cruise/deceleration vibration

Daspanka

Registered User
I know this has been hashed over before, but I'm hoping someone else has had this and has a new idea. I just put a set of Cobra R's and Kumho 255x45 17' on. As best as I can tell, the tires are balanced properly and there is no runnout in the wheels. Starting at about 70MPH, I get a slight vibration under power that gets worse at cruise and dramatically worse under deceleration. If I just kick it into neutral, it stops. I get NO vibration at all under 70. I checked the flange bolts on the diff, they are tight. The U-joints don't have any visible play either (The car has about 95k on it). There is a little slack in my diff, I get a audible "clank" when rotating the shaft back and forth, but I've had that since the 3:73's were installed. Could the U-joints be bad and I just couldn't feel it with the other tires and stock wheels? I'm tempted to order a Dynotech driveshaft and be done with it.
Thanks in advance for any help!

Daspanka
 
I have the exact same wheels and tires as you, and I have the vibration too, but only under acceleration and noticably when loaded in 5th gear.

I was told by a fellow SC'er that the wheels are hub-centric vs. lug-centric and there are some hub sleeves or inserts that you can use to center the wheels better.
I don't think this is the case with these wheels because the hub and wheel center are a tight fit.

Same guy also mentioned that you need to pay attention when putting the wheels on to get them oriented correctly. He said there's a mark on the hub or rotor that you are supposed to align with the valve stem or something.

Personally I call BS (not by my buddy, but where he must have heard it) on both but wondering if anybody else heard of this?
 
One of the front wheel studs on each side has yellow paint on the tip. My understanding is that you align the valve stem as best you can with the yellow stud. Not true of the rear, though. I had heard of the Hub centric rings and have been looking for a set. But, wouldn't a wheel that was not centered properly vibrate all the time? Irregardless of acceleration/deleceration, etc?
 
I thought so too, but another thing I found out that was perplexing.

I had a slight front bumper hit and it pushed my IC back a little. This tweaked both of the IC tubes and caused it to make a vibration and banging noise at certain RPM.

Maybe not related at all, but loosening my tubes and tightening them back up in a logical order also helped my tranny shift better. It shifted fine before the wreck but was a real grinder until I loosened the tubes up. My mounts are new all around so I am sure it's not a bad mount on mine, but heard that will cause issues like this also.
 
Well maybe I stand corrected regarding my earlier post. I found this on another board.

"assuming that the hubs are stock and studs haven’t been replaced, there should be one stud on each hub that is painted yellow on the top. That is the lightest side of the hub (I guess) so that’s where the valve stem of the rim should end up (from what I was told.) "

I also read alot about the driveshafts being dual tubes with rubber as a damper. Replacing stock with aluminum DS seems to be the common fix.
 
Actually, my undestanding is that all the red under the hood creates an inbalance with the magnetic field of the North Pole. They say chrome restores the correct polarity under the hood and eliminates all inbalance and vibration and returns the universe to its correct order:D

Actually you will likely find the wheels are not balanced correctly. I had all the same problems and posted many times about it. I was told to find someone with a Hunter balancer machine. My so call balanced wheels were way out. Just because someone said they balanced out everything when installing tires dont mean its so.

Ken
 
Actually, my undestanding is that all the red under the hood creates an inbalance with the magnetic field of the North Pole. They say chrome restores the correct polarity under the hood and eliminates all inbalance and vibration and returns the universe to its correct order:D

Actually you will likely find the wheels are not balanced correctly. I had all the same problems and posted many times about it. I was told to find someone with a Hunter balancer machine. My so call balanced wheels were way out. Just because someone said they balanced out everything when installing tires dont mean its so.

Ken


Hehe. I was wondering if my duplicolor was causing trouble.

I didn't want to blame the tire balance because of the change when decelerating. But that actually makes as much sense as anything else. My tire balance guy is always perfect. But, he doesn't like the stick-on wieghts and told me flat out about it. I guess that the changing toe-in in the back from acceleration to deceleration might account for the amplifying of the vibration. Did your vibration increase when decelerating?

Daspanka
 
Well maybe I stand corrected regarding my earlier post. I found this on another board.

"assuming that the hubs are stock and studs haven’t been replaced, there should be one stud on each hub that is painted yellow on the top. That is the lightest side of the hub (I guess) so that’s where the valve stem of the rim should end up (from what I was told.) "

I also read alot about the driveshafts being dual tubes with rubber as a damper. Replacing stock with aluminum DS seems to be the common fix.


