How does this suspension work?

[anamgee]

I recently purchased a Mercedes ML350 (W164) and as usual I started to see how to tweak the handling and was very surprised with what I saw. Attached are pictures of the Sway bar and the bushing used to connect the sway bar to the body. How would this sway bar work? In all my other vehicles, the sway bar would be able to swing freely in the bushings, but in this case the bushing is clamped to the sway bar, and then the bushing is clamped to the body. How does this even work?

 

[anamgee]

Here are the images

 

[Tmoose]

It's hard to say with great confidence from the pictures, but i'm thinking the clean steel clamp that grabs the bar extends axially and has the rubber element bonded to it. The relative motion between bar and large main clamp is meant to be handled by rubber-in-shear (no lube required), not by the rubber sliding either in the clamp or on the bar.

The grease suggests the rubber tore loose and someone tried to quiet a squeak with lube.

 

[anamgee]

That is what I was thinking too. Like an engine rubber mount. Which also means that the sway bar is not relaying the roll motion to the other side as it is restricted/terminated at the clamp, as if the middle section of the bar did not even exist.

 

[JayMaechtlen]

looks pretty normal to me - each end of the bar is linked to moving parts of the suspension, and it rotates in bushings mounted to the frame or chassis.
Looks like those clamps just keep the bar from sliding side-to-side in the bushings.
What part of this do you consider mysterious or unusual?


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[JayMaechtlen]

Oh-you're referring to the bushing design?
Well, does the car move in jounce? Then you can be sure that the bar turns.
What exactly happens in a factory bushing when it is in good condition? I guess the rubber shears (deforms).
Does the service literature say anything about the replacement procedure for the bushing, or if it should be lubed?

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[BrianPetersen]

The rubber shears (deforms torsionally) to allow the antiroll bar to rotate. Most suspension bushings are like that nowadays.

 

[anamgee]

This is the first one I have seen like this. Interestingly, if I were to replace these bushings, would I be replacing them with the vehicle up on the lift or on the ground? In others words, would this need to be preloaded? In conventional sway bar bushings, I'd replace them with the vehicle on the ground, but since this design appears to defy that conventional design, I am questioning the whole concept.

 

[GregLocock]

The bush is soft enough in torsion that it does not terminate the torsional loads in the bar. It does, of course, contribute to the springing of the car, and basically acts in parallel to the road springs. The reasons for using this design are several, the primary one being that it eliminates the sliding joint between the bar and bush, which is a cause of squeaks. The lubrication of that interface is not satisfactory over long periods of time, even with fancy oil retaining liners.

It's known as a grippy D block around here.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Normally when you replace that type of bushing, you do whatever you need to do to change the parts with the vehicle up on a lift except you leave the clamp bolts (#350 in the diagram) loose until you have the weight of the car on its wheels so that the suspension is compressed to nominal ride height - with the bolts loose, the bushing will find the place that it wants to be - then you tighten the bolts with the suspension loaded.

I'll just about guarantee that this is not the first time you've seen this design, you just might not have recognized it before. Most shock-absorber eye mountings are rubber-in-shear, also. That's why you're not supposed to tighten the bolts until the car is sitting on its wheels at nominal ride height.

 

[anamgee]

Jay, Greg, Brian - thanks for the information.

Interesting, if you delve a bit deeper, you can see this design should provide an infinite/analog amount of preload(add to spring rate) depending upon how high the car was WHEN you tighten the bushing to the rod & clamp - right from nominal load when the vehicle is on the ground on its own weight to when it is high up in the air on a lift.

I will try out various settings during the week and see which one provides the best ride and control. Stay tuned.

 

[BrianPetersen]

You do not want to do that. You want the rubber to be at (or near) no load at normal static ride height. If you set it otherwise, it will shorten the life of the bushing. It's not meant to be used as a ride-height adjuster.

 

[anamgee]

After some trials during the week, I have found some things that are very interesting.

