posted 08-20-2008 12:01 AM ET (US)        

We drove onto the CAT Scale platform. The platform has three scales, which are easily deduced by their markings. I positioned the truck and trailer so that the steering axle of the truck was on the front platform, the drive axle of the truck was on the second platform, and the trailer axles were on the third platform. The spacing is really proportioned for a larger rig, but with careful positioning you can get most vehicles and boat trailers on the three scale platforms.

I had to get out of the truck to go to the intercom panel, as it was several feet in front of our truck. It was also several feet above my head. Standing on tip-toes I could just reach the button to summon the operator.

The operator responded: "Is this your first weigh?"

I said, "Yes, I am weighing my truck and trailer, and then I will get a re-weigh after I drop the trailer."

The operator replied in the affirmative, then told me she had my weights. At this point I was careful that I was not standing on any scale platform. Thus my weight was not recorded in this first pass. Chris was in the car, so she would have to stay for the second pass.

Next, we pulled off the scale and dropped off the trailer. The parking lot had plenty of room, as it is sized to accommodate many large trucks. Then we pulled back onto the scale with just the truck. Again, I positioned the truck on the scale with the steer axle on the first platform and the drive axle on the second. I also parked in about the same position as the first measurement so as to reduce any influence on the weight from the axle position on the platform.

I got out and went to the intercom. I called and asked for my re-weigh. And I again was careful to stay off the scale itself. The operator told me she had the weights, and I was to come to the office to get the tickets.

Here are the weights:

FIRST PASS
Steer Axle = 2,360-lbs
Drive Axle = 3660-lbs
Trailer Axle = 5140-lbs
Gross Weight = 11,160-lbs

SECOND PASS
Steer Axle = 2,560-lbs
Drive Axle = 3,000-lbs
Gross Weight = 5,560-lbs

Calculations

Trailer and Boat Combined
Now for the calculations. First we want to find the combined weight of the trailer and boat. This is easy: it is the difference in the two gross weights, or

11,160 - 5,560 = 5,600-lbs

This is very straight forward. One total has the trailer, the other does not. You just subtract them. This is how much weight we are towing with the truck. I'll come back to this later.

Boat Weight
Next, we deduce the boat weight by subtracting the trailer weight.

5,600 - 1,250 = 4,350-lbs

How'd I know the trailer weight? From the manufacturer's certificate of origin, which stated 1,200-lbs, I added 50-lbs for the spare tire, the jack stand, and the tie downs I added.

To my boat weight of 4,350-lbs I will later have to add the weight of any fuel I add, as well as my own weight and any crew weight. We added about 40-gallons of gasoline before we launched, which is about 250-lbs. We also added 370-lbs of captain and crew. We already had on board all of our other gear that we'd be using, so we don't need to allow for that weight. Thus, when the boat was underway later that day it probably weighed

4,350 + 250 + 370 = 4,970-lbs

Tongue Weight
We are also interested in knowing the tongue weight. This is actually easy to figure. Consider this: we have deduced the trailer and boat combined weight at 5,600-lbs. That weight bears on only two places: the trailer axles and the trailer tongue. We already have a measured weight on the trailer axles from our first pass: 5,140-lbs. Therefore, we can deduce the tongue weight as the trailer weight minus the trailer axle load:

5,600 - 5,140 = 460-lbs

This has to be the tongue weight; there is no other place for that load to bear.

Now we compare the tongue weight with the total trailer weight to see if it is in the recommended range of 5- to 10-percent of the total weight:

460/5600 = 8.2-percent

We also have to check the rating of the hitch. My hitch can handle 500-lbs of weight. It has a 40-lb margin.

