Rear suspension.
Owners of the 'A' Class will be fully aware that there are few references that the owner can draw on to find out not only how this vehicle is constructed but also what each component part does. Some of the components speak for them selves others are less conspicuous, not only in their design but what their full function is.
The rear suspension consists of a sub-frame Part number 16835 05700 1 199 0255 04 Mounted and bolted onto the underside of the body work, Chassis, almost certainly rubber mounting pads are positioned between the sub-frame and the bodywork the rears being mounted in the metal housing of the sub-frame A There are two mountings at the front and two at the rear, although the fronts are not so easy to find. I have outlined the sub-frame where it can be seen, The large oval cross member B forms the mechanical link between the N/S & O/S wings forming a large ridged 'U' shaped frame on which the other suspension components, swinging arms and rear wheel hubs are mounted.
The mountings and securing bolts are displayed in these photographs , the fronts are difficult to see and suspect even more difficult to get at and work on should it become necessary.
3 Fronts 4 Rears
Situated between the static wing of the sub-frame and the movable swinging arm is the coil spring which makes up the rear suspension
It should be noted that the spring lays at a slight angle in relation to the rear of the car, as weight is applied so the springs straighten the load being applied directly over the swinging arm.
C Swinging Arm
D Rear Wheel Hub Assembly
E Rear Suspension Coil Spring.
1
The trailing arm securing bolt.
2 One of the front sub frame securing bolt.
The coil spring is held in position by the tension it is under, There are recorded incidents where these springs have broken, which I have been given to under stand is at the bottom of the spring close to the lower location cap. Where a spring has failed, expect to see your car sitting very lop sided and avoid driving any further than is necessary. This defect will dramatically affect the stability of the vehicle, reduce the load on the affected side as much as possible, drive at a safe speed particularly when cornering .
Replacing the spring is not a difficult job! When replacing the spring ensure that the cap and rubber cushion is in a sound clean, a smear of grease around the bottom edge of the spring will do no harm avoid getting grease on the rubber cushion.
The caps have been known to rot out being made of quite thin metal so it is worth checking them prior to its M.O.T.
1 Shows the fitting at the head of the spring.
2 The fitting at the bottom.
F The thin metal cap.
G The rubber cushion.
The rear spring fitted on earlier A class are known to break on the lower 4" of the spring, this can allow the the cap to ride up inside the spring and fall out.
The spring has been modified twice in recent years , the spring shown in this photo is the latest available and does away with both the dome cap and the rubber , sitting straight onto the dome on the suspension arm.
If you are purchasing from other than MB
outlets there is every chance you will be sold the old style spring. To get the most up to date version Go to MB parts they may be more expensive but at least you will have to correct and most up to date spare part for your car MB part No. 168 320 0408. ensure you get the most up to date spring, as old stock may still be held by some branches.
Also note it is a waste of good money to put New springs on the car if the metal around them is left untreated against rust , spray with 'Swarfega' Duck oil designed to protect the underside and vulnerable parts of your car.
Two examples of broken rear springs, Note the cup inside the coil of one of the broken springs. In the other example the cup is missing. Note the exposed hole in the casting central to the base of the spring. A
The only saving grace about this problem is that the springs are very easy to replace and are not that expensive as car spares go. £21.00 plus vat august 2006.
The new Springs like the old are colour coded for the various models of 'A' Class and you will therefore need your Vin/chassis number to obtain the correct springs.
It is advisable to always fit springs in pairs to maintain the overall balance of the car.
Replacing Rear Springs.
This method avoids the need to purchase a spring compressor.
Having taken a safety precautions outlined on mypage 5 'Working around and under your Car'
Slacken the wheel bolts
Jack up the car sufficiently to be able to get at and remove the spring.
Place blocks or another jack under the trailing arm as a precaution against the main jack slipping or in the case of hydraulic jacks dropping while under load.
Slacken and Remove the 18mm Lower shock absorber bolt
Lower the jack/remove 1 block from under the arm
Push down on the trailing arm and remove old spring
Reposition the blocks or or 2nd jack
Remove the metal cup and rubber from the spigot on the trailing arm, clean area as necessary.
If fitting the modified spring supplied by Mercedes Parts the cap and rubber is no longer required
Plug the hole left in the arm using the Bung Part Number A168 997 0686.
Lower 2nd jack slightly to allow you to fit the new spring. Position the new spring, the top of the spring will determine it's rotational alignment.
Apply a small amount of grease to the shank of the shock absorber bolt and ensure the thread is clean.
Slowly jack up the trailing arm until the holes in arm and shock absorber re-align, Fit bolt
Apply a small amount of loc-tite to the thread of the bolt and fit, tighten nut
Lower the trailing arm
Re fit the wheel and wheel bolts
Lower the car
Torque the road wheel bolts and shock absorber nut.(see page 25 for torque settings )
If you have a spring compressor there is no need to remove the the shock absorber bolts. All other items apply.
record repair spring change in you service records.
