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Open Roads Forum  >  Truck Campers

 > Approximating Truck/TC Vertical CG - dhackney

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BradW

Mayor of Flat Rock

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Posted: 06/06/07 02:23pm Link  |  Print  |  Notify Moderator

Note: This information was originally posted by dhackney on 6/6/07.
BradW - moderator



I used the XL model posted on the Jeep forum at: http://www.jeepaholics.com/tech/cog/


My methodology for the lift was:
1. Locate and mark the horizontal centerline of front axle. This ensured I would always be using the loaded tire radius for the calculations. Since both tires are the same circumference, I didn’t have to worry about averaging those as you would on a race car set up for stagger.
2. When we lifted the front axle by driving it up on the tilt bed, I used the mark on the front axle and a laser level to obtain the lifted front axle centerline height of the loaded front tires.
3. I did not replace the shocks with solid bars to eliminate suspension height change, as this would have been impractical in my circumstances. I made no allowance for the compression of the rear suspension, the corresponding lift of the front suspension and the resulting weight transfer rearward.
4. I made no allowances for the shift of liquids in the tanks, specifically the fuel tanks, which are long and narrow. The estimated 185 lbs. of fuel in those tanks would have shifted towards the rear.
5. I only took one measurement and one weight at one height.
6. We used a moving company truck scale that allowed us to weigh each wheel, each side and each axle individually.
7. We used a tilt bed wrecker as a ramp to drive the front axle up to obtain the required front contact patch lift.
8. Total cost was $10 for all the weights (amazing!) and $75 for the wrecker.


Test conditions:
• Fuel tanks: ~25 gallons. The tanks are saddle tanks located under the garage along the frame rails. Equal amounts in both tanks via a crossover. There are no baffles, so this weight would have all shifted back. That’s 180 lbs @7.2 lbs/gallon.
• Raw water: ~35 gallons. The tank is mounted between the frame rails aft of the garage. No baffles. That’s 291.5 lbs. but since the tank was ~75% full there would have been minimal shifting.
• Fresh water: 33 gallons. The tank is mounted under the dinette, left side of camper centerline, aft of the rear axle. Full load, 100% full. That’s 266.56 lbs.
• Gray water: 0
• Black water: 0
• Front axle centerline, static: 14.375” / 365.13mm
• Front axle centerline, raised for test: 42.875” / 1,089.03mm
• Amount front axle centerline raised for test: 28.5” / 723.9mm
• Front contact patch elevation raised for test: 28.5” / 723.9mm
• Front track: 71.75
• Rear track: 79
• Wheelbase: 154

On the front axle lift amount, I had previously contacted the author of the Jeep spreadsheet and asked if I needed to go higher than 24” since our FG was a longer wheelbase. He responded that any higher lift would not significantly improve the accuracy of the model.

In summary, I did not use nearly enough process rigor for the tilt test to obtain a valid result. At a minimum I should have had full tanks and I now believe the only way to get an accurate test is to replace the shocks with bars to eliminate suspension travel.

Because we have a 3 point pivot frame, I believe the only way we will ever accurately know our left and right roll angle limits is to put the truck on a tilt table. I am currently seeking a resource to perform that test so we will have a definitive answer using empirical data.

Shots of the tilt test:
[image]

[image]

[image]

Doug


Wake Up America
1996 Lance 500 and 2006 F-350 PSD 4X4 DRW
Our Truck Camper Photos


EricCO

Colorado

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Posted: 06/06/07 02:57pm Link  |  Print  |  Notify Moderator

Why?


2002 Ford F-350 CC LB PSD
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BradW

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Posted: 06/06/07 03:03pm Link  |  Print  |  Notify Moderator

Because. [emoticon]

dhackney

San Diego

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Posted: 06/06/07 05:36pm Link  |  Print  |  Notify Moderator

EricCO wrote:

Why?


This information is probably only relevant or desirable if you are heading off of pavement or off-road with your TC.

We need to know where our center of gravity (CG) is and what our resulting roll angle limits are because we will be using this vehicle for a good portion of its service life in unpaved and off-road environments.

Our particular truck chassis has a pretty narrow track and we built a vehicle with a relatively high CG. If, by driving on an inclined road where the left or right side is higher than the opposite side and we get our CG outside the track width, the truck will tip over. That's a real concern where we are headed, so we are putting energy and effort into determining what the actual limits of the vehicle are before we get ourselves into that situation.

From a practical standpoint, in a cab over design you are perched up so high and over the front axle, the pucker factor sets in very early, especially for my wife. If I can obtain a real, empirical roll angle limit number I can prevent some unnecessary stress for both of us by comparing our pucker alarms with the roll angle guage on the dash.

Again, if you are staying on pavement or level roads, it's more a matter of curiosity than a requirement.


2007 Mitsubishi Fuso FG 140 4x4
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Rig buildup site: http://www.hackneys.com/mitsu/
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dhackney

San Diego

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Posted: 06/06/07 05:57pm Link  |  Print  |  Notify Moderator

To give you an idea of what the test produces, here's the results of the tilt test after compensating for shifting liquids and suspension flex.

