Tandem axle weight sharing - Escape Trailer Owners Community

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Old 08-06-2015, 06:53 PM   #1
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Tandem axle weight sharing

Looking at a five year old thread of a Frederick weighing at Bandon as reported by Rick M 7/25/2010.

"Total weight 3,370! Wow! Here's the weight for my 19 footer from Bandon:
Left front axle 1,340, rear axle 500,
Right front axle 1,180, rear axle 420 "

I always thought the axles pretty much shared the load. This shows that the front axle is more than double that of the read axle.

The explanations I can offer is that the front axle being closer to the hitch acts like a single axle carrying much of the load with rear axle taking what's left over.

If this is typical, it goes against what I always believed. Amazing what attaching a number can do.

This makes an excellent case for keeping the hitch pretty much level unlike with a single axle that needs the hitch lower.

Anyone know where one could get all four wheels weighed ? Short of running into Frederick.

Can you say, more tire and bearing wear on the front axle !


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Old 08-06-2015, 07:02 PM   #2
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It'll never be even between the front and rear axles because they're slightly toward the back which puts the front axle near the center, where hopefully, the center of gravity/balancing point is. But, these numbers are a bit out of whack. I weighed our unit near Boise a few months ago and total was 3245 lbs, with about 300 lbs more weight on the front axle than the rear. That trailer might benefit by shifting more cargo weight to the rear.

Having said that, having enough tongue weight is just as important from a handling perspective, so you don't want to make the tongue too light in the process.
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Old 08-06-2015, 07:18 PM   #3
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Robert, I didn't think the axles would be even but the distribution I saw reported was unbelievable. Glad yours is way better. Thanks.

I wonder how a stock 21's left to right weight distribution fares. Knowing that through actual measurements (weighing them) of course, would tell us which side could use more or less weight when adding heavy equipment such as batteries under the seats, etc.

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Old 08-06-2015, 08:24 PM   #4
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I would not put too much faith in those numbers as Frederick used a portable scale and weighed each wheel separately which would shift weight to the other axle. I took my trailer to a truck scale where the axles were measured as a unit, which is the standard used. My 21' weighed 3800 at the axles, both. I tow my trailer level so there should not be any difference from front to rear.
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Old 08-07-2015, 12:47 AM   #5
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Actually, now that I have thought about it, a balanced trailer that evenly distributed its weight over both axles would theoretically have zero tongue weight ! The fact that there is 400-500 pounds of tongue weight would tell me the axle closest to the hitch would bear a higher load.

I have also wondered for many years if the distribution from left to right was somewhat even on varying models I have run across and if floor plan designers purposely spread the internal appliance, tank, etc loads to distribute weight between the right and left wheels more evenly.

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Old 08-07-2015, 01:15 AM   #6
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Quote:
Originally Posted by Santiago View Post
"Total weight 3,370! Wow! Here's the weight for my 19 footer from Bandon:
Left front axle 1,340, rear axle 500,
Right front axle 1,180, rear axle 420 "

I always thought the axles pretty much shared the load. This shows that the front axle is more than double that of the read axle.
Yes, the axles should split the load approximately evenly, but they don't do it automatically. That's wildly bad distribution, and in the wrong direction for stability. Perhaps the tongue was far too low at the time of this weighing... hopefully it wasn't towed that way.

Quote:
Originally Posted by Santiago View Post
The explanations I can offer is that the front axle being closer to the hitch acts like a single axle carrying much of the load with rear axle taking what's left over.

If this is typical, it goes against what I always believed. Amazing what attaching a number can do.
The position of the axles does not determine the distribution of load between them. They're on springs - the one which is compressed more will carry more load, regardless of location. Numbers are great; out of context they can be misleading.

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Originally Posted by Santiago View Post
This makes an excellent case for keeping the hitch pretty much level unlike with a single axle that needs the hitch lower.
Right - any trailer should be level, but if slightly off level then a single-axle trailer should have the tongue lower than level, slightly shifting the centre of mass forward (which increases weight on the hitch) for stability. A tandem-axle trailer get the same mass shift, but a far bigger effect (especially with Torflex suspension) of being off-level is that the axle load is shifted to the leading axle if the tongue is dropped, so err a little on the high side if anything, because more load on the trailing axle (and hitch) is more stable.

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Originally Posted by Santiago View Post
Anyone know where one could get all four wheels weighed ? Short of running into Frederick.
The weighing platform of any truck scale should fit both axles at the same time... they're designed to take both axles of a big truck tandem set (which as much further apart) or occasionally have two weighing sections together to get each axle separately. I use the government-run (and free to use) scales on highways which are there for commercial truck rule enforcement.

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Originally Posted by Santiago View Post
Can you say, more tire and bearing wear on the front axle !
With those numbers, if it were actually towed this way, the front axle rubber (which acts as the springs) would be overloaded. Fortunately, the bearings are good for 3500 pounds per axle (lots of room there), and the tires are rated even higher (if sufficiently inflated).
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Old 08-07-2015, 01:18 AM   #7
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Originally Posted by cpaharley2008 View Post
I would not put too much faith in those numbers as Frederick used a portable scale and weighed each wheel separately which would shift weight to the other axle.
I believe Frederick weighed one axle at a time (two scales) and used spacers - equal to the thickness of the scales, under the other axle's tires. Without the spacers the numbers would be wrong, but both axles would get excessively high numbers.

