[BearInTheCorner]

I am about to begin a RUF conversion on my 413 tintop widetrack. Will likely be doing a combined chassis extension and re-drilling the rear hanger. I have ordered 4x OME rear leaf packs and potentially adding some leaves from the stock packs if required as I have a bullbar with winch and a camping setup the the rear. Nothing too heavy but there is a deep cycle battery, fridge, drawers, etc. I would say all up 100kg permanent rear payload.

My question is what would be the best shocks to use on this setup? When I originally planned the RUF I was thinking of just using 4x OME rear shocks as I had heard the Gabriel commodore/Hilux shocks (81147 and 81340) were not the best option due to the valving. However since I am having a heavier load, perhaps the Gabriel are the better option?

I am reasonably sure the stock shocks are cooked because since installing the camping setup in the rear the rear suspension has sagged significantly and when driving over 70kmh it develops an uncontrollable death wobble that seems unrelated to the steering, as though the chassis seems to rock from side to side while the wheels are tracking straight. Usually triggered by a small undulation in the road. This did not happen before adding the ~100kg payload so I figured there is now some extra momentum to wobble about and that the busted shocks went unnoticed without this load. I am hopeful that new shocks will sort this out.

[Gwagensteve]

There's a few different things gong on here we need to discuss and flesh out. I'll discuss shocks last.

Firstly, in real terms, with the car fuelled, packed and with driver (and passenger?), it's likely at or over GVM, so ~1300kg. A sierra is generally considered to have 450kg of payload, but payload is a slippery term as it's based on a dry weight which no car can achieve. Best to compare what your car actually weighs to GVM which is either ~1350 or 1410kg. Regardless, my point is your car is basically fully laden at all times.

The sag in the rear end has nothing to do with shocks, that's 100% the spring. Shocks have no effect on ride height (they do in extreme cases but that's not what we're discussing here)

Death wobble is it's own thing and it's 100% steering. If you have experienced death wobble you'll know, it's not "rocking from side to side"- the front wheels will leave the road surface at 40km/h+ in time with wheel speed. It's incredibly violent and you genuinely feel like you're going to die. There may be issues with you shocks, but also bushes, shackle wear (this is common), loose U bolts - there's lots of other issues that can induce "loosenes" in the suspension. Also, if you have draws and a fridge, you'll have a fairly high rear centre of gravity which won't be helping.

I need to ask some questions about the rest of the car and use.

What size tyre?
Does the tyre size you want to run require a bump stop spacer in the front?
What's the motivation for RUF? Do you have experience with suspension and chassis fabrication/welding? Lots of RUF conversions don't work very well and they are harder to get right than people make out.
It sounds like you don't want to move the axle forward, and I'm sort of picking up you want to retain stock shock towers/mounts. This is difficult as there isn't space for a long enough shock to work with the camber of the rear spring when used in the front. (reference the thread about EFS shock lengths here: viewtopic.php?f=5&t=57113)

As a light, leaf sprung vehicle, sierras don't need much in the way of shocks. Leaf springs are inherently self damping and a Sierra will pass the "bounce test" with no shocks at all. Whilst you car is heavy, it will also require plenty of leaves to support the weight, and with more leaves there's more internal friction so less need for shock valving. Many US pickups with huge rear load ratings run practically no shock valving.

[BearInTheCorner]

Hi Steve, you were just the person I was hoping to comment on this.

When I mentioned "payload", I basically used it as a substitute for "extra weight over stock". I would say I have about 100kg in the rear and about 30ish kg on the front with the bar and winch.

I know the shocks aren't responsible for the sagging, but I thought they may be responsible for the rocking side to side that I observe when encountering a bump. I have not had the rear shocks off the car yet to check them. My logic was that if they were no good, it would easily go unnoticed with no load as you correctly say there is enough interleaf friction for it to pass the bounce test with no shocks. However perhaps the extra weight is now presenting a problem as there is more mass (and therefore energy) being transferred into the suspension when encountering a bump in the road. This is made worse by the higher CoG.

Perhaps death wobble isn't the correct term. I have read a lot of posts about death wobble and none of them describe what I am experiencing. I have mounted a GoPro under the car and there is no oscillation of the steering when experiencing it. But it is definitely frightening.

The ubolts, shackles and bushes will all be replaced with the suspension overhaul so if any of those are the culprit then it will be sorted at the same time.

To answer your questions about the car:
Currently running 235/75/15s and have no immediate plans to go bigger as they have plenty of tread left, although I am also about to put 4.16 gears in the transfer so I may have capacity to go an inch larger at a later date.
I won't know if I require a bump stop spacer until I do the work, however you may be able to advise from your experience.
Motivation is mainly a better ride and my stock springs are inverted (even without winch - it isn't currently mounted until I do the springs) so I was going to have to replace them anyway. The added flex will also be welcomed.
I have lots of fab experience and have already done the extension, I just need to cut and re-weld the front shackle mounts.
I will likely move the front axle forward a little (maybe 20mm) as the extra wheelbase can't be a bad thing, but I don't require any extra clearance from the firewall with my current wheels so won't be going too extreme. Still undecided on this one.
If I can I would like to retain the stock towers. I will take a look at the thread you shared to get some more insight on getting a long enough shock in there. I assume you mean that I won't be able to achieve the full droop that the spring will allow because the travel will be limited by the shock length. Is the only way to do it correctly to put larger towers in?

[Gwagensteve]

I'd weigh your car, either at a weighbridge or you can buy 3X bathroom scales from bunnings for $8 each and with some wood/steel you can weigh the car one corner at a time pretty readily. A stock Sierra is fairly evenly balanced front to rear. My car is nose heavy (and very heavy), I suspect your car will be on or over GVM when fully packed and with driver/passenger and I bet that weight balance is largely rearwards. It's an interesting thing to understand. Many people are in the same boat, but knowing how close to GVM you are can help guide your choices and understanding weight distribution can change the way you pack the car or where you carry heavy load.

The shocks may be contributing to the sway you're experiencing, but it's not a common issue. I suspect it's a culmination of wear across the suspension, and as you point out, a heavy car with a high rear COG.

I guess where I'm heading is that based on what you're experiencing and what it sounds like your use is, I'm not sure the complication and possible compromises of RUF are required for the job you need to do. Mostly, I think you need new springs with a higher rate and replacement shocks.

RUF with OME rears will require you to derate the front substantially to get the car level. This results in a very low spring rate which is great for maximum offroad performance but It feels to me like your looking for more of a distance/camping/moderate terrain user and aren't going to want to drive a car with lots of body roll/brake dive/loose steering feel etc characteristic of cars with very low spring rates.

With basically little or no requirement for a bump stop spacer and wanting to run shocks in the stock shock tower your maximum droop will be within the range a standard aftermarket spring could deliver without the fabrication, potential steering clearance issues etc.

If I've misread the application and you definitely want to take on the fabrication work, more power to you, but the less fabrication you try to do (minimum wheelbase stretch, stock bump stop and shock location etc) the less benefit you get out of RUF. There is no question it provides a great ride, but dropping from 220lb/" front spring rate to ~100lb/" is a big step.

Just some food for thought.