| The CAT Scale, 2002
If you want to know what you're towing and whether or not your rig is within specifications/tolerances, you must weigh it on scales. Trailer and tow vehicle weighing begins at a certified truck stop scale, known as a CAT (Certified Automated Truck) scale, a multi-platform mega-scale which weighs each vehicle axle. To begin, you drive on, positioning the tow vehicle's two axles on each of two adjacent platforms and the trailer's axle on a third platform.
This scale is designed for a tractor towing up to two trailers, and it's found at truck stops. You self-serve drive on and position your rig, get out and push a button which alerts the inside attendant, who confirms with you via intercom. You'll want both tow vehicle axles on a single pad on one of the passes. Original weighing cost $7.50; subsequent weighings in the same session were $1/each.
I invested a solid half a day in obtaining the numbers: filling the trailer with water, towing 25 miles to the nearest facility, spending roughly 90 minutes there on Labor Day morning (I pretty much had the scale to myself; time spent doing this for the first time including unhooking/rehooking to obtain scale's tongue weights every time I passed through the scales -- which I did 4 times, plus waiting in line repeatedly for the _sole_ cashier besieged by all manner of truck drivers in for gas/showers/general griping, waiting for the water to drain once I decided to weigh it dry -- even though I know you can subtract 8.35 lbs/gallon, which part of the trailer would I subtract how much from, the tongue or the axle? -- and towing the trailer back to storage. Note: the pads were too long to allow me to obtain separate tongue/trailer axle weights without unhitching and pulling the Jeep away.
I weighed the trailer 'wet' and 'dry'. Wet means with a full water load, in my case a 20 gallon fresh water tank straddling the axle, a 6 gallon water heater in the trunk, and aprx 8 gallons toted in the trunk (2 6gal green skinny totes 2/3 filled each). Dry means all those tanks (not the potti's) emptied as much as is normal stationery on level ground via gravity draining (opening the 3 under-floor valves), meaning that several gallons remained in the 20 gal tank due to the drain opening height. In retrospect I realize I did _not_ weigh the Jeep's rear axle dry, only wet.
Other load factors: the Jeep carries aprx. 350-400 lbs of cartop carriers with windsurfing gear plus windsurfing support items and tools filling the cargo bay to roughly the seat top. The LP tank was filled minus whatever was used during a partial 45-degree night's heating. The Jeep has Timbren rubber load assist springs installed, and I don't know whether or not this affects any per-axle weights. The Cheyenne and Jeep were fully loaded for camping as they have been for months. As I write this up I realize that several weights were missing from the scales:
- me (160 lbs)
- cooler w/food
- dry foods
- my clothing tote
- a 2nd human
- the last 1/4 tank gasoline (the Jeep's tank was ~3/4 full).
So to be utterly precise we would need to add ~250+ lbs in the Jeep for me, a cooler and clothes, and gasoline somehow distributed among the axles, and consider adding more weight for one or two more people (who don't typically camp; their presence is the exception not the rule).
MANUFACTURER'S SPECS (all numbers = pounds):
1998 Jeep Grand Cherokee 4L 4WD:
- gross vehicle weight rating 5300
- gross combined vehicle weight rating 9550
- gross front axle rating 2750
- gross rear axle rating 2950 (axle ratings with P215 tires; mine are P235; I'm unsure if this affects capacity)
- max trailer weight 5000
- max tongue load 750
2002 Coleman Cheyenne:
- gross vehicle weight 3070 +/-5% (~150)
- gross axle weight aprx/average 2720
WHAT I FOUND THE (un)SKINNY:
I placed the trailer tongue/axle on the same pads for both the wet and dry weighing passes. Oddly, on the 3rd pass I believe the scale lied: the printout showed a combined trailer weight (3080) on pad 3, but I am about as certain as I can be that I placed the tongue on pad 3 and the trailer axle on pad 4 (the Jeep was entirely on pad 2 -- I purposefully 'backed up' one pad just as a verification attempt) -- perhaps I neglected to unhitch (I can be forgetful -- but I was there to unhitch to derive various axle weights, so I don't know what happened; this is one reason I decided on a 4th pass and then decided to make that one dry).
