The 65 MPH "Shimmy"

This is fairly common although it is usually corrected fairly easily.

First and foremost, be sure your tires are properly balanced. It must be done by a competent technician using a dynamic balancer. Some shops perform dynamic balancing, but they don't do it correctly. It should be done using a more sensitive setting and the technician should properly install the balancing weights. It should also be re-checked after weight installation. In addition, most major tire manufacturers put a marking on each new tire to indicate the "heavy spot" on the tire. Kevin Lakkis, field development engineer from Toyo Tire, notes:

Toyo measure uniformity – Radial Force Variation, Lateral Force Variation, Balance, etc. AND no tire leaves the factory beyond certain limits. (That is why Toyo has so few vibration complaints).

Every Toyo tire is marked at the low balance point with a yellow dot (it is not always a yellow dot with all tire manufacturers, but it is with Toyo). OE tires are also marked with a red dot, marking the low RFV spot (so they can be match-mounted with the rim).

A general rule is to match the yellow dot to the valve stem, however, many wheel manufacturers drill the valve stem hole at the light spot of the wheel.

In any case, when I mount my own tire, I spin up the wheel alone, mark the heavy spot, and match that to the yellow dot on the tire. In my limited experience, the heavy spot of the wheel is not always near the valve stem.

BTW, there is a new balance machine that puts a load roller wheel on the tire and actually measures RFV as well as balance. We are ordering one very soon. That type of machine should help with the 65mph shimmy. I plan to work with my local chapters with people who can’t seem to get rid of the vibration, and note exactly how sensitive the Miata is.

Check out this link: www.GSP9700.com

We (Toyo) have been using one of these machines for some time with excellent results.

We use it to diagnose vibration complaints.

There is a dealer locator which will help Miata owners find a tire dealer who has one of these special balancing machines. Since the GSP9700 is significantly more expensive than standard balance machines, it is safe to assume a tire dealer who owns one is an expert on diagnosing vibration problems.

Finally, if all else fails, take the tires to a specialist and have them "shaved" to 100% true. Manufacturing anomalies are not as uncommon as one would think. Shaving the tires will bring them back into the "round."

Next, the car should be properly aligned by an alignment specialist. Just because the car is new, don't assume that it has been properly aligned at the factory or by the dealer. Many dealers are not very good at alignment. Go to a specialist! If you want to get a very accurate alignment, tell them to let you sit in the driver's seat while they're doing it. The additional weight on one side of the car affects the alignment dramatically.

Consider installing aftermarket bracing to help reduce frame flexing. A rear subframe brace was standard from '92 to '96(?), but can be added to earlier models. A front subframe brace can be added as well as a shock tower brace. The combination of steps should eliminate the shakes.

If that doesn't take care of it, check out Dunlop Tire's description on troubleshooting vibrations.

Here's another tip that was sent in by Brent Tomerlin (fastshadow@juno.com) :

I work with a large tire company and over the years I have had plenty of people with "uncurable" balance problems. These were  on all cars, not just Miatas. They best solution I have found is to have the tires match mounted. This can be done with a balance machine with a match mount program. What happens is through  a process of certain steps the machine figures out the high and low spots of both the wheel and the tire. It then instructs you where to line them both up. This will decrease the weight needed  and also give the smoothest ride possible. I've also found that once you air the tire up the final time it's best to deflate once more just in case any air is trapped between the tire and wheel. This is a tedious process as you must semount the tire 3 times for each wheel and usually takes  about an hour or so for 4 tires. Maybe two or three times in 6 1/2 years have I had somebody  still have a vibration problem. Any questions please feel free to email me.

Greg A. Lloyd (galloyd@micron.net) sent in this detailed analysis:

Thoughts regarding WHEEL VIBRATION with the stock SF-325 Bridgestone tires.

Like many Miata owners, I received my car with the stock Bridgestone tires. My Miata is a '97 R-package equipped with the aluminum wheels and 185/60HR14 Bridgestones SF-325 tires. The car was also equipped with the special locking lug nut on each wheel. I believe BOTH THE LOCKING LUG NUT AND THE BRIDGESTONE TIRES can be the source of the 65 MPH vibration.

THE SF-325 BRIDGESTONE TIRE (Imbalance issue #1 of 2)

I thought many readers might appreciate hearing of an experiment I performed regarding the 65 MPG vibration. I am a R&D mechanical engineer working on alternative energy sources and appreciate reproducible data produced by good experiments. I suffered for nearly two full summers with the 65 MPG wheel vibration and recently forked over $40 to have the wheels computer balanced. Immediately after getting the car back from the shop, they still vibrated. The Miata is sensitive to vibration because it has such a tight, low isolation, suspension. I recently noticed that when I drove at an true 64.5 MPH (indicated 68 MPH) immediately after I got onto the freeway, that there was no observable vibration for the first three or four miles. After that point, the vibration became perceptible and slowly increased to the full amount I normally experience.

