1987 TDT keeps breaking sway bar brackets - Page 4 - PeachParts Mercedes-Benz Forum

vstech · #46 07-12-2010, 10:41 PM

HOLY!!!
no wonder that thing is breaking! wow, what a poor design!
I TOTALLY see how the thing would be twisting! now, how can we fix it?
I'll get under my wagon tomorrow and see what I can see. that thing really needs some triangles in the support design!

babymog · #47 07-12-2010, 10:58 PM

Poor design? Based on your finite-element modal analysis? Pardon my scepticism.

It works, on a million 124s, but this one fails. I'd say that the evidence points to something flawed in this car, not the design.

The loading on the bar at the bracket, by design, is vertical. The top of the bracket is connected to the frame tips.

Did you get brackets and grommets for the 124 wagon or sedan? I measured the swaybar diameter on my '87 wagon, it is significantly thicker than on my '95 sedan. Seems that putting sedan hardware (smaller dia) on a wagon swaybar might cause stress in the mount. Also, if the bushing were clamped tighter (wrong bushing or clamp) than the design, rotation of the bar in the bushing would cause the bracket to stress at the top bolt-hole instead of allowing the bar to rotate in the bushing (which appears to be the design).

I don't have access to a part breakdown, is the wagon swaybar hardware a different part number than what you have received/installed? Different than the sedan?

leathermang · #48 07-12-2010, 11:08 PM

What are ' frame tips ' ?

babymog · #49 07-13-2010, 09:52 AM

There is a subframe that supports the engine, the ends/tips of those frame members is where the swaybar mount hangs.

Rick Miley · #50 07-13-2010, 10:26 AM

Quote:

Originally Posted by babymog

I don't have access to a part breakdown, is the wagon swaybar hardware a different part number than what you have received/installed? Different than the sedan?

Online EPC shows different numbers for the swaybar and bushing on 300TD vs. 300E. It was late last night and I don't remember about the bracket. But I'd say that's enough for the OP to call a qualified parts person and make sure they get the right stuff. And if they don't know the full history of the car, then taking a measurement of the torsion bar (MB's term) would be in order since an incorrect part may have been installed in the past.

babymog · #51 07-13-2010, 10:38 AM

Thank you for looking it up, and this is my point. If a 300E swaybar bushing is used, smaller diameter, on the larger 300TD swaybar, it seems that it would grip the swaybar and cause the bracket to twist and eventually fatigue it at the stress riser (hole).

whunter · #52 07-13-2010, 12:17 PM

Quote:

Originally Posted by whunter

Have you inspected the spring and perch on both sides?

W210 DANGEROUS FLAW please read (cross linked in post #1 to all on topic data)
W210 DANGEROUS FLAW please read (crosslinked in post #1 to all on topic data)

W124 SPRING PERCH = Front Spring Bracing

Have a great day.

Damaged spring perch, weak or broken springs will cause off center/eccentric/excessive flexing and extreme stress on the bracket.

Note:
Bad lower control arm bushings, ball joints and/or struts magnify bracket stress, including bad rear suspension and total vehicle load.

This is a tuned suspension, designed for absolute minimum material/weight/mass = minimum tolerance. Any motion outside design/ engineering does cumulative damage.

babymog · #53 07-13-2010, 12:30 PM

Provided the control arm has good bushings and is not damaged, the 124 swaybar should not be affected by broken springs nor spring perches as the swaybar ends travel with the control arm.

Your point to check control arm bushings is a good one IMO, however those too would have little or no effect as the swaybar is allowed to slide in the bushing.

vstech · #54 07-13-2010, 12:47 PM

mog, you are disagreeing with Whunter on a vehicle design problem... um, you know who he is right? he's designed vehicles himself, and has been employed for decades by national car and truck manufacturers. I would trust something he says without hesitation on the design characteristics of a vehicle flaw.
it's true that the antiswaybar is attached to the control arm, but if a spring was broken on one side, couldn't you see how the entire assembly would tilt and stress all parts of the system?

babymog · #55 07-13-2010, 01:10 PM

I'm disagreeing as a life-long automotive Engineer also, who has worked directly with Mercedes' Engineers, and as a former SAE member.

Reasonable people can and do disagree. I spent some time under two 124s last night studying the problem and its possible causes, and it is on this which I base my stated opinion. I respect Mr. Hunter a good deal, and his opinions, but there is a reason that even learned Engineers consult with each other and try to pick up on any missed details (and we enjoy it), the reason that an Engineer on a program almost never signs-off on his own FMEAs.

