Jump to content


Photo
- - - - -

Grandby actual weight and cg FYI


  • Please log in to reply
9 replies to this topic

#1 Jon R

Jon R

    Senior Member

  • Members
  • 905 posts
  • LocationWashington State

Posted 08 October 2021 - 08:51 PM

I just weighed my truck with and without my 2021 Grandby front dinette slide in. I have hot water with outside shower only and the 130 liter 2 way fridge, 50 lbs of solar panels between the vents, a 45 lb lithium battery in the original compartment at the rear, flush sink and stove, and a few extra pounds of external LED light options. Water tanks and propane tanks were full, no other water, food, or equipment in the camper except for a portapotti with 3 gallons of water in it in the rear portapotti cabinet.

The camper weighed 1740 lbs, of which 100 was on the front axle and the rest was on the rear axle. (CCLB with 172” wheel base). This puts the cg 43 inches back from the front of the camper front wall bumpers, or about 39.5” back from the front wall of the camper. I was surprised the cg wasn’t further forward given the overhang and water lank location.
  • 1

Jon R

2021 Grandby Slide-in Front Dinette

2021 GMC 3500HD CC LB SRW Gas

1993 Chevrolet K2500 Ext Cab LB


#2 EYEMLOST

EYEMLOST

    Advanced Member

  • Gone Traveling
  • PipPipPip
  • 71 posts
  • LocationPNW

Posted 08 October 2021 - 09:08 PM

I suck at math.

 

 

How did you come up with where the CG is?


  • 0
1998 FWC Grandby
1994 Ford Bronco 5.0 XL 4X4
Sky's ORD 6" Lift / Sterling 10.25 Dually 5.13 Gear Detroit Locker / '99.5 Front F-350 Leaf Springs at Rear / HMMWVtires

#3 Jon R

Jon R

    Senior Member

  • Members
  • 905 posts
  • LocationWashington State

Posted 08 October 2021 - 11:49 PM

It’s a little challenging to explain without making a diagram. Think of the truck in side view as a simple beam spanning two supports. The axles are the supports. We want to figure out the point along that beam where, if all of the weight of the camper was concentrated at that point, it would produce the same axle loads as the measured axle weight effects from the camper. I measured a 100 lbs increase in front axle load and a 1640 lbs increase in rear axle load from the camper, for a total load of 1740 lbs.

One way to calculate the location of the equivalent camper point load location is to think of the camper and support forces as applying moments (an engineering term) or torque around one of the support points, which function as hinges. Think of the front axle support point as a hinge. The truck is not accelerating in rotation around that hinge, so torque or moment around that hinge from the rear axle support and from the point load of the camper must be equal and opposite.

The torque from the rear axle support in the upward direction is 1640 lbs times 172 inches, or 282,080 inch pounds. The moment or torque about the front axle support from the camper pushing the beam downward must be equal and opposite, so it’s 282,080 inch pounds also. So how far out along the beam does 1740 lbs need to be applied to create that torque? 282,080 inch pounds divided by 1740 pound is 162.1 inches. That is the distance of the cg of the camper from the front axle. (A similar calculation using the rear axle as the hinge point will give the same location in distance from the rear axle.) You can relate that location on the truck to the location on the camper by simple measurements.

Make sense?

Edited by Jon R, 08 October 2021 - 11:50 PM.

  • 0

Jon R

2021 Grandby Slide-in Front Dinette

2021 GMC 3500HD CC LB SRW Gas

1993 Chevrolet K2500 Ext Cab LB


#4 EYEMLOST

EYEMLOST

    Advanced Member

  • Gone Traveling
  • PipPipPip
  • 71 posts
  • LocationPNW

Posted 09 October 2021 - 01:09 AM

It’s a little challenging to explain without making a diagram. Think of the truck in side view as a simple beam spanning two supports. The axles are the supports. We want to figure out the point along that beam where, if all of the weight of the camper was concentrated at that point, it would produce the same axle loads as the measured axle weight effects from the camper. I measured a 100 lbs increase in front axle load and a 1640 lbs increase in rear axle load from the camper, for a total load of 1740 lbs.

One way to calculate the location of the equivalent camper point load location is to think of the camper and support forces as applying moments (an engineering term) or torque around one of the support points, which function as hinges. Think of the front axle support point as a hinge. The truck is not accelerating in rotation around that hinge, so torque or moment around that hinge from the rear axle support and from the point load of the camper must be equal and opposite.

The torque from the rear axle support in the upward direction is 1640 lbs times 172 inches, or 282,080 inch pounds. The moment or torque about the front axle support from the camper pushing the beam downward must be equal and opposite, so it’s 282,080 inch pounds also. So how far out along the beam does 1740 lbs need to be applied to create that torque? 282,080 inch pounds divided by 1740 pound is 162.1 inches. That is the distance of the cg of the camper from the front axle. (A similar calculation using the rear axle as the hinge point will give the same location in distance from the rear axle.) You can relate that location on the truck to the location on the camper by simple measurements.

Make sense?

So; is the number 282,080 a constant?

 

I can use it to plug in my own weight?

 

 

I spent a lot of time at CAT scale weighing different set ups, so I have accurate camper weight.


