C3 Rear Suspension question

[TimP]

I'm working on the rear suspension at the moment and whilst access if good I'm just making sure that everything is ok before the body goes back on. I have a question about the movement of the rear road wheels relative to the differential and the stress it puts on the diff and/or half shafts. I've been thinking about it for a while and my head is starting to hurt so perhaps someone can help put me straight ..... Here's my question....

The differential is effectively fixed to the body via the cross member.
When a rear road wheel hits a bump, the half shaft and strut move around fixed points on the differential and the strut mounting underneath.
The trailing arm rotates around its spindle at the forward end and the spindle is mounted in a bush on the trailing arm.
When going over humps or bumps, the road wheel goes up and down in the vertical plane, controlled by the trailing arm.


When a bump is hit, the road wheel goes up and down vertically (and as it swings around the spindle, the wheel centre presumably moves back a bit too), the half shaft and the strut rod rotate around the differential and thus the distance between the diff and the road wheel changes, the distance being at a maximum when the half shaft is at 90 degrees to the diff. The diff is fixed and thus something has to give to accommodate the changing distance between the diff and the road wheel, otherwise there will be a massive stress on the half shaft, the trailing arm or the components in the differential. Is this correct?

The only compliance in the arrangement is in the training arm spindle bush and the bushes on each end of the strut, so when going over a bump does the trailing arm twist on the spindle (and/or move outward) resulting in the camber of the wheel changing?

 

[Oneball]

The camber and toe of the wheel changes due to the different arcs of the 3 suspension links. The arm twists slightly and moves inwards/outwards slightly. There’s no great force and nothing “gives” (there is some movement in the rubber bushes but it’s very small, there doesn’t need to be any compliance as race cars replace these with Rose Joints and it works the same way). The change in camber is a design feature you don’t want constant geometry as the suspension moves.

 

[TimP]

Great - thanks for the explanation.

 

[Roscobbc]

Lots of confusion 'out there' relating to Rose joints and vehicle applications. Brits and Aussies call them Rose joints, the septics call them Heim joints. Both are named after earlier suppliers and technically known as 'spherical' joints. Superb accuracy for the suspension geometry requirements of a race car and complicated carb and injection application but with 'metal to metal' contact they may be too harsh in terms of ride and noise for a street vehicle. The Americans also seem to have a version of the spherical/Rose/Heim joint known as a 'Johnny' joint which I believe is PTFE/poly lined to remove some of that harshness and ride noise and improve service life.
Johnny Joints - Custom & Off Road Johnny Joint Kit

 

[Forrest Gump]

When you Jack-up the rear of a C3 and the suspension drops you can see a few nasty things happening - large camber change, spring link bolts get pulled across and start binding, the lower strut rod is subject to torsion. None of this is too much of a problem in reality though because the suspension travel is relatively small when driving.
If you have a rear sway bay, this arcs in the opposite direction to the trailing arm, so this too induces some binding in theory.
My strut rods are poly bushed and threaded adjustment type. It has been a recurring problem preventing these from undoing their lock-nuts (rose joints would be a solution but might break for road use). This is because of the torsion they experience. Stock strut rods must just twist a bit.
I think the rear suspension is somewhat flawed technically, but it works well enough.

 

[Roscobbc]

When you Jack-up the rear of a C3 and the suspension drops you can see a few nasty things happening - large camber change, spring link bolts get pulled across and start binding, the lower strut rod is subject to torsion. None of this is too much of a problem in reality though because the suspension travel is relatively small when driving.
If you have a rear sway bay, this arcs in the opposite direction to the trailing arm, so this too induces some binding in theory.
My strut rods are poly bushed and threaded adjustment type. It has been a recurring problem preventing these from undoing their lock-nuts (rose joints would be a solution but might break for road use). This is because of the torsion they experience. Stock strut rods must just twist a bit.
I think the rear suspension is somewhat flawed technically, but it works well enough.

For a relatively 'simple' independent rear suspension system designed over 60 years ago it works well (within reason) certainly as compared against a 'live' axle set-up.
Think I'd rather have the C2/C3's suspension 'flaws' as compared to the issues that a period 'live' axle set-up could bring.

 

[antijam]

My strut rods are poly bushed and threaded adjustment type. It has been a recurring problem preventing these from undoing their lock-nuts .........

Replacing the original fixed length strut rods (due to their difficult and rather unreliable adjustment mechanism) is a popular mod to the C3 suspension. I've done it to my car and have the same problem as FG - The lock nuts tend to unlock due to the torsion applied to the rod with suspension travel.



The rod ends are rubber or poly bushed which absorbs motion but still applies torque to the rod. I did toy with the idea of replacing one poly-bushed rod-end with a Rose joint while keeping the compliance in the other. This would obviate the undoing while helping to avoid harshness in the suspension. So far it's still on the to-do list.......

 

[Roscobbc]

Question.
Would the addition of a lock-nut on the strut rod end help to 'secure' adjustment settings?

 

[Forrest Gump]

Question.
Would the addition of a lock-nut on the strut rod end help to 'secure' adjustment settings?

I‘m not sure it would actually, because the threads at each end of the strut are opposite handing. A second lock nut would hold the first lock nut in place on the rod end, but the threaded strut would still be able to undo itself.
Ive got loctite on mine which seems to help. In practice, the lock nut at one end only will ever come loose which then allows that slight twisting motion of the strut rod to happen via rotating on the thread. This is obviously only a tiny, tiny amount but not really how it’s supposed to be. The strut rod will only adjust its length if the locknut at both ends came loose and the strut rotated.

 

[Forrest Gump]

For a relatively 'simple' independent rear suspension system designed over 60 years ago it works well (within reason) certainly as compared against a 'live' axle set-up.
Think I'd rather have the C2/C3's suspension 'flaws' as compared to the issues that a period 'live' axle set-up could bring.

Agree. They decided not to design new chassis and suspension for the C3, meaning that by the 80’s it was very agricultural. Night and day difference when the C4 came along.