I think the problem is that Frederick wanted to eliminate the effect of weight-distribution hitches, since his purpose was to determine the weight of the trailer, not to assess load distribution between axles. To do this, he had the trailer owner disconnect the WD before hitting the scales, and as a result rigs using WD were weighed with the rear of the tug sagging, which lowered the hitch and changed the distribution between axles. He then separately weighed the trailer tongue after the trailer was parked in the campsite, so the owner would not need to unhitch for weighing then hitch up again to get to the site. This means the tongue weights for all WD-using vehicles are wrong... meaning not like they are when fully hitched up.

Normally Frederick only reports (publicly) the total axle load; I suspect that all WD-using tandem-axle trailer weighing results are similarly problematic.

Quote:
Originally Posted by cpaharley2008 View Post
I took my trailer to a truck scale where the axles were measured as a unit, which is the standard used. My 21' weighed 3800 at the axles, both. I tow my trailer level so there should not be any difference from front to rear.
You can get separate axle loads at a truck scale by either stopping twice (the scale should be level so you get valid readings) or by using a scale with multiple sections.
I agree that if the frame is level the suspensions will be equally loaded.
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Old 08-07-2015, 01:21 AM   #8
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Originally Posted by rbryan4 View Post
It'll never be even between the front and rear axles because they're slightly toward the back which puts the front axle near the center, where hopefully, the center of gravity/balancing point is.
It doesn't work that way. There are three support points (hitch and two axles) so the distribution between them is not sufficiently defined by just the location of the centre of mass. In mathematical terms: if the trailer total mass (weight), mass distribution, tongue location, and axle locations are all known - but nothing else - it is still an under-defined system so tongue weight and individual axle loads are not known.

The point in the middle (leading axle) can support no load, and the ones on the ends (hitch and trailing axle) just support more load, getting the same result; Escape sends their tandem-axle trailers through the factory this way (with no wheels on the leading axle). Conversely, the trailing axle can lifted off the ground (or nearly so, if the centre of mass is behind the leading axle location), and the leading axle just takes all the load.

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Originally Posted by rbryan4 View Post
But, these numbers are a bit out of whack. I weighed our unit near Boise a few months ago and total was 3245 lbs, with about 300 lbs more weight on the front axle than the rear. That trailer might benefit by shifting more cargo weight to the rear.
That's a more reasonable difference, but it still indicates that the tongue was a little bit low. Lift it up slightly, and the axle loads will even out.

Shifting a bit a cargo will make very little difference to axle load distribution - adjusting the level is much more effective.

With a tandem-axle trailer, if you are not dead level it is better to err slightly on the nose-high side, so the trailing (rearward) axle gets the larger share of the axle load, which is more stable.

Quote:
Originally Posted by rbryan4 View Post
Having said that, having enough tongue weight is just as important from a handling perspective, so you don't want to make the tongue too light in the process.
Increasing the trailing axle load by raising the tongue will also increase the load on the hitch (and reduce the load on the leading axle).
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Old 08-07-2015, 01:32 AM   #9
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Originally Posted by Santiago View Post
Actually, now that I have thought about it, a balanced trailer that evenly distributed its weight over both axles would theoretically have zero tongue weight ! The fact that there is 400-500 pounds of tongue weight would tell me the axle closest to the hitch would bear a higher load.
No, it doesn't work this way. Raise the tongue enough to lift the leading tires right off the ground and you'll have at least several hundred pounds on the hitch, with the rest on the trailing axle... without moving the trailer's weight at all.

If you keep the frame level so that the two axle suspensions are both compressed the same amount, then the two axles each take the same load. In that case, you can think of the tandem axles as effectively one axle, located midway between the two axle (wheel) centrelines. Then the distribution between the axle set and the tongue is determined by the location of the centre of mass (trailer weight) just like with a single axle trailer.

Quote:
Originally Posted by Santiago View Post
I have also wondered for many years if the distribution from left to right was somewhat even on varying models I have run across and if floor plan designers purposely spread the internal appliance, tank, etc loads to distribute weight between the right and left wheels more evenly.
Yes, side-to-side load distribution is an element of good trailer design.
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Old 08-07-2015, 07:30 AM   #10
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Quote:
Originally Posted by Brian B-P View Post
. I use the government-run (and free to use) scales on highways which are there for commercial truck rule enforcement.
Something I learned taking Highway Design when in Civil Engineering, is that one of the main purposes of the scales is to provide information on how much load that particular highway gets, which assists in the design of the road structure for future improvements, from the base materials and thickness, to the pavement thickness. Passenger vehicles are so light, that their effects are not taken into account at all. But, these scales do function well to ensure that the design weight limits are not exceeded, stopping excessive wear of the road structure.
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