1. How do you ensure a 4632 lbs. SUV handles quick and tight?
2. How do you get a soft smooth supple ride while attaining #1?
3. how do ensure that the sway bar does not slide around?
4. How do you get a sway bar work like a bigger bar without using a bigger one?

The answer is BIG BAR SOFT SPRING SET UP + some Mercedes ingenuity.

In order to get the bar act like a bigger one than it really is, it has to be preloaded. One of the test I did was to loosen the bar and tighten it when the vehicle is on the ground, and it drove horrible because the soft springs on their own are not capable of carrying the weight of the vehicle. The optimum setting I discovered was raising the vehicle by 2 inches and then tightening the sway bar and bushings. When I dropped the vehicle back to the ground, the ride height was still the same, but the sway bar was now preloaded and contributing as a spring.

At this setting, it takes turns like a sports tuned vehicle, and when on the highway I cannot even feel the road cracks and joints. In addition, the confidence grows as I go faster and faster as it becomes extremely stable which is the highlight of the BBSS setup.

 

[Nereth1]

Layman here, and I may misunderstand what you have done, so take this with a grain of salt:

1) You haven't got the bar acting like a bigger one than it is just by preloading the rubber bushings. Roughly speaking, the roll bar system will not be stiffer in any loading case (roll, two wheel jounce, or single wheel jounce), it just has an offset starting point so it handles some extra forces throughout.
2) If you had actually got the bar acting like a bigger one - what would happen now to your road holding, with both front wheels linked stiffly together, against single wheel jounce/rebound?
3) I don't know what your cars suspension is like, but as per what Brian Petersen said, let's say it's a big SUV and normally has 6" of travel in jounce, but you've offset those bushings to 2" in the starting position. When you hit your bump stops now, they will be at 8" from their free state - 33% over design load, no? Same potentially goes for all the mounting hardware nearby them.

What you have basically done is the equivalent of replacing your stock springs with less stiff, longer free state springs. You can do that without compromising the rubber bushings.

 

[GregLocock]

1) Yes
3) No, longer travel reduces impact loads
Last sentence No, the rate is unchanged


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Nereth1]

Greg,

Please correct me if I'm wrong, but:

3) He doesn't actually have longer travel in the suspension itself, after he offset his bushings, he just has his bushings moving to longer travel points under the same loads that used to move them to shorter travel loads - thus bushings always operating under increased stress/strain (except in rebound).

Last sentence: He said he replaced his normal springs with softer ones and ended up losing some ride height (until he moved the bushings) which means he simply did not compensate the reduced stiffness main springs with longer free length to increase the spring deflection at nominal ride height. Lower stiffness main springs + same stiffness bushings, but bushings preloaded a few inches means the total rate is lower as per the main springs, no?

 

[GregLocock]

O, sorry, I missed the bit about softer springs.

I don't understand your explanation of your comment on travel. If the wheel travels further before it strikes the jounce bumper then in general the force will be less than with a shorter travel, as the shock absorbers will have more time to absorb energy.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[RossABQ]

I don't see where he actually says he replaced the springs?

 

[Nereth1]

RossABQ,

He said he had a "big bar soft spring" set up, and " the soft springs on their own are not capable of carrying the weight of the vehicle" which I assume means he fitted softer than stock springs. Could be wrong,

In light of that:

Greg,

I don't think he increased his travel with his changes - I think he first fitted softer than normal springs with no corresponding increase in length (thus lowering the vehicle and decreasing his available jounce travel) and then to recover his normal ride height, he then jacked it back up to 2" higher than the stock ride height, released the rubber sway bar mounts, then retightened them in that position. Then he removed the jack and his car settled back to the stock ride height, but with the rubber sway bar mounts deflected 2".

Anamgee, can you confirm the above? At this point I start to wander if I am the only one reading it that way.

 

[anamgee]

#1.Everything is stock here - Springs, shock, swaybar and bushings.

What I was attempting to do is understand how Mercedes Benz designed this front suspension. All said and done, whether I preload or not, the ride height is still the same. But the swaybar us now preloaded by the twist lock action.