Center of gravity thread

[bebe]

Also lifting IFS is hurting your COG unless the trackwidth is brought out or flex is gained?

If you go up you have to go out to maintain COG.

For instance - when you crank the t-bars and add 315 75 R16, you are doing just that. When you add a rancho 4" and decrease the backspacing and add width to the tire you are doing just that.

My track width increased an inch but my tire width increased by 3 inches. So I'm 4 inches wider and 6 inches taller. However the added travel 9" makes up for the 2 inches difference in height and width.

 

[HUMMER INVESTMENTS]

Heard from one of my good friends, that when Bebe took him up the escalator he was beyond scared! Seemed like she got him scared without scaring herself:thumbs:

 

[ReconH3]

Recon:One of the toughest rigs to build is a daily driver. Comp rigs can be torn down and put stuff wherever, it really makes life easier. DD's need to have space for multiple passengers, trunk space for the crap we need and all the other stuff that can hinder the COG. Btw where do you keep your hi lift? I doubt you need one but there is no way I can store my 60" hi lift inside, so thats why its weight up top on the rack.

Also to recon: what is your trackwidth? do run the same full widths that the ORE JK's run. They are around 88in wide

I have no clue what my track width is and won't be able to measure it till I get back after new years.

My HiLift sits in my trunk, right behind the rear seats. I got the one that fits perfectly from side to side. It's strapped down to the OEM tie down loops.



"Ex Umbris Venimus"

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[Ron B]

I think bebe once said start getting scared at around 38.

good advice -- I wouldn't go much past that in an h2 or 3.

 

[Portager]

I found a useful site for determining your center of gravity.

The best site I've found is the jeepaholics site :link: which provides a long, relatively clear and very detailed discussion of all the math involved in calculating the 3 dimensional (x, y & z) CoG components and the 4 roll over angles, drivers side, passengers side, forward and rearward roll over angles. Note the driver and passenger side roll over angles will be different depending on the presence and weight of passengers and gear. Just substitute “Hummer” anywhere you see “Jeep” in your mind. Also, the sketches of Hummer’s aren’t very good! There is also a link near the top where you can download an Excel spreadsheet that does all the calculations for you. If you can guess the password to unprotect it you can replace all the occurrences of “Jeep” with “Hummer”.

As you will see as you read through the discussion, you can use a standard scale to determine the vertical (Zcg) but only if you weigh one axel and raise the other axel of the vehicle. You could also accomplish the same thing if you were using 4 portable scales on a slope, but if it was a side slope you would have to re-derive the equations and you couldn't use the spreadsheet.

 

[06 H3]

Bumping this thread: This is kind of COG related but thought I would throw it here.

The topic is weight...

Why do some feel the ligher your rig is the better off you are? How true is this? I mean I feel the H1 is an exception. In stock form I feel its one of the best rigs you can get without any modifications.

How about adding weight to the front? I feel its good and bad. The good is that for IFS especially it can force more traction but at the risk of more stress and wouldnt it also contribute to sliding in muddy situations vs. floating on top as a lighter rig would?

 

[ReconH3]

Lighter is always better. Especially in low traction terrain. That's why I have an H3 over the other two Hummer models. Even the H3 I would consider heavy. You don't want to add too much weight to the front or you might get a bad surprise the day you have to go down a steep hill. Always aim for weight as much in the middle and low as possible. The engine, winch and bumper create more than enough front downforce.


"Ex Umbris Venimus"

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[3Hummers]

Generally you want the weight load evenly distributed. Lower is better than higher. 50/50 weight distribution is desired but rarely achieved.

 

[cgalpha08]

06h3 how is your COG with lift and dual spares and gobi doin?

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[ReconH3]

Generally you want the weight load evenly distributed. Lower is better than higher. 50/50 weight distribution is desired but rarely achieved.

That's nearly 100% correct. Not truly applicable in a world with gravity though. When you climb, gravity works against you so having weight forward helps with traction. Going down, you don't have to worry about so much. Just enough to not flip forward. Priority goes to climbing since its where most things are working against you. Just the fact that you have the engine up front works in your favor. 4x4 is one of those motorsports where rear mounted engines would not be an advantage.


"Ex Umbris Venimus"

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[3Hummers]

I thought about that but with our up/down cross slope, etc it was a general statement. Most of my experience is from the sports car world were even weight distribution front to back is highly desirable.

 

[elripster]

One other thing higher COG's do is discourage IFS flex. As your vehicle crosses up over an obstacle it will often lean one way or another. As the COG leans it is often moving away from the wheel you want to stuff and on to the wheel you want to extend. The result is less stuffing and more leaning. I'm sure you have all see an IFS rig crawling along, lift a wheel a little bit, then lift it a lot. That is the effect of the COG moving to the lower wheel exacerbated by the fact the the force of gravity is no longer inline with your direction of suspension travel. Building the lowest best clearance 4X4 usually yields the most stability and traction since wheels spend more time on the ground than they otherwise would.

06, I think you asked a question about body roll and shocks. Shocks don't limit the magnitude of roll unless you are at the very end of travel at rest (which is bad for a lot of other reasons). They can in some instances limit the rate of roll but not usually very much since roll doesn't usually happen that quickly (compared to hitting a bump for example). Shock force is a function of speed so the faster you try to move the shock the more force it exerts against the movement. To reduce roll the best way is a thicker stability bar. You can also do it with stiffer springs at the expense of ride quality and off road flex.

Frank

 

[06 H3]

Thanks for all the input guys. I posted this because I was thinking about moab and my lack of front end weight compared to the rear. I have two 82lb tires (tire/rim combined) in the back and a lot of crap in the back...well not crap its useful stuff. I have added no front end weight. No bumpers, winches etc. I can feel it but I wouldnt say it has made me get stuck vs. not getting stuck without it.

I want your guys opinions on what I should do with the front end in the future. I can add lots of weight or try to minimize it. Tim and I have had conversations about this and back in xmas of 09 when I purchased the rack I had little "off road science" knowledge. I didnt think about COG or suspensions and all the other technical aspects. Back in 09 I was leaning more towards the overland/camping expeditions with a trip here or there with some rocks but since then my build has changed. I am more towards the performance on tougher trails perspective, I plan to eventually make changes to the rig but will probably still hold onto the rack when I do overland someday. I wanna do both but am more towards harder trails then overland. While I became interested in tougher trails I became super interested in types of suspensions, spring rates, COG, etc. It really interests me, I can sit down and read about that kinda stuff all day. So I am now trying to decide what to do in terms of a front bumper. The bumper will not be done or on before moab and wont even be started till after. I just want advice before I look into bumpers. The front end weight will help for steep climbs but will also hurt in the mud because instead of floating I will be sinking right? What is a good weight to aim for?

As for lighter vehicles I think its true to an extent. EVO1 is over 6000k lbs (stock JKs are under 4500) and that thing is a monster off road! H1's are heavy but they can do some extreme stuff too. A group of H1's ran pritchett canyon, thats tough.

 

[06 H3]

For moab though, I will be able to put everything in the middle and on the floor because I wont have any passengers in the back. I will be bringing the 2nd spare but once there I may take it off or put it in the trunk.

 

[elripster]

Weight = more breakage. The lighter the truck the longer everything that has to carry that weight will last.

I suggest adding what you need. Be it a winch and bumper then you have also added some weight up front. Since adding weight always means more fuel consumption, wear, and tear on the truck you generally want to make sure the trade off is worth it.

Frank