very slight vibration at 90+ mph....question

[neckdemon]

i finally decided to get this '03 cobra i was looking at. the car has the stock eagle f1 tires on it. at around 90 mph i notice a very very slight vibration. it's almost not noticeable but i am very meticulous about things, and was paying very close attention. i am assuming it is the crap tires, because i don't feel it in ther shifter at all. what do you guys think? also i was thinknign of going with the nitto 555R's in the back and the 555's in the front. you think that will be a good tire choice for a car that will be mostly daily driven with minimal trips to the track?

[cobrabitn]

Congrats on the purchase! :thumbsup:

It's hard to diagnose a "slight" vibration as it could be anyone of several things. If it is a "seat of the pants" vibration then it is most likely the index of your driveshaft. If it is a shake then it's probably a balancing issue with the tires. I would start with the simple thing first, go get the new tires or balance the old ones and see if that corrects the problem. (Although I doubt you'll be running 90 mph on the highway all the time, at least I hope not.)

If the problem persists then it could be a warped or damaged wheel that is not noticeable. Lastly, you would have to take on the duty of driveshaft indexing, warped driveshaft, or rear bearing/axle/driveline problems.

Every car has a slight vibration but they are at different speeds/rpms, those vibrations can be moved up or down on the rpm chart but that would require indexing and alignment of the driveshaft. You not only have to have the right place to insert the yoke in the transmission but you have to have the shaft at the correct angle. It can get very complicated and tedious to fix this problem or then again it could be something as simple as a new set of tires and balancing. :D

[neckdemon]

thanks. i really love the car and am pretty happy with the purchase so far. the tires would be the first thing to go no matter what, because the traction is terrible. what do think of the tire choices i mentioned? any size recommendations? like i said the vibration was pretty minimal and it seemed to get a little better when i pushed up to 110. normally i won't be driving at these speeds obviously, but i just wanted to get a feel for the car. i also noticed that the car likes to "feel" the road a little bit. i'm assuming thats the irs hugging the road combined with the relatively tight suspension. overall i'm happy with the car though. i can see it feeling slow relatively quickly and will be saving for some minor mods. if i can get the car the kind of power it will need to be in the 11's i think i will be set. my objective is just to have it be a pretty fast street car that will be seeing the drag strip occasionally.

[ausie]

A good choice in vehicle :thumbsup:
I have had a similar situation with new tires on the car. It occured after finding out it does not have a speed govener in it :D (only did that once and have not attempted it again:eek: ). After that, when I wind up with speeds in excess of 110 I get a slight vibration (probably threw a wheel weight). I had all of the wheels re-balanced which improved the vibration considerably but I still get it but is barely noticable. When I changed to the snow tires this winter and at the same speeds I do not have any vibration (different tires, and different wheels).

I would recommend when you get the wheels balanced, tell the shop you do not want to feel any vibration at speeds above 130. If the vibration does not go away, it may be wheel alignment or more than likely it is the tires (may be a sign of tread separation or weakness if it only occurs at higher speeds than normal, this is one of the indicators that Firestone listed as a warning sign but for SUV tires). Also if the tire has steel belts imbeded in the tread compound they also may create some vibration if the tire has experienced hitting a pothole or shoulder of the road which may result in a bend of the braided steel belt. Damage to the support structure of the tire may lead to hot spots, or a change in characteristic of tire rotation when driving. (best to avoid potholes since this could cause tire damage that may lead to a worse case issue like loss of control). Much of this may sound far fetched, but over the years of ownership of a 96 Ford Explorer you tend to learn more than you want to.

[neckdemon]

thnx. it sounds reasonable. and, imo, it FEELS like it's coming from the tires. either way the tires were going to go. i was looking at the nittos. in the back they have 275/40/17 and a 315/35/17 to choose from. i'm not sure if the 315's will fit, maybe you guys can clue me in on that. otherwise i'll probably just go with the 275's. in the front i was thinking of going with the nitto 555's. the widest size it seems they make is 245. so i guess i'll go with that. unless you guys have any other recommendations. imo, it doesn't make much sense to put DR's in the front. so i figure those 555's will be a pretty good z rated tire. awith the 555R's in the back i should get decent traction and still be bale to drive in the rain. does that sound about right?