Drive line vibration.

[Sam Powell]

I don't feel this until about 75MPH, and then it is a subtle undulating period at about 2 CPS. But it is there. Under 75 is quite smooth. So, I am asking this without doing any research up front, partly because the NA section has been kind of idle for a bit. Is there a standard drive shaft service on these cars? I don;t even know if, or what kind of U joints they have. Is there a slip joint like an old RWD American car. How hard are they to get out? Are there other things that can cause this kind of vibration in the IRS?

On my old Dodges, I would remove the shaft, and take it down to a place called Northern Virginia Supply in Rockville that services trucks. The have a balance machine for drive shafts, and do a pretty nice job. For about $110 bucks they would replace both U joints and spin balance the shaft. Then they would paint it while it was still spinning. It likely cost more now. If the Miata has a standard, old fashioned drive shaft, I was going to pull it and have this done.

But, I don;t know what the drive shaft arrangement is on these cars. Any insight here?

Sam

[ChristianL]

Unfortunately, the driveshaft and u-joints aren't separate on the Miata.

At what RPM does this occur when you're doing 75mph? If you're at that rpm at a lower speed, do you still notice the vibration? If not, it's probably suspension or tire related.

[IH82BL8]

At what RPM does this occur when you're doing 75mph?

Wait! I wanna guess...3700rpm.

I was just reading yesterday about this in the Performance Handbook by NGIII. He says "Beginning in 1992 Mazda added a rear subframe brace as an evolutionary improvement to the rear subframe. This brace had some effect on reducing the Miata's sensitivity to that infamous 65mph vibration introduced by imperfect tires."

So, good news, Sam. Now you can get those new tires you've been wanting AND a subframe brace.

Lewis

[Sam Powell]

Thanks for the replies. I'm pretty sure this is not tires though. I know what that feels like.These tires only have 5K miles on them, and were balanced on a Hunter Road Force machine. I just had not taken it over 70MPH until recently. Although, I am open to that possibility that it is tires. It is not what you don't know that hurts you, but what you know for sure.

It does not make that vibration at other speeds with the same engine RPM. But, the drive shaft speed is tied to MPH of the car, and not RPM of the engine. The out put shaft speed is related to engine RPM only by the factor of the gear the tranny is in. So there is still the possibility of drive line vibration that is not tire, suspension, or engine related.

So if the u-joints are not separable from the shaft, do you just buy a new shaft? Are there U-joints? I cannot imagine a RWD car without U-joints. Even though it is IRS you would think there would be flex in the body and torque shaft, (or whatever Mazda calls the connector between the tyranny and pumpkin) that would require a flexible union there. I have not crawled under there to look, but I guess I will have to track down my cheap generic shop manual and take a look.

Again, it is very subtle. But it is there. BTW, I am very open to buying the new whatsit that I can bolt onto the frame and rear to make it stiffer. Doesn't the 97'M have a factory cheapo version of that though?

Sam

[IH82BL8]

the drive shaft speed is tied to MPH of the car

The drive shaft would be my next guess. I assume there is no accompanying noise? I had a rear hub go out at 130k miles. It MADE NOISE but NO VIBRATION. It also had noticable play. So I'd guess that's not what your problem is. Read this:
viewtopic.php?f=5&t=8115&p=77751&hilit=driveshaft#p77751
If the dealer is asking $750 for a new assembly, I'd look for a lower mileage one from a salvage yard.

[prostwest]

Although, I am open to that possibility that it is tires. It is not what you don't know that hurts you, but what you know for sure.

Exactly- I'm not 100% sure whether people always bring up tires in discussions about Miata discussions because: A) It's the number one culprit, or B) People always bring it up, so that makes it stick in peoples heads when the topic comes up.

Considering there's another fresh thread talking about crappy balancing practices, and you have freshly mounted tires that you have only just tested at speed, I'd say that's a hell of a place to start.

The easiest test you can to will be to swap off a set that has proven to be vibration free. (eg at tech day). A really good balancing could solve your problem. Also, on a lark, let me mention that Chris Windsor claims his new shaving machine can true tires, although that problem would likely manifest at all speeds.

[Sam Powell]

Thanks for that link. Carbon fiber sounds nice. But, I think it would be worth pulling the shaft and at least taking it to my guy in Rockville. I have had good luck with him in the past. I would think he could rebuild the existing unit nicely. I agree that the dealers would likely rather charge you big bucks to unbolt and bolt in a new unit than rebuild an old one, but that does not mean it is the only way to go, or even the best way to go. Maybe it is. I will do more reading. Thanks again for the link.

Sam

[prostwest]

Oh, and Mazda doesn't break the driveshaft down into sub-assemblies in the parts catalog, but I think there was recent discussion about sourcing replacement joints.

[IH82BL8]

Carbon fiber sounds nice.

Sorry, I guess I should have actually read that.

[Sam Powell]

Carbon fiber sounds nice.

Sorry, I guess I should have actually read that.

