I’ve not even bought one – yet, but I’ve been thinking about the back suspension issues on the Libra/Spyder.
It strikes me that when cornering the outside wheel is tending towards negative camber – I’ve seen this mentioned elsewhere on the forum. This means that the top arm is being pulled outwards from the car while the lower is being forced inwards. The “give” in the system allowing the wheel to twist towards this negative camber.
Now, do we know whether it’s the arms or the body tub that is/are distorting to allow this? The Z-cars system sees the same force at the wheel, but only having single planar fixing at the body end they see the force at the body mainly as rotational – which GRP can resist more comfortably, while the original twin arm setup sees forces perpendicular to the face of the bulkhead – which isn’t what GRP isn’t good at resisting. It only needs slight distortion at the body end to produce significant movement at the hub end.
If it is the arms that distort there’s nothing for it but to beef up or redesign the arms themselves - which isn’t great from an unsprung weight perspective.
But if it’s the tub, it just needs stiffening to lessen the distortion. I’ve pondered either a plate welded to the existing (inner) tub fixing brackets with a tube between the two (four) of them that is clamped to the tub somewhere in the middle, or simple spreader plates in the area of the mounts.
Bearing in mind that I’ve never closely looked at this area of a Libra in real life so it probably isn’t that simple.
Thoughts anyone?

I think the general consensus is that it's a bit of both, the wishbone design is pretty poor and there is some inherent flex in the bulkhead, hence the Z cars kit addresses both issues. I know a few people have tried to retrofit some kind of crossmember to the bulkhead in the tank recess but this was reported to not do an awful lot.

I've often thought that a decent setup could be a rose jointed set of MK1.5 wishbones (flat not tubular like the mk2 but with joined up tub fixings unlike the early mk1 setup) fixed via rose joints to the tub mounts, with some bracing from side to side. You'd still be stuck with two arms though and I've never liked the control arm setup either. The Z cars kit is so much simpler/better. Such a shame the factory never came up with something similar.

Always good to ponder.
In terms of the bulkhead flexing its worth remembering that there is a grp box on the cockpit side of the bulkhead such that you have a horizontal slab of thick grp (either the floor or the top of the box) close to the mounting points of the trailing arms to the bulkhead. As such I can't imaging the bulkhead is going to flex a lot. The trailing arms themselves are lattices and whilst there will be a bit of stretch I can't see that as being huge. There will inevitably be a bit of give in the pivot mountings, whether its the earlier land rover track rod ends or the later pillow joints.
In reality there will be a small amount in each of these locations and they'll add up such that small+small+small=not so small! This gives rise to a change in camber but as the geometry gives an increase in camber with compression this should correct itself and as long as you start off with sufficient negative it should never go into positive. Same with rear toe. In my opinion this is more important as they seem to very sensitive to even a tiny amount of rear toe out giving a lot of instability. The same forces/flex also changes the toe from in to out and this is added to by any slop in the toelink. I've found I need a fair bit of toe in to keep it stable and this does lead to a bit of tyre wear.
I'm guessing the Z-cars should deal with all of this and I am tempted but right now I can't justify the cost.
My next upgrade will be a front ARB to try to keep the whole thing flatter in the bends without having to set the dampers silly hard which makes it respond badly to mid corner bumps.
All of this only really applies to track days as once you've got the geometry set nice it'll handle great on the road whatever.

Some years ago some clever people, one being a suspension engineer for Jaguar did FEA analysis of the rearms, joints etc. I don't have the figures but combined over the 7? joints the deflection was quite high. It was also noted that especially the mk2/2.5 arms didn't hold their geometry and would go very quickly out of alignment after being set. Some time was also spent looking at the rear bulkhead. Over the years of production it was found that the thickness varied a lot. When the suspension was put under modest lateral load deflection in the rear arms was quite dramatic, most of this coming from bulkhead flex.

I tested mine in a less scientific way. My mate big Dave got a grip of the back of the rear wheel and we saw more than 25mm of movement. Again the bulkhead was the bigger issue. This really is what lead me to speak to Chris at Z Cars.

Others have done other things inc:

Rose Joints on mk1 suspension instead of ball joints
Braced between the bulkhead brackets, then welded a tube across the bulkhead to the brackets.

This worked very well but wasn't easy to fit rose joints of the required size. 5/8" thread 1/2" from memory.

Finally transplant of a full MGTF rear subframe. Works well but is quite heavy.

I did a similar test on mine but I didn't have big Dave so I bolted six foot of unistrut to the hub bolts and little Dave leaned on that. I was only able to generate around 3mm of movement at the wheel with the leverage in the horizontal plane to simulate toe change. I couldn't get the car high enough off the ground to use a long lever vertically to test for camber change but I could get around 5mm of movement in that plane with a lever about 2 foot long. Maybe quite a bit of the additional 30kg of grp on the Spyder tub went into the bulkhead! Mine is Mk1.5 (I think). Its got the single bulkhead brackets (4 rather than 8 ) and the land rover TREs.

I'm set up with 5mm of rear toe in per side so even under full load I should stay toe in and 11mm of negative camber so should stay negative.

If you are moving 25mm with all the bits in good condition then I'd definitely spend the money on the Z cars

Gents can we all just take a step back for a second. I know we all mean well but for us to truly understand the forces at work here, we really need to know how big "big Dave" actually is and what he had for breakfast that day.

At least 3 shredded wheat from the sounds of it......probably spinach for
lunch....

All you need to know about Big Dave is when tightens something it stays tight.

What's the torque on that Dave?

Don't worry Rog it's "Dave tight!"

Oh and mostly beans with sausages!

So torque = Big Dave x distance?

I've been thunking again....

Are the rear arms parallel along their length? If they are, has anyone thought to link the two together at the wheel end - or at least as near to the wheel as possible, to reduce the relative horizontal movement between the two?

Yes, it'd be fiddly, it might greatly increase the friction in the system if not designed properly, and I wouldn't be comfortable taking a welder to the rear arms, but it's a thought.

But don't you need relative horizontal movement between the two to produce more camber in bump/roll to counteract the loss of camber from the roll itself? The problem is that the increase is decreased by the slop but there is still some resultant additional camber. If they were fixed all the body roll would become camber loss. Also, they couldn't be rigidly fixed cos that would lock up the suspension. Would have to be something that could pivot top and bottom - a metro front upright on two ball joints maybe???