Scary Front End Vibration
 

Yesterday while driving up I-95 from Philly, I hit one of winter's surprises aka a pothole at about 75mph. This was one of those 4" deep, sharp edged jobs that was too small for me to see until it was too late to avoid it.

Immediately after striking the pothole the entire front end started shaking 4-6" in a nearly uncontrolable fashion. Braking, as I obviously did to get the car off to the side, only made it worse. Fearing a blowout or worse, I pulled to the shoulder to investigate - no signs of any damage to the tire, wheel, or any easily visible suspension linkages on the side that hit the pothole (right).

Thankful to still have both my person and my car in one piece, I ventured back into traffic (which happened to come to an immediate stop due to some fool who had JUST flipped his Lexus SUV - looked rather serious, perhaps fatal :( ). Once I was able to get up to speed, the car handled just fine even up to 85mph.

Is this type of response normal? Is it possible that some hidden damage was done even though the car still drives normally?

jlc

Had a similar problem with my '88 190E 2.6 last year. It developed a vibration that I eventually traced to a tire out of round situation, but in the process of doing some testing the car went into a violent front end shimmy, the likes of which I had never experienced on ANY car. I knew the steering damper was seeping, so I pulled it off, and it offered virtually no resistance.

I purchased and installed a new damper and a check of the 205/55ZR-15 Dunlop D40M2s on a Hunter 9700 showed excess radial force variation on three with the fourth marginal, so I got an adjustment toward a new set of Sport 8000s in the same size being as how the D40M2s were out of production. The forcing function of the tire radial force variation combined with the worn steering damper was enough to set off a runaway front end shimmy. The stiff sidewall 55 series tires relative to the soft sidewall OEM 65s could also contribute to such a problem, especially with a deteriorated damper.

Mercedes have a lot of front caster (about 10.5 degrees on my 190), and apparently a steering damper, which doesn't last forever, is absolutely necessary to damp out runaway oscillations. As another example, sixties vintage Corvettes have a steering damper, except those models with mechanical lifter Special High Performance or Fuel Injection engines. The longer deep section of the oil pan on these engines precluded installation of the damper, so GM just left it off, and it doesn't cause any adverse affects in the steering system. However those cars only run about 2 degrees caster.

I think the damper cost about forty to fifty bucks and it's easy to change - just two bolts, but you have to remove the belly pan first.

Duke

My vote goes to the steering damper
 

Duke is right.

I experienced the same thing a couple of days ago.

I had just entered the highway and at 70km/h, I hit a small bump and there was a slight instantaneous vibration felt at the steering wheel.

Accelerating to 100km/h, I hit another small bump but this time, the steering vibration was much more pronounced and the vibrations lasted about 3 seconds.

I pulled to the side and did a quick visual inspection.Nothing was amiss.

I am going under the car to check the steering damper (that's my guess coz it doesn't seem to be absorbing the forces transferred to the steering).

On my car, the damper is connected by 2 bolts.I believe, a good way to test it is to remove the bolt from the drag link end (either side is ok i guess) of the damper and test the damper for resistance. It should be like a miniature shock absorber. If there is no resistance in the damper, it's shot.

It's a cheap part for my car (like 20-30 bucks) and it looks like a quick DIYer.

I'll find out soon enough.

Hope it helps.

Azhari
Babybenz 1991 190e 1.8

Thanks folks. The steering damper was on my list of eventual to-do's. I guess it will get done a bit sooner.

BTW, while I'm there, how difficult is replacing the three links of the steering linkage and how would one determine if they are shot?

jlc

Just grab the tie rods and try to push and pull them in all directions while looking for play other than rotation of the tie rod ends. Same with the relay rod and idler arm, which should only move around it's axis of rotation. If your steering doesn't have any noticeable on-center play, then the linkage is probably okay.

Duke

Thanks Duke 2.6 - the tie rods seem fine so I just replaced the damper. While I haven't gone looking for potholes to hit at 70mph;) , the car does seem better.

jlc

Does the old damper show evidence of leaking and how much resistance did it have to stroking relative to the new damper?

Duke

Duke,

The old damper did have some resistance but much less than the new one. It's hard to tell if it was leaking or not as I've had a power steering leak that coats much of the underside of the car at times.

jlc

Duke,

The old damper did have some resistance but much less than the new one. It's hard to tell if it was leaking or not as I've had a power steering leak that coats much of the underside of the car at times.

jlc

Another good and quick test of steering linkage integrity is to rock the steering wheel back and forth with the engine off but with the key in the accessory position. If there is something loose it will often be evidenced by doing this.

Haasman