You know I've always heard this to be true. My car doesn't have the paint on the rears, though. I've seen others post the same. I've always lined the fronts up with the valve stem and wondered about the rears. Interesting about the drive shafts. I've known that the early Mark VIII shafts had a rubber sleeve in them, but did not that about the SC's. I know that there are issues with the 4r70w driveshafts and high RPM "whip" so I need to repace it anyway.
 
Hehe. I was wondering if my duplicolor was causing trouble.

I didn't want to blame the tire balance because of the change when decelerating. But that actually makes as much sense as anything else. My tire balance guy is always perfect. But, he doesn't like the stick-on wieghts and told me flat out about it. I guess that the changing toe-in in the back from acceleration to deceleration might account for the amplifying of the vibration. Did your vibration increase when decelerating?

Daspanka


I dont remember for sure if it did it on deceleration. It would vibrate my rear view mirror and 60 to 70 was the worse. At 70 it would smooth out a little. It just seems logical that if you didnt have the problem before the new wheels than it is a balance problem. My wheels were balanced when the tires were mounted and the vibration drove me nuts. I thought it was the rear end and did the rotating of the yoke one hole at a time. Tried a hose clamp on diffrent parts of the drive shaft to see if it was out of balance. Finally went to a shop with the Hunter balance machine and found the rears were way out and the front just so so. Vibration is gone and I moved on to the next mystery, weres all my missing HP

Ken
 
Thanks Ken.
That does make the most sense. I'm switching the fronts and rears to see if there's any change. Think I will thow some u-joints in too. Noticed one of the bearing caps has a shiny spot where it's obviously moving around in the yoke. Since those are pressed in, it really shouldn't be. Did you just call tire shops and ask if they had a Hunter machine?
I didn't want to ask, but how DID the dyno at the shootout turn out?

Daspanka
 
Thanks Ken.
That does make the most sense. I'm switching the fronts and rears to see if there's any change. Think I will thow some u-joints in too. Noticed one of the bearing caps has a shiny spot where it's obviously moving around in the yoke. Since those are pressed in, it really shouldn't be. Did you just call tire shops and ask if they had a Hunter machine?
I didn't want to ask, but how DID the dyno at the shootout turn out?

Daspanka

Sucked, Ive been on the dyno 5 times this year. At the shootout I replaced 2 DIS, had a drop of pressure on the fuel rail. Had to pull the new fuel pump on the dyno repair it and stick it back in. Car would not make HP after 4000 RPM. Maf readings all over the place. It was decided by smarter people than me that the IRCM and or the EEC was bad. Car made 275RWHP, about 100 less than expected. Took it to the track Sunday at the shootout and managed a 13.6 lost the first round to a real nice CMac89. I replaced the IRCM just recently and sent the old one to Duffy. He is testing it and I will hopefully know today if it was bad. I am thinking about going to the dyno Saturday for a couple of pulls to see if anything changed. Shootout was everthing I hoped it would be. Funiest people on the planet


Ken
 
for me it was the ujoint bolts that hold it to the yoke.. tehy had backed themselves out.... i pulled them out with my FINGERS! put some loctite on them, and no more problemo :)
 
Sucked, Ive been on the dyno 5 times this year. At the shootout I replaced 2 DIS, had a drop of pressure on the fuel rail. Had to pull the new fuel pump on the dyno repair it and stick it back in. Car would not make HP after 4000 RPM. Maf readings all over the place. It was decided by smarter people than me that the IRCM and or the EEC was bad. Car made 275RWHP, about 100 less than expected. Took it to the track Sunday at the shootout and managed a 13.6 lost the first round to a real nice CMac89. I replaced the IRCM just recently and sent the old one to Duffy. He is testing it and I will hopefully know today if it was bad. I am thinking about going to the dyno Saturday for a couple of pulls to see if anything changed. Shootout was everthing I hoped it would be. Funiest people on the planet


Ken

Hate to hear that the gremlins followed you to the shootout. I think that a phantom electrical/computer problem like that is what we all fear with these cars. A bad sensor, etc is usually pretty easy to nail down and fix, but something like that can be a bear to track down. What happened to your fuel pump? I remember you found gunk in the screen of the old one.
I really wanted to go to the shootout. But, there was no way my car would be ready in time. I'm already planning on attending next year and am looking forward to meeting a lot of the sccoa crew in person.

Daspanka
 
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