350SL Rear Suspension Instability

[BiggusDickkus]

My 2009 350SL is suffering from a rear suspension anomaly. I have owned the car for 3 years and it has always had a tendency for the rear of the car to step sideways when driving over a bump or a rough road surface. Its worse on the nearside than the off side. The car is low mileage, as in 23,000 miles from new. Recently I had a garage change all 4 dampers for new Bilstein OEM types, and in doing this it has made this problem much worse. On a straight flat road the car behaves well including round corners, but as soon as the road surface is rough or if there is a manhole or pothole the car lurches towards the side of the road where the poor surface is. It does the same on both sides but the near side is much worse than the off side. The car is in excellent condition and I am a little mystified about this problem. I had a similar problem on an E46 M3 about 10 years ago. Three BMW garages could not find it, but a suspension specialist found the problem to be a faulty bush in one of the rear wishbones. Because of this I am suspecting that it could be one of, or a few of, the rear suspension bushes, or even a subframe bush(s). Has anyone else encountered this problem and if so, what was the solution? Forgot to say my car is the sports version with 19" AMG wheels. The tyres (Continental ContiSportContact 6) are relatively new, having only done about 2000 miles. I would welcome any advice...

 

[AndrewOl]

I think you are on the right track looking for a knackered bush. I would have thought shocks too but you have ruled that out. If it was something bent you would probably see some weird tyre wear. Have you had a four wheel alignment done at a decent garage?

 

[BiggusDickkus]

I think you are on the right track looking for a knackered bush. I would have thought shocks too but you have ruled that out. If it was something bent you would probably see some weird tyre wear. Have you had a four wheel alignment done at a decent garage?

Thanks, I was going to get a full alignment done after getting the dampers replaced but I need to get this problem sorted out first. I can't see it being an alignment issue as the car lurches about too much at the rear to be alignment, plus I believe there is only very small adjustments that can be made to rear suspension on these cars.

 

[MrGreedy]

The symptoms you've describe sound exactly like a rear shock issue, which I've had in the past. And replacement shocks cured that up straight away, so at least that's ruled out and at 12 years, it wouldn't be unusual for shocks to be getting way past their best so I wouldn't class putting the new shocks on as a waste of money.

I've had a severe alignment issue in the past due to a slightly bent suspension component, but this manifest itself in always kicking out in one direction, whereas the old worn shocks were either direction as you describe.

I wouldn't rule out a bent component, but it is sounding like more than 1 worn bushing. I'd be expecting the same bush on both sides. Now, which bush, will take a specialist who knows the car a bit of digging.


What I will say is that on my 2004 w203 V6, I had very twitch rear end around 2016 , and it wasn't shocks (I replaced them like you, suspecting this to be the sole cause like I have had on other cars). I suspected rear subframe bushes, and given that changing those is a bit involved, I opted to get all rear suspension arms and bushes replaced at the same time.

My trusted garage couldn't definitively locate the 'problem' bush, so all out replacement seemed the way. To keep costs down, he allowed me to source all Lemforder parts and supply. Where I could get arms with bushes in, I opted for that rather than just replace the bush. He also did a Hunter 4 wheel alignment as part of the job, and I supplied the oval concentric/eccentric(?) bolts that allow proper adjustment of the rear.

This includes replacement of a stupid design front upper suspension arm/link that can only be changed with the rear subframe removed. The rear bulkhead blocks the path to allow the bolt to be removed, so with the subframe dropped the suspension arm bolt can be removed.

The only exception was on the large arms at the bottom that seat the spring and connect to a bush in the hub, I bought the bushes as a new hub and that large arm would have jacked the cost up significantly.


This totally transformed the car and twitchy rear end problems had gone.


I bought from Mister Auto France at the time, but maybe Autodoc is the most competitive these days.
I've recently found if you register on Autodoc, put the bits you want in the basket, then close your browser, over the next few hours you will get special offer emails with up to 40% off (I think I lost my nerve at 39% and bought at that point, but I've since had 40% emails, but they are very time limited).

 

[Will]

Even if you don’t plan on getting the alignment done straight away (if you’re expecting to be replacing parts) it may not be a bad idea to get a free alignment check and that may help point you towards a worn or damaged part (eg bent/loose suspension arm/bush etc.)