Axle Loading
We check all the axle weights to see if they are within the ratings of the maximum gross axle weight ratings for the truck and trailer. You can usually find these ratings on a sticker on the driver's side door of your vehicle or on the manufacturer's rating sticker on the trailer:

TRUCK
GAWR-Front = 3,400
Measured = 2,560
Margin = 840-lbs

GAWR-Rear =3,968-lbs
Measured = 3,660-lbs
Margin = 308-lbs

TRAILER
GAWR = (TBA--trailer is not on-site as I am writing this)
Measured = 2,780-lbs (i.e., 5,560-lbs / 2)
Margin = (TBA)

Note that I should add my weight back into the truck axle loading. I weigh 225-lbs, and you can figure as the driver I will be mainly on the front axle. Let's use a 75-25-split, or 169-lbs on the front axle and 56-lbs on the rear axle. That reduces the margins to 671-lbs (front) and 252-lbs (rear).

Gross Vehicle Weight Rating
We also want to check the gross vehicle weights. Again, you find these ratings on stickers on the vehicles.

TRUCK
GVWR = 6,800-lbs
Measured = 2,360 (front) + 3660 (rear) + 225 (me) = 6,245-lbs
Margin = 555-lbs

TRAILER
GVWR = 6,800-lbs
Deduced = 5,600-lbs
Margin = 1,200-lbs

Cost
The first weighing cost $8.50. The policy of CAT Certified Scales is that a re-weigh of the same vehicle within 24-hours only costs $1. Thus the total cost of these two weighing was $9.50.

Comments
My truck is rated for towing a maximum of 7,000-lbs. The listed hull weight of my boat is only 2,600-lbs. At first glance one can easily underestimate the total and true load of towing this boat. As these measurements have shown, in several areas (such as tongue weight and rear axle weight) I am approaching the maximum allowed loads.

I don't know where you can get more information for only $9.50. The entire process of weighing only took about ten minutes. Most of that was in unhitching the trailer.

You can locate a CAT Certified Scale by using their website at http://www.catscale.com/ . The scale operator was very friendly and seemed happy to work with us, even though we weren't professional truck drivers.

posted 08-20-2008 12:44 AM ET (US)            

Tongue Weight  = [STEER + DRIVE](Hitched) - [STEER + DRIVE](Unhitched)
               = (2360 + 3660) - (2560 + 3000)
               = 460

posted 08-20-2008 04:48 PM ET (US)            

The front axle was unloaded by 200 lbs, the rear axle loaded by an extra 660 lbs, ergo there is a total of 460 lbs of tongue weight (660-200).

posted 08-20-2008 05:56 PM ET (US)            

This is an interesting topic. Thanks for starting the thread.

With the discussion of tongue weights, and front and rear tow axle vehicle loading, I'm curious if any of the engineers out there can mathematically calculate the effect of tongue weight on the two axles.

For example: jimh's 460 lb tongue weight translated into 660 lbs on the rear axle and minus 200 on the front.

posted 08-20-2008 08:03 PM ET (US)            

The rear truck axle is a fulcrum for a lever. One side of
the lever is of length WB (wheel base of truck), the other
side of length L (distance from rear axle to hitch). Tongue
weight TW was 460 pounds. We can calculate FW (front axle
weight change) and RW (rear axle weight change) as follows:

FW = -TW*L/WB (note that this is a negative number)
RW= TW-FW

Now in this case, we know:

FW = -200 lbs.
RW = 660 lbs.
TW = 460 lbs.

flipping things around, we get:

L/WB = FW/-TW = 0.434782609

JimH: how about some measurements on your truck, and tell us
what you get for L and WB.

And Mr. Dobosh (high school physics teacher): the units cancel
out, so both L/WB and FW/-TW are dimensionless.

posted 08-20-2008 08:19 PM ET (US)            

WB = 132
L = 60

L/WB = 60/132 = 0.45

posted 08-20-2008 09:04 PM ET (US)            

Chuck, I can't believe you remember this stuff from high school physics. I remember taking the class, and that Mr. Tiernan taught it, but that's about it.

So, If I understand what you've posted, and using my information, this is what I come up with.

