Is it my tires? NA miata wheel problem.

I bought a used set of konig rewinds a few years ago practically new with new dunlop (205 50 15 dz102) tires. The composite hub centric rings were a little loose and the whole car picked up a bad shake over 60mph and made it basically undrivable on the highway (like to an autocross...) This tire and wheel combo has been balanced, re-mounted and rebalanced a few times. 

I put my stock daisy's back on with worn out dunlop (185 60 14 zII's) and the shake disappeared.  I assumed it was the hub centric rings being loose, put off doing anything for a year then ordered metal hub centric last year.

The aluminum hub centric rings fit the wheels tight, and sit a little loose on the hub. By my inaccurate measurements, the hub centric ring's ID is 0.008" larger than the OD of the miata's hubs.  I have tried multiple times with different lug tightening techniques, rings or no rings, but the vibration is still there and this car can't be driven on the highway with these wheels. 

I got the idea of measuring the roundness and measured a radial run out (vertical run out) of 0.035" on average on each wheel . I have been through many stages of measuring setups, and am confident that these numbers are not supremely accurate, but they are measurable and repeatable.  Here is a picture of an early setup, I have since switched to a rigid micrometer magnetic stand.

From what I can find, 0.035" is about the maximum radial run out allowed within spec. The old snap on wheel balancer is hub centric, it uses 2 cones to center the wheel on the axle and has 0.001" radial run out from what I can measure. 

I was pretty confident the wheels were not bored on center perfectly causing a radial run out from stacked tolerances and thus my vibration, but I was waiting to get a rigid micrometer stand to confirm this theory.  Unfortunately I put the stock daisys on the wheel balancer tonight and checked them, only to find 0.030-0.040" radial run out AND some lateral run out from a little bend or 2. The stock daisy's are in worse shape, but don't vibrate.

I like the grip of 205's vs the 185's, however its not of much use if I can't get anywhere without worrying about the car shaking itself apart. I hate the idea of buying 4 new tires without being confident this will fix my problem.

If I put the back of the car on jack stands and run through the gears there is visible radial run out and the vibration occurs at speed with the 15's, if I swap the 14's back on the vibration disappears.

Is it obviously tires? Do the 60 series sidewall absorb the run out better and thus the larger wheels can't be fixed without being true?

Thanks,
Mike

I'm wondering if one of the 15" tires that you have is defective.  A broken belt or something that causes the tire to distort under load.

Does the shake go away at a higher speed? Both of my Miatas had/have a bad shake, for about a 4-7 mph range somewhere between 60-75. The variances depended on which wheels/tires, as well as other mods. For instance, the shake moved from 68-71 to about 65-68 when I installed my trailer hitch on the NB. I think it was something to do with harmonic balance. 

Centering rings are just there to ease installation. The lugs center the wheel. 

Miatas are very sensitive to tires being out of round or having busted belts. They'll shake at the resonant frequency of the chassis, which is about 65 mph. Get the problem wheels load force balanced and see what that tells you.

Following on what APEowner said, could you try putting two of the shaky tires on, say up front in case of LSD and for better feel? (EDIT: it was late, and I got two different ideas mixed up; was thinking of even trying one at a time, hence worrying about the LSD...) If it doesn't shake, try the other two. At that point swap out one or the other for one from the non-shaky pair... That should allow you to look for a single bad apple.

Er, wait... Was just re-reading... Have you done the rear-on-jackstands things with all four of the shaky tires? And do you mean that not only do you see the runout, but you feel that vibration, even as it's up in the air? Is it on one in particular? Two, three, all four? I'm guessing up in the air you'd be able to isolate if only, say, the left was shaking, though I guess those vibes might shake the opposite side.

If you're feeling that vibration while it's up in the air, then I don't think it's a matter of the sidewall absorbing it, as it's not supporting anything. Under those conditions, I also think I'm dubious of the balancing, no matter how many times it's been done.

I'd want to sanity check by looking for vibes in one pair at a time of the 15s. Or if you've got a safe place, even one at a time.

Keith Tanner said:

Centering rings are just there to ease installation. The lugs center the wheel. 

Miatas are very sensitive to tires being out of round or having busted belts. They'll shake at the resonant frequency of the chassis, which is about 65 mph. Get the problem wheels load force balanced and see what that tells you.

