Odd vibration, I'm stumped!

[zeeboost]

About a week ago I had a blowout in my zx. Because I have Ross C's 5-lug conversion on it, I don't have a matching spare. So, call a friend over to pick me up and take me to discount tire so I can have them change out my tire. They were closed, so I went to work and tried to find some used tire to fit - no dice. My last resort was Wal-Mart. They had a tire that would fit, but was slightly larger. Fine, I thought, as long as it will get me home. BTW -- all 4 tire sizes are 245/40/17 and they were replacing the blown-out tire with a 245/45/17. Well, when I was driving back home with it, I noticed it had a rhythmic vibration, like it would happen every 2 seconds. I figured it was something to do with the harmonics of the two different-sized tires spinning, so I figured I wouldn't worry about it until I get the tire sizes matching.

 

Today, I mounted two brand new BFG T/A G-force 255/40/17 tires in the back. BOTH the same size, BOTH identical tires, BOTH well-balanced and only showing 11-13 lbs. of road force, and it STILL has the vibration. It's tolerable at 55, annoying at 65, and downright scary at 75. My CD player will skip if I drive at 75 -- it shakes the living piss out of it. I'm used to working with vibration at work all the time, but I've never messed with one that only vibrated every 2 seconds.

 

I'm hoping some of you can help direct me where or what to check out that would cause the problem, because it's really getting scary to drive it. The only thing a search pulled up was that the tranny mounts or u-joints could possibly do this, but considering this only happened after the blowout, I doubt it's the problem. Thanks

[spork]

Well, try this, jack up the back of your car and place several jack stands underneath. Have a friend handy and have them put the car in gear and engage the drive wheels. Watch your wheels/tires for a possible bent rim. With the car not running, check wheel bearing play and grab all your driveshatf and half shafts and check for play. Honestly, it sounds lke you may have a bent wheel. More than once I've takena set of wheels some place and they balance them and make no mention to me that they're bent...until I approach them about it and ask why they place weights in certain spots that normally shouldn't have weights (like diagonally across from each other). I don't know if that will help or not, but hopefully you can track it down while its on jack stands. Fair warning tho...sounds travel through metal and can really deceive you on which component is actually making the noise.

[zeeboost]

I too, suspected a bent wheel, but I measured the rim runout at work and it passed, so I assume it's not a problem (unless the machine needs to be recalibrated). I should've checked the wheel bearings while I had it up in the air today, but then again, I was assuming the new tires would fix the problem. I can't think of anything that I may have damaged during the blowout, other than the rim. It wasn't a wild, out-of-control blowout -- I didn't even know I had one until I pulled over to see what why I was suddenly having so much resistance. The sidewalls on the old tires were pretty damn stiff, so despite not being inflated, it still did a fairly decent job of keeping the rim off of the ground.

 

Thanks so far for the replies!

[JMortensen]

Did they balance the tires with weights only on the inside of the rim? That would be a STATIC balance. Shops do this a lot of times when they see a pretty set of wheels, and don't want to ruin their appearance by putting weights on them. I've NEVER had a static balance on any of my vehicles that didn't have vibrations. DYNAMIC balance is where it's at. Weights on both the inside and outside of the rim is the only way I've found to cure vibrations at high speed.

[Guest 305240]

Just for piece of mind, you may want to check out your driveshaft. A friend of mine had a vibrationin his 79 that was so bad the wheels hopped about 2" while up on a hoist. They investagated and found the front U joint was froze. It had went on for so long that it actually put a little warp in the driveshaft. A new driveshaft and u joints cured the problem.

[zeeboost]

I'm the one that balanced them (at work), and yes, I did a dynamic balance. I don't have a lip on the outside, so I used clip-on weights on the inner lip and sticky weights on the inner rim right next to the spoke.

 

I'll check out the driveshaft and u-joints when I put her back up on the lift. The only thing is that I've felt driveshaft and u-joint vibrations before, and they weren't like this, but nevertheless, I can't rule them out just because I haven't seen them do it before. It's just so weird how the vibration comes and goes every 2 seconds. I'm always dealing with vibration problems at ford (especially with the new F-150s) and this one's new to me.

[Tim240z]

Is the vibration coming through the steering wheel, or through the car. At least you can narrow it down to the front or rear.

[zeeboost]

Honestly, it feels like it starts in the rear and progresses to the front. First it's a butt vibration (feels like you're driving over a cattle guard) and ends up in the steering wheel. But, it could just be my mind playing tricks on me -- I'm thinking the vibration in the rear may just be exaggerating the one that I already have in the front, because one (or both) of my front tires are out-of-round. I know this because I have a bad vibration in the steering wheel at 45 mph, and then at 70 mph. I've had it for a while now, though -- long before this started happening. I was going to re-index and rebalance the front wheels when I had the car on the lift yesterday, but I couldn't find my lug nut key for the two locking lug nuts I have in the front, so I wasn't able to remove them from the car. Hopefully after I take care of the front tires, I may have some luck into trying to pinpoint the problem a bit better.