Anyone with a W123 LCA and/or trailing arm handy? - PeachParts Mercedes-Benz Forum

Secondaries · #1 08-29-2013, 11:53 PM

If so I'd like a few measurements if you would be so kind!

Lower control arm:
- length from center of inner bushing to center of ball joint
- length from center of ball joint to center of spring perch

Rear trailing arm:
- as many dimensions from the following image as possible!

For those with knowledge of the subject, I am attempting to calculate motion ratios for the front and rear suspension in order to figure out ideal spring rates/frequencies for racing.

Related thread of mine located here in Performance Paddock.

Stretch · #2 08-30-2013, 02:56 AM

I can do this this weekend for you - do you need the dimensions of the subframe too?

Stretch · #3 08-30-2013, 02:58 AM

Nice trailing arms (in the picture) by the way what are they from?

{Oh and bear in mind the brake stay and LCA on the front of the W123 can be moved by about 5cm to may be 7 cm within the wheel arch of a W123 <= guess based on my experience of doing my own alignments}

{Oh and this thread might be of interest to you Trying to fit an aluminium W126 / 107 trailing arm to a W123 - a question of compatib even though the aluminium arms are heavier than the steel}

Secondaries · #4 08-30-2013, 08:53 AM

Trailing arms pictured ate from a BMW E30. I used the picture because it showed all the measurements I need to calculate motion ratio - a bit harder than the front!

I would be very grateful for measurements, Stretch! Also, could you elaborate on the movement of front LCA and brake stays?

Stretch · #5 08-30-2013, 09:07 AM

I'll crack on with it all tomorrow - what about the subframe?

As for the front suspension it isn't "movement" of the LCA and brake stay that I'm talking about but the adjustment.

The W123 front suspension is a bit of a weird beast. There's an eccentric bushing on the inboard side of the LCA that adjusts the camber - well 90% of the time it adjusts castor but because of the geometry (weird triangle) it also adjusts the castor a bit. Likewise if you change the length of the brake stay you mostly get a change in castor but the camber is also changed (weird triangle geometry again you see?).

If you do your own alignments you will get the impression that as you adjust one thing - say castor - you are effectively moving the point of contact of the wheel around in a circle (positioned on the floor in the lateral : longitudinal plane) as you then need to go back and adjust the castor once you've set the camber - and then bloody hell! The camber gets changed again!

If you have the time I recommend a bit of a play about with your current set up and see what you make of it. It would be very educational for your future set ups. I can send a link on how to do it if you like.

Secondaries · #6 08-30-2013, 10:57 AM

Re: subframe measurements, I'm not sure which dimensions would benefit me in my current project but they may prove useful. Which measurements in particular are you referring to?

Re: alignment, I intended on taking the car to a reputable alignment shop that doesn't mind aligning to the specs I give them (versus what their computer says is correct). However, that may be easier said than done and I might just try my hand at a DIY alignment. I think have seen (or can find) the thread in question but please, if you have it offhand then post it here for reference.

Stretch · #7 08-30-2013, 11:22 AM

Quote:

Originally Posted by Secondaries

Re: subframe measurements, I'm not sure which dimensions would benefit me in my current project but they may prove useful. Which measurements in particular are you referring to?

Re: alignment, I intended on taking the car to a reputable alignment shop that doesn't mind aligning to the specs I give them (versus what their computer says is correct). However, that may be easier said than done and I might just try my hand at a DIY alignment. I think have seen (or can find) the thread in question but please, if you have it offhand then post it here for reference.

Well here's the DIY thread

How I adjusted the toe in / out, camber and caster on my W123 300D

As for the subframe - well as the camber gets changed when the rubber components on it go south; then you might be able to do something snazzy with the ride height that way too - though be warned the idea with this system is that the single UJ in the propshaft stays straight so you don't get vibrations...

DeliveryValve · #8 08-30-2013, 02:19 PM

I can also measure the pieces if stretch somehow can not get to it.

.

Stretch · #9 08-31-2013, 11:21 AM

Quote:

Originally Posted by DeliveryValve

I can also measure the pieces if stretch somehow can not get to it.

.

Yep my garage is a mess - now it is also full of W201 bits as well as the W123 ones. It just so happens, however, that I've recently moved the W123 trailing arms, wheels and subframe to the little shed so there is only recycling and old TVs in the way (yes once the car is fixed I'm off to the tip).

I got the bits out alright...

...but I'm not sure what would be a useful measurement.

[Then I remembered]

Chapter 35-410 and 35-420

This is what I'm up against =>

^^^That's the whole subframe and trailing arms mounted on a special surface table with lots of gucci jigs to make sure everything is aligned...

^^^ and that's just the trailing arm with the gucci jigs and the surface table.

Note how the attachment eyes are at a different height from the hub centre.

I reckon I can attempt to replicate this rig but seriously if I can manage to get measurements within half a centimeter then I'd be bloody impressed. I have a wooden table and a sheet of 0.8mm steel and some blocks of wood - I mean heaven help us!

Alternatively I might be able to get some where with digital photographs and an autocad app I have heard about but again with optical distortion and so so... we're probably back to a guess-timate rather than a measurement.

Secondaries · #10 08-31-2013, 04:59 PM

Wow, Stretch, going above and beyond as always! I really just need good approximations of those lengths and angles involved so I can have some idea of motion ratio. But if you intend to get as precise as possible for the sake of the rest of the community then more power to you!

Stretch · #11 09-01-2013, 01:31 AM

Well I'm a bit busy with my own stuff at the moment so I'll give this autodesk app a go first and see if the approximate picture based measurements are good enough for you.

Stretch · #12 09-02-2013, 06:16 AM

...sorry to report that I'm not cut out for modern life. I've downloaded an app called "Force effect" which is meant to be able to lots of wonderful things.

I think it does but with pig's tits for fingers, and all that Star Trek the Next Generation tapping and whafting of fingers and arms I can't get it to work as I'd like it to.

Moving lines and points on an image is difficult with this app and a small screen.

So I then tried LibreCAD - freeware 2D drawing package and I got a bit further. But then I thought - hang about you can do this too - and you know what you want!

So here are some pictures for you =>

If you bang this dimension into LibreCAD or ForceEffect or something like that you should get a rough estimation of the other dimensions =>

41.5 cm is the magic number - gap between the arms as shown above.

Stretch · #13 09-02-2013, 06:18 AM

I plan to persist with these apps and the drawing package but I've got other stuff on at the moment so it might be a while => so give it a go yourself if you don't want to wait.

Secondaries · #14 09-04-2013, 12:04 PM

OK well the plan is to use the second schematic drawing to mark out and measure angles and assign arbitrary but scale lengths as accurately as I can with a protractor and straight edge. This should get me an accurate enough answer to proceed with choosing spring rates for a given wheel frequency. Do you happen to have the same type of drawings for the front suspension?

Stretch · #15 09-04-2013, 03:09 PM

The best I can get is this one.

Probably not much use - there's that tricky twist in it

You see these measurements are pretty 3D based.

Do you need more pictures?