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Vibration at 70 mph from S500?

I have an S500 that has a small vibration at high speeds. I’m fairly certain it’s not wheel or alignment related. I thought it could be the motor mounts but the vibration doesn’t occur while at rest with the motor at high rpm’s. It feels like it’s coming from under or behind the drivers seat. Any ideas? Thank you?

don’t be so sure it isn’t wheel or alignment related, especially since these are the most common causes of vibrations at certain speed ranges. Have the tires rotated and balanced. wheel wieghts can slip of fall completely off if not applied correctly… or if you hit a big enough pothole

Wheel or tire problems are the most common causes of vibration problems at speed, but alignment problems will NOT cause a vibration. Worn suspension or chassis parts can cause a vibration, and an alignment will often need to be done after parts are replaced, but alignment itself will not cause a vibration. This does seem to be a common misconception which often needs to be debunked.

Is this a rear wheel drive car? Have you checked your U-joints and driveshaft?

Not to be rude, but improper alignment can cause uneven tire wear which can cause a vibration at speed. Especially if the tires haven’t been properly rotated.

That is no misconception.

Find a place with a Hunter wheel balancer and hopefully tech who knows what they are doing. They can identify if is a tire and/or wheel issue as the machine reads this.

Thanks for the input so far fellas–the main reason I don’t think it’s tire or alignment related is because I just got new tires after 50,000+ from a very reputable shop and asked them to check alignment and balance and they said it was good to go. They could have easily charged to do that work if it needed to be done.

A thought I had is that the understide of the entire car has an airdam that covers everything to keep it aerodynamic–I wonder if it’s possible that could be causing a vibration at high speed? Also, the vibration is more pronounced when I’m accelerating from say 60-80. If I’m cruising at a speed and there’s not much torque on the drivetrain, it’s not nearly as prominant.

Dry and worn U-joints can cause vibration that comes from the middle or rear area of a front engine rear wheel drive car. This vibration might start with a click/clunk sound when the car is first put into gear, depending on how far gone the joints are. At speed it will vary with engine speed as the transmission is shifted from gear to gear. Eventually it will wear out the rear transmission seal causing lubricant leakage. I have seen drive shafts that have seperated at the U-joint and it is not a pretty sight. You need to have the U-joints checked out if just for peace of mind. but it sounds to me more like a rear wheel ballance problem than U-joints.

Sounds like a driveline problem, tire balance problem, or possibly a rear suspension or loose wheel bearing problem. The latter two are not likely.

It’s kind of strange that your reputable tire shop has the remarkable ability to align a car with a visual inspection; going by the way you phrased this.

Did this problem crop up with the new tires? One might have a defect causing the vibration that doesnt’ cause an imbalance. Can they swap tires front to rear?

Tire balancing is not always cut and dry, There are different machines and some operators and techs are much better than others. Also, one of the new tires could have defect.

Since the vibration changes when the car’s drivetrain is under more stress that points to U joints, CV joints (independant rear suspension I assume), and mounts (motor, trans, and rear end - any or a combo).

This stuff isn’t easy to track down. If you are using aftermarket wheels you could even have a centering ring that is worn or fell out of one of the wheels. Start with wheels and tires first and then look at driveline components next.

The S500 uses flex disks, not U-joints. From the stated symptoms I would look at both the front and rear disks. There are factory wear standards that can be determined by a visual inspection.

Replacement is fairly straight forward, but a hoist is almost certainly required. The transmission mount will have to be removed, so you’ll have to support it from below.

A new set of flex disks seem to cure a lot of vibration issues.