Rear suspension flex

[xMAGNUMx]

Hey folks,

I've got a stock MY19 PS which serves as a family car and for weekend thrills. I'm trying to figure out how to get the best flex out of the rear suspension so I've done a lot of reading on here and have learned a lot (thanks!), but I haven't been able to find answers to all my questions.

First I took some measurements:
Rear EBH 550

Then I drove onto my homemade flex ramp:
Rear EBH at full compression 420 and rear shock 370
Rear EBH at full droop 690 and rear shock 465

I tried my best to ensure the EBH measurements on the flex ramp were accurate, but due to the circular motion of the rear axle under articulation they may be off by 10. Nevertheless, from what I understand, the OEM shocks travel range is 315 to 500. They are not being maxed out at either end based on my measurements.

Clearly MM have designed the rear suspension to avoid shock damage under compression and I noticed the axle was resting on the cone bump stop inside the rear coil under max compression. It is not my intention to increase up travel - I only want an increase in down travel. I measured the rear spring and on the long side it was 320 long at full droop. I understand OEM spring size is 380 so it's a long way from falling out.

So how do I get more flex out of the rear end? What is restricting down travel in the OEM setup? Is it the sway bar? I don't have a lift to measure the rear EBH when the car is lifted off the ground and the axle is level. This could help answer the question...

[old Jack]

The Pajero Sport rear suspension is identical to the PB/PC Challenger and I have spent a lot of time and money getting my rear suspension to do what I want it to do, which is to catrry heavy loads over rough tracks at speed and for extended periods of time.

The limiting factor in regard to down travel is firstly, spring rate to overcome the effect of the swaybar, if the sway bar is disconnected the you require lower spring rates. Then it is the shocker length, 525mm is the longest safest shocker length, Bilstein B6 are 530mm but unless you have low spring rates the coils can unseat off the lower coil pads. Then comes the brake line length, with Bilstiens the brake lines will go tight, so you need to fit extended length brake lines.

The limiting factor on compression are the articulation stops on the axles, even the replacement rubber cones springs that I recommend, which are longer and a greater spring rate, do not effect the compression travel.

OJ.

[xMAGNUMx]

Hmmm, cheers OJ. So get stronger springs to bend the sway bar. Then get longer shocks so they are not the limiting factor. Finally extend lines as required.

Based on what you've said, what would be a better pick: long linear springs (like the Dobinson C43-203 you've mentioned in another post) or a slightly shorter progressive one (like the KCRR-38)? Which one would provide better flex? I should note that I have no intention of loading up my PS for an outback trip anytime soon so load carrying ability is not part of my requirements.

[old Jack]

Hmmm, cheers OJ. So get stronger springs to bend the sway bar. Then get longer shocks so they are not the limiting factor. Finally extend lines as required.

Based on what you've said, what would be a better pick: long linear springs (like the Dobinson C43-203 you've mentioned in another post) or a slightly shorter progressive one (like the KCRR-38)? Which one would provide better flex? I should note that I have no intention of loading up my PS for an outback trip anytime soon so load carrying ability is not part of my requirements.

If you are not carrying any extra weight in the rear the a longer softer (lower spring rate) coils will give you more articulation if you disconnect the rear sway bar.

Some coil data to ponder over;

Factory 5 seater, 374mm free length, 2.36-3.54kg/mm spring rate.
Factory 7 seater, 380mm free length, 2.36-3.54kg/mm spring rate.
King KCRS-33, 375mm free length, 2.4 to 4.1kg/mm spring rate.
King KCRR-33, 395mm free length, 2.4 to 4.1kg/mm spring rate.
King KCRR-38, 400mm free length, 2.4 to 4.1kg/mm spring rate.
Dobinson C43-099, 385mm free length, 2.7kg/mm spring rate.
Dobinson C43-097, 405mm free length, 2.7kg/mm spring rate.
Dobinson C43-203, 425mm free length, 2.96kg/mm spring rate.

If you weigh your rear axle weight then I can do a comparison of calculated/predictive ride heights.

OJ.

[xMAGNUMx]

After a busy week with lots on the calendar I finally got some time to do a bit more testing. Results are:

With sway bar in place:
Rear EBH at full droop 690 and rear shock 465

With sway bar removed:
Rear EBH at full droop 715 and rear shock 490
Rear spring on extended side was about 345

The nonlinear behaviour of the shock is quite interesting. That's one I'll have to figure out. More noteworthy from a flex point of view is that the shock isn't maxed out at 500. I'm guessing this means some line is tight stopping the the axle from drooping more.

Looks like a number of components need to be changed for more flex:
1. Install sway bar disconnects
2. Install longer springs
3. Install longer shocks
4. Extend brake lines as necessary

A project that will have to wait for another time. I don't even know which springs would better suit my use: linear or progressive...

[old Jack]

Because the shocker top mount is set forward of the coil centre line the shocker travel per mm of suspension travel varies but it is less than a 1:1 ratio.

At full droop on one side and full compression the other side the bushes of the trailing arms will have quite a bit of side load so this will start reducing the droop travel.

There will always be multiple successive limits, it is just how much time and money you want to throw at the problem.
Normally it is a case of escalating cost for minimal benefit.

OJ

[denisp]

Just had the 30,000km service on my PS.

Nothing said to me when I picked up the vehicle after service, but written on the report is a mention that the rear LH shocker is showing "sweat" signs.

Tomorrow I am going to ring and argue that this must be a fault and the shocker needs replacing under warranty. Don't want to be a couple of hundred km away from help and have a collapse. Might ask for both to be replaced.

What do folk think?