aerodynamics maybe?

I recently changed tires and all wheels are properly balanced. Over the last days I have been wondering about this noise. Getting the car up to 193-194 km/h it starts to shake in the front a little, this goes away when you take it to more than 240 km/h. It feels like the air is not flowing through like it should. Anyone else experience this? What could be the cause? It is normal like that?

Shakes and noise that is speed dependent is very commonly tire balance problem. Given you had your tires changed recently, it remains the main suspect. Maybe it's not "properly balanced". I'd bring it back to the tire dealer first for a rebalancing, or even a second tire shop, before I start looking somewhere else.

Obviously tire balance is the place to start but you say you've already checked that. If you have, then have your wheel bearings checked. They will also cause a speed sensitive vibration.

had the same problem with my Viper. 82MPH - 98MPH vibration. Dealer balanced it twice. No improvement. Took it to Discount Tire, they spent some time with the balancer, and that cured it. Seems if you pay $25 - $35 they care more. Try a 3rd disinterested party and stand there and watch while they balance the tires. Hope that solves it.

My experience is that about 1 in 10 tire shops can properly balance tires. Make sure that when then spin them and get them in balance that they rotate the wheel 180 degrees and check balance again. If it is out at all, the wheel is NOT balanced. All balancing machines specifically instruct the operator to do this, but they all agrue with you that it is not necessary. WATCH them do it too.

I'll take the car for a re-balancing this week end and give it another run. I'll keep you posted. How does a failed wheel bearing influence the car at speed?

[QUOTE=How does a failed wheel bearing influence the car at speed?[/QUOTE] It sounds like a low moaning wobble at slow speeds, progressively getting higher pitched as the speed picks up. It usually does NOT disappear, usually no shake. If the bearing is REALLY rotten, you'll get a rattle sound, then a shake, then a loud screech as the wheel falls off .

Palting, your explanation sounds like a wheel bearing on most cars but not the way we've seen them go on my 996 race car. Both times we've had bearing go bad the first symptom has been a small vibration around 70 mph. Both times I assumed balance problems. I don't know why it's speed sensitive. Jim

JimB: How long after the vibration did the bearing really go? What happened when the bearing go? Kam

I wouldn't worry about it being the bearings until you positively eleminate whell balance problems. That is unless you LIKE to worry about things. MY guess it is improper balancing. Have them balanced as I described above while you watch.

Kam, I drove the car on the track at Putnam for a full weekend trying to figure out the vibration before it got bad. As our bearings get worse they go from a subtle vibration to worse noises like Palting described (hard to hear in my car) to flexing bad enough that they will push your pads back and you need to double pump your brakes. This happened to me at Road America. I don't think you have to worry about a sudden failure. They come on slow. "I wouldn't worry about it being the bearings until you positively eleminate whell balance problems. That is unless you LIKE to worry about things. MY guess it is improper balancing. Have them balanced as I described above while you watch." Porsche 911, I have no idea what you are talking about. I agree that it is more likely a balance problem than a wheel bearing but it takes about 1/10 the time to check a bearing. Maybe you shouldn't state things so forcefully unless you are sure you know what you're talking about. BTW, bearing failures are common enough in 996s that I carry two spares with me to races. Jim

Jim, If I was fortunate enough to have the skills to drive as well and and as hard as you do, I'd be bringing spare bearings, too. I'd love to have a race car!! Anyway, last time I had my tires done, the tires came to me directly first. I took them to my neighborhood tire shop, and had them mounted and balanced "properly". Got shimmy and vibration at about 60mph, goes away at about 80mph. Brought it back, and asked them to do Dynamic Balance. I was told the Porsche wheels could only be static balanced! After they fooled around placing the weights and what-not, I drove the car to a better shop about 45 minutes away. Shake and shimmy at higher speeds. Again I was told by the desk guy they can't do dynamic on alloy wheels. I asked for the manager, who said he didn't know but he'll ask the head tech (smart fellow, that manager). Head tech came by, looked at the wheels, said sure we can do dynamic. Less than an hour later, I was happily breaking speed limits with nary a shimmy, shake or howl. Wait a minute. 195 to 240 kph??? That's 120 MPH to 150 MPH!!!!! Holy shmokes!! I would think balance problems would show up before you'd get to those speeds. I would think bearing problems would as well. Maybe IT IS aerodynamics

