Suspension springs interchangeability - Page 2 - PeachParts Mercedes-Benz Forum

Brian Carlton · #16 08-16-2009, 10:27 AM

Quote:

Originally Posted by Brandon_SLC

I've wondered the same thing about spring rates. Many manufacturers vary spring rates depending on options. But I see springs for sale that say they fit many years and models of the 123. But there's a big difference in weight between a low optioned '78 240D and an '85 300DT. So, I've often wondered if those "1 size fits all" springs are going to give anywhere near factory original ride quality.

Like most on this thread, the statements are sheer conjecture and have no data associated with the spring rates and the lengths of the various candidates.

Therefore, unless someone has already done the exact swap, you're basically on your own with the result.

If you wish to do a proper conversion, you need the spring rate and length of the existing spring and compare it to the spring rate and length of the replacement spring.

As a general statement, there are usually two springs for each of the vehicles............one with a stiffer spring rate and one with a softer spring rate. These two springs are then matched to the vehicle with the use of the rubber spacers.

If the change to the vehicle is not radical, the rubber spacers should give you all that you require, unless you're already at the thinnest spacer.

I recently found that the SD had the four nub, 24mm spacers on the front end..........not to specification. I replaced them with the 8mm single nub spacers. The vehicle is still under repair, but I expect a lowering of the front end by 1.25" from this change. Earlier SD's always tend to look too high in the nose and the cause might be the use of erroneous front pads.

juanesoto · #17 08-17-2009, 10:26 AM

Thanks for the input Brian and Brandon.

As Brian said, we are just guessing here. We would need all the technical specs (length, diameter, number of coils, wire diameter, coil steps, etc.) of the different spring types of all the W123 ad W126 chassis in order to make a proper decision. Any idea of where to get this information??

If we could find this info and compare it to the tech specs of the Vogtland springs, we might find a winner!

Anyone knows the specs of the Vogtlands?

juanesoto · #18 08-17-2009, 12:15 PM

I found the W123 spring specs in my service manual. I transferred that info to excel and made some calculations to find out the spring constant and the required energy needed to compress a spring to a given length (CE@XXXmm given in Newtons).

You can see the results in the attached documents. You'll notice that even though some springs have a lower constant than others, the energy required to compress them to a given length is higher due to their larger length. This means that stiffer doesn't means taller in terms of vehicle ride height. Ride height depends on both factors...

Can someone send me the specs of the W126 springs maybe in .pdf? With this info, I can make the same calculations. In the W123 manual is the 32-250 section, just for reference.

I feel we are coming closer to something...

juanesoto · #19 08-17-2009, 12:32 PM

I forgot to mention another important thing!! If you know which springs the car originally had and the original ride height of the car, it would be really easy to make some estimations around this.

With the part number of the original springs, its possible to figure out the amount of energy contained in the springs at the original ride height. Just jack the car to have the original height (for the ones sitting low), measure the distance between the spring seats and make the calculations to get the energy.

Knowing this, it will be possible to know which of the stiffer (and usually shorter) springs can be fitted in there while knowing the possible effect it will have in ride height.

Please correct me if I'm making a mistake in my considerations...

ColLonewolf · #20 08-17-2009, 02:21 PM

Quote:

Originally Posted by juanesoto

Can someone send me the specs of the W126 springs maybe in .pdf? With this info, I can make the same calculations. In the W123 manual is the 32-25 section, just for reference.

I feel we are coming closer to something...

I have been looking for some intel on the W126 springs as well. If they are availale they would come in very handy. Thanks.

Brian Carlton · #21 08-17-2009, 02:44 PM

The early W126 used one of two springs in the front:

126 321 17 04 with a spring rate of 17.4mm/1000N. Length of 475 mm

126 321 18 04 with a spring rate of 16.3 mm/1000N. Length of 478 mm.

The stiffer spring was used on the diesels, the gassers had the softer spring.
All other height adjustments were done with the spring pads.

juanesoto · #22 08-17-2009, 04:09 PM

Thanks for the info Brian. With that, the number are:

126 321 17 04 k:57.5 CE@300mm 10057
126 321 18 04 k:61.3 CE@300mm 10920

Those figures are higher to the ones of a W123 mainly because of the spring length.

After doing some research, our main concern here is the k value. The higher the value, the stiffer the spring.

Now, so far the stiffest W123 spring is 123 321 45 04 with a k: 58.5 . Considering this, the W126 spring used in diesel models is about 5% stiffer in terms of k. However, if the W126 spring was cut to the same lenght of the W123 springs, you would need 9202 Newtons to compress it to 300 mm. That's 431 Newstons more than the W123 springs in discussion...

I think we need more information on the W126 springs... The .pdf pages from the manual under "Test Values of Springs"

juanesoto · #23 08-17-2009, 07:26 PM

BTW, does anyone knows the proper ride height for the W123 measured from the rim center up to the fender lip?? Both front and rear...

