someone school me this physics please, 1995 E300D coil springs

[Zulfiqar]

Its spring/shock related.

I have been looking up parts info and confirmed that dave aka gsxr page for the springs of W124 is spot on. In that case my car has original springs and uses the shortest front springs of the W124 diesel lineup, even the 2.5 turbo ones are one coil longer with the same gauge wire.

So - are shorter springs stiffer or softer (bearing in mind the OM606 has one more hole than the OM602) and what sort of strut would work. Do I need a stiff jounce/rebound? or soft.

At the moment I have bilstein comfort struts on the car, they dont like to handle small potholes at slow speed.

[sixto]

What do you mean they dont like to handle small potholes at slow speed?

If both are linear springs and have the same spring constant, the springs that's more compressed from its natural length will provide more resistance so it will be stiffer. I can't speculate how much a 2.5 compresses its longer spring. compared your E300 and its shorter springs. We'd have to know the load on the spring and how much it's compressed, or at least how much load is on the springs.

I'm sorry I tried

Sixto
87 300D

[Zulfiqar]

cant handle pot holes at low speed means...

as you are driving along and you get to part of a road where the slabs are not level, the next one sits lower than the first, The car drops hard instead of the wheels just moving, If I approach this the opposite way - as in the road is higher on the next slab, its not a problem.

Is this the same problem Im discussing or is it something else. I have tested the struts as the WIS tells, and they are perfect.

The reason I posted about springs is that I beleive the car sits lower than normal in driving condition, I looked hard for any information and found that the springs are original to my car and are the shortest, compared to your car which is an 87 300D, the 95 300D is short two coils.

[Brian Carlton]

Originally Posted by Zulfiqar

as you are driving along and you get to part of a road where the slabs are not level, the next one sits lower than the first, The car drops hard instead of the wheels just moving, If I approach this the opposite way - as in the road is higher on the next slab, its not a problem.

What you have described is a situation where the rebound damping is too stiff. The entire front end of the vehicle drops faster than the strut extends.

This definitely points to a problem with the strut, despite your assurance that it is to spec.

Or, this phenomenon is typical of the vehicle by design. You'd need to try another 124 on the same road for a direct comparison.

[Zulfiqar]

Im sure thw w124 is not supposed to drive like this, even a civic can handle that sort of road.

So you believe its a strut issue... and not spring related. btw the struts are bilstein comforts.. I think I need to contact a good benz indy mechanic to compare

[Brian Carlton]

Originally Posted by Zulfiqar

Im sure thw w124 is not supposed to drive like this, even a civic can handle that sort of road.

So you believe its a strut issue... and not spring related. btw the struts are bilstein comforts.. I think I need to contact a good benz indy mechanic to compare

I'm quite sure it is not spring related. Whether you feel the road or not is always related to shock damping.............unless you drive an 18 wheeler.

[sixto]

Brian, does the damper lead or lag the spring in that situation? I get the sense the spring will beat the damper during a drop and the damper's job is in fact to slow down the spring. Is it possible that Z's front springs are tired and thus lazy to extend as quickly as necessary?

The Catch 22 is HDs are stiffer in both directions. While an HD should extend more quickly to catch a fall, it might not compress quickly enough to buffer a rise.

Sixto
87 300D

[t walgamuth]

Stiffer shocks should give more control both on compression and rebound.

[Brian Carlton]

Originally Posted by sixto

Brian, does the damper lead or lag the spring in that situation? I get the sense the spring will beat the damper during a drop and the damper's job is in fact to slow down the spring. Is it possible that Z's front springs are tired and thus lazy to extend as quickly as necessary?

The Catch 22 is HDs are stiffer in both directions. While an HD should extend more quickly to catch a fall, it might not compress quickly enough to buffer a rise.

Sixto
87 300D

The damper always lags the spring. Without any damper and without any unsprung weight, the wheel will instantly rebound to the lower pavement height (and unfortunately continue to bounce, but that's another discussion).

Since the wheel has weight and the strut slows motion of the wheel, it cannot fall to the new pavement height quickly enough and the vehicle falls slightly.

The question is always "how much is slightly"?

If the damper is really stiff (HD shocks for example), the wheel takes its time to fall and the vehicle must do most of the drop.

If the damper is soft, the wheel moves relatively fast and the vehicle hardly drops at all.

If the damper is very soft, the vehicle will bounce several times on the spring after the event.

[Zulfiqar]

After reading this I believe the WIS is a bit wrong on checking the struts - here is how it goes.

remove strut - check

work strut up and down till piston sound diminishes - check

hold strut 85 or 100mm (need to read again) compressed on bathroom scales - check

read scales after 1 minute of holding compressed to read 25 kg or minimum of 14 kg - check (at 25 kg).

It does not speak of checking the damping - this apparently is checking the oil reserve.. And with experience with other cars - these struts appear soft to me, I can recall trying to work a VW golf strut and that was quite stiffer to move that this one. Same on an old corolla strut too - really hard to move either way.

for comparison, I am a bit heavyweight, to compress the VW golf struts I had to bend my knees and use my body weight to get it down smoothly, with the MB strut I can do it with my arms strength. I have also pushed down on new W126 front shocks and they were a bear to move too. It hit me quite odd that the W124 struts are so soft - so I cross checked with the WIS. then I got stumped and made this tread.

the 85 or 100mm compression is normal driving position. So me thinks that the struts are jamming on the upstroke (wheel moving down) at that level - because they can compress themselves (wheel moving up) easily.

[Codifex Maximus]

The physics of springs are very involved. You have lots of things affecting their efficiency. You have metal composition, type of coil, length of spring material, temper etc...

I would say that a spring that has been shortened from it's original length would be stiffer. If it was cut with a torch, the spring is likely ruined in my opinion as it's temper has been changed.

The spring and the shock work together. They should be matched for the car. Modify them at your own risk.

[Zulfiqar]

No they have not been cut, they are original springs. The original springs on a W124 E300D are the shortest of all the diesel line up of the W124.

[moon161]

The rate (stiffness) of a helical spring (in axial deformation) is pretty straightforward. To determine it, you need to know:

Material (shear modulus- how stiff is the material in torsion)
Helix diameter
Wire diameter / cross section
Pitch of coils (limits travel)
# of active coils
Active length (active coils * pitch)

Here's an online calculator
Calculator for Designing Compression Springs

Simplified:
Bigger helix->softer
Bigger wire diameter-> stiffer
More active coils -> softer

All other things held equal, cutting a coil out of a spring makes it stiffer.

This issue was beaten to death on this forum, by myself, twalgamuth and leathermang, and britishbulldog, I think. More details lie in engineering design and/or strength of materials guides and texts.

To preserve this and future lunchtimes, I shall say no more.