WET WEIGHTS (weighing #1/wet)
Jeep front axle 2040
Jeep rear axle 2780
Pup axle 2580
Gross axle weights (combined vehicle weight) 7400
- then I unhooked and drove the Jeep off the scale (weighing #2/wet)
Pup tongue 540
Pup axle 2520
yielding a gross trailer weight of 3060
- tongue weight as a percentage of trailer weight 17.6%
- then I rehooked and positioned the Jeep with both axles on a single pad (weighing #3/wet)
Jeep 4340
Pup 3080
yielding a combined vehicle weight of 7420 (I'll use 7400 for any further calculations)
DRY WEIGHTS (weighing #4)
Pup tongue 420
Pup axle 2360
yielding a gross trailer weight of 2780
Combined vehicle weight 7120
- tongue weight as a percentage of trailer weight 15%
DIFFERENCE BETWEEN WET & DRY WEIGHTS
Pup tongue difference 160
Pup axle difference 60
Combined vehicle weight difference 300
yielding a calculated water weight of:
- 34 original gallons 284
- water left in 20 gal tank 34 (4 gal?)
- water emptied 250 (30 gal)
unfactored weight:
200-350 lbs human/s, clothes, food/cooler, typically in the tow vehicle. Add 125 for a 3rd human/clothes
SUBSEQUENT BALANCE BEAM TONGUE WEIGHTS (see pix below)
Dry (with typical trunk cargo load) 333
Dry plus 2nd LP tank (that's two LP gas tanks and two heavy G31 batteries) 375
Dry plus 2nd LP tank plus 17 gallons of water in trunk (6 gallons in heater; 11 in totes) 410
Dry plus 20 gallons water in white water tank (straddling the axle, if memory serves)…unknown; I'll have to obtain this in the spring!
WHAT DO THESE RESULTS MEAN?
[Important note: the following interpretation was written immediately after the CAT scale weighing and before the balance beam tongue weighing (see below). Since the CAT scale misweighed me by 5% (I stood on a free pad during one pass), I began to mistrust its accuracy with featherweights -- which is how I view the tongue weight range on a scale built to measure 80,000 trucks. This assessment has been borne out, as the figures above testify.]
1. using the CAT readings my "tongue" weight is maxxed out to exceeded (I use quotes because this was not a true tongue weight, with the trailer supported at the ball cup; the weight was taken at the dolly jack wheel, well back of the ball cup). Since a 2nd full LP tank weighs 40lbs plus a switchover, I'm not sure I can add this unless I were to store the 8 spare gallons of toted water in the trailer (I could do this but it would make loading/unloading it more troublesome than simply putting the totes in the trunk, where I have them arranged where they cannot tip over or spill through their vents -- loading two 33 lb water containers into a crate and into the popped-down and carpeted trailer living area is not a trivial physical endeavor).
2a. Despite the (CAT) 17.6% wet tongue weight, this trailer has never swayed), period (I use a sway control). This included flatlands, Rocky Mountains, crosswinds, trucks forward/aft in either direction, drive speeds 65-70, and with the trailer out-of-level by ~5" (5" higher in the rear; measurements taken from ground to bottom of frame) -- although now, with the addition of a high-rise hitch ball, the trailer is visually level and if it's out of level at all front-to-back it can't be more than 1-2" over the length of the frame.
2b. The good news is that the balance beam tongue weight readings, which I believe to be more accurate, place my dry tongue weight right within the manufacuturer's specification.
3. It appears that the wet trailer is virtually at its maximum gross vehicle weight capacity, altho the Jeep is well below its capacity, primarily on its front axle. Meaning that were I to attempt towing a heavier load (I wouldn't with the 4L 6 cylinder unless I lived in the flatlands) I would be wise to investigate using a weight distributing hitch.
3. Axle weights appear to be well within manufacturer's tolerances. Overall, the cost of carrying water is that I max out my weight bearing capacity -- and this doesn't take into account the weight of additional passengers, and the unknown factors regarding another one or two humans and their stuff is how much is carried by the Jeeps front vs rear axles.
4. Regarding the Rocky Mountains: this scenario does not make me particularly happy, as I would prefer to operate at a maximum load representing ~85% capacity for the overhead required to tackle 10,000' mountain passes with less strain on the tow vehicle (due to decreased performance at 1-2 miles altitude).
5. All in all, this is a sobering assessment of how weights add up, and it focuses on the trailer more than the tow vehicle. Interestingly, I don't feel as if I've loaded the trailer with pounds that are 'empty of calories', that is, it seems to me that the stuff that's in there is reasonable for camp living (including items several not strictly necessary that might be used depending upon the nature of the site and/or trip) and/or safety (tools/parts).
