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PostPosted: Thu Jun 09, 2016 3:34 pm 
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This is worth giving a bump, I have no contribution to make right now but do have a new V8 Z31 circuit car project and am thinking about keeping the standard type suspension. Rather than change over to S chassis suspension. Still doing more research on the topic which is how this thread came up. Z31 is a series 2 with five stud wheels.

Any progress anyone?

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PostPosted: Thu Jun 09, 2016 3:54 pm 
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Check out my build...
I've changed the sweep angle and raised the inner pivot to keep roll centres in check.
Gone from 25deg to 17deg.
Haven't had a chance to hit the track at the moment, but going around the street it feels the same :lol:

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PostPosted: Fri Jun 10, 2016 9:16 am 
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Thanks for the two graphs based on the now defunct BMW info link you did, very useful.

Once I find out the sweep angle of the Z31 suspension and have a good look at it then a decision can be made as to retaining it or not. Am fairly sure the angle is relatively low but in any case I'd want to change it to what the race BMW's used, mine will be race only so spherical bearings for all pivots are likely as well as solidly mounting the subframe. Ideally it all would need to be setup so the wheels go into toe in under heavy acceleration and braking.

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PostPosted: Fri Jun 10, 2016 9:47 am 
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Sounds interesting. I'd be interested to see what you end up doing.
Let me know if you want any advice on calculation of roll centres etc.

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PostPosted: Fri Jun 10, 2016 2:35 pm 
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Thanks for the calculations offer, my maths is rubbish :)

My idea on the rear roll centre is to move it as high as possible with provision for adjustment down. Practically, it looks like the centre can't be raised that much with semi trailing arms so maybe the highest would work out best anyway. As for adjustment hardware, I'll be looking at what's available for BMW race cars, no point in reinventing the wheel. Alternatively, there are lots of hardware options available through race chassis shops, all sorts of parts located by various means. I don't have access to machine tools so for me it's going to have to be a matter of search, imagination and adapt. If that makes sense.

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PostPosted: Sun Jun 12, 2016 2:03 pm 
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Further thoughts are to fabricate a new subframe starting with a length of straight RHS. This would suit my equipment and skills, the pivot brackets and diff mounting platform would be straight forward fabrications welded to the RHS after calculating the desired sweep angle which would be different to the stock Z31's. Adjustment would be provided by rod ends attached to the arms and forming the pivot points.

Before any of this is done though I'd like to run some figures through a preferably free version of something like Susprog (sp?). One thing that has me intrigued is what the effect would be of using angled pivot points ie looking from the back the four pivot points would be in the form of a shallow V, two each side. While reducing the sweep will reduce toe change on deflection it's still a factor but I guess if there was anything to be gained from the V treatment then it would have been used a long time ago. Still........

Lots of reading here http://www.the510realm.com/viewtopic.php?f=29&t=18585

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PostPosted: Mon Jun 13, 2016 12:21 pm 
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Well bugger me, trust the Pommies to come up with something out of the box :shock: A bit of motorsport history first. Back in the Group A days the Sierra RS500 went pretty well, semi trailing arm rear suspension and all. Their trick there was to be able to within the rules change the sweep angle from 16 to 9 degrees, yes, down to 9 bloody degrees. But there's more, the aftermarket now offers what they call a modified beam (subframe) with 6 degrees sweep, do a google for 'ahmed bajoo bar' if you want more info. He is one of several who offer 6 degree sweep subframes for the Sierra and derivatives.

And according to the Brit Sierra racers these subframes are the duck's nuts and are a essential basic necessity for a proper rear suspension setup. So, do I go with this info for the Z31 rather than with other options like using an entire S chassis rear transplant????

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PostPosted: Wed Jun 15, 2016 9:49 am 
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That is very interesting. Looks like an interesting bit of kit.
It's the Ahmed Bayjoo Bar (It's the F&r(ken Catalina wine mixer!)

The change that I made was specifically based on what I could fit in without going too crazy.
If I ended up going to less of an angle, the rear guards would need to be tubbed as the camber change really helps the 'tuck'
There's a compromise between how much body roll you have, suspension travel and your required roll centre height. The 6 degrees would sit the roll centre lower to the ground. Basically a trailing arm and the roll centre is on the ground.
You need to match the front to the back too. Change the rear, it's probably a good idea to modify the front to suit (whether that is stronger springs, or modification to the roll centre)

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PostPosted: Wed Jun 15, 2016 11:55 am 
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The 6 degree info will probably be what persuades me to stay with the semi trailing arm suspension. I love trying something new with a bit of a challenge proving the odds are with me. We should keep in mind that BMW were very successful on the track with their 3 series, I can remember driving a 325 along mountain roads and how responsive it was in the rear end, power down rear steer which felt great.