Here's what I think is going on (read: "theory"). I believe the Bridgestone tires have a construction that asymmetrically softens as the tires warms up to operating temperature. A "bulge" develops in the loaded rolling radius on one side of the tire after it is warm. Stiffness-wise, the tire may be "round" only when it is cold. Conversely, you could think of it as a "softness dent" on the other side of the tire. Either way you look at it, all the elastomers in the tire soften as the tire warms. Its just that one area of the tire softens at a different rate than another. There are circumferential stiffness asymmetries. This discontinuity in the loaded stiffness of the tire only becomes noticeable when you drive at 65 MPG where it then has the opportunity to excite the suspension at one of its natural frequencies (32.5 MPH is another). Centrifugal force (out-of-balance) isn't the only force which can excite a sprung oscillator -- which is what a suspension system is.

I understand there are special computerized tire lathes that measure the out-of-roundness of tires by spinning them while load against a drum. A computer locates and memorizes the location of high (or stiff) spots and and then shaves them down. The machine therefore doesn't simply "round" the classic high spots off of tires but also removes tread over high resistance areas that don't deflect as much under load. It can actually "unround" tires. Its been over a year since I was told how these machines work and I don't have first hand knowledge that they really exist. As of two years ago, there was only one shop in my town that purportedly had such a machine. I was told they are used to salvage (or improve) retreaded tires.

If circumferential stiffness asymmetries are the true problem with the Bridgestones, then there is no perfect solution. If you were to try to solve the problem using some sort of special, high-speed computerized tire lathe, you would have to first run the tire for five to ten minutes at 60 MPG just to get the tire up to operating temperature before making the computerized measurements. THEN you cut. I don't know if the tire lathes (if they exist) are even capable of turning this fast. Also, even if one did lathe a tire using such a technique, the tire would STILL vibrate for four or five miles after getting onto the freeway before it warms to operating temperature.

Preventive Medicine? The Tire Rack offers their competition tires with an optional "heat conditioned" state. For extra money, they mount the tires on a wheel and spin it a high speeds between two drums until it warms to operating temperature. They then let the tire cool for 24 to 48 hours. The theory is that this allows the weaker molecular bonds to break and to then recombine, making for a stronger, longer lasting, better performing tire. To me, this sounds like a post-process cure of sorts. Several tire manufacturers take the position that this is the preferred way to break-in their competition tires and advise against racers going straight to the track before first breaking in their competition tires one way or another (either on or off the car). I wonder if the test drives many Miatas get at the dealership (before the ultimate purchaser buys the car) harms the Bridgestones because they haven't been properly broken in. Still, I'm sure that many buyers of aftermarket, non-Bridgestone performance tires fail to properly break in their tires and go straight to four wheel drifts the day they receive them. It seems that many of these people later write reviews about how their vibration problems disappeared after getting rid of their Bridgestones. So it may just be that the Bridgestones would behave as poorly as they do regardless of how they are broken in.

THE LOCKING LUG NUT (Imbalance issue #2 of 2)

Simply put, these can be a source of imbalance. My '97 Miata came with one locking lug nut on each "alloy" (aluminum) wheel. Each wheel had three normal lug nuts. I measured the weight of the locking lug nut as being about three-quarters of an ounce (20 g) heavier than the normal lug nuts. The bolt pattern on the Miata is a four hole pattern with centers on a 100 mm diameter circle. The 14 inch rims places the wheel weights on a 370 mm circle. This means the locking lug nut's extra weight is equivalent to a rim-mounted wheel weight that is 100/370ths (27%) of 3/4 ounce -- 1/5th ounce (6 g). This is A LOT for a sports car. I used to work in an auto garage when I was younger and had access to a computerized spin balancer, a bubble balancer, tire mounting machines, etc. I used to balance the wheels on my old '78 Mazda GLC to 1/8 ounce and thought I could easily feel anything greater than 1/4 ounce of imbalance. And this is with an old, soft-suspension, family car that is as FAR from a high-strung sports car as one can get. Another interesting note: I was told by my dealer that the older Miata locking lug nuts used to be even heavier than mine. According to the dealer, the reason for the weight reduction was customer complaints. Well folks, if the new weights are still 3/4 oz overweight, they didn't fix the problem, they just minimized it.

My vibration problems used to be worse with my Bridgestones until I got rid of the locking lug nuts. I went to the dealer who had extra standard lug nuts laying around and picked up four for free. Alternatively, I could have installed two opposing locking lug nuts on the rear wheels and had all normal lug nuts on the front. Another alternative is to have the wheels spin balanced on the car. Only some shops can do this, it is more expensive than spin balancing, and is usually only used on crappy old Detroit-type cars that have out-of-balance drums or rotors.

Finally, why do some Miata owners experience worse vibration problems than other? Part of the reason undoubtedly lies in that some tires have circumferential stiffness asymmetries that are worse than others. However, another reason can be constructive interference between the locking lug nuts on the wheel and circumferential stiffness asymmetries in the tire. With four lug nut positions, there is a 25% probability that the heavier locking lug nut will be within +/- 45 degrees of being directly opposite the stiffness bulge. When this occurs, the stiffness bulge would be hitting the road at the same time the heavier locking lug nut is straight up; worse possible arrangement. My advise: If you have locking lug nuts, AND stock Bridgestone tires, AND vibration problems at 65 MPH that can not be cured by spin balancing, then first try swapping the location of the locking lug nuts with the normal lug nuts located 180 degrees on the other side of the bolt pattern. If that doesn't work, then start making more aggressive changes. I gave up after trying all that. This week, I bought some Yokohama AVS Intermediates even though I have only 15,000 miles on the damned Bridgestones.