How to determine a safe angle on a side hill?

m_lars

m_lars

Rank V
Launch Member

Off-Road Ranger I

2,041
Heber City, Utah
First Name
Matt
Last Name
Larson
Member #

8212

Ham/GMRS Callsign
KJ7ATX
I was out today on a trail right near my house, I was traveling on a section that left me feeling uncomfortable with how much I was leaning. This got me thinking, how do I know what is unsafe and what is OK? Obviously there are many variables depending on the rig and how it's loaded, but how would one determine what is safe?
 
I wonder about that too. Outside of the obvious answer above, I think it might just be through experience. I’ve seen guys installing meters but I wonder if you can really go by those.
 
So here’s the mathematical answer to your question


That said, the potential of rolling my vehicle has always been my biggest fear. I’ve been in some pretty uncomfortable situations off-road where I’ve felt close to tipping, but in reality I probably wasn’t even close. The problem is not knowing. Because of this I err on the side of caution and don’t unnecessarily raise my CG by putting gear on my roof.
 
  • Like
Reactions: Lindenwood and In Itinere
The NHTSA publishes rollover-related data that compares CG height to track width. Even SUVs of the 1990s that were most prone to rollovers actually could handle greater than 45 degrees without rolling.

For example, the factory 3rd gen 4Runner has a CG height of about 28" (unsure if this is with fuel or not). However, from the center of the vehicle to the outer edge of the factory tires is about 34". That means it would take a hair over 50 degrees to roll the vehicle, or perhaps a little less due to tire deformation. Still, if you only go to the center of the tire instead if the outer edge, that is still about 46 degrees before rolling.

On my rig, I measure about 35 inches from the center of the vehicle to the outer edge of the rim. I am lifted about 1.5-2" over stock, plus 1" for the 33s, but honestly I dont think I have changed the body CG much since most of my added weight--skids, bumpers, wheels / tires--is at or below the factory CG height. The weight of the driver and a full tank of gas move the CG left of center by about 1.5 to 2". Then, lets assume that my own weight and my tools and such raise my CG by as much as 3" (being quite generous). that puts me at 34" max CG height right now. So, in this roughly worst-case scenario, with a left-biased CG (and leaning left), my rig still requires roughly 44 degrees to actually roll. If you measure to the center of the tires instead of the edge, we still get 41 degrees as the limit.

All that said, anything more than about 20 degrees is quite unsettling to me, heh.
 
Last edited:
  • Like
Reactions: 4wheelspulling and Boort
This is one of the reasons I am cautious about roof racks (the other is because they are, IME, a PITA to use).

Lets take a 4500lb vehicle with a static CG height of, say, 30". Then, add 400lb at a height of 76" (assuming most offroad SUVs put the base of the rack at over 6ft). That raises your CG by almost 4"! If I did this to my rig, the above lean tolerance numbers go down to 42 and 38.5ish degrees, or essenially a 10% decrease in rollover resistance.

I built a rear swing-out tray, and am about to attach a tow hitch to my front bumper so I can build a small, elevated rack for a single heavy item like a mid-sized plastic box (that fits between the headlights). Between the two (and my cargo drawers), I anticipate zero issues carrying anything I would need for the camping we do, will maintain easy access to my stuff, and won't dramatically affect CG or gas mileage like stuff on the roof.

*edit*

Of course, all this math assumes a 100% stable platform. Once you start adding suspension sway, rocking, shifting soil, low-pressure tires, etc, these numbers probably go down a bit. Still, I think a careful driver could easily get within, say, 25% (about 10 degrees) of these theoretical lean angles without major concerns. For me, that would put me around 33-35 degrees as my safe limit, which is actually pretty close to @madcratebuilder 's instincts :) .
 
Last edited:
  • Like
Reactions: madcratebuilder
Lindenwood's comment on "suspension sway, rocking, shifting soil, low-pressure tires, etc," is very important IMHO. Many of our rigs have soft suspensions that can compress a considerable amount, down hill tire rolls in to a small depression as uphill tire rolls over a slight raise. It can cause serious pucker factor.
 
  • Like
Reactions: 4wheelspulling
I ran some more numbers today. Without my sway bars connected, if I lean hard enough to put all the weight on one side of the wheels, the wheels move up in their travel a theoretical 7" (not sure I actually have that much up-travel, but stick with me). In doing so, the body rolls toward the low side around the upper connection of the panhard bar, which is about 24" high. With an assumed 34" CG height, this means the CG moves left about 2.4". In turn, this makes my actual static rollover numbers about 42 degrees, or 38.5 degrees if I measure to the center of the tires instead of the outer edge.
 
Unfortunately is truly just something you have to try over and over again to get comfortable with a point where you know you won't roll over but something to think about even being less than a point where your vehicle normally roll over if you hit a bump or something it can easily tossed the vehicle over you could do it at less than half of the normal rollover point
 
  • Like
Reactions: 4wheelspulling
Oh!

I hit the trails today with my phone in its dash mount. Usually I use ut for maps, but today I split the time between maps and this new angle-finding app. I forgot to have it up on my most extreme obstacles, but even ones that Id have sworn have me leaning over 20 degrees were usually like 13-15, including body roll!
 
  • Like
Reactions: 4wheelspulling
We should all drive over to a Land Rover dealership and play on their rock demo setup. That thing scares me... Tire pressure, tire ply, speed and the type of suspension play an important of rolling. I rolled my Tacoma and it was crazy how much it leaned before it finally let go. To help with lean angle - you can install wheel spacers to widen your track, not that I recommend it but if you are truly worried and do a lot of sidehill trails then go for it.
 
If you start feeling sketched out and can do so safely, just get out and look at your rig. It's almost never as bad as it feels from behind the wheel.
 
The thing is, given the above statements, you have to wonder how the hell did that guy feel like whatever he was doing was safe?! :P .
 
  • Like
Reactions: Kevin108
Kevin108 said:
I left "almost" because, as seen above, some people still manage to flop. Had to leave some room for error. :wink:
Click to expand...

You weren’t wrong, I just think it’s funny how it feels the worst from in the vehicle. It looks like even less steep if you take a picture of it!
 
  • Like
Reactions: Kevin108
Lindenwood said:
The thing is, given the above statements, you have to wonder how the hell did that guy feel like whatever he was doing was safe?! :P .
Click to expand...
I think speed was a bigger factor than angle.

m_lars said:
You weren’t wrong, I just think it’s funny how it feels the worst from in the vehicle. It looks like even less steep if you take a picture of it!
Click to expand...
Pictures and video ruin it, for sure. You can easily drive up something too steep to walk up, but if you capture it...suddenly it's just a dirt road.

eg:

 
Last edited:
  • Like
Reactions: 4wheelspulling, Lindenwood and Graeman
well the flop was caused by that large stump behind the jeep , came over the little sand burm, in 2nd gear 4 low so maybe 10 mph and as soon as I got to the top of the berm I saw the log but it was to late to turn . The front tire drove right over it but it hit the rocker and rolled the stump up higher and it was to much for the rear suspension and slowly flopped over . Thankfully everybody was safe and all the gear is always very secure so nothing shifted , only damage was to the rocker where the stump hit and a dent in the drivers door .
 
  • Like
Reactions: Lindenwood
You must log in or register to reply here.
Top Bottom