Broken spring? Subframe bush? Stuff like that should be relatively straightforward for a decent specialist to check up on the ramp

Be good to know what you find

 

[Bellow]

I would look for two things - both of them bush related.
I'd look for the bush that is most tasked with restraint when the upward motion is resisted by the damper. That is, which bush is most loaded as the assembly tries to pivot around the damper's (inevitably inboard and possibly longitudinally offset) lower mount.
I'd also look for the bush that is most tasked with controlling toe. This could be a gyroscopic effect - but I'm more inclined to the former as the change of dampers worsened the problem.

 

[BiggusDickkus]

The symptoms you've describe sound exactly like a rear shock issue, which I've had in the past. And replacement shocks cured that up straight away, so at least that's ruled out and at 12 years, it wouldn't be unusual for shocks to be getting way past their best so I wouldn't class putting the new shocks on as a waste of money.

I've had a severe alignment issue in the past due to a slightly bent suspension component, but this manifest itself in always kicking out in one direction, whereas the old worn shocks were either direction as you describe.

I wouldn't rule out a bent component, but it is sounding like more than 1 worn bushing. I'd be expecting the same bush on both sides. Now, which bush, will take a specialist who knows the car a bit of digging.


What I will say is that on my 2004 w203 V6, I had very twitch rear end around 2016 , and it wasn't shocks (I replaced them like you, suspecting this to be the sole cause like I have had on other cars). I suspected rear subframe bushes, and given that changing those is a bit involved, I opted to get all rear suspension arms and bushes replaced at the same time.

My trusted garage couldn't definitively locate the 'problem' bush, so all out replacement seemed the way. To keep costs down, he allowed me to source all Lemforder parts and supply. Where I could get arms with bushes in, I opted for that rather than just replace the bush. He also did a Hunter 4 wheel alignment as part of the job, and I supplied the oval concentric/eccentric(?) bolts that allow proper adjustment of the rear.

This includes replacement of a stupid design front upper suspension arm/link that can only be changed with the rear subframe removed. The rear bulkhead blocks the path to allow the bolt to be removed, so with the subframe dropped the suspension arm bolt can be removed.

The only exception was on the large arms at the bottom that seat the spring and connect to a bush in the hub, I bought the bushes as a new hub and that large arm would have jacked the cost up significantly.


This totally transformed the car and twitchy rear end problems had gone.


I bought from Mister Auto France at the time, but maybe Autodoc is the most competitive these days.
I've recently found if you register on Autodoc, put the bits you want in the basket, then close your browser, over the next few hours you will get special offer emails with up to 40% off (I think I lost my nerve at 39% and bought at that point, but I've since had 40% emails, but they are very time limited).

Many thanks, The ride improvement was noticed straight away with the new Bilstein dampers fitted, quieter too, that was until I hit the first rough bit of road surface where the car lurched about depending on what side the rough bit was, as in rough road on the near side and the car lurches to the near side after it has gone over the surface...

I have just been under the car and it looks like new, and no signs of any bush deterioration, but then again I know that is not necessarily the whole story. I am inclined to take it to an MB main agent and see what they say as it looks like you need some specialist tools to do a lot of that work. I also noted the bolt you mention very close to the rear subframe, but I would be concerned whether any current MB tech would be familiar with a car this old now...

When you see how new the underside of the car looks and with only 23K miles, it does make me wonder what could have failed in that short amount of miles. Is there a good independent MB specialist in the south east who would have the knowledge to fix this problem and who wouldn't bugger the car up in the process? Something that most garages seem to achieve with regularity in my experience.

 

[MrGreedy]

I'd go specialist and NOT main dealer for this. Every time.

I'm not massively familiar with Bucks, but Wayne Gates in Harrow (inside M25) gets loads of recommendations, depending on which end of Bucks you're at.
I've travelled hours to have my car serviced as PCS in Horndean, the owner of which is very helpful on this forum and I believe a forum sponsor, but obviously a further trek.

