Rear Beam Geometry

[dch1950 37 1 Cars]

Hi all,

I refurbished my rear beam almost exactly 2 years ago now and feel it's time to get it on the stands and check out the beam seals and generally do maintaining stuff . Also when I set the ride height (315mm pin to pin) it turned out to be a bit too low when I put the wheels back on, so I need to raise it a bit.

Here's a piccy - ( it's too low)

 

 

 

That got me to thinking about the value I should be looking to reset it at. (obviously higher) so greater than 315mm (p2p).

I then started drawing it up (the layout) to see if I could calculate (p2p) for a known ride height.

My first stab is shown here,

 

https://dl-web.dropb....JPG?w=ec5acf7d

 

have a look and tell me what you think - all suggestions/mods gratefully received.

My loaded suspension drops around 25mm so that must allowed for when working out the final for D1 (i.e. the torsion bars are in and active)

regards

Dave

Edited October 17, 2011 by dch1950

[Anthony 1,002]

For standard front springs and standard GTi torsion bars, just set it to ~322mm and it'll be right.

 

That link doesn't work btw.

[dch1950 37 1 Cars]

Cheers Anthony thanx for the setting for mine.

When I did my beam I searched around for ages to see if there was a table of pin to pin VS ride height (nope) so as looking at it again cropped up I thought I'd draw up a schematic of the arm layout and see if I could do the calculation. You would only then need to allow for the drop in height caused by a loaded torsion bar (bar dependant of course and for mine is around 25mm) Mind you had to look up the cosine rule as I couldn't remember it . Funny that dropbox link doesn't work - it's OK when I use it. The link for ATM track pdf works fine - curious.

Thanx again Anthony

Dave

[welshpug 1,646]

that would be useful to have a table to work out heights for a given damper bolt spacing, especially if a rough guide could be given for varying torsion bar stiffnesses as well

 

link is asking for a login btw.

Edited October 18, 2011 by welshpug

[Tom Fenton 1,536]

I would guess it can be calculated, however like many others I have a few setting distances that suit different front springs and so use these, typically 322mm for standard, 312mm for Eibach front springs.

 

As for your link, I would suggest the "https" at the start is the culprit for it not working.

[Cameron 16]

that would be useful to have a table to work out heights for a given damper bolt spacing, especially if a rough guide could be given for varying torsion bar stiffnesses as well

 

link is asking for a login btw.

 

It can be done but shotgun not being the one to do it.

[dch1950 37 1 Cars]

Hi Tom,

yes it could be that - I'm not up to speed with dropbox Mk2 quite yet.

 

Maybe this will do it.

 

Beam layout schematic

 

Of course, when I re do mine I could just write a whole sequence of values and post them on here. But I do like the idea of a calculation , You would only then need to know the axle weight to calculate the the static load drop for any one diameter of torsion bar.

 

Dave

Edited October 18, 2011 by dch1950

[Cameron 16]

It's a very good idea, it just won't be at all simple.

 

Ed and I have done a lot of work into torsion bar spring / wheel rates in another thread, can't remember the name at the mo but a search should bring it up.

[EdCherry 34 1 Cars]

I will get around to making this table. The unemployed need something to do

[dch1950 37 1 Cars]

It's a very good idea, it just won't be at all simple.

 

Ed and I have done a lot of work into torsion bar spring / wheel rates in another thread, can't remember the name at the mo but a search should bring it up.

Hi Cameron - yes recall the thread 'twas a good un. The geometry part should be fairly simple though.. We are not looking at rates, but the displacement under a static fixed load to get the initial equilibrium position of the bar. Admittedley this can only be a best calculation as it clearly depends on the stiffness of the bar itself.

Dave

[Cameron 16]

You'll need to know the spring rate to do that (deflection = force / spring rate) unless I'm misunderstanding you.

[dch1950 37 1 Cars]

You'll need to know the spring rate to do that (deflection = force / spring rate) unless I'm misunderstanding you.

Hi Cameron - in as much as I want to calculate the angular deflection of a torsion bar subjected to a known load - yes.

But I thought generally the term applied to helical coil springs and so the calculation is of course different.

regards

Dave

[Cameron 16]

Yeah the calculation is different, but that's why I mentioned that Ed and I had worked out the spring rates for a few different sizes of torsion bars already. Should save you a little bit of work at least!

 

Of course if you're anything like me (super anal) you'll just do the whole lot yourself anyway.

Edited October 18, 2011 by Cameron

[dch1950 37 1 Cars]

Yeah the calculation is different, but that's why I mentioned that Ed and I had worked out the spring rates for a few different sizes of torsion bars already. Should save you a little bit of work at least!

 

Of course if you're anything like me (super anal) you'll just do the whole lot yourself anyway.

 

Yes you're probably right - I do like detail. Where are your figures then - I had a quick flit through your "torsion bar rates" thread was it the first table quite early on?

What is interesting is the torsion bar position relative to the radius arm shaft. One being in front and the other behind this main axis. We then have a flexing affect to take account of as well. I tried to explain this to Tom (Fenton) and this is the 97%/3% split he refers to.

I shall recheck my measurements and have another go. Any idea of what the unsprung weight(s) would be per corner.

regards

Dave

Empiricism is good, but a nice equation is so much better.

 

D

Edited October 18, 2011 by dch1950

[Cameron 16]

Yes the calculations that we did (or at least the ones I did) ignored the bending of the shaft as it's only a very small percentage of the actual spring rate. It would be very interesting to see a spreadsheet or similar that could calculate the ride height from the TB diameter and the damper eye-eye distance (with the wheels off the ground), once you have the corner weights it's just a case of a bit (read: quite a lot) of trig.

 

I'm not sure what the unsprung weight would be, I'd guess about 30kg per corner at the rear. If you're after actual corner weights there's a few threads that have listed them over the years, although they can vary a fair bit from car to car.

 

Edit: Just had a skim back through, looks like I didn't put the rates on that thread! Will dig them out tomorrow if I get a chance.

Edited October 18, 2011 by Cameron

[dch1950 37 1 Cars]

Hi all,

Just dug my spare arms out of the shed to have a quick measure in order to get the lever part of the rear suspension right.

I'm still not convinced that the flexing of the TB will be insignificant though.See attached sketch.

any views ?

Dave

 

Cameron - I'm using a shear modulus value of 75 GPa. for the bars. (reasonable ?)

D

[Cameron 16]

Yep sounds reasonable, I think I used 79GPa for my calculations.. will have to check later.

[dch1950 37 1 Cars]

Hi,

I've finalised the static bar deflection calculations for the same range(s) as in Cameron's original rates table.(19-30mm). I just need to refine the relative positional geometry of the arm and shock pins and that should be it. The only unknown in the equation is the old TB settle figure, i.e. the drop due to twist distortion of an old bar. This was about 25mm on my beam refurb and refit in 2009.

regards

Dave