Downhill Stability Assessment

taylor1

taylor1

Well-Known Member
Joined
Jan 16, 2006
Location
Charlotte
I knew I had to compromise some trail capability for better road manners when rebuilding the front and rear suspension in my 1985 4runner a while back. The compromise has been obvious in two very specific situations on the last two wheeling trips.

First near tipover was going down the backside of Daniel. During a slow descent, pointed straight down the trail, suspension was crossed up, the 4runner gently came to a rest on the passenger rock slider atleast 45 degrees over. I engaged the front elocker to get power to the passenger front wheel then moved forward and she leveled out.

I tipped over, slow and soft to the drivers side at the Flats Sunday afternoon. Deja Vu sort of thing... It was the small dirt hill trail next to the retention pond, by the gate and ticket tent. I was in the same crossed up suspension scenario in the staggered ruts. Cut it off this time and couple guys, both named Riley, helped me winch myself forward and level.

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Factor1: suspension travel, I had double shackles in the rear previously. Single shackle now with much heavier 63 Chevy pack. 12" travel 5125 in rear 14" 5150 in front, maybe 2" made a difference but this is same in both phases.
Factor2: wheelbase. I'm running 17 fj cruiser Steel wheels with 285 70 17 versus 15x10 35x1250x15 before.
Factor3: weight distribution.
Between bumper tire carrier and fiberglass top the rear is obese and center of gravity poorly placed.

Please add any additional comments or insight to the discussion. I will take and post some driveway measurements soon. Then maybe find a ramp or forklift in Charlotte area to get another perspective.

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This is gonna sound crazy but I think your going down the hill to slow. You fill it start to tip and hit the brakes harder and that stops forward motion and exaggerates the rear end lifting and the noose dropping causing you to
Tip. I went nearly on my top because I hit my brake when I should have just let it go.


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Loganwayne said:
This is gonna sound crazy but I think your going down the hill to slow. You fill it start to tip and hit the brakes harder and that stops forward motion and exaggerates the rear end lifting and the noose dropping causing you to
Tip. I went nearly on my top because I hit my brake when I should have just let it go.
Click to expand...

That can definitely be true, about the braking. Any time you have a change in acceleration (braking, etc) you can get a fairly dramatic weight transfer when you have a high CG and soft springs with long travel.
 
How stiff are your rear springs compared to your front springs or vise versa? Also how much lift over stock are you? It could be the angle but it looks tall.


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I understand the momentum and braking. It might have prevented the tipover and will be in my mind next time for sure.

My approach to descending has typically been 4 lo and off the brakes, but as you both said it's often counter intuitive and on road reaction is strong. Others have mentioned even rolling through in neutral.

Exculding driver experience or maybe lack of, anything else jump out?


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May be the angle of the picture, but with it tipped up, it looks like the front axle is turned or pushed backward, on the driver side. And the driver spring look like it has pushed into a negative arch, like the old Blazers. Are the shackles or springs doing something odd? Maybe the front is too soft?
 
NickMaul said:
How stiff are your rear springs compared to your front springs or vise versa? Also how much lift over stock are you? It could be the angle but it looks tall.


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Click to expand...
Front and rear are pretty similar stiffness, but only based on my highly scientific tailgate and bumper bouncing with my body weight. It's lower in the current setup than before. Guessing 4-5 inches over stock, front 44044 pack with added leaves, trail gear hanger, and shackles I forget how long. 63 Chevy rear from 2500 and added leaves. I've thought about re drilling the front shackle to lower a bit, as first simple idea.

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rodney eppes said:
May be the angle of the picture, but with it tipped up, it looks like the front axle is turned or pushed backward, on the driver side. And the driver spring look like it has pushed into a negative arch, like the old Blazers. Are the shackles or springs doing something odd? Maybe the front is too soft?
Click to expand...
The angle is strange. I had to zoom way in to check for myself, but front driver spring is compressed about flat and passenger is droopped without proper spring clips ( on my list with the shackles).

So I think your observation is technically correct in this extreme example, natural motion of the spring and shackle arch is compressed longer spring axle driver back, extension shorter spring axle passenger forward.

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I always descend obstacles in gear, low range, with the clutch in. I use the brakes to control speed.

I always pick a higher gear than normal, just for these reasons.

I almost always use 3rd gear in low range for two reasons.

