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center of gravity and weight distribution

Topdown

Topdown

1/2 ton status
Joined
Nov 26, 2002
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Vancouver, Washington, U.S.A
In building my truggy I have come to the very obvious conclusion that it will be nose heavy... for those of you that have built them or driven them or both, what have you done to redistribute the weight? I dont have to be 50/50 but I would like to be better than 80/20

Hanging my spare & jack as far back as possible, mounting my gas tank as far back as safely possible... what else should I consider?

Also, what has anyone done to lower the center of gravity on thier lifted rig? I think the first roll-over would be fun but I am certain that it would only be fun for the first few minutes after it was over...

-Ryan
 
rear mounted radiator, battery in the back, headers, aluminum heads and intake(lighten the front end) thats what pops up off the top of my head
 
Push the engine & trans back as far as possible.

Move the seats back.

Move the cooler back.

Put the tool box in the back.

Partly fill the rear bumper with #9 lead shot.

Move the rear axle back.

Dump the iron m/c and put an aluminum one on.

Dump the brake booster.
 
Topdown said:
I had considered relocating the battery since I was going to narrow the nose a bit... I guess it would make sense to put it "out back"
Click to expand...
Put it down low and offset to the right, too, to help with the CG... of course, not so low it becomes a rock magnet or an anchor. IMO, ahead of the rear axle but lower, is better than behind the rear axle but higher.
When I re-do the floors in my K5, I plan to make a dual-battery compartment under the cargo floor, behind the pass. seat, to help offset the weight of the 6.2 that's getting swapped-in.
 
Find a local race shop that has 4-corner scales and have them weigh your truck as it sits now..... I'll BET you it really isn't 80/20.

In fact, I was very surprised to see that my K5 (with top installed) had almost perfect 50/50 weight distribution!...Of course that was back in the 1/2-Ton axle days and I've made a lot of changes since then....but I think if there's a way to get a measurement NOW, you will have a better idea about how "mental" you will need to get about moving that front-end weight around.....it might be less than you expected??? :cool1:
 
In my S-TON build, I moved the cab back as far as I could in the center of the frame so to speak. I will also be removing some of the leaves from my lift springs to maintain a lower CG. Lastly, the longer wheelbase which usually takes away your Breakover angle - I will make the bellypan completely flat and flush with the frame. My battery will be located out back on the right rear just ahead of the fuel cell. Not that the weight of the battery will offset my fat arse, but anything helps. I think that is a great idea to weigh the rig with race car scales. The Midwest is full of circle track racers - I might steal the idea and weigh mine. Good Topic. What is optimal weight bias?
 
An alternate way to measure corner weights is a tool I have and tried & failed to find a pic of. Basically it is a lever with a pivoting foot. The foot is adjustable for height and the lever has a couple holes for different leverage ratios. The wheel end of the lever has a tab that fits into either the safety bead groove or where the clip-on wheel weights go. The opposite end has a 'socket' welded to it. I leave the one I have on the 10:1 leverage setting.

In use you set the foot's height so that the lever is level when the tab is touching the underside of the top of the wheel. Plug a beam type torque wrench into the 'socket' and lift the tire off the ground. Have a helper place a piece of paper under the tire and set the tire back down. While the helper is lightly pulling on the paper push down on the torque wrench. Take the reading when the paper slips out and multiply by the leverage ratio.

It's not easy like with a set of scales, but with practice it can be pretty accurate and fast.
 
ntsqd listed pretty much what I did.

Gas tank all the way back, tool box and cooler stacked on top. Battery just in front of the tank along with heavy spare parts. My "cab" went back only 1", but that is all I could do with the other goals competing for space.

Pushing the axle back is not going to change the unsprung CoG, but it will reduce the weight on the rear wheels and change the gross CoG.
 
I weighed my burb at the truck scales and it was 2900# front and #2500 rear with tools and 42" spare. I mounted my winch receiver style and built a rear mount that puts the winch flush with the end of the framerails. Spare, fuel, and duall batterys mounted as far back and low as possible. Canned the core support and made one out of tubing. Oh and as much aluminum on the motor as you can afford. The aluminum block in my burb only weighed 80 lbs.
 
well im REAL nose heavy. and im not 80%/20%

im:
3780 front
2500 rear
 
Pushing the axle back is not going to change the unsprung CoG, but it will reduce the weight on the rear wheels and change the gross CoG.
Click to expand...
Been thinking about this, moving rear axle back would move both the gross CG and the unsprung CG back.
CG is the sumation of all of the component weights and it's location is the sumation of all of their distances from each other. Moving any one thing affects the CG. Obviously a 14bff is going to have more of an effect than a 10b, but they both will affect it. It wouldn't be limited to just the weight of the axle, more than likely you'd also be moving the rear springs with the axle.
 
Ok, I'll concede that the springs and longer drive shaft would have some effect on unsprung CoG, but it would be very small I think, though the moments could be larger than I realize... It would also make a diff when the travel is maxed and the axle is figured back into the unsprung CoG (just before an endo) such that it might be just enough to let you scoot out before the point of no return, though the only diff would be due to the incresed moment arm.
 
Effect of the driveshaft, def small.
Effect of springs and axle would depend on the their percentage of the total weight. On a full bodied truck it would be small. On a tube buggy it could be a different story.
 
Well, you know more about the details of this than I do, but this is how I see it.

It seems the unsprung weight is of no consequence in this scenario until the sprung CoG is over the forward pivot point. At that point you are already pretty far nose down, and the weight of the truck that is over the pivot is going to be pulling you further over, stretching out the back springs. If you go very far over, your gross CoG is also going to be over the pivot, and you either power out or get real up-close-and-personal with the ground in front of you. Once you're that far nose-down, the moment arm of the axle hanging from the springs is not that long (in the simplified view) so the gross CoG may well be too far gone. But it's more complicated than that since it's not even just gross CoG, but does not consider the front axle weight among other adjustments that would be needed to be accurate.

At least that's what is on my mind, though I may well be missing something. Stuck in Boston watching the snow and freezing rain, time to get another beer...
 
Now I see where you're going with this.

I'm looking at it sitting flat and level, and then moving parts around in my mind to see where they move the CG. The in-use dynamics I'm letting take care of itself. If we get the CG in the right place it will.

The unsprung weight is a bit difficult when it comes to CG since it can move relative to the frame. Which moves the CG around a little. I think you can make yourself needlessly crazy trying to deal with that. Think about it as a Static situation first, since that is much simpler to work with. Once you've got the Static CG where you think it should be, then tilt it and what-not to see how that affects what you think is the ideal CG location.

Boston, very carefully drove way around that place last time I was on the Right Coast.
 
1UAVRockpile.jpg


I dont know about you, but I think you are all over thinking this :grin:
 
Mine is pretty nose heavy and it is loose going down something steep but it does climb real well. Since most of us try to go up stuff, engineering some nose heaviness is not a bad idea IMO.
 
Not to be argumentitive, but my favorite thing to do in the Dune Buggy is to drop off steep stuff. Better if I drop the fronts over the edge and slide a bit on the bellypan......
 
One more thing to do for centering the CG better: move the front axle forward. Past that, all the relocation stuff works too. Miniwally is about 55% front with the rad. in front. Wally is 60/40 and is a bit sketchy on downhills. I think I figured moving the front axle forward 6" would get me 3% or so change to the weight bias. Alum heads on the bbc will help too.

You don't want to move the rear axle back. Think about an extreme case, if you move the rear back a bunch (10 feet or more) sooner or later you're going to lose all the weight on the rear tires, this is the opposite direction from what we want. Ideally you'd move the front and rear axles forward at the same time.
 
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