When fitting the modified spring it will be necessary to remove the metal dome & cap along with the rubber from the swinging arm. However on early 'A' Class models a hole in the casting of the arm central to the base of the spring will be exposed. This hole should be plugged to stop water and contamination getting inside the arm.
On the later models the hole has been re-located in the casting to the rear of the stub axle, see photo. B This plug should be fitted on the early models and replaced if missing or perished on later models. The part number applies to the plug fitted in both instances mentioned. Part no.A168 997 0686 @ approx .50p each
So how can we prevent our springs failing as in the photos above?
I use Duck which comes complete with a mini hand sprayer keep your springs sprayed and like me you will have no problems, it will also protect all of the under parts of your car exposed to rock salt applied to our roads during the winter months.
Shock absorber marked with a part number. E4SR4 Z007A00 The top of the shock absorber is bolted to the wings of the sub-frame and the bottom to the swinging arms Their function is as their name implies to reduce shock through the chassis & body of the vehicle when travelling over rough terrain, thereby reducing erratic spring movement of the vehicle, which in turn stabilizes the movement of the car. Defective units will show themselves in the that they invariably leak the hydraulic fluid contained within them and then allow a lot of bounce on the corner where the defective unit is located. When replacing shock absorbers, unless a new unit fails it is normal to replace both units i.e. fronts or rears. This of course does depend on the age of the units but the front or rear of the car need to remain balanced, shock absorbers do after a period of use become less effective than when new, hence the reason to fit replacements in pairs.
Both top and bottom
H and 5 shock absorbers are secured by bolts, The ones at the bottom are easier to get at than the top but both can be reached with ring spanners. Access to the outer shock and trailing arm bolts Both top and bottom
6
is via the wheel arch by removing the wheel arch liner.
Also Secured by the same fixing is the end of the Rear Torsion bar, one being located either side. This unit was removed from Sergio's car it having done approx 50,000 miles, as can be seen the bar it's not only hollow but has sheared through, judging from the metal the break has been hanging on by a very small amount for some time however unlike the front springs this was heard to finally break by the owner who has obviously replaced it.
His comments like mine are not very complimentary about Mercedes-Benz for fitting a hollow torsion bar, something neither of us have ever heard of before.
A contradiction of terms because these bars are normally solid sprung steel, this component is nothing other than a piece of hollow tube and it's difficult to see where the term torsion is applied to this unit, the continuous twisting or torque of this unit must cause fatigue and after time failure. I can only think that in the case of the 'A' Class this component does little work or we would I'm sure have heard of more defects of this nature and this in my experience is the first to come to my attention.
Never the less when checking your car for M.O.T (Ministry Of Transport Test )or if you have rear suspension problems, including unusual rear tyre wear, Check the rear torsion units (bars).
Having said that Sergio reminds me that things aren't quite as simple as just checking because the plastic under floor pans at the rear of the car have to lowered to see and get at the fixings fully, owners will be aware that that is not easy due to the pans being hinged on the n/side and therefore the vehicle has to be elevated to a reasonable height before the pans will drop low enough to be able to inspect and get at the fixings etc.
Remember always work safe
Remember when checking shock absorbers apart from looking for leaking, the bushes need to be in a sound serviceable condition.
This information received from an owner may help those of you who cannot locate or get a spanner onto the top shock-absorber outer bolt head.
Hi Lofty,
Have owned our A170 for a few years and have used your FAQ a few times - many thanks.
Read you advise on changing rear shockers - the top bolt is completely hidden between sub-frame and chassis.
I've just replaced mine and found the easiest way was to undo the two rear sub-frame bolts and lower the rear sub-frame on a jack until it exposes the top shocker bolts (with the rear of the car on axle stands). Hope this can be of some help
Thanks(Although i managed to photograph the one on my car it isn't easy to get at, the inner wing lining is semi flexible, I was able to locate an open ended spanner on it, but you may need two pairs of hands to undo the bolt and keep the spanner located. (In the case of older cars, where sub frame bolts show signs of rust, lubricate prior to attempting to slacken/remove, take great care not to shear these off)
You may well find that useful but take great care to ensure the car is stable whilst you work, axle stands or blocks are essential, never rely on jack's alone.
Not technically part of the suspension but while we are here we might as well look at the components on the back plate of the hub, on the lower edge of the back plate we can see the sensor for the ESP/ABS systems A which obviously pick up there readings from the stub axle. Also the handbrake cable connection. . B
One word I noticed that the outer casing of the of the wire carrier for the ESP was wearing against the swinging arm and so placed another rubber buffer 1 around the casing effectively moving it away from the rough casting of the swinging arm.
