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2000 S, rear vibration, dealer no help


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I purchased my 2000 Boxster S with 16,500 miles on it 7 months ago. It now has 20,000 miles on it. It has a slight vibration that feels like it is coming from the rear end. You feel it in the seat of your pants, not the steering wheel. You notice it at speeds as low as 25 mph. It doesn't get any better or worse with speed. My local Porsche dealer doesn't have any tire equipment so they sent me to a local tire shop. They road force balanced my tires using their Hunter 9700 machine, software revision 4.3. All of the tires could be perfectly balanced, were not out of round and were true. The front right tire had a road force reading of 30 lbs. I was told. He said I might feel some vibration through the steering wheel. The car is now no better. I was out of town for 4 days and left the car at the dealer. I told them I had the tires balanced by the shop they recommended. When I returned, they never put the car on a lift, they were only concerned about the tire balance. Could this problem be some type of engine/trans mount, CV joint or suspension problem. It appears that my dealer is not going to put forth any effort on my car until I buy new tires. If I spend $1000 and the problem still exists, I will be furious.

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I purchased my 2000 Boxster S with 16,500 miles on it 7 months ago. It now has 20,000 miles on it. It has a slight vibration that feels like it is coming from the rear end. You feel it in the seat of your pants, not the steering wheel. You notice it at speeds as low as 25 mph. It doesn't get any better or worse with speed. My local Porsche dealer doesn't have any tire equipment so they sent me to a local tire shop. They road force balanced my tires using their Hunter 9700 machine, software revision 4.3. All of the tires could be perfectly balanced, were not out of round and were true. The front right tire had a road force reading of 30 lbs. I was told. He said I might feel some vibration through the steering wheel. The car is now no better. I was out of town for 4 days and left the car at the dealer. I told them I had the tires balanced by the shop they recommended. When I returned, they never put the car on a lift, they were only concerned about the tire balance. Could this problem be some type of engine/trans mount, CV joint or suspension problem. It appears that my dealer is not going to put forth any effort on my car until I buy new tires. If I spend $1000 and the problem still exists, I will be furious.

Is this vibration you are talking about at 3500 rpm?

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I purchased my 2000 Boxster S with 16,500 miles on it 7 months ago. It now has 20,000 miles on it. It has a slight vibration that feels like it is coming from the rear end. You feel it in the seat of your pants, not the steering wheel. You notice it at speeds as low as 25 mph. It doesn't get any better or worse with speed. My local Porsche dealer doesn't have any tire equipment so they sent me to a local tire shop. They road force balanced my tires using their Hunter 9700 machine, software revision 4.3. All of the tires could be perfectly balanced, were not out of round and were true. The front right tire had a road force reading of 30 lbs. I was told. He said I might feel some vibration through the steering wheel. The car is now no better. I was out of town for 4 days and left the car at the dealer. I told them I had the tires balanced by the shop they recommended. When I returned, they never put the car on a lift, they were only concerned about the tire balance. Could this problem be some type of engine/trans mount, CV joint or suspension problem. It appears that my dealer is not going to put forth any effort on my car until I buy new tires. If I spend $1000 and the problem still exists, I will be furious.

Is this vibration you are talking about at 3500 rpm?

It has the vibration even if I push the clutch in and let it idle and coast.

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It appears that my dealer is not going to put forth any effort on my car until I buy new tires. If I spend $1000 and the problem still exists, I will be furious.

Once the tires and wheels have checked OK on the road force test, the dealer should stop worrying about them!

I would investigate the rear wheel bearings. Usually bad wheel bearings produce noises but not much vibration. However, I just replaced a badly worn front bearing and it cleaned up a vibration I was feeling in the steering. There were a couple of spots on the bearing's races which were badly scored.

Are you hearing any noises from the rear?

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If the problem does not change with speed, it is not your tires (or rotors, etc.)

Tires typically have a number of force-stiffness/uniformity harmonics that manifest themselves as vibration with varying speed. The first harmonic will usually develop a vibration commonly known in the industry as 'shake' at between 10-15Hz between 55-70mph, depending on tire size. The 2nd and 3rd harmonics contribute to a vibration usually known as 'roughness.' The steering column in many, many vehicles has a natural frequency at about 30Hz, which is usually affected by the 2nd and/or 3rd order harmonics, sometimes at speeds as low as 45mph. Higher order harmonics, 4th through 9th, are very vehicle dependent (some vehicles may have a tire-sensitive natural frequency of the suspension, cowl, firewall, etc.), but fortunately uncommon.

Shake at the front axle can create dashboard and floorpan vibrations as well as a steering wheel vibration condition known as 'nibble' (the steering wheel oscillates as if being turned left/right :drive: ). Shake at the rear is usually manifested in the floorpan/seat track. Roughness at the front can affect the dash and floorpan, but usually vibrates the steering column and steering wheel (not like 'nibble'--more of a buzzing) the most. Rear tire roughness, frankly, is usually damped-out and not felt.

For the lower order harmonics, to trained ears (or if its bad enough) there may be an audible cue. What's really important here, in this situation, is that the issue seems constant with speed, which is contrary to the noise and vibration elements typically attributed to tire uniformity parameters. With tire vibrations, especially shake, you can get the vibration to phase in and out by easing through the speed range where the vibration is greatest.

The Hunter 9700 at the tire shop you visited was measuring the force uniformity of the tire (the "30 lbs" "road force" measurement was 1st harmonic). These machines do not provide very accurate readings (i.e. don't focus on the number "30 lbs"), but the readings can be correlated to laboratory grade machines that do. They were correct to mention the possibility of steering wheel vibration from your right front, but if you're not feeling it there, then all the better.

You have to investigate things that don't change (much) with speed. From what you have mentioned, I would not buy tires. I would ask the dealer to swap-out known good tire/wheels from another vehicle to get them to rule it out. Best of luck.

--Brian

Edited by Q-Ship986
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