Yes, having one spring shorter than the other for any reason, front or rear, will cause a static load on the swaybar. This load is within its design I feel, and not at any un-natural angle. Were the swaybar bent asymmetrically, if something is binding, other possibilities exist that can cause a failure. I would like to look for a reason that the bracket is trying to rotate with the swaybar, which would in theory cause a failure like the one pictures.

whunter · #56 07-13-2010, 01:24 PM

Quote:

Originally Posted by babymog

Provided the control arm has good bushings and is not damaged, the sway bar cannot be affected by broken springs nor spring perches as the sway bar ends travel with the control arm.

Your point to check control arm bushings is however a good one IMO.

Uneven spring load directly impacts sway motion and load.

This is a balanced four wheel independent suspension platform = disrupt the balance and the stress points move from design/engineered locations.

The calculations are EXTREMELY complex, and made worse due to the dynamic nature of the unibody and four wheel independent suspension platform.

Suspension (vehicle)
http://en.wikipedia.org/wiki/Suspension_(vehicle)

whunter · #57 07-13-2010, 01:42 PM

Quote:

Originally Posted by babymog

I'm disagreeing as a life-long automotive Engineer also, who has worked directly with Mercedes' Engineers, and as a former SAE member.

Reasonable people can and do disagree. I spent some time under two 124s last night studying the problem and its possible causes, and it is on this which I base my stated opinion. I respect Mr. Hunter a good deal, and his opinions, but there is a reason that even learned Engineers consult with each other and try to pick up on any missed details (and we enjoy it), the reason that an Engineer on a program almost never signs-off on his own FMEAs.

Yes, having one spring shorter than the other for any reason, front or rear, will cause a static load on the sway bar. This load is within its design I feel, and not at any unnatural angle. Were the sway bar bent asymmetrically, if something is binding, other possibilities exist that can cause a failure. I would like to look for a reason that the bracket is trying to rotate with the sway bar, which would in theory cause a failure like the one pictures.

There is no way I could take this technical discussion personally.

FYI: For members that don't know what it is.

FMEA - Failure Modes and Effects Analysis
http://www.google.com/search?hl=en&rls=com.microsoft%3Aen-us&q=FMEA+-+Failure+Modes+and+Effects+Analysis&btnG=Search&aq=f&aqi=&aql=&oq=&gs_rfai=&gs_upl=1279042805843%2C1279042805843%2C0%2C0%2CNaN%2C0%2C

Rick Miley · #58 07-13-2010, 01:50 PM

Quote:

Originally Posted by whunter

There is no way I could take this technical discussion personally.

I think people may be a little on edge after what happened last night.

You guys continue to debate suspension design while the rest of us look on from the side. Meanwhile, the original poster hasn't been back in days.

leathermang · #59 07-13-2010, 02:26 PM

Are you actual engineers saying that the break was caused by something acting through the sway bar ...
or have you agreed with my analysis that the long end of the bracket is being moved by something and it is not designed to move... thus causing the break at the location in question ?

babymog · #60 07-13-2010, 03:10 PM

Quote:

Originally Posted by whunter

There is no way I could take this technical discussion personally.

I suspected as much, you sound like one who's more interested in exploring possibilities, always a challenge.

I agree that asymmetrical wheel heights will cause a load on the swaybar, that's what it is designed to do. Whether they are beyond the design limit is another question.

If the long-end of the swaybar is moving, or the swaybar is moving fore/aft, it seems that the fatigue would be at the top of the bracket at the 90* bend. As it is at the bolt hole, it seems to me that the stress/flex of the swaybar is at the bolt hole. To flex the bracket at the bolt hole you would need to rotate the bracket at the point between the two lower bolt holes, which would be about the axis of the swaybar. If the swaybar were being rotated and allowed to move fore-aft (abnormal behaviour) then again the fatigue point would be at the top of the bracket, the 90* bend.

This rotation would occur whether the wheels are at the same extension, or uneven. The significant difference would be the vertical load on the bracket, or pre-load.

The same thing holds true for any un-natural movement transverse to the car, it would either slide the bar in its bushings, or bind and pull the bushings sideways, which would likely cause it to fail at the upper bolt-holes where it mounts to the car subframe.

It seems that the bar is designed to rotate in the bushings on this bracket, it is a smooth bar, perhaps some rust has caused it to bind or the bushing/bracket are not the correct ones for the diameter of bar being used. I do hope that the findings will be posted, I'm curious.