Shocking madness? {Looking for shock absorber data} - PeachParts Mercedes-Benz Forum

Stretch · #1 09-18-2013, 02:56 PM

G'day Folks,

My W201 looks like it still has its original shocks - they are pretty expensive to replace.

Cheapest I've seen for Sachs touring is just over 70 euros each - which isn't a bad price - but that's probably more than the cost of some new shock oil and a recharge of nitrogen.

I've been influenced by this

Refilling shock absorber?

Problem is I don't have the specifications.

Does anyone know if they are published somewhere?

So far the interweb has produced these for me

http://www.bilsteinus.com/fileadmin/user_upload/user_upload_us/pdfs/BlackHawk9300SeriesTechnicalManual-March06.pdf

http://www.bilsteinus.com/fileadmin/user_upload/user_upload_us/pdfs/motorsport/en/valvingmanual_1to10.pdf

http://www.zf.com/media/media/img_1/brands_1/sachs/technical_and_service_sachs/pdfs_2/pdf_technical_brochure_suspension_EN_SACHS.pdf

Not quite what I want but a punt in the right direction.

Zulfiqar · #2 09-18-2013, 03:49 PM

In another life I lived in countries where this is actually done and their results were great. The major hiccup for those shops were cheap shocks which have badly made parts in them. They absolutely loved Bilstein and Boge shocks because their internals were very nicely made.

Mölyapina · #3 09-18-2013, 06:33 PM

Quote:

Originally Posted by Zulfiqar

In another life I lived in countries where this is actually done and their results were great. The major hiccup for those shops were cheap shocks which have badly made parts in them. They absolutely loved Bilstein and Boge shocks because their internals were very nicely made.

What countries were those? That might help...

Stretch · #4 09-19-2013, 01:53 AM

Repairing struts and shocks seems to be a normal thing in the motor cycle world - especially the off road stuff.

It doesn't look too too difficult to do - so long as the units haven't been designed as sealed units (I haven't looked yet so I don't know).

What I'd like to find is the official gas pressure required and the amount of oil you are meant to put in there. With that information I have a fighting chance. Without that it could be disastrous...

Stretch · #5 09-30-2013, 12:34 AM

How about a Monday bump for this one?

Mölyapina · #6 09-30-2013, 08:01 AM

Hmm. I think I'll call MBUSA today and see if I get anywhere.

Mölyapina · #7 01-30-2014, 08:42 PM

Yo Stretch-O! This came to mind recently and it occurred to me that -- hey! -- if we don't have specs, let's just get our own! Let me be more specific.

Pressure = Force/Area (force of piston, area of said piston in this case).

Therefore, if we can measure the force needed to move the piston from its resting point on a new shock, we should be able to derive the pressure quite easily! What do you think?

I've been trying to figure out a way to measure the force applied and have so far come up with two different ways, but I feel that there must be a better one.

1) Get a sturdy spring with a known spring constant, place it on top of the shock, and measure the amount that the spring compresses before the shock begins to move.

2) Use a weird torque wrench setup.

Both situations drawn out on a brown paper bag (figures I would misspell something that simple once someone else is actually going to see it

):

Mölyapina · #8 01-30-2014, 11:08 PM

Now I'm wondering if a bathroom or kitchen scale could somehow be used to measure force applied to a shock. Unfortunately, I am too tired to think clearly about it and am going to bed, so I leave you with that

.

Stretch · #9 01-31-2014, 03:50 AM

I agree the bathroom scales approach - a direct measurement of force would work but only on new shocks. (meaning I don't really want to calculate the pressure charge in 20 year old shocks!) I'd also have to have a nice measurement of the piston area and an idea of the design requirements of the internal friction through the restrictions of the piston - this isn't a hydraulic ram, it is a hydraulic ram with holes in the piston!

Direct measurement of the damping constant is also possible but them you need to measure the return travel as a function of time. In my university days it would be a nice LVDT and a data acquisition system to the rescue and an answer before lunch. These days I'm stuck with my bathroom scales and the random junk in my shed...

Zulfiqar · #10 01-31-2014, 10:51 AM

The W124 FSM specifies a bathroom scale to measure the working life of the shock absorbers. It says to press the piston rod so that 100 mm is exposed while the entire unit is on a bathroom scale.

a new strut/shock should measure 25 kg, limit for it is 15 kg.

before doing this - they say to work the strut till the hissing sound is not heard anymore.

Zulfiqar · #11 01-31-2014, 10:52 AM

Quote:

Originally Posted by Jooseppi Luna

What countries were those? That might help...

India, Pakistan, Iran, Turkey.

Mölyapina · #12 01-31-2014, 10:36 PM

Quote:

Originally Posted by Stretch

I agree the bathroom scales approach - a direct measurement of force would work but only on new shocks. (meaning I don't really want to calculate the pressure charge in 20 year old shocks!)

Well, this is certainly true. I was thinking I might try the technique with the new rear shocks I have for my 'SD.

Quote:

Originally Posted by Stretch

I'd also have to have a nice measurement of the piston area and an idea of the design requirements of the internal friction through the restrictions of the piston - this isn't a hydraulic ram, it is a hydraulic ram with holes in the piston!

I could blow apart one of the worn shocks for the 'SD to get the piston area measurement. To get the force imposed by friction, we could attempt to measure the force required to move the piston when there is no pressure in there.

Quote:

Originally Posted by Stretch

Direct measurement of the damping constant is also possible but them you need to measure the return travel as a function of time. In my university days it would be a nice LVDT and a data acquisition system to the rescue and an answer before lunch. These days I'm stuck with my bathroom scales and the random junk in my shed...

Hm. Interesting idea, but it sounds harder... then again, maybe I'm not getting something.

Mölyapina · #13 01-31-2014, 11:24 PM

OK, here is my fine-tuned idea on how to measure the pressure in the shock. I wanted to compress it horizontally so as to mostly eliminate the pull of gravity.

This is what I ended up with -- two lengths of wood somehow attached to each other and turning on an axis. The lengths of wood are centered on the axis so that each side is equally weighted and not causing the contraption to move from a standstill. A scale is then set up as shown and when a shock is pressed into an arm of the contraption, the contraption transmits that force right to the scale so that it can be measured.

That was a really bad description, but hopefully you get the idea.

Mölyapina · #14 01-31-2014, 11:33 PM

Picture added to above post.

Stretch · #15 02-01-2014, 08:22 AM

I wouldn't go that far, a direct push onto the bathroom scales would be fine. They are "calibrated" (I use the term loosely!) for that orientation.

Have a look at chapter 32-120 for how the FSM says to check 'em I think that'll give you some idea of what's involved.

If you get to the stage of taking some shock absorbers to bits (before me) make sure you don't injure yourself - they are pressurised and could potentially put a hole in you.

If you can measure the volume of oil in a shock that would be great information to have - but as measuring cups are often a bit out of whack an additional measurement of weight could prove to be useful.