Truck Wheelbase: 172.4
Length from C/L of rear axle to C/L of hitch ball (with hitch extension and ball mount): 99.5

172.4/99.5=.578

.578x350(tongue weight)=202.3

So the front axle's weight is reduced by 202 lbs and the rear axles weight is increased by 350+202 = 552 lbs when towing the boat.

posted 08-20-2008 10:17 PM ET (US)            

Lets play with the unknowns

What's a certified weight? by whom and to what standard ?.

All calibrated scales have an accuracy factor over it's operating range

Trailer:
We are assuming the trailer manufacture weighed trailers is identical to your one.

I would presume/guess that the manufacturer may of weighed it once ( i.e. the first one he ever made) or worse one generic trailer and adjusted the weight to suit the various models he makes.

I doubt very much that he weights a random sample or ever updates the weight should he have changed any parts or suppliers over the years.

Although it would be reasonable to assume that he has ensured some additional margin or contingency to allow for such deviations including the accuracy of his original weighing.

Therefore this weight could easily be +/- 5%, but I would assume on the plus side as it would be reasonable to assume he has been conservative for safety.

JimH you have not listed the maximum permissible tongue weight of the trailer, not what's stamped on the coupling but what the trailer manufacture has designed to. This could be a limiting factor (i.e. less than your vehicles hitch capacity)

Cat Scale:

Cat scale does not state by whom or what standard they are calibrated to and what method they use to weigh. Assuming they have used a recognised body for certification we are left with what is there accuracy over the operating range.

It would be fair to say that they should be more accurate at the higher end as usually this is the case or worse they have used an average calibration.

Given that you're set up is much lighter than trucks and the maxima load Cat Scale can weigh, it is more likely to decrease the accuracy of your results.

Additionally we can assume they are usually conservative else they would not be so confident on say "else we pay the fine" as they would only be caught out if their scales had not been recently re-tested/calibrated.

Therefore this weight could easily be +/- 3% for lighter loads, but I would assume a plus 1.5% would be reasonable assumption.

Truck:

Unfortunately your truck is not a true ridged body so simple static's will not be that accurate. Without loading it with many variations we are unable to determine accurately how it distributes the load between the two axels. So there is likely to be a margin of inaccuracies there.

JimH overall you have a reasonable weight that should be conservative for the total load you are pulling.

However given that the first limiting factors are the trailers maximum designed nose weight and the car hitch, which ever is the least.

I strongly recommend that you simply weigh the nose weight of the trailer at its normal towing height to limit/omit the inaccuracies/margins that are accumulated by deducing hitch weight by axel weighing subtraction methods.

posted 08-20-2008 11:17 PM ET (US)            

This is an area overlooked by many. The stock tires on my truck were rated at 3,415 lbs each. I've since upgraded them to tires rated at 4,300 lbs each.

Steve, Aren't you splitting hairs? While the vehicles frame may flex a bit which might effect the exact weights on the various parts, it's not really practical to figure out many different loading scenarios. I think a "certified scale" is about as good as one can reasonably do.

While I certainly am not an engineer, every engineer I've ever met calculates things on the conservative side.

posted 08-21-2008 02:00 AM ET (US)            

I agree a lot of this is down to degrees of accuracy.

You should weigh something on a certified scale that is ideally working around 80% of its capacity.

The truck also has a suspension system that will also influence the results. I would never assume that any of us would go to the trouble of trying various loadings :).

Simply we can cut out all of this and just get the trailer nose weight directly.

The weigh bridge is only good for checking your overall mass you are pulling and axel loading vs tyre rating

It should not be for deducing your nose/hitch weight is my point as too many unknowns/variables/allowances are in the summations, even if you consider them to all be equally wrong by the same percentage.

It's true that there are safety margins in many things. However give an insurance investigator an inch or lbs over and he will wriggle his way out of a claim.

As I previously noted Jim missed out the trailer capacity. Especially nose weight. This is not the capacity of the actual hitch cast/pressed head on the trailer but the design capacity of the frame and trailer as a whole, carrying it's maximum permissible load.

posted 08-21-2008 08:29 AM ET (US)            

Here is the weigh ticket. It contains various statements of accuracy and guarantee.

