Flexing the rear of a 100 Series Landcruiser

Hard numbers on rear suspension travel for my HZJ105.

When reading articles in magazines, I see this statement all the time: “We broke a shock absorber” and then remarkable stories of welding it up with batteries or bubblegum.

Myself on the other hand have never broken a shock absorber. Not in the rally car, not on Cape York corrugations, not playing silly buggers at Ormeo, not whilst breaking diff’s, not whilst launching 4wd’s or race cars airborne, not whilst towing heavy loads, not whilst hot, cold etc etc. I have tried good factory shocks, worn out crap factory shocks, Monroe’s, Koni’s, Bilsteins, Old Man Emu, yet none of them have broken. Get the drift.

Let me introduce you to the poor underappreciated Bump Stop. You see, when a suspension system reaches the limits of it’s travel, up or down, it has to stop. If it stops gently, all are happy, if it slams hard, steel on steel, something may give. If it slams hard time and again, fatigue will make sure something will give. Most importantly, in MOST cases, shock absorbers are NOT bump stops. Even if they are uprated “I have a bigger piston than you” shocks, the mounts still aren’t built like bump stops. Something will break.

Here are the general scenarios:

• The very common – shock is too long when compressed – it will become the bump stop over large bumps – Very Bad (Broken Jeep again Adam?)
• If the shock is too long when fully open, the spring will become free and will rattle / may fall out – Kinda Bad (and really irritating isn’t it Mr AdNic)
• The other not so common scenario is a really long stiff spring and a short shock. In this case the spring keeps trying to extend, but the shock stops it early. It is common for the shock to limit the downward travel and act as a bottom bump stop in Live Axle vehicles, but a really long spring or heavy spring will overload the normal behavior – presto – broken shock. The spring free length should only be 10 – 20mm longer than the shock, just enough to keep it captive.

Aftermarket suspension suppliers are generally very quiet on this particular topic. Trying to get numbers is next to impossible. Even the Internet discussion groups are generally ignorant or won’t discuss the travel numbers for their vehicles.

Well here are the numbers for the rear of a 100 Series Landcruiser. The rear suspension design is apparently not the same for the IFS and Live Axle vehicles. Mine happens to be a HZJ105 (live Axle) fitted with Old Man Emu aftermarket suspension. In these pic’s I am in the process of swapping the Old Man Emu OME863 rear springs which were too high / stiff for my use, with a set of King Springs KTRS-70.

All measurements are shock measurements from centre of pin to base of top mounting plate.

• Full Droop, no springs, no shocks, sway bar  – 640mm
• Full Droop, no springs, no shocks, no sway bar  – 640mm
• Fully Compressed, no springs, no shocks, no sway bar  – 400mm
• Full Droop, King Springs, no shocks, no sway bar  – 655mm
• Full Droop, King Springs, OME Shocks, no sway bar  – 640mm
• One side Fully Compressed, King Springs, No Shocks, no sway bar – 505mm

The following pic’s document and explain the above some more

  
 
Fully Compressed on bump stops. The cruiser is a great candidate for polyairs looking at this pic. I normally don’t like them as cars with the bumpstop inside the spring will have heavily limited up travel trying to compress the bag to zero size. In this case the bag would only be compressed approx 50%, making it highly effective at carrying additional load whilst not stopping travel (other than the increased spring rate).

Exhaust touches when on bump stops. Toyota got things very tucked away up there. The panhard rod is just above the horizontal. Lowering the mount would significantly equalize the Left – Right travel of the diff. The panhard rod ideally should be at horizontal when the suspension is at rest.

Sway Bar still not horizontal at full compression. There is some discussion that sway bar mount extension will improve wheel travel. Disconnecting it definitely improves side to side flex, but the actual design in itself does NOT limit down travel. The numbers above prove that. Note the upward direction of the bottom control arm, but the downward slope of the sway bar.

 
Full droop is limited by control arm bushes. (shock is disconnected at bottom). Diff is hard to RHS from Panhard Rod. Note the top control arm angled to the right and the large angle on the panhard rod. The compliance of the upper / lower control arm bushes are limiting down travel. A longer panhard rod would reduce this effect. The axle would spring back up to this spot even when pushed down further.

Same pic on the RHS. Not the top control arm angled to the right.

One side (LHS) only carrying full rear weight with King Spring. The RHS is at full droop. The vehicle is empty. When I first did this test with the jack the front LH wheel was on the ground taking some of the weight. To test it properly, you need to have the car on just the opposite wheels. These springs are possibly a little soft, but I plan to install polyairs. I would like to see about 10 – 20mm clearance from the bumpstop here.

So it looks like in a 100 Series, suspension droop is limited by the bush compliance. This is loaded by the panhard rod. The rod works in an arc, and in the case of the Landcruiser, this arc is mostly downward  below the horizontal. This effect is to move the diff significantly to the right as the suspension droops. As the rod is working at an angle most of the time the bushes are pushed to one side only, binding them, and limiting travel.

The next mod for me will be a panhard rod drop bracket. You could extend the rod, but this is simply patching the problem. Lowering the body mount point will still leave it up out of the way, but make the arc work above and below the horizontal. This will reduce the effect of the Left / Right movement and keep the dif closer to the centreline for longer. Presto, less bush stress and more travel. There is some discussion on effects on roll centre’s. When I know more I’ll update.

These results mean the factory sway bar setup does not limit droop. Whilst I would disconnect it off-road to get maximum flex at the rear, it is the panhard rod / control arm bushes that limit droop. No need to make crop brackets yet.

It looks like Mr ARB / OME has his shock lengths pretty well perfect. I was quite surprised. I am not normally an OME fan, but have been quite impressed with them in this vehicle. Acceptable on road, great off road. The maximum free droop is normally 640mm in and that is the length of the shock. This keeps the spring captive, but within 15mm of the end of its travel. A slightly longer shock would give slightly more travel, adding lever effect over the axle, possibly another 20mm at the tyre. The compressed length is less then 400mm, so they aren’t acting as a top bump stop and going to be damaged.

Now all I need is for someone to try the same with the front. Cmon guys, give me some numbers. I don’t have an excuse to do the front yet myself.

If you are looking at aftermarket suspension you can use the above to determine if what you are doing is of benefit. There is no point in going for longer shocks that 640mm unless you drop the panhard rod a bit, or force the bushes with longer springs. If the compressed length exceeds 400mm, you need to extend the bump-stops. Now I just have to figure out how to flex up like the rear of this Rangie.