  • 0
1998 FWC Grandby
1994 Ford Bronco 5.0 XL 4X4
Sky's ORD 6" Lift / Sterling 10.25 Dually 5.13 Gear Detroit Locker / '99.5 Front F-350 Leaf Springs at Rear / HMMWVtires

#5 Jon R

Jon R

    Senior Member

  • Members
  • 905 posts
  • LocationWashington State

Posted 09 October 2021 - 01:25 AM

282,080 inch pounds was for my particular measured axle weight (contributed by the camper) and wheelbase.

You need to have your camper weight contribution to each axle weight to calculate cg. To get that you would measure each axle weight with and without the camper. (Or total weight minus one axle weight to get the other axle weight)

If you would like to provide your axle weights with and without the camper and your wheelbase measurement I will calculate cg for you. Alternatively, the formula is:

(Camper weight on rear axle)x(wheelbase)/(total camper weight)=(cg distance from front axle)
  • 0

Jon R

2021 Grandby Slide-in Front Dinette

2021 GMC 3500HD CC LB SRW Gas

1993 Chevrolet K2500 Ext Cab LB


#6 EYEMLOST

EYEMLOST

    Advanced Member

  • Gone Traveling
  • PipPipPip
  • 71 posts
  • LocationPNW

Posted 09 October 2021 - 01:59 AM

282,080 inch pounds was for my particular measured axle weight (contributed by the camper) and wheelbase.

You need to have your camper weight contribution to each axle weight to calculate cg. To get that you would measure each axle weight with and without the camper. (Or total weight minus one axle weight to get the other axle weight)

If you would like to provide your axle weights with and without the camper and your wheelbase measurement I will calculate cg for you. Alternatively, the formula is:

(Camper weight on rear axle)x(wheelbase)/(total camper weight)=(cg distance from front axle)

Cool; here are the numbers:

 

Without camper

Front axle: 2,820lbs

Rear axle: 2,420lbs

 

With camper

Front axle: 2,820lbs

Rear axle: 3,360lbs

 

Wheelbase: 110"


Edited by EYEMLOST, 09 October 2021 - 02:03 AM.

  • 0
1998 FWC Grandby
1994 Ford Bronco 5.0 XL 4X4
Sky's ORD 6" Lift / Sterling 10.25 Dually 5.13 Gear Detroit Locker / '99.5 Front F-350 Leaf Springs at Rear / HMMWVtires

#7 Jon R

Jon R

    Senior Member

  • Members
  • 905 posts
  • LocationWashington State

Posted 09 October 2021 - 04:29 AM

You don’t need to do any calculations for your case assuming the front axle weight not changing isn’t a typo. If the front axle weight is unchanged when you load the camper, the cg of your camper is exactly above the rear axle.
  • 0

Jon R

2021 Grandby Slide-in Front Dinette

2021 GMC 3500HD CC LB SRW Gas

1993 Chevrolet K2500 Ext Cab LB


#8 EYEMLOST

EYEMLOST

    Advanced Member

  • Gone Traveling
  • PipPipPip
  • 71 posts
  • LocationPNW

Posted 09 October 2021 - 04:02 PM

Cool; here are the numbers:

 

Without camper

Front axle: 2,820lbs

Rear axle: 2,420lbs

 

With camper

Front axle: 2,820lbs

Rear axle: 3,360lbs

 

Wheelbase: 110"

 

You don’t need to do any calculations for your case assuming the front axle weight not changing isn’t a typo. If the front axle weight is unchanged when you load the camper, the cg of your camper is exactly above the rear axle.

The actual weight with Bronco and camper ONLY was -

Front axle: 2,560lbs

Rear axle: 3,360lbs

 

 

After discussion here about weight distribution in another thread, I added four water jerry cans to the front to get the front axle back to 2,820lbs.

 

215292755_4466947396703136_3572274325422

 

You will notice there is blue tape on the side of the camper.

 

That represents the camper new CG, while FWC's CG point (white mark) is just to the right of said blue tape. 

 

I put a piece electric tape directly above the rear axle, on the top bed rail to further use as a reference point. 

 

As it sits; the camper's CG is 11" behind the rear axle.

 

 

My sig has my Bronco current set up.


  • 0
1998 FWC Grandby
1994 Ford Bronco 5.0 XL 4X4
Sky's ORD 6" Lift / Sterling 10.25 Dually 5.13 Gear Detroit Locker / '99.5 Front F-350 Leaf Springs at Rear / HMMWVtires

#9 Jon R

Jon R

    Senior Member

  • Members
  • 905 posts
  • LocationWashington State

Posted 09 October 2021 - 05:30 PM

Using numbers for axle weight changes due to changes in addition to the camper installation prevents correct calculation of camper cg using the method I described.
  • 0

Jon R

2021 Grandby Slide-in Front Dinette

2021 GMC 3500HD CC LB SRW Gas

1993 Chevrolet K2500 Ext Cab LB


#10 EYEMLOST

EYEMLOST

    Advanced Member

  • Gone Traveling
  • PipPipPip
  • 71 posts
  • LocationPNW

Posted 10 October 2021 - 09:48 PM

I google-fu'ed this topic and found a calculator.

 

According to a website calculator, my longitudinal CG is near center of wheelbase:

60" from front axle. (2,820lbs)

50" from rear axle. (3,360lbs)

 

110" wheelbase


  • 0
1998 FWC Grandby
1994 Ford Bronco 5.0 XL 4X4
Sky's ORD 6" Lift / Sterling 10.25 Dually 5.13 Gear Detroit Locker / '99.5 Front F-350 Leaf Springs at Rear / HMMWVtires




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users