Changing vibrations per road and speed - shocks or tires?

JHZR2

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7296B2A4-B606-4380-BAA0-81A3B4F85DE7.jpg

70 mph on 225/60-16 tires @ 758 revolutions per mile. ~14.8Hz if vibration is once per tire rotation.

I have a lot of older cars that sometimes ride perfect, sometimes I feel a lot of road. On my 93 300SD, I found a great deal on some original equipment wheels with brand new tires.

My issue is this - depending on the road surface I will feel like an unbalanced tire. What Is strange is that it seems to be very dependent upon the road surface, not just depending on being within a certain speed range. I could be going along at a certain speed, and one minute roll smooth, the next minute shaking the steering wheel and floorboard (slightly) enough to notice a difference.

Sometimes I wonder if losing has something to do with it, like it’s more likely to happen when decelerating or lightly loaded. I assume this would affect the position of the car a bit. But there’s nothing definite relative to a phenomena observed.

There are no obvious issues with any rubber bushings, but I can’t really tell if the shocks are OK. Is there a good way to determine if this is due to shocks or tire balance?

Road force balancing arbitrarily is $100, two bolstering shocks is $200, and I should probably do all corners, so $400. Would like to avoid throwing money at this stuff.
 
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also might be a loose drive shaft coupler, these often fail on Mercedes, they are made of plastic/composite

 
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I've dealt with road force issues several times with a few cars. Typical RFB issues will occur around 69 - 72 mph, and feel like the vibration is moving around in the car. Sometimes in the seat, sometimes in the steering wheel, etc.
Anyway, find a shop that will work with you and has a Hunter RFB machine. Not all tires will need to be unmounted and reindexed on the rim, but some might. They should also SHOW you the rfb # in pounds for each tire. It has been my experience that you can feel any rfb issue above 10 pounds. Most shops will tell you that 15# is fine, but that's just being convenient for them.
 

Kestas

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I second the thought of examining the flex coupling. There should be two... one toward the rear and one near the shifter. It's the one by the shifter that usually goes. I experienced a vibration that came and went. I also felt it in the shifter.

I attached the bad vs. good from my 95 E320.
flex disc.JPG
 
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Your primary vibration is the one at 14.8Hz, as you calculated points to the tires at the particular speed you felt it at (70 mph). It's unlikely the guibo as the driveshaft rotates at a much higher speed.

Did you have all 4 new wheels and tires installed and felt the vibration?

If you still have you old wheels and tires, may be worth the hassle of reinstalling them one by one and narrow down the offending new wheel/tire. I doubt every wheel and tire is causing the vibration, it's probably just one. Focus on finding that and dealing with it will be way cheaper.
 

JHZR2

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also might be a loose drive shaft coupler, these often fail on Mercedes, they are made of plastic/composite


I’m aware of guibo failures but mine have always been perfect. Every time I’ve looked they look perfect, not cracked, chunked, etc.

I had this car up on a lift before I bought it a month ago and know these are fine.

I've dealt with road force issues several times with a few cars. Typical RFB issues will occur around 69 - 72 mph, and feel like the vibration is moving around in the car. Sometimes in the seat, sometimes in the steering wheel, etc.
Anyway, find a shop that will work with you and has a Hunter RFB machine. Not all tires will need to be unmounted and reindexed on the rim, but some might. They should also SHOW you the rfb # in pounds for each tire. It has been my experience that you can feel any rfb issue above 10 pounds. Most shops will tell you that 15# is fine, but that's just being convenient for them.

Yes, I have two good shops I’ve used for this. But again, it’s a $100 endeavor. So I don’t just want to throw a RFB at this any more than I want to throw any other parts. In fact, if anything I’d rather throw shocks because they’re probably the original ones!

I second the thought of examining the flex coupling. There should be two... one toward the rear and one near the shifter. It's the one by the shifter that usually goes. I experienced a vibration that came and went. I also felt it in the shifter.

I attached the bad vs. good from my 95 E320.
View attachment 90892

My two look like the one on the right.

Your primary vibration is the one at 14.8Hz, as you calculated points to the tires at the particular speed you felt it at (70 mph). It's unlikely the guibo as the driveshaft rotates at a much higher speed.

Did you have all 4 new wheels and tires installed and felt the vibration?

If you still have you old wheels and tires, may be worth the hassle of reinstalling them one by one and narrow down the offending new wheel/tire. I doubt every wheel and tire is causing the vibration, it's probably just one. Focus on finding that and dealing with it will be way cheaper.

The interesting thing is the set of peaks after 14.8. There certainly could be something with the driveshaft, at the 3x point (I think the diff ratio is 2.82, not 100% certain), but if it was that it should be happening all the time it’s at some speed, not sometimes present sometimes not depending upon the road. That’s the strange wild card.

The wheels and nearly new tires were bought from someone locally who was upgrading their w140 to larger AMG wheels. The tires are nearly new and look it, verified by date code.

I had the old tires removed from the old wheels. I kept one that I may put in service as a spare. I suppose I could install it on a suspect corner and see what changes. But for that effort and energy I might be better off just taking it to a shop for a RFB.
 
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The second peak at 28-29Hz is secondary harmonics caused by the same source as the first peak (2X multiple). The third peak is a bit odd as it's showing a bit over 40Hz. This may be the driveshaft if the diff ratio is 2.82, so 14.8 X 2.82 = 41.7Hz. But the third peak is also much lower than the first peak, so it's definitely a much lower contributor to what you are feeling.

My money is still on the wheel and tire rotational assembly. It could be an issue with movement of the steel belts inside the tire, might shift a bit when you corner and cause the imbalance and then shifts back again.
 

JHZR2

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The second peak at 28-29Hz is secondary harmonics caused by the same source as the first peak (2X multiple). The third peak is a bit odd as it's showing a bit over 40Hz. This may be the driveshaft if the diff ratio is 2.82, so 14.8 X 2.82 = 41.7Hz. But the third peak is also much lower than the first peak, so it's definitely a much lower contributor to what you are feeling.

My money is still on the wheel and tire rotational assembly. It could be an issue with movement of the steel belts inside the tire, might shift a bit when you corner and cause the imbalance and then shifts back again.
That would be a shame if it is belts within the tire. These (Turkish) tires are worth what I paid ($125 for the tires and wheels) and are almost brand new. No reason to think they were dumped due to balance (need wheels and I was told upgrading), I’d hate to waste the tires.

But I agree that RFB is the best first bet.
 
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