Yes, it's definately racecar with spherical bearings but it's pioneering work like yours which gets some of us thinking about the next step. Most mods are unoriginal and copies of what has been done before a million times, all this being built on what someone pioneered often years ago. Sure it's one way to get known performance but that's more like picking a new car off the showroom floor although of course there is the presentation factor, a quality build is a quality build so credit there is well deserved.

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PostPosted: Mon Jul 11, 2016 3:45 pm 
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Some self correction plus clarification of roll centre adjustment :oops:

There are two adjustments which can change roll centre height, track and semi trailing arm (STA) sweep angle. By itself, the wider the track the higher the roll centre, and, the closer to zero sweep angle the lower the roll centre. Vice versa for both. That's it, the height of the arm's pivot points by themselves make no difference to the roll centre. The only diagram that illustrates this that I know of is in Puhn's How to Make Your Car Handle, if I see it online some time it will be posted up.

A fully trailing arm system's roll centre is at ground level so if we think about it, the less sweep angle used with a STA suspension the lower the roll centre and if the sweep angle is reduced until it reaches zero the STA suspension becomes a trailing arm suspension.

But just because the STA's roll centre cannot be changed by using adjustable vertical pivots it does not follow that they are of no potential use. Consider dynamic camber change and rear grip.

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PostPosted: Mon Jul 11, 2016 4:56 pm 
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Richard,

You're heading in the right direction.

There are a few adjustments that can change roll centre height of an STA suspension:
1. Track
1.5 Tyre contact patch (as your car goes through camber, the contact patch changes, so does your track and so does your Roll centre...)
2. Sweep angle
3. Pivot height


The height of the pivots do make a difference to roll centre.
The image I have attached was from Gustave's E30 site, which now doesn't exist?
http://www.e30m3project.com/e30m3perfor ... /index.htm
It shows if you raise the outer pivot, you lower the roll centre.
Raise the inner pivot, you raise the roll centre.
And vice-versa too.
Lower the outer pivot, raise the roll centre etc.
Can be a handy way of undoing what you have tried to do. Change one thing and another changes.
The difficulty when changing toe or camber on an STA you are doing these things. You are changing the STA geometry by either changing the sweep angle, or raising/lowering the pivot.


Attachments:
outer_up.gif
outer_up.gif [ 10.95 KiB | Viewed 2704 times ]

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PostPosted: Tue Jul 12, 2016 11:16 am 
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Thanks for the correction Nik, now to work out what is the potentially best setup for a circuit car. It will be a while before mine is physically modded but it's best to get the theory right first. I'm thinking that a rear roll centre of around 150 to 200mm would be suitable with the front a bit lower. I don't want to use a rear anti roll bar if possible which means that the rear roll centre will have to be on the high side, Quaife LSD, V8 torque.

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PostPosted: Fri Aug 19, 2016 6:29 pm 
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http://www.susprog.com/ caters for our semi trailing arm suspension but at $200 to use just once is a bit much, they do have a trial option but STA compatible?

The thing that has got me thinking lately is why the big sweep angles? Which progressively became smaller (narrower) in later versions. There had to be a reason for the bigger angles but what was it? I've tried to come up with something more than a guess but nothing.

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PostPosted: Fri Aug 19, 2016 10:26 pm 
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Potentially due to the need for increased strength on the mounting points when the lower sweep angles are used??

The outer semi trailing arm mount would have to be significantly stronger to cope with the lateral forces from the wheel.

Don't think this would be much of a cost increase for the Datsun engineers so im not sure??

Does anyone know what the sweep angle is for a commodore?

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PostPosted: Sat Aug 20, 2016 9:31 am 
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By 'mounting points' lampy do you mean pivot/pickup points?

Changing the sweep angle by extending the two inner pivot points forward (which would require lengthening the two inner arms to match) would take up a bit more space under the car, that's all I would think. Perhaps the reason why sweep angles were progressively reduced has more to do with an appreciation by the designers of the advantages by doing so, just like front caster was increased progressively. Perhaps, as sometimes happens, I'm over thinking :)

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