels balanced but it still does it. It's very low mileage (30k) and other than that in very good condition.
AnswerAll Miatas pre 2005 have an inherent vibration issue to some degree at or around 60-65 MPH.
I would first rotate your tires to see if the vibration follows a particular tire. If the vibration is in the rear and moves to the front then you have it (or some of it) solved. Experimenting with air pressure can help isolate a tire with an issue.
A common Miata flaw comes in the the underslung design of the rear suspension cross member and body mount points. In later years an increase in sub structure reinforcement was thrown at the design to help combat this problem. The addition of a rear sub frame brace helps minimize the vibrations, they sell for about $90. Sub-structure vibration issues tend to harmonize at 60-65MPH and go away below and above this speed. If the vibration keeps on beyond this speed then look elsewhere.
Now, not all vibration problems are due to sub frame issues, tires though balanced can have circumferential stiffness anomalies. Also not all shops take the time to balance out a tire properly some some variant came be had here also. Because of the responsive design of the suspension (performance) these anomalies can translate into vibration.
So how to avoid these, buy a well established brand of tire. Dunlop, Yokohama, Toyo all closely watch for variances in their tires prior to shipment. Buy performance tires not all season and a step up a half size over stock also helps since it places more rubber between the wheel and the road.
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