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PostPosted: Sat Aug 20, 2016 6:56 pm 
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Yep i mean the pickup, pivot points

260DET i think your explanation is probably the closest. It would be quite a complex subframe to mount the diff and have a low sweep angle.

The 510/610 was build with simplicity and functionality in mind

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PostPosted: Sun Aug 21, 2016 12:02 am 
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I did a bit of a sketch of the possible roll center of the group C bluebird. Difficult to estimate distance and angles of the lateral arms but gives you an idea where the roll center might be.
Attachment:
[ attachment ]
Bluebird roll center.JPG [ 80.82 KiB | Viewed 3016 times ]
I would guess that the center of mass for a 1600 would be just above the rear diff?
This would mean the roll couple is quite small. Wonder what spring rates they ran?

Did you ever measure the roll center height of a standard 1600 semi trailing arm and the modified version nick?

It would be interesting to compare the two

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PostPosted: Sun Aug 21, 2016 10:31 am 
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The Bluebird Group C suspension is a viable alternative I think, thanks for the roll centre sketch lampy because I was just wondering about that. While it may look like a complicated mod most of it seems pretty straight forward except perhaps for locating the vertical link arms to the wheel hub bracket arrangement. Back to the roll centre, it would be adjustable by using alternative mounting holes at the diff end I think and would involve some of the same design considerations as a double wishbone suspension wouldn't it?

Susprog representation below, what is this suspension type called I wonder.


Attachments:
[ attachment ]
Bluebird STA mod.jpg [ 26.91 KiB | Viewed 3006 times ]

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PostPosted: Sun Aug 21, 2016 11:19 pm 
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I might have to download that susprog program. That looks exactly like the rear suspension setup in the bluebird.
I had a bit of a play with the angles of the arms in my diagram and it is quite easy to move the roll center up and down.
From the photo of the bluebird there is evidence of where they have mounted the arms in different positions.

It would be really nice at the track to be able to adjust the roll center easily. With that setup you could move the lower lateral link up one hole on the diff/ re-camber the wheels and off you go for some testing with a higher rear roll center

I don't think the attachments on the hub would be too hard. an few well placed and designed brackets

The issues come from the rear diff mount. The lateral arms go right through the area the moustache bar mounts on to the body.
I think I have come up with a solution which would remove the moustache bar and the studs on the body and then use the two alloy plates on the diff to bolt to the body.

Also the exhaust becomes an issue. The bluebird had a side exit exhaust. There is not much room to run a single 3" exhaust around the lateral links

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PostPosted: Mon Aug 22, 2016 10:21 am 
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Agree with your thoughts lampy, for me it's a bit easier with a Z31 because the diff mounting arrangement is quite different to the 1600's. Another advantage with the Bluey setup should be camber change, properly set up it would work for you just like a twin A arm does, unequal length arms I would say.

But what about toe? That would have to involve making the forward arm pivot points adjustable sideways wouldn't it? I'm thinking about Susprog too but not ready yet and at $200.........wonder if the trial version would do it.

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PostPosted: Tue Aug 23, 2016 9:27 am 
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It looks like our friends at Honda have been there done that, see pic below. The small link (compensator arm) at the front end of the trailing arm would act as a toe corrector while the pivot bush in the arm would be compliant enough to allow the link to hold the arm in at the front during suspension movement. Using compliant bushes is out for me so ideally an alternative toe correcter setup is needed, assuming it's all mechanically doable. Still. if that is not an option it does not rule out using the system given the other adjustability available.

Any ideas on a 'practical for us' toe correcter anyone?


Attachments:
[ attachment ]
Honda rear trailing arm.jpg [ 21.36 KiB | Viewed 2971 times ]

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PostPosted: Tue Aug 23, 2016 12:44 pm 
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Hi guys,

I didn't calculate the roll centre of the factory rear end. Instead I did it for my car which is probably an inch or two lower than factory and I don't have the details in front of me right now, but from memory it's about 180-200mm off the road. Not sure how that correlates to the diff position. Probably near the bottom or on it? I wanted to keep the roll centre near the bottom of the diff, but was more interested in how it changed with bump and roll. There's an image of my sketch on my build somewhere...
The more you decrease the angle with the STA, the closer you go to a fully trailing arm suspension which lowers the roll centre to the ground.

Richard, regarding what the bluebird group C suspension is called; it's similar to a Chapman strut, but I can't remember what the extra links on the back make it called...

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PostPosted: Wed Aug 24, 2016 6:29 am 
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Thanks nick for the info. I found your drawing of the roll centers with the reduced sweep angle.
Seems like just below the diff is around where it needs to be.

The sierra's must have a low roll center if they are running around 6 degrees of sweep.
That is strange for a 4wd car that easily suffer from under steer if the rear end is not stiff enough. FWD's usually run massive rear spring rates and sway bars to get the things to turn in.
Edit: just read that the early sierras were rear wheel drive, particularly the ones that raced in group A.

Interesting idea on the Hondas Richard.
I was thinking the inner leg of the trailing arm could be cut off for weight savings and boxed and the outer arm could be converted to a rose joint attached to the standard outer cross member mount. The rose joint would need to be spaced using bushes either side. The bushing lengths could be changed to move the rose joint sideways and thus changing toe.
Not sure how much adjustment you could get within the outer cross member bracket, but it would be simple to cut off the existing bracket and weld on a wider one.