NO apologies needed. Carbon fiber does sound nice. I believe what they claim is probably true.

At 75 MPH the RPM of the engine is about 3700, as put forward by a previous poster. You figure the output shaft of the tranny is around . 6 of that or 2220 per minute. Divide that by 60 to get the CPS and you have a frequency of 37 CPS.

Tires at that speed are turning about 555RPM depending on the rear ratio, and then divide that by 60 seconds to get a frequency of 9.25 CPS. As a piano tuner by trade, I can recognize by memory beat cycles ranging from 1CPS to about 13 CPS. That is tool of the trade, memorizing beat speeds in this range. The vibration in the Miata is much faster than 9CPS. I don't have 37CPS memorized. But the lowest A on the piano is 27CPS and the A one octave up from that is 55 CPS. 37 CPS would thus produce a pitch somewhere in the bottom octave of the piano, maybe about an F. I could look it up to find out which note, but am too lazy right now. Anyway, the vibration in the Miata is producing a pitch somewhere in that bottom octave of the piano for sure. That is too high for tires.

However, keeping an open mind, and not wanting to be open to "I told you so" later on, I am still open to the possibility that maybe it is tires. but, I am checking the drive shaft sometime soon. Was that a nerdy enough response?

Sam

[travism]

Tires at that speed are turning about 555RPM depending on the rear ratio

Care to show your math on that one? I'm not following with my own calculations.

[Sam Powell]

There are several ways to get there. Take the tire height x 3.14 =inches rolled per revolution and divide that into the number of inches per mile times the number of miles traveled in a minute at 75 MPH. I hope I did not do any math dyslexia there. I did not use that formula, but I have in the past when trying figure if a speedo was correct. (That was before radar guns)

I took the RPM of the drive shaft (2200) and divided it by 4 which is roughly the ratio of the rear. For every 4 revolutions of the drive shaft, you would get one revolution of the tire. My number of 555 also assumes the .6 overdrive, which I don;t have actual numbers on, but is likely close. The Mustang 5 speed in my Dart has an OD of .69, but the Corvette had an OD of about .6. I also don;t know the real numbers of the rear, but I suspect 4:1 is close enough for our logic here.

One of the intriguing things about this vibration is that it has an occilation to it that sounds like two close but different frequencies which come in and out of phase at about 2 CPS; as if one frequency is around 39 CPS and the other one is 37CPS. What I visualize though is that there is a slight amount of play in one of the U joints enough that it moves in and out of alignment as it spins and gets into a rhythm. I will crawl under there before too long and see if there are zirk fittings on the joints. If so I will start by greasing them. Just for the record, I have not written the possibility of tires problems off the list. It is just lower at this point. Forgetting all the other logic, and input, it seems like new tires, balanced on a Hunter machine are less likely to be acting up than a drive shaft with 118K miles on it.

Sam

[prostwest]

Tires at that speed are turning about 555RPM depending on the rear ratio,

The speed the tires turn at a given speed is unrelated to gear ratio. Calculating from the other direction, a common Miata tire has about a 5.9 foot circumference, and 75 MPH= 6600 ft./min. Divide 6600 by 5.9 and find 1188 to be the wheel RPM, about twice what you thought. Taller wheel/tire combos would reduce this by a couple percent, but not 50%.

Edit: Haven't read your last post yet.

[prostwest]

There are several ways to get there. Take the tire height x 3.14 =inches rolled per revolution and divide that into the number of inches per mile times the number of miles traveled in a minute at 75 MPH. I hope I did not do any math dyslexia there.

Don't forget to include tire sidewall in your diameter, or just cheat and follow the link I posted.

[travism]

I just measured my wheel/tires and did the calculation and got 1069 Tire RPM. Driveline and gearing ratios have no bearing on the formula.
((75 Miles/hour)*(1hour/60 minutes)*(5280 ft/mile)*(12 inch/foot))/(11.5inches (measurement of radius for my wheel/ tire combo)*2*3.1415926)=1069RPM
Divide that by 60 gives you 18.26

[Sam Powell]

I just measured my wheel/tires and did the calculation and got 1069 Tire RPM. Driveline and gearing ratios have no bearing on the formula.
((75 Miles/hour)*(1hour/60 minutes)*(5280 ft/mile)*(12 inch/foot))/(11.5inches (measurement of radius for my wheel/ tire combo)*2*3.1415926)=1069RPM
Divide that by 60 gives you 18.26

Formual for circumference is pixD not RxD. Your tires are rolling twice as far as you have calculated. Divide your result by two and you come up with 535RPM which is not far off from my calculations. And gearing does effect tire speed. You can only calculate wheel RPM from it if you know the RPM of the engine at a given speed, which I did know.

Sam

[prostwest]

Formual for circumference is pixD not RxD.

ΠD or 2 ΠR (as I learned the formula) are equivalent- look for the *2 in his calc below.