On the top edge, the body of the wheel cylinder can be seen C along with the bleed valve assembly, worth checking to see the bleed valve protective cover is in place. . D
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The rear suspension consists of a sub-frame Part number 16835 05700 1 199 0255 04 Mounted and bolted onto the underside of the body work, Chassis, almost certainly rubber mounting pads are positioned between the sub-frame and the bodywork the rears being mounted in the metal housing of the sub-frame A There are two mountings at the front and two at the rear, although the fronts are not so easy to find. I have outlined the sub-frame where it can be seen, The large oval cross member B forms the mechanical link between the N/S & O/S wings forming a large ridged 'U' shaped frame on which the other suspension components, swinging arms and rear wheel hubs are mounted.
The mountings and securing bolts are displayed in these photographs , the fronts are difficult to see and suspect even more difficult to get at and work on should it become necessary.
3 Fronts 4 Rears
Situated between the static wing of the sub-frame and the movable swinging arm is the coil spring which makes up the rear suspension It should be noted that the spring lays at a slight angle in relation to the rear of the car, as weight is applied so the springs straighten the load being applied directly over the swinging arm.
C Swinging Arm
D Rear Wheel Hub Assembly
E Rear Suspension Coil Spring.
1
The trailing arm securing bolt. 2 One of the front sub frame securing bolt.
The coil spring is held in position by the tension it is under, There are recorded incidents where these springs have broken, which I have been given to under stand is at the bottom of the spring close to the lower location cap. Where a spring has failed, expect to see your car sitting very lop sided and avoid driving any further than is necessary. This defect will dramatically affect the stability of the vehicle, reduce the load on the affected side as much as possible, drive at a safe speed particularly when cornering . Replacing the spring is not a difficult job! When replacing the spring ensure that the cap and rubber cushion is in a sound clean, a smear of grease around the bottom edge of the spring will do no harm avoid getting grease on the rubber cushion.
The caps have been known to rot out being made of quite thin metal so it is worth checking them prior to its M.O.T.1 Shows the fitting at the head of the spring.
2 The fitting at the bottom.
F The thin metal cap.
G The rubber cushion.
The rear spring fitted on earlier A class are known to break on the lower 4" of the spring, this can allow the the cap to ride up inside the spring and fall out. The spring has been modified twice in recent years , the spring shown in this photo is the latest available and does away with both the dome cap and the rubber , sitting straight onto the dome on the suspension arm.
If you are purchasing from other than MBoutlets there is every chance you will be sold the old style spring. To get the most up to date version Go to MB parts they may be more expensive but at least you will have to correct and most up to date spare part for your car MB part No. 168 320 0408. ensure you get the most up to date spring, as old stock may still be held by some branches.
Also note it is a waste of good money to put New springs on the car if the metal around them is left untreated against rust , spray with 'Swarfega' Duck oil designed to protect the underside and vulnerable parts of your car.
Two examples of broken rear springs, Note the cup inside the coil of one of the broken springs. In the other example the cup is missing. Note the exposed hole in the casting central to the base of the spring. A
The only saving grace about this problem is that the springs are very easy to replace and are not that expensive as car spares go. £21.00 plus vat august 2006.
The new Springs like the old are colour coded for the various models of 'A' Class and you will therefore need your Vin/chassis number to obtain the correct springs.
It is advisable to always fit springs in pairs to maintain the overall balance of the car.
Replacing Rear Springs.
This method avoids the need to purchase a spring compressor.
Having taken a safety precautions outlined on mypage 5 'Working around and under your Car'
Slacken the wheel bolts
Jack up the car sufficiently to be able to get at and remove the spring.
Place blocks or another jack under the trailing arm as a precaution against the main jack slipping or in the case of hydraulic jacks dropping while under load.
Slacken and Remove the 18mm Lower shock absorber bolt
Lower the jack/remove 1 block from under the arm
Push down on the trailing arm and remove old spring
Reposition the blocks or or 2nd jack
Remove the metal cup and rubber from the spigot on the trailing arm, clean area as necessary.
If fitting the modified spring supplied by Mercedes Parts the cap and rubber is no longer required
Plug the hole left in the arm using the Bung Part Number A168 997 0686.
Lower 2nd jack slightly to allow you to fit the new spring. Position the new spring, the top of the spring will determine it's rotational alignment.
Apply a small amount of grease to the shank of the shock absorber bolt and ensure the thread is clean.
Slowly jack up the trailing arm until the holes in arm and shock absorber re-align, Fit bolt
Apply a small amount of loc-tite to the thread of the bolt and fit, tighten nut
Lower the trailing arm
Re fit the wheel and wheel bolts
Lower the car
Torque the road wheel bolts and shock absorber nut.(see page 25 for torque settings )
If you have a spring compressor there is no need to remove the the shock absorber bolts. All other items apply.
record repair spring change in you service records.