Thanks for the confirmation on the hubcentric rings, I felt like I was on a wild goose chase with those.

It will shake at 60mph, and just gets worse with speed, I haven't tried much over 80mph, but it is violent that fast.   I don't think its the "miata resonance" as I had that with an older set of 14" tires and it only occurred in the infamous 60-70 mph range. 

The setup was allegedly road force balanced, but by the PO. 

As far as I can tell all 4 are bad, I swapped front to rear and tried running on jack stands and noticed no improvement. Vibration is felt and seen with the rear wheels in the air. I have rotated and swapped ad nauseam, all 4 are equally bad, I can't isolate the problem. 

I was convinced it was the wheels/tires, I was trying to check measurements to see if it was the tires or wheels to see if I could get away with just new tires, or if I needed to scrap the wheels and get a new set. What is throwing me off is the fact the 14 daisys measure as bad and worse when looking at radial and lateral run out yet have no vibration.  I can see the radial and lateral run out on the daisys, I had just never been looking before. 

Thanks 

Mike

I've had four sets of 15x7 Konig Rewinds with the same size tires. For a long time, I've been saying that I actually prefer the way Miatas feel on stock 14 inch wheels, but I'm starting to wonder if maybe I just don't like the way the cars feel on those wheels.

I also had a '95 M-Edition that came from the factory on 15x6 BBS wheels and (I think) 195 series tires. That car felt much better on the stock BBS wheels than the cars on the 15x7 Konigs.

It sounds more like a tire problem than an issue with the wheels. I have had tires that appeared fine, but when dismounted it was apparent there were broken belts that caused the vibration even though they balanced ok.

I'm wondering if 15x7 is wrong for the street on such a light car.

Thousands of Miata owners have proven that 15x7 works just fine on the street.

The actual tire choice makes a big difference. I used to have a set of 14x6.5 with Falken Azenis RT215s and a set of 17x7 with...something black and round on them. The car rode much, much better on the 17x7s because the RT215 might as well be full of concrete when it comes to sidewall deflection. My CRX got much happier when I went from the original 13" to a set of 15x7 with S.drives.

In this case, I'd set the car up with 14" on one end and 15" on the other. If it still has the shakes, swap to the other two 15". Then take the pair that causes the shake and install just one of them along with a known good 15. After 3 tire changes (at most) you should have isolated a bad wheel/tire if one exists.

Also, make sure they're road force balanced. Simply spinning the problem tire won't tell you if it's got a broken belt. Just how old are the 15" Dunlops by this point?

you are not alone on this...

https://www.miata.net/garage/65_mph.html

they are susceptible to harmonic oscillation at those speeds, balancing is key.

Problem seems to be worse on tires with stiff sidewalls, which you generally want for good cornering. Its a compromise territory. 

I never have a problem on my snow tires, but my summer tires...   fine at initial installation and the slightest bit of flatspotting can go nuts.  

Update:

It was the previous tire balancer....

I decided to static balance my tires with a 1950's snap on static balancer. I also decided to fix a coolant leak and oil leak "while I was in there," thus the time gap.

I stripped off all the weights and stuck them in different spots. 

The vibration is gone, I can now go on the highway without worrying about a traumatic retina detachment.

I'm a little frustrated that this tire wheel combo was balanced, then rebalanced on a road force machine and still had an out of balance problem. I hate being reminded that it's not worth paying other people to do work for you, as they will probably screw it up...

My father always said "why pay someone to screw it up when you can screw it up yourself"

At least the car is back in rotation!

I'm glad you got to the bottom of that.  Good job.

That's both a relief and frustrating. Good reminder that "professionally double-checked"  does not mean "beyond reproach". Congrats!

I've had rewinds for 13 years on my 2 NA's never an issue.

You need to rule out the tires. 

  I worked in the tire business in the 70's during the bad old days of US steel belted radials falling apart.   Remember the Firestone 500?  We made lots of money replacing those pieces of doggie do-do.

Firestone and Uniroyal were the worst.   Goodrich the best.    None could touch Michelin, Continental or Pirelli by the way.

Anyway people were constantly coming to my grandpa's store wanting an alignment or tires rebalanced.  I would put the car on the lift and spin the tires - almost always there was some huge or subtle visible anomaly in the belt  or sidewall causing the issue.