These revised calcs are based on a very minor (relative to vehicle weight and weight of fluids) allowance for rearward shift of liquids and suspension travel during the tilt test.

Center of Gravity (CG) is probably lower, but I'm going to stay conservative.

Right and left roll angles are based on the front track width. Values using the rear track width are ~two degrees higher.

Due to the 3 point pivot frame used for the camper & rear storage boxes, I don't think there's any way to know the real roll angles until we get the vehicle on a tilt table.

CG: 58.92" / 149.66cm high, 98.14" / 249.28cm behind front axle centerline
Roll angle left: 31.12 degrees
Roll angle right: 31.56 degrees
Roll angle back: 43.48 degrees
Roll angle forward: 59.02 degrees

(the yellow star in these photos should more properly be labeled "Center of Gravity")

[image]

[image]

[image]

[image]

[image]

[image]

Surferkim

Sunset Cliffs,San Diego,Ca.

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Posted: 06/06/07 07:03pm Link  |  Print  |  Notify Moderator

dhackney wrote:

EricCO wrote:

Why?


This information is probably only relevant or desirable if you are heading off of pavement or off-road with your TC.

We need to know where our center of gravity (CG) is and what our resulting roll angle limits are because we will be using this vehicle for a good portion of its service life in unpaved and off-road environments.

Our particular truck chassis has a pretty narrow track and we built a vehicle with a relatively high CG. If, by driving on an inclined road where the left or right side is higher than the opposite side and we get our CG outside the track width, the truck will tip over. That's a real concern where we are headed, so we are putting energy and effort into determining what the actual limits of the vehicle are before we get ourselves into that situation.

From a practical standpoint, in a cab over design you are perched up so high and over the front axle, the pucker factor sets in very early, especially for my wife. If I can obtain a real, empirical roll angle limit number I can prevent some unnecessary stress for both of us by comparing our pucker alarms with the roll angle guage on the dash.

Again, if you are staying on pavement or level roads, it's more a matter of curiosity than a requirement.

All I can say is " I want to go with you!" and bring my Surfboard[emoticon]


2005 Dodge 2500 SLT+,CTD 610,Auto,4x4,LWB,QC,Leather
00/915 Lance,AC
LCD/DVD,custom surf racks,Hellwig S/B,Rancho 9000's,Lifeline AGM,PD 9245C
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One Beautiful Wife,4 year old Son & 9 week old Son">">


4SEASON

Thousand Oaks, CA. USA

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Posted: 06/06/07 08:12pm Link  |  Print  |  Notify Moderator

Doug,

That's all fascinating and elegant mathematics...
I'm kind of a seat of the pants practical guy.... I would recommend:

Loading the truck for a trip.

Put a fork lift under the drivers' side.

Rig some stops so the truck can't tip over.

Both you and your wife get in the cab and permanently stick an angle level to the dash.

Have your buddy slowly raise the driver's side with the forklift , tipping it toward the passenger's side to see where the balance point is.

Read the angle gauge on the dash and record the balance point.

Lower the truck, put the forklift on the passenger side and repeat the process.

(I'm not try'n to be a smartass... that's just how I would do it [emoticon] )

dhackney

San Diego

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Posted: 06/06/07 08:28pm Link  |  Print  |  Notify Moderator

4SEASON wrote:

I'm kind of a seat of the pants practical guy


A man after my own true heart! [emoticon] I'm in total agreement with your approach. I believe in science, math and all the rest, but to me there's just no substitute for putting this thing on a big tilt table and finding the real limits. Especially with the 3 point pivot frame back there going back and forth, I don't really see a way to do this mathematically, although I'm sure it's possible if we jack the frame all the way over to each side and repeat the tilt test properly.

I'm too chicken to do it with the fork lift. I'd rather pay somebody with a big tilt table. That way if they drop it on its side there's somebody between me an my wife with some responsibility other than my buddy with the fork lift. [emoticon]


4SEASON wrote:

(I'm not try'n to be a smartass... that's just how I would do it [emoticon] )


No offense taken. I'm in full agreement. Except for the part where I'm sitting next to her when the chain breaks and we tip it over on its side...

4SEASON

Thousand Oaks, CA. USA

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Posted: 06/06/07 08:48pm Link  |  Print  |  Notify Moderator

[emoticon]

really look'n forward to reading about your travels...

Godspeed.

Harald

Sumner, WA

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Posted: 06/07/07 10:02am Link  |  Print  |  Notify Moderator

Although I've taken my truck and camper off road, I've never had to think about tipping over. You must go on some seriously rough roads.

Check out this truck and camper.
http://img295.imageshack.us/my.php?image=f550camperyz1.jpg
I wonder what the roll limits are on this one?


Harald
------------------
2000 Ford F350 CC 4x4 DRW with Superhitch/Rancho RS9000 shocks/Firestone Ride-Rite airbags
1998 Alpenlite Pendelton 11'10" camper

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