 

[Petrol Pete]

OP . It might not be the mileage but the car is 12 years old , so (presumably) are all the 'rubber bits' in the suspension links. What you describe - loss of traction/control on bumpy surface points to worn shock absorbers . But you have new ones fitted ?? . So unless you can see cracks in the subframe or broken springs I can only think a worn link (or two is to blame) . Best of luck with this one

 

[BiggusDickkus]

OP . It might not be the mileage but the car is 12 years old , so (presumably) are all the 'rubber bits' in the suspension links. What you describe - loss of traction/control on bumpy surface points to worn shock absorbers . But you have new ones fitted ?? . So unless you can see cracks in the subframe or broken springs I can only think a worn link (or two is to blame) . Best of luck with this one

I have been taking a good look at the nearside rear suspension and there appears to be some movement in the track bar bushes. Looking at this, if these are not firm, as in the rubber bushes then there could be potential for the rear hub to "steer" the rear of the car on either side... The track bar has one bush fitted into the bar, but the other end bolts into a bush in the hub. Both of these are a starting point I suspect.

I can't see any deterioration of the rubber in the bushes, but unless you take these out its very difficult to see for cracking and this is a big job without getting the car on a proper lift. The rubber on the track bar bushes seemed to allow movement which I suspect is more than it should be which could easily be the cause, especially after 12 years...

 

[MikeInWimbledon]

It might be a bit of a distance but Micheldever Tyre have a very sophisticated operation just north of Winchester. They would be the best I know to diagnose this.

As you say, the suspicion is that it’s not mileage but age. The result of standing still for 12 years - as happens with most SL’s

it won’t help that you’ve got big wheels on a Sports suspension, but then you’ve seen that before with your M3. British roads simply aren’t maintained to German or French standards, sadly.

 

[Bellow]

I have been taking a good look at the nearside rear suspension and there appears to be some movement in the track bar bushes. Looking at this, if these are not firm, as in the rubber bushes then there could be potential for the rear hub to "steer" the rear of the car on either side... The track bar has one bush fitted into the bar, but the other end bolts into a bush in the hub. Both of these are a starting point I suspect.

Remember that the suspension may be configured not just for vertical movement but also some rearward movement when encountering a bump. Any legitimate 'softness' in a bush should be to facilitate that but will not permit steer effects. Tendency to steer effects will be entirely absent or countered by another link.
Don't forget gyroscopic effects (where a bump will try and force toe out IIRC) which have to be controlled by track arms.

 

[BiggusDickkus]

Update: Over the last few days I have driven about 250 miles on all different types of roads including motorways and I can report back that the car has settled down nicely. The original problem I experienced driving the 7 miles back from the garage has lessened to the point that it is now no worse than it was originally. The Bilsten OE dampers (the black ones, not the yellow sports ones) are working very well and there is a lot less noise coming into the car. This is to be expected seeing at the original dampers were 12 years old. I was perhaps a little anxious during the initial drive of the car which prompted me to raise this issue. Now the car only appears to move about slightly at the rear when a large bump is experienced. The car is also booked in to have a full suspension check and alignment in a couple of weeks time so I will report back more fully after that.
Many thanks for all the advice and constructive comments.

 

[BiggusDickkus]

Update 2: Finally I got the handling problem sorted on my 350 SL. I took the car to Northampton Motorsport on Monday, a company I have used in the past many times and had excellent experiences with, and Monday's visit was no exception. They drove the car first and confirmed that there was definitely something not right with the handling.

Firstly, they found the front offside camber was out, so that was adjusted into spec.
Secondly, they found the "toe" on both sides at the rear was out, so this was realigned so the front and rear now meets the factory alignment specifications.

I was a little sceptical that alignment would fix the handling issue, but driving it home it was like a different car. Now there is no bump steer and less tyre noise from the rear and its much quieter inside the car. The SL must be quite critical on alignment, because with it set up correctly it drives amazingly well.
I hope this helps anyone with a similar issue.

 

[AndrewOl]

It is amazing what having all four wheels pointing in the right direction does!

 

[QUTSELL]

Well, our car C Clas 09 Reg going for all replacements new front shocks plus new rears, plus new rear springs, done more miles than I would care to walk. Job booked for the 29th so once done I will report back.

 

[QUTSELL]

Well, our car C Clas 09 Reg going for all replacements (Bilstean)new front shocks plus new rears, plus new rear springs, done more miles than I would care to walk. Job booked for the 29th so once done I will report back.