Intermittent Vibration Causes?

[Russ Dickey]

Hadn't had the Cobra out for a cruise in about 10 days, so last night the wife and I go out for ice cream. Much to my dismay, we notice an intermittent vibration at speeds above 60mph. Vibration lasts for a second, then stops. Then starts, then stops. Then starts, then stops. You get the picture. Can feel it the strongest under my seat, but the steering wheel vibrates a little, too.

Happens in 4th and 5th gear at highway speeds. No vibration below 60mph. Very weird for something like this to develop as the car was just fine when I drove it 10 days ago. Can you guys give me some quick tips on the things I should check which could be causing this??


Thanks,

Russ

P.S. Tried to do a search on this topic, but the database gave me an error message. So don't flame me too bad if this is a repost.

[Mr.Fixit]

Try to figure out if it is an RPM or MPH related vibration.

[CobraEd]

A slipping / failing harmonic dampner can cause exactly this symptom. It comes and goes and can cause the whole car to shake. I had it happen to me. Supprisingly, even though the dampner is way at the front of the engine, the shaking and vibration goes thru the whole driveline.

Ed

[Lowell W]

I'm gonna take a wild-a$$ed guess at this and say it sounds like two different small vibrations, neither of which is very noticeable alone but which resonate or "beat" together to cause something which feels big. So a small change in frequency of one vibration might be enough to get the two to resonate. Please don't all you engineers jump all over me if I'm wrong: I'm no engineer and I'm just guessin'. As Mr. Fixit suggested: try driving at 60+, then shift to neutral and let the motor idle to determine whether this changes with engine speed. Does the vibration in the steering wheel come and go, also, or does it remain constant at speed? Look at your wheels to see whether you've lost a wheelweight. Check your lug nuts or knock-off's. Check your tire pressure. Have you picked up something in a tire? When did you last have your wheels balanced? Check your driveshaft to see whether you've lost a clip on one of your U-joints, or whether there's a mark from a lost balance weight or something stuck on there.

Usually it's something simple but the strangest things can and do happen. A friend had a newly-reassembled motor start up great, then get sicker and sicker with only a few hours of running. Finally, drawing the obvious conclusion that the motor would have to come apart again, he tore everything down, only to find the rings stuck tightly in the pistons. Further examination revealed the remains of mud-dauber nests in the intake manifold. He had cleaned the manifold and set it aside, giving the wasps an ideal place to nest. When he started the motor, it sucked in what he estimated to be half a cup to a cup of fine grit, which packed in between the pistons and rings.

So...Good luck and let us know what you find.

Lowell

ed: Just hit "Submit" then checked back and read the post about the vibration damper. I haven't seen one go bad but it sounds like a good call to me. If you drive along at 60+, push in the clutch and let the motor idle and the vibration goes away, look there.

[Russ Dickey]

Thanks for the input. I just performed a road test, and the vibration occurs only between 60-70mph, regardless of what RPM or gear I am in. Like a dumba$$, I forgot to push in the clutch to see if it does it in neutral. Oh well, another excuse to do drive around on a beautiful day...

I will doublecheck all of the aforementioned items. Any other suggestions are appreciated.

Thanks!

Russ

[CobraEd]

Might have thrown a balance weight off your driveshaft. They are only spot welded on.

[Bob In Ct]

It's the tires! They are not as round as you think. I'm also willing to bet that your steering system has no damper between the steering wheel and the tires.

Bob

[rdorman]

Tires are my bet. The speeds in which it occurs is the norm for a dynamic balance issue. Only the best of machines can fix and diagnose the issue. Do a search, when it becomes available again, and the machine and process are described. In the mean time you can look for a missing weight. And the vibration with this issue does vary in intensity. Some times you can barely feel it, others it is pronounced. When the vibration occurs, look at your wheel, it will be shaking back and forth!
Rick

[Bob In Ct]

Rick:
No offense, but it's not balance. Most Cobra owners have the problem with balanced wheels. Even a square tire can be dynamically or statically balanced!

I've been chasing this problem for 3 years. I have had my tires dynamically balanced 3 or 4 times (they're always perfect no matter who does them) with no improvement. I've had people drive along side, the tires look fine. The biggest improvement I got was after I had the tires "trued". Even a slight out of round will cause harmonics to come and go (see Lowell's post above).

The problem exists for many cars, but on most cars the steering wheel is connected to the wheels through a damper (rag joint) and much of the vibration is eliminated before it arrives at the steering wheel. Even a little movement in the system is a lot due to the diameter of the steering wheel.

Bob

[Lowell W]

A lot of good suggestions here.

Bob: In the meantime, I've been out to your neighborhood and back. Had a dinner or two at the Cove, a lunch at the Hitchin' Post in Charlestown, RI, lobster at Abbott's and a ton of seafood on the Cape. Now, about this "truing" thing: where do you go to get this done? What's involved? Which tires do you have?

Russ: Any progress on this vibration problem? Inquiring minds want to know.

Thanks,

Lowell

[CobraEd]

OK, OK, OK, here goes. Now I don't want any of you guys to laugh at me here, but this is a way cool thing that you can all do.

I had tire vibration problems with my BFG Radial TA's for a long time. Had them balanced again and again at a shop that I know does a good job. The problem was that they were not perfectly true (round). So no matter how good they were from a balance perspective, if they are out of round or "lumpy" you get a shake when driving.

Here is the simple zero cost solution: . . . . . . Buy a cheap belt sander, . . . Put the tire/wheel in question on the front of the car. Jack up the front and spin the tire while holding the belt sander firmly against the floor facing up. Every time a high spot on the tire passes the sander it is shaved off. Do this untill the tire is perfectly true. then GET THE TIRE REBALANCED.

Your car will rid like it is on glass!!

You can all thank me later!!

Ed

[ROSS WEAVER]

Or you can take it to a shop that shaves tires. I'm thinking about doing that to mine, as I have done the balancing thing many times to no avail. I was told my tires are very out of round. I have Goodyear Eagle 1's.

Ross

[Ron61]

Ed,

In the case of a tire that I had done at one of the shops that have the machine to do this, it should have ridden like glass. At any speed above 30 MPH the front end looked as if I had a football for a tire. By the time they got it trued, I had two spots with no tread left at all. Took it back to the dealer and he wanted to argue so I just told him call the tire place and talk to them. As it turned out he had gotten in a whole shipment of that type tire that was really bad and had to replace all of them.

Ron

[ROSS WEAVER]

Ron,

What did the dealer do about your tires? Did he replace them?

Ross

[Ron61]

Ross,

Yes he replaced all 4 of my tires plus about 50 others that he had sold from that bunch and were coming back in to him. It wasn't the dealers fault as the tires just came that way from the factory when they deliverd them. I had tried the balance, change rears to front routine and everything else I could think of until one of the balance shops said lets put it on the machine to make sure the tire is true. And as I said, two spots actually had to be ground down to where there was no tread to make the tire round. This was a few years back and I think they were Firestone tires but can't remember for sure as I keep trying different brands and styles.

Ron