When fitting the modified spring it will be necessary to remove the metal dome & cap along with the rubber from the swinging arm. However on early 'A' Class models a hole in the casting of the arm central to the base of the spring will be exposed. This hole should be plugged to stop water and contamination getting inside the arm.
On the later models the hole has been re-located in the casting to the rear of the stub axle, see photo. B This plug should be fitted on the early models and replaced if missing or perished on later models. The part number applies to the plug fitted in both instances mentioned. Part no.A168 997 0686 @ approx .50p each
So how can we prevent our springs failing as in the photos above?
I use Duck which comes complete with a mini hand sprayer keep your springs sprayed and like me you will have no problems, it will also protect all of the under parts of your car exposed to rock salt applied to our roads during the winter months.
Shock absorber marked with a part number. E4SR4 Z007A00 The top of the shock absorber is bolted to the wings of the sub-frame and the bottom to the swinging arms Their function is as their name implies to reduce shock through the chassis & body of the vehicle when travelling over rough terrain, thereby reducing erratic spring movement of the vehicle, which in turn stabilizes the movement of the car. Defective units will show themselves in the that they invariably leak the hydraulic fluid contained within them and then allow a lot of bounce on the corner where the defective unit is located. When replacing shock absorbers, unless a new unit fails it is normal to replace both units i.e. fronts or rears. This of course does depend on the age of the units but the front or rear of the car need to remain balanced, shock absorbers do after a period of use become less effective than when new, hence the reason to fit replacements in pairs.
Both top and bottom
H and 5 shock absorbers are secured by bolts, The ones at the bottom are easier to get at than the top but both can be reached with ring spanners. Access to the outer shock and trailing arm bolts Both top and bottom
6
is via the wheel arch by removing the wheel arch liner.
Also Secured by the same fixing is the end of the Rear Torsion bar, one being located either side. This unit was removed from Sergio's car it having done approx 50,000 miles, as can be seen the bar it's not only hollow but has sheared through, judging from the metal the break has been hanging on by a very small amount for some time however unlike the front springs this was heard to finally break by the owner who has obviously replaced it.His comments like mine are not very complimentary about Mercedes-Benz for fitting a hollow torsion bar, something neither of us have ever heard of before.
A contradiction of terms because these bars are normally solid sprung steel, this component is nothing other than a piece of hollow tube and it's difficult to see where the term torsion is applied to this unit, the continuous twisting or torque of this unit must cause fatigue and after time failure. I can only think that in the case of the 'A' Class this component does little work or we would I'm sure have heard of more defects of this nature and this in my experience is the first to come to my attention.
Never the less when checking your car for M.O.T (Ministry Of Transport Test )or if you have rear suspension problems, including unusual rear tyre wear, Check the rear torsion units (bars).
Having said that Sergio reminds me that things aren't quite as simple as just checking because the plastic under floor pans at the rear of the car have to lowered to see and get at the fixings fully, owners will be aware that that is not easy due to the pans being hinged on the n/side and therefore the vehicle has to be elevated to a reasonable height before the pans will drop low enough to be able to inspect and get at the fixings etc.
Remember always work safe
Remember when checking shock absorbers apart from looking for leaking, the bushes need to be in a sound serviceable condition.
This information received from an owner may help those of you who cannot locate or get a spanner onto the top shock-absorber outer bolt head.
Hi Lofty,
Have owned our A170 for a few years and have used your FAQ a few times - many thanks.
Read you advise on changing rear shockers - the top bolt is completely hidden between sub-frame and chassis.
I've just replaced mine and found the easiest way was to undo the two rear sub-frame bolts and lower the rear sub-frame on a jack until it exposes the top shocker bolts (with the rear of the car on axle stands). Hope this can be of some help
Thanks(Although i managed to photograph the one on my car it isn't easy to get at, the inner wing lining is semi flexible, I was able to locate an open ended spanner on it, but you may need two pairs of hands to undo the bolt and keep the spanner located. (In the case of older cars, where sub frame bolts show signs of rust, lubricate prior to attempting to slacken/remove, take great care not to shear these off)
You may well find that useful but take great care to ensure the car is stable whilst you work, axle stands or blocks are essential, never rely on jack's alone.
Not technically part of the suspension but while we are here we might as well look at the components on the back plate of the hub, on the lower edge of the back plate we can see the sensor for the ESP/ABS systems A which obviously pick up there readings from the stub axle. Also the handbrake cable connection. . B
One word I noticed that the outer casing of the of the wire carrier for the ESP was wearing against the swinging arm and so placed another rubber buffer 1 around the casing effectively moving it away from the rough casting of the swinging arm.
On the top edge, the body of the wheel cylinder can be seen C along with the bleed valve assembly, worth checking to see the bleed valve protective cover is in place. . D
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