The Stoneyard Scale, 2004
Altho not ideal in that I was unable to obtain a Jeep rear axle weight on this single pad scale without totally unhitching, weighing the rig 2 years later was interesting:
DRY WEIGHTS
Pup Axle: 2440
Pup Gross (uncoupled): 2780
Pup Tonuge (via balance beam): 340 (no surprise)
Jeep Front Axle: 2230
Jeep Gross with Pup hitched: 5040
This time the Jeep had close to a full, rather than half, tank of gas. The pup was dru except for about 4.5 gallons of fresh water in a 7 gallon Aquatainer in the trunk which I like to keep on the basis that it's wise to have some H20 in supply. Pup tongue loading has stabilized at 2 LP tanks and a single G31 battery. One tank was full; one tank about 1/3 full.
The scale operator and I deduced the scale as off by 10# -- this because we weighed the pup Gross with me on the scale and then I stepped off; the reduction was 160#; I weigh 165 and was clothed, for aprox. 170#.
In retrospect I had a hard time believing the CAT scale's 420# tongue weight reading. Tho I don't recall my exact cargo from 2002, the good news is that my cargo load, very consistent over the seasons as I've settled into the stuff I keep aboard, has not grown. With a Gross payload of 2780# (mostly dry) I am running about 90% of trailer GVWR. Were I to run completely wet with full freshwater, hot water heater, and Aquatainers (with 4.5 gals each rather than 7 so they don't leak), the Gross weight would be 3040, or 99% of GVWR. While I prefer running with 10-15% headroom at elevation, I take comfort in knowing that I will not overload the trailer given my nominal use patterns.
I estimate the following:
UVW (unloaded vehicle weight at per Fleetwood): 2110
Major appliances and OEM items: 300
Personal Cargo of all kinds: 370 (this is all permanently stowed cargo including ~40# water, thus ~#330 of cargo ranging from BAL leveler, chocks, ground covers, tools and supplies to sheets, storage items, and nominal grocery/toiletry items stored permanently).
In my case I stow clothes, project bin, and cooler in the tow vehicle when travelling.
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The Balance Beam
The CAT scale is designed to accurately weigh tractor trailer rigs weighing many tens of thousands of pounds; it may not be sensitive enough to accurately weigh the relative featherweight trailer tongue. and besides: on the CAT scale the recorded weight is from the dolly jack, not the hitch ball cup! Two accurate tongue weighing options exist: a specialty scale costing ~$75, or a balance beam which splits the weight between a platform scale (typically from your bathroom) and a support. This is necessary because the bath scale will top out at 300 lbs, lower than a typical tongue weight of ~350 to 400 lbs. If the ball upright is midway between supports, multiply the scale reading by 2; if it's 2/3 way from the scale, multiply the reading by 3. I estimate the beam's accuracy +/– 5%.
Safety has got to be the priority with a system like this to avoid broken toes and/or a broken trailer tongue. Thanks to the PopUPTimes forum member Geotexz71 for alerting me to the finer points of engineering a balance beam:
– use rollers to reduce friction for a truer reading
– cut half-circles under the beam to contain the rollers, so the beam doesn't slip off to the side (this wizardry is referred to as bearing seats by engineers)
– I opted to keep the dolly wheel just off the ground so even if the beam broke the rig would only 'crash' down a fraction of an inch.
The first pix shows the balance beam kit parts: scale, block support (an object the same height as the scale), two rollers (1.25" pipe nipples), a 4x4 beam, a 1.25" vertical pipe screwed into a foot which is screwed securely into the beam, and a spare ball which slips into the pipe. The pipe must be just the right length so the trailer will be level during weighing.
The second pix shows the beam in use. I took numerous readings, using full water totes to simulate the weight of a 2nd LP gas tank. I raised the trailer using the dolly jack, turning just a few cranks to lower the wheel, release the weight from and zero the scale for each weighing. Reversing the cranks raised the wheel, letting the balance beam take the trailer's weight. It's likely you'll want to take various readings as you simulate or adjust tongue weight options (number of batteries and/or LP tanks and their positions; amount of cargo in the trunk if you have one).
Here's how I cut the bearing seats: I clamped a 2x4 to the 4x4. Since their corners are slightly rounded I had a built-in notch by which to center a hole boring bit (I used the same bit used to drill the shower drain hose access through the trailer floor). I drilled as far as the bit would go on one side. Then I used a pilot drill to complete the pilot hole, and turned the board over, completing the hole bore. Of course, the bearing seats must end up a slightly smaller depth than the diameter of the rollers. This arrangement lets the beam ride relatively friction-free for a more accurate reading, but captures the rollers so the whole thing won't come crashing down. Now, you might say it wouldn't happen anyway given the way I screwed the vertical pipe into the beam with a foot, and had chocked both trailer tires. I'd rather not test that hypothesis.
The third pix shows one reading of 120 lbs and my instruction notes on the beam. Do the math and this tongue weight reading is 360 lbs. |
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