Yes, I agree the weights involved here are far below the scale's capacity. And the load is not centered on the scale platform. On the other hand, I don't have a scale with any reasonable calibration that can weigh 460-lbs of tongue weight. The hitch manufacturers make some tongue weight scales, but I think they are 0-300-lbs range. So it is not simple to just weigh the tongue with a small scale.

posted 08-21-2008 08:41 AM ET (US)            

Model 6 Actuator with 2 5/16" Ball Coupler

7,500 pound capacity, Max. G.V.W.R., 600 pound tongue load, G.V.W.R. limited to ball rating.

My deduced trailer weight is 5,600-lbs, thus the margin is 1,900-lbs on GVWR. The tongue load is 460, so margin on that is 140-lbs.

posted 08-21-2008 09:56 AM ET (US)            

Centering two 1 inch by 6 inch steel pipe nipples on both the scale and the platform, I laid the wood beam so all my centering makes lined up across the two and rested the trailer tongue on the center of the beam giving me a 2 to 1 ratio on the bathroom scale.

posted 08-21-2008 10:21 AM ET (US)            

WW: you got one BIG truck. And

172.4/99.5=.578

should read

99.5/172.4 = 0.577146172

I suspect the left side is a transcription error. But you
came up with the right answer.

posted 08-21-2008 02:18 PM ET (US)            

1987 Outrage 18 with 1987 Johnson 150 HP motor sitting on an aluminum and steel trailer.

Trailer with boat weighed 3440#

Axle of trailer with boat 3060#

Tongue weight is 380# then.

Trailer only 900#

Boat, motor, fuel, and stuff 2540#

I had about 25 gallons of fuel aboard.

The whole package is about 500# more than my estimate.

I also learned that a new acronym of FORD is found on ramp dead. Starter went south with trailer well submersed in the water.

posted 08-21-2008 02:46 PM ET (US)            

Thank you both for the information and corrections. Thanks to CW I can now figure out the effect on the axle weights of adding a trailer with a bathroom scale and some simple math. Even I can do that!

Chuck it is a big truck. Full of fuel with no tailgate, passenger, driver or load it weighs 7,720 lbs. For carrying and/or towing a heavy load it is awesome.

posted 08-22-2008 11:14 AM ET (US)            

Examples:
Jimh's Revenge 22 hull weight at 2600#, loaded vessel at 4970#.
ConB's Outrage 18 hull weight at 1250#, loaded vessel at 2540#.

posted 08-22-2008 04:57 PM ET (US)            

I agree this is a very good thread in highlighting how simple it can be to add hundreds of lbs as the accumulated affect of adding gear to your boat easy to misjudge or over look.

The Cat Scale is clear indication of what you're pulling behind your truck and gives an easy way to check axel loading, tyre rating and braked (hopefully) towing capacity.

Twin bathroom scales and spreader beam is as simple and easy as you can get for checking trailer nose weight.

I have seen compression struts a bit like fish scales in reverse, reminiscent of the old bull workers that you place in the hitch and chock to your tow height.

Here's a UK version (sim to one have used) but in the US as you tow with much greater weights so you would need a bigger version http://www.towsure.com/product/406-Nose_Weight_Gauge

I think a young UK female Eng has won an award for incorporating such a device into caravan hitches.

Manufactures balls and hitches usually have a capacity far greater of your towing vehicle capacity or the trailer design. These should never be used as guidance over other limiting factors.

JimH rule of 5-10% of towed load as a nose weight (providing it is does not exceed your vehicle or trailer capacity) is a good rule as this increase the stability of the tow and reduces pitching.

Snaking (ie the tail wagging the dog) is usually caused by outside influences (wind, passing trucks etc) or high speed or excessive braking on declines, it is a distinct advantage to optimise nose weight for long distance tows to ensure maximum safety and limit wear on your vehicle.

Another simple rule/guideline is 80% ratio between tow weight and kerb weight of vehicle for inexperienced towers and no more than 95%.