(11.5inches (measurement of radius for my wheel/ tire combo)*2*3.1415926)=1069RPM

[travism]

prostwest beat me to it...also, I said gearing had no bearing on the formula, not that it doesn't dictate tire speed.

[prostwest]

You figure the output shaft of the tranny is around . 6

Just looked it up- .84 overdrive 5th gear in NAs. Final drive is 4.10. This link shows an overall 3.34:1 gearing in 5th, which, at 3700 engine rpm, puts wheel rpm at 1107.

I think there was recent discussion about sourcing replacement joints.

Duh, in the thread linked 2 posts above where I said that.

[Sam Powell]

OOPS. I don't see any errors in my logic of determining tire RPM from engine RPM, but I had significant errors in my assumptions about the various ratios of the gears. It would have been smarter of me to go look those up before making the first post. However, since I have an automatic, everything is different from most Miatas, and maybe a little harder to track down.

If I had bothered to measure the tires, and work the formula, I would have realized that. Remember what I said about "It is not what don't know that hurts you but what you know for sure". I will measure my tire height and get back to you. I have 15" wheels for what it is worth. We'll see what that does to the math. The thing is my Dart has 205-60 15 tires and my Miata has 205 15 also, but don't remember the aspect ratio. Maybe it is 55. Tomorrow I will go out and look at it, and measure the height.

Sorry I missed the 2R in your formula. Thanks for being so nice about it. There might have been flame throwers out on the Miata.net forum.

While none of this proves anything really,except for what assume does to you and me, It was interesting to force the brain to work this way.

Sam

[bill_keksz]

Did you swap tires around?

[Sam Powell]

You can always count on Bill to pull us back to earth with the shortest possible sentences. Thanks. That is a good idea.

I thought with the marvelous display of accuracy I exhibited there, it became abundantly clear I missed my calling. I should have been an economist. I hope I did not offend any economists in the group. (But not enough not to type the joke).

Sam

[IH82BL8]

but don't remember the aspect ratio.

According to my calculations the aspect ratio should be 47.9.....hmmm.

[prostwest]

Sorry I missed the 2R in your formula. Thanks for being so nice about it. There might have been flame throwers out on the Miata.net forum.

Honestly, I was afraid you might feel like we were piling on. In fact, I find this kind of mental exercise to be quite entertaining, so thanks for hanging in there.

[Sam Powell]

It's not the first time I have shot from the hip, and felt embarrassed when a more thoughtful person asked a simple question. The tone of this forum is usually pretty gentlemanly.

So, I went and measured my tires. They are 22" high, and working the math, I came up with a rotations per second of 21. That is quite a bit higher than the 9.5 RPS I estimated with flawed gearing data. But, it still seems too low for the frequency of the vibration I am feeling. Since my gearing estimate put my tire rotation way off, it also put my drive shaft estimate way off. I am not going to make the SAME dumb mistake twice in a row, so I am not going to guess about the gearing of the rear or OD in this car. But, the shaft must be turning faster than my gearing assumptions calculated.

But, swapping tires is an easy thing to try, as Bill suggested.

I went on line and found out why Mazda says the drive shaft is not serviceable. The u-joints are staked in place, and this staking must be destroyed to remove the u-joints. However, there are replacements available with the standard snap ring. You have to be careful when removing the old ones not to deform the slip joint yoke.

I will check with my drive line folks at NVS to see if they have these u-joints available to them, and then let them do this if they are. In the past, I have examined noisy u-joints that appeared to be good, but still needed to replaced, as proven by the fact that replacing them eliminated the vibration or noise.

Another issue is any possible mismatch of front and rear angles. I don;t know if this is possible with the Miata IRS, but with a live rear it certainly is critical. Raising or lowering a car with a live rear can introduce drive line shake unless you compensate with the appropriate angle shims. To anyone's knowledge, is this an area of tuning on a Miata? Has this ever come up in anyone's experience? I have had cars where bad motor mounts caused drive line shake, that went away with new motor mounts. The culprit was mismatched front and rear angle.

Sam

[Sam Powell]

but don't remember the aspect ratio.

According to my calculations the aspect ratio should be 47.9.....hmmm.

That is much too accurate to qualify you as an economist.

I am going to work now, and hope nothing super interesting gets posted before I get back. Keep it dull please.
Sam

[bill_keksz]

Precision and accuracy are two different things.

[Sam Powell]

Excellent observation. I was precisely inaccurate. (I cheated and looked at the forum while at work.)
Sam

[prostwest]

Precision and accuracy are two different things.

In fact, an increase in precision decreases the likelihood of accuracy. Fun!

Sam, your axle ratio should be around 4, give or take a few tenths.

I really like the aural tool you're bringing to the diagnosis, but I'm skeptical that it can go very far in isolating a suspect component. There are so many parts of the car with their own harmonic frequencies,like springs, tire sidewalls, body cavities, eg, it seems it would cloud the picture too much to draw much of a conclusion. I'm not doubting your ear, I'm just unclear on how much we can glean from that data.

[IH82BL8]

Now I'm unclear.