That's funny Palting and I agree. At those speeds my car is doing all kinds of shaking and vibrating. At 150 my Lexan windshield starts buffeting and I stop worrying about vibrations. Finding a good tire shop is like finding gold. I've never been told that they couldn't do dynamic balancing on Porsche wheels but I have had it done incorrectly may times. Oddly, it doesn't matter much on a race car. My Jongbloed wheels are near perfect and Hoosiers tend to be pretty round. At the Road America club race last week I had some new tires mounted at the track. We marked the stems and all four tires rotated between 90 and 180 degrees in the first two sessions. So much for balancing. Have a good weekend, Jim

Jan_M - just check the temp of the hubs with you hand after driving awhile. If you have a bad bearing on one side, it will be a lot hotter than the other side. Additionally, if you have a flat bearing and you're doing 130 mph, chances are it would be screaming. Jack the car up and try to move the wheels in and out. A loose bearing that could cause a shimmy would definately be noticable.

Jim, I have to ask ... are you saying that the tires rotated on the rims while you were driving it?

I know tires could rotate on our crossbikes, thus the use of gripsters, but on cars, and that much... yikes! Car is due late on monday for re-balancing, I'll check bearings at that time as well. I'll get back to you.

Pete, Yea, the tires rotate on the rims. This happens to track cars under braking. It is especially bad right after you mount the tires. Good luck Jan. Jim

Hi gents, The car's wheels were put to the test today. No faulty bearing, which is nice; wheels are not oval which is also nice. Just needed some rebalancing mostly on front left inner. Strange as this was done recently and no weight had dropped off. They also found a little screw that had lodged itself in my right rear, no puncture though. This tire shop is great, wash wheels properly, put copper paste on nuts and hubs, use "newton wrench" (or whatever the name is) to put nuts back on. Love it when you see the mechanics do a great job. Brake pads look good, no oil between engine and gearbox... just back on the roads again.

it may not be just a tire balance you need, but a high-speed tire balance. there are a few shops around in atlanta that do this

I wish it I could get to 120 MPH before my problem sets in. At speeds greater than 70 my tires feel like they are going to jump off the car! I have been to three shops to have the tires balanced three times with no improvement. Vibration is in the seat and the steering wheel and is enough that my passangers give me a funny look when I get just above freeway speeds. The last tire shop was one I found in my local PCA newsletter. I asked if they used the Hunter Roadforce etc. etc. When I left the last shop they said all the tires zeroed out on their machine - perfect balance so they didn't need to change anything......And they told be that if I had a bent wheel it would have shown up on the machine. My last resort is to take it to the dealer and pay three times the $$ to have them take a crack at it. I'm just frustrated as hell that my Explorer handles better than my P-car and that THREE SHOPS can't even begin to tell me what is wrong! The tires are Perelli (sp.?) w/10k miles on them.

wheels could be bent, does not necessairily show on a balancing machine unless they looked for it. Tire could have had a hit (pothole) and is not really round anymore. I am not sure if a bad alignment would start shakes like that. Good luck.

I went to a local PCA tech session last weekend. One of the more technical oriented guy said that he balance the wheel. He reasoning is that tires might rotate on the wheel as described earlier in this thread.

You guys have jinxed me!!! My left front bearing is now kaput!! Sounds just like what I said in my previous post. A low pitched wobbling moan, progressing to a whine as you speed up. Too much late braking, too much trail braking, too many hot right hand turns. My last trip to VIR probably did me in. Aaaarghhhh

Palting: At least you had fun wearing out the bearing. Kam

Palting, You should swing by VIR at the end of the month for the GrandAm Rolex/PCA club race. There will be much bearing abuse going on. Jim

You gonna be racing, Jim? Good luck and have fun if you are. VIR was a ton of fun, Kam. Long, fast and curvy . Too bad it's too far for me to make it regular. Me, they can't get the car in 'till next week. They say it's OK to drive, but I'd rather not since excess bearing heat may do something to the bearing housing. So, I'm Porsche-less