If things go bad, I can release brake, dump clutch and get on throttle to get the front end to pull quickly. The extra wheel speed also helps to clean the tires out and fight for traction.

Driven out of several bicycles with the talent tank low this way.

Another option to prevent it from happening that is fairly cheap is to mount an atv winch as a rear suck down. You can ditch the cable and use a tree saver strap on the narrow drum also.

That's my .02. A little more planning before hand for exit maneuvers and $50-$100 for a rear suck down. Before going down hill, suck the suspension down, let it out once at the bottom.
 
Loganwayne said:
This is gonna sound crazy but I think your going down the hill to slow.
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Errrbody has covered it, and how to avoid it (see Mac's post) but when you watch poor Morgan go over, you can see what takes place (al-be-it very QUICKLY)

Now we can ALL sit back and armchair quarterback, but me personally? It looks like if he had gased it and dealt with the JOLT, he MAY have avoided the roll. But....I could be wrong, and of COURSE I'm not second guessing Morgans actions, I'm simply speculating what MIGHT have saved it.


I also would have likely panicked and hit the brake and rolled :rolleyes:
 
CasterTroy said:
Errrbody has covered it, and how to avoid it (see Mac's post) but when you watch poor Morgan go over, you can see what takes place (al-be-it very QUICKLY)

Now we can ALL sit back and armchair quarterback, but me personally? It looks like if he had gased it and dealt with the JOLT, he MAY have avoided the roll. But....I could be wrong, and of COURSE I'm not second guessing Morgans actions, I'm simply speculating what MIGHT have saved it.


I also would have likely panicked and hit the brake and rolled :rolleyes:
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Terrible line choice. I wasn't there, and can only speculate from the videos.

There looks to be room to stay to pass side more and continue up farther to where that drop is less significant.

Or back up and pick the line to the left to begin with.

I blame that one completely on gross driver error. Could have been easily avoided. There are a lot of times I can't see over the hood especially when going down hill and to pass side. I always have my pass tell me what they see. If it's on my side, I lean out and look out the window.

My dad has wheeled two very large rigs with me for years, and managed to avoid situations like that with better driving line choice.
Some of it luck, mostly better situational awareness.

I watch the videos and just think, what was he thinking?
 
As for the OP, I'm curious to know what the rig looks like on level ground. Also curious to know how it looks crossed up. For example, put the pass front and the driver rear on equal-height blocks 8-12" high, see how it moves. Are the springs compressing equally, does the body stay level? Or do differences in the front/rear suspension cause the body to rotate? Are there sway bars on one end or the other? It looks like both axles have the shackles in compression. That lowers the roll center, but I think that's stock for the 4Runner, so it would be difficult/impossible to fix.
 
shawn said:
As for the OP, I'm curious to know what the rig looks like on level ground. Also curious to know how it looks crossed up. For example, put the pass front and the driver rear on equal-height blocks 8-12" high, see how it moves. Are the springs compressing equally, does the body stay level? Or do differences in the front/rear suspension cause the body to rotate?
Click to expand...

x2........

I'm not convinced you need a full 2500 pack out of a chevy for the rear. Maybe take one of the bottom leafs out? How is your rear shackle angle?

Perhaps you might have found a limit on your suspension. My XJ doesn't have a lot of articulation with short control arms so I understand that feeling going downhill and maxing out suspension travel and the rig starts to teeter-tot. But I have also confirmed my rear leafs are just barely at negative arch when I hit bump stops. If it feels dicey I have to back up and figure out a new approach.


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I have to agree with Nick in that it looks tall for the tire. Having great stability on a full leaf spring rig will be a fight.
 
:lol: I know it as "Unsympathetic, stubborn, most hardheaded person you've ever met"

This fit's to the TEE only 2 individuals I've ever encountered:

1) My bull dog "Atlas"
2) Shawn

Both own it....and wear it like a badge, and have to be respected, if not admired, for owning it with no apologies. Both slobber uncontrollably, and sort of look like Samuel L Jackson when they've encountered the stupidity of others
Samuel-L-Jackson-Bedtime.jpg
 
The other Webster:

Obdurate implies an active resistance of the heart and will against the pleadings of compassion and humanity.
 
when in doubt, throttle out
 
No sway bars Front or rear. She is tall on the 285s, this suspension Height Started on 315 duractracs. I'll play with the service ramps on the drive way tomorrow morning.

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I'll get flat ground pictures too. No wild shackle angle in the rear.
 
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