Thread: Longer rear trailing arms on a defender.

Originally Posted by Judo

The A frame arms are shorter than the link arms, so the arc is smaller. It would be interesting to see the position of the ball joint at full axle droop. As far as I can tell, the further the axle is away from the body/chassis, the closer you get to max ball joint angle, before the axle perhaps twists or moves forward (crushing your link arm bushes).

A question that may also be relevant - do you have an anti-sway bar?

No sway bars. I had to remove front and rear long time ago. I kinda like the tinny in a storm handling. Keeps you honest and on your toes.

The other effect of the shorter A frame arms is the affect that at full drop the arc pulls the top of the diff housing forward rotating the housing and pinion down towards the ground. As the trailing arms are longer (thus less axle walk forward) the rotation may be exaggerated compared to the factory set up. Whether that is problematic in real life i have no idea. A CAD drawing will give you an idea of the results.

MLD

There is normally a nominal relationship between the A-frame and lower links, but there is no correct suspension setup. In relation to the setup, you would set the mounts on the chassis for the lower links to provide the correct pinion angle at ride height. When you deviate from normal ride height, much like with any setup you allow the universal joint to deal with the changes in pinion angle. To minimise the changes in angle you would be looking towards equal length upper and lowers and as you increase the difference then the more the pinion angle will alter as the axle as a whole deviates from ride height. The reason for this ratio between upper and lower links is to change the amount of anti-squat as the suspension moves.


Put into really basic terms, the real reason that longer lower links are used in comp 4WD's is because the bodywork is generally set high, and when the lower link which are generally at axle level push forward, shorter links in terms of vectors will push forward and at the same time push the body up (anti-squat). If there is too much anti-squat and you're in a sticky spot then as you try to put power to the ground the body will lift, then you lose traction, then the body drops, then you gain traction, then the body lifts again and you lose traction, then the body drops and then you break an axle. Running longer lower links allow you to reduce the amount of anti-squat in the suspension, so you get less pig-rooting and smoother drive and power down. Don't forget there is also a reaction happening in the upper links (A-frame) too.

In terms of bush wear, all things being equal apart from longer lower links it should be better, but I suspect a solution is also out there that doesn't involve longer lower links and fabrication.

MLD, I am curious to know why you wouldn't try an option with rose joint or similar at both ends? Is the thinking that longer arms might be overall better if you need to cut and weld the chassis bracket either way?

Originally Posted by Judo

MLD, I am curious to know why you wouldn't try an option with rose joint or similar at both ends? Is the thinking that longer arms might be overall better if you need to cut and weld the chassis bracket either way?

Hi Judo,

My default is the TerraFirma or Raptor 4x4 johnny joint trailing arms which are a bolt in solution. I don't like the TF because it shortens the arm by about 20 to 30mm by mounting the pivot on the rearward side of the chassis frame. I like the Raptor idea of mounting the pivot forward of the chassis frame but don't care for the dual bends in the arm and the weakness that it introduces. Thus neither meet my idea of a neat solution.

Rose joints are prone to wearing prematurely especially at the axle end which is subject to dirt, mud, water etc etc. While Johnny joints are a better design they too cannot avoid the ill-effects of a hard life at the axle.

The thought of longer arms being a better and longer term solution to my problem is a combination of things. It's a heavy 130 with 35" rubber, 3" rear lift and 11.5" travel shocks. The cranked gwyn lewis trailing arms at rest are stressing the bush and when the axle drops the wheel walks forward by significant visible margin. On-road i get rear steer when on and off the gas (coming off the gas quickly at 100km'h can really unsettle the truck). If I can reduce the effect of the negative characteristics I ought to end up with a truck that is more compliant on road and less maintenance and breakages off-road. I'm way past a modest budget on this truck and has succumbed to the trappings of the domino effect. That said I could go the other way and remove the coils for shorter coils (say +1.5" to 2" lift), go to shorter shocks (say 10" travel) and bring the suspension geometry back into reasonable parameters. The cost will be the same when i do all 4 corners.

MLD

I'm no expert on this stuff, but you can change the A frame geometry without changing the A frame; have you seen the Wildbear A frame joint?



The effective length is adjustable via the rose joint thread and the joint itself must be a good 1-2" lower than stock. I think something like this should be able to get you back to being reasonably close to stock in terms of the relationship between A frame arc and trailing arm arc; even though the position of the two will have changed.

I would always question putting a side load through a thread like that. People do it by just beefing things up, but that design is setting the joint up for failure because the thread acts as a major stress raiser which can, and I have seen, lead to a failure of the part.

Take a look at the coresetti arms out of Italy .

Originally Posted by Pateyw

Take a look at the coresetti arms out of Italy .

Thanks Pateyw, these are similar to the terrafirma product and, right or wrong, I don't like the shortening of the trailing arm. That would likely exaggerate my axle walk, rear steer problem.

MLD