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PostPosted: Thu Aug 25, 2016 9:20 am 
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Yes lampy, that's how I imagine it being done too. With perhaps the spherical joint for toe adjustment being mounted in a block that could be moved sideways and locked into the desired position.

The Honda example has dynamic toe adjustment though which I like very much, which is why I was wondering if anyone could come up with the equivalent of their compensator arm but without using a compliant rubber bush. I just can't get my head around all the varying angles and movement required, it would be nice to think that a spherical joint in place of the rubber bush would work but...........

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PostPosted: Sun Aug 28, 2016 8:45 pm 
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No dynamic toe change compensator that would work with a Bluebird Gp C modded suspension? :(

To complete what can be done with this mod, the diagram below illustrates two significant advantages which can be designed into it, dynamic negative camber gain and a suitable roll centre. The first of course comes from using unequal length arms, the second from the various adjustments which can be built in. :thumbsup:

Edit. Just a thought but it looks like some dynamic toe in can be designed into this by adjusting the arm operating angles, how that would fit in with the rest of the requirements is another question but my inadequate brain is telling me that something like the arm arrangement in the diagram below would do it.

Second Edit. To elaborate on the dynamic toe in possibilities with this suspension, the remaining STA outer arm would need to be converted to have something like a spherical bearing so, unlike the Honda version above, there would be no lateral movement at the pivot. Which would result in the rear end of the arm at the hub moving in and out in bump and rebound. That movement would result in dynamic toe changes.


Attachments:
[ attachment ]
Bluebird susp.gif [ 8.88 KiB | Viewed 2928 times ]

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PostPosted: Fri Oct 07, 2016 6:59 pm 
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Well I'm going ahead with the rear suspension mods as discssed, no interest? It's pretty ace in potential but preferably a suspension program is needed to set the dimensions up for the best result.

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PostPosted: Sun Oct 09, 2016 11:32 am 
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So your going ahead with the bluebird style suspension Richard?
I am very interested in how you go.

I am going to go the same direction but am a bit busy with a new house to start at the moment

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PostPosted: Fri Oct 14, 2016 12:04 am 
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Definately, I like the idea that there is potentially a lot of adjustability available, incidentially I found out the other day that E39 BMW's had the same type of suspension at the rear. Will be a while before I get to it, just about to start making a two engine stand rotisserie.

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PostPosted: Fri Oct 21, 2016 7:27 am 
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260DET wrote:
Definately, I like the idea that there is potentially a lot of adjustability available, incidentially I found out the other day that E39 BMW's had the same type of suspension at the rear. Will be a while before I get to it, just about to start making a two engine stand rotisserie.
Hello, I am new to this forum and to be honest, I don't even have a Datsun, but I do have a car with a semi-trailing arm rear suspension, a Ford Sierra based Merkur XR4Ti. I stumbled upon this discussion when searching for different ways to modify and improve the basic semi-trailing arm suspension.

Coincidentally, I had seriously considered modifying my suspension to look like the Bluebird pictures in this thread. You are correct that several BMW's have this type of suspension, namely the E36 and E46 3 series. The E39 5 series does NOT. But, I joined the forum just to give you a heads up that it does have a problem in that the concept inherently causes a lot of toe change and if one is not careful with the angles of the lateral links it could even toe out under compression which is bad. If one does the calculations, you will find out that it actually has more toe change then a semi-trailing arm with a smaller sweep angle like the E30 BMW's. If you want to do some reading, look at the toe change that the C2 and C3 Corvettes have, as they basically also have the same suspension layout with one of the links being the drive shafts.

Even though it does provide the easy ability to adjust the camber curve and roll center, I decided against this configuration in my car due to the toe change.

I am not quite sure which direction to go yet. The stock Sierra sweep angle is 18 degress, less then the 25 degress of the 510 but still way to much by modern standards. As mentioned in a previous post, I common mod for a the Sierra is to modify the rear beam to change the sweep angle to 6 degrees. This seems a bit low to me, but it is proven. I could easily do that, but it would not fix the other problem, which is the lack of anti-squat and the resultant low roll center. To fix the anti-squat in my application I would need to raise the pivot points about 4inches (100mm) which of course fouls the floor :lol: Lately my thinking has been to simply replace it with a double A arm setup of either my own design or from another car like a 350Z.

Tim
XR4X4Ti


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PostPosted: Fri Oct 21, 2016 8:57 am 
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Hi Tim, welcome to Ozdat and thanks for your post. It's always great to get information from other sources and someone who obviously has experience with the Sierra.

Perhaps a compromise would be required. The existing sweep angle for a Datsun 510 is approx 25deg and I ended up changing it to approximately 17deg. I raised the inner pivot towards the floor (by about 25mm) to get the roll centre back to approximately where it would have been based on a few calculations I did.
My suggestion is to find out what sweep angle you could get away with by figuring out how much clearance you can modify your inner pivot.
It may not be the sweep of 6 degrees, but I think the benefit outweighs the downside.
The other thing with raising the inner pivot (or any pivot for that matter) is you get a small amount of anti-squat.

Fitting a multilink rear end is certainly tempting, but I didn't want to cut up the floor of my car!

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