Also, can someone provide me the camber, caster and toe in specs for the W123 both front and rear?? I couldn't find them in my service manual. I only found the rear wheel position checking procedure, which states 1* negative camber and 3* toe which I know is wrong...

I'm making some calculations regarding spring rates, ride height and rear wheel alignment.

winmutt · #24 08-17-2009, 09:41 PM

A few measurements for you:

juanesoto · #25 08-17-2009, 10:43 PM

Thanks Winmutt!! With this info I now have a starting point for my research. I can compare the current height of my car to the factory specs. Also I can take the car to the alignment shop to find out how low the car can sit without compromising rear camber and toe in. I beleive something in the order of -0.5* of rear camber will be nice for corner handling without affecting the toe in that much. Anyone with experience on this?? I have used -2* in a N-class rally car in the past and even with body roll, the car sticked to the road in corners...

But that would be way to much for a daily driver... Also in that car, toe in could be adjusted separately...

I think we are on track here...

We just need some more specs on W126 springs.

winmutt · #26 08-18-2009, 04:39 AM

I am around -1.5 in the rear

juanesoto · #27 08-18-2009, 10:02 AM

Quote:

Originally Posted by winmutt

I am around -1.5 in the rear

Did you noticed any effects in the toe in? How is your tire wear?

BTW, yesterday I measured my car with the blueprint you provided me and the front is sitting at 830 mm and the rear is seating at 750 mm, both measured at the 760 mm points. I really need to lower the front...

juanesoto · #28 08-19-2009, 12:24 AM

I think I found a valid way of comparing springs of different characteristics and determining the way the will behave at a given length. Using simple factors as unsprung length and travel per load, you can find out if by cutting a spring you'll get a given behavior. Of course this only applies when the springs to be modified or fitted are longer than the ones they will replace.

In an old thread I found the specs of the Vogtland springs from the group buy. With this info I made a two charts comparing their characteristics and the OEM springs behavior if cutted to the Vogtland's unsprung length.

If you take a look a the chart, you'll find the usual characteristics for automotive springs. You'll notice I included something I called a deflection factor. This is a number that represents the current travel of the spring at a given length, divided by the sprung length on the spring. This reflects the amount of deflection provided by every mm of the spring length at a given load. Once you find this factor, you multiply it by the desired length (in this case the Vogtland's length) to find out the deflection the studied spring will provide under the given load if cutted to a certain length.

Some might think that the behavior of a spring is determined by the amount of coils, coil caliber, mean diameter, etc., and that's totally right. You need those numbers when you want to determine the travel per load before you build the spring. However, in this case we already know the mechanical characteristics of the spring. We just need to compare them in equal conditions (length).

I really would like to hear you feedback on this matter...

juanesoto · #29 08-19-2009, 01:05 AM

BTW, if my guesstimation is correct then anyone could get the proper springs according to the part number highlighted in the chart, cut them of the exact same length of the Vogtlands and get very similar performance...

Also, I have to mention I found the W126 springs specs in a FSM I downloaded. I'll include them in the chart for comparison purposes.

juanesoto · #30 08-19-2009, 01:38 PM

Hello everyone! I have some good news regarding spring interchangeability. Thanks to the information gathered in this thread and some in depth research I did, I'm happy to announce we now have two charts showing the specs of both W123 and W126 factory suspension springs. The chart compares both models with the Vogtland springs from the group buy as the control parameter.

Here is the explanation of the comparison method and its principle posted in the original thread:

Quote:

Originally Posted by juanesoto

If you take a look a the chart, you'll find the usual characteristics for automotive springs. You'll notice I included something I called a deflection factor. This is a number that represents the current travel of the spring at a given load, divided by the sprung length on the spring. This reflects the amount of deflection provided by every mm of the spring length at a given load. Once you find this factor, you multiply it by the desired length (in this case the Vogtland's length) to find out the deflection the studied spring will provide under the given load if cutted to a certain length.

Some might think that the behavior of a spring is determined by the amount of coils, coil caliber, mean diameter, etc., and that's totally right. You need those numbers when you want to determine the travel per load before you build the spring. However, in this case we already know the mechanical characteristics of the spring. We just need to compare them in equal conditions (length).

In the charts you'll see a highlighted line which represents the W123 spring that has the closest characteristics to the Vogtland springs if cut to the Vogtlands length. Also you'll see some lines in red font, which are the OEM springs all the W126 models had for US market. In the case of the rears, the red fonts on top of the highlighted line are the non-self leveling equipped chassis springs and the ones below the red fonts are the self leveling equipped chassis springs.

I'm attaching the W126 springs application guide for further reference.

As a final word, W123 springs are not an option, since the stiffer ones are fitted in the police version, the heavy duty suspension for poor roads version, the T-wagon with higher load capacity and the limo version...

Please provide me you feedback on this matter!! I really would like to hear your opinion about this!