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Old 12-20-2015, 08:13 AM   #1
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Axle weight--four corners????

A fellow Alpiner and I were recently discussing axle weight, ride height, and tire pressures. After 13 years and 165K miles, its probably a bit late but I decided to find out just what my 2003 40MDTS coach weighed at each corner--the result were interesting to say the least.

First the methodology: I didnt have access to individual weighing scales so I used the truck scales at the local Pilot. These scales are divided into three segments and there was enough margin on either side of the scale so I could weight just the drivers or passengers side at one time. During each weight cycle, I tried to position the coach in approx. the same location on the scale segments.

The results: Drivers side front--5160 lbs; rear--10240 lbs. Passenger side front--6160 lbs; rear--8400 lbs. Combine axle: front--11,320; rear--18,640. Total weight--29,960. Front axle difference--1000 lbs. Rear axle difference--1840 lbs.

Post weigh-in analysis: Fuel tank was nearly full, water tank 1/4 full; propane 1/4 full; holding tanks empty; no unusual cargo placement in basement. Ride heights were close to specs--rear corners were good; passsenger side front corner, slightly high. In each case, axles were within 10 percent of balanced distribution but the front was a bit of a surprise--with propane tank and kitchen/living room slide on drivers side, might of expected the drivers side to be a bit heavier?

As always--thoughts and analysis are welcome.

Merry Christmas!
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Old 12-20-2015, 09:08 AM   #2
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Quote:
Originally Posted by Old Scout View Post
A fellow Alpiner and I were recently discussing axle weight, ride height, and tire pressures. After 13 years and 165K miles, its probably a bit late but I decided to find out just what my 2003 40MDTS coach weighed at each corner--the result were interesting to say the least.

First the methodology: I didnt have access to individual weighing scales so I used the truck scales at the local Pilot. These scales are divided into three segments and there was enough margin on either side of the scale so I could weight just the drivers or passengers side at one time. During each weight cycle, I tried to position the coach in approx. the same location on the scale segments.

The results: Drivers side front--5160 lbs; rear--10240 lbs. Passenger side front--6160 lbs; rear--8400 lbs. Combine axle: front--11,320; rear--18,640. Total weight--29,960. Front axle difference--1000 lbs. Rear axle difference--1840 lbs.

Post weigh-in analysis: Fuel tank was nearly full, water tank 1/4 full; propane 1/4 full; holding tanks empty; no unusual cargo placement in basement. Ride heights were close to specs--rear corners were good; passsenger side front corner, slightly high. In each case, axles were within 10 percent of balanced distribution but the front was a bit of a surprise--with propane tank and kitchen/living room slide on drivers side, might of expected the drivers side to be a bit heavier?

As always--thoughts and analysis are welcome.

Merry Christmas!

Years ago, I read an article by an industry expert about weighing technique with air suspensions. According to this article, the 3 point ride height valves are meant to be accurate in the dynamic environment of driving down the highway. They are not especially good at "Snap shots" like sitting on a scale.

Again, according to this author, the way to get an accurate measure of your loaded coach is to setup the suspension just before driving onto the scale in this manner.

1. Line up with the scale.
2. Turn off you engine and "air down" your suspension until the air system is empty.
3. Crank up the engine & allow the coach to return to ride height.
4. Pull directly onto the scale and take your weights.

The important thing here is to not make any turns during the procedure. If you have to make a loop around & weigh the other side, then you should perform this procedure just before each weighing.
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Old 12-20-2015, 09:46 AM   #3
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Steve--appreciate the input. I have read/heard any number of descriptions about the impact of air bag adjustment on weight distribution--ranging from "not much" to "a lot." Given the dynamic nature of the air bags as they adjust/compensate for changes in on-board weight and road conditions, I suspect that weight distribution will vary each time you drive on the scales. Notwithstanding this dynamic, I was expecting the axles to be fairly balanced, or perhaps a modest biased toward the drivers side [the big slide].
PS--I was entertaining the idea of dumping the bags once I got on the scales to see what the dead-weight distro was but am not sure how that would enter into the overall bag adjustment equation.
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Old 12-20-2015, 10:20 AM   #4
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Dean,

Start by verifying ride height-- out of adjustment ride heights can transfer a lot of weight side to side.

AFTER verifying ride height on all four corners, and, rechecking wheel position weights, I would suggest minor "fine-tuning" to ride height to help balance side to side weights.

As an example:


Front ride height: bottom of frame rail to top of axle 10 5/16”. So: Heavy side 10 3/16”. Light side 10 7/16”

Rear ride height: bottom of frame rail to midline weld seam of rear axle 7 ”. So Heavy side: 7 3/8”. Light side 7 5/8”


I have foundthese adjustable links a real help in fine-tuning ride heights: https://www.heightcontrolvalve.com/shopexd.asp?id=109


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Old 12-20-2015, 10:31 AM   #5
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Thanks Brett--interesting product. The underlying point of my original post is that I have read alot over the years about adjusting our Alpines to the recommended ride heights of 7 1/2 and 10 5/16 but dont recall anyone linking the fine-tuning of the height to weight distribution. This discussion then begs the question about whether weight distribution impacts on front-end alignment and overall driveability [eg "my coach pulls to the right"]?????
PS--mine doesnt pull, just saying......
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Old 12-20-2015, 10:39 AM   #6
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Old Scout, your surprise concerning the weights on the front tires is understandable. Based on your floor plan I would expect the left front to weigh more that the right front also. I believe that I can explain why the weights are as you measured them. For background information to this discussion I have been involved in both circle track and road racing for just about my entire life and I am also an engineer. The reason I mentioned this is that weight distribution on the 4 tires of a race care is absolutely critical in its handling characteristics and understanding some of the factors that control that is applicable to explaining what your weight data shows. From your data it appears that the right front (RF) weight is high because, as you stated, the right front ride height is high. What happens on a vehicle is that as you raise or lower a particular corner the weights on all 4 corners change. The changes occur across the chassis in an X pattern. As an example, if you raise the RF ride height by adding air to the RF air bag the weight of both the right front tire and the left rear (LR) tire will increase. Also the weight on the LF and RR tires will decrease. Also, the total weight on the front axle will not change and the total weight on the rear axle will not change. Likewise the total weights on the left side and right side tires will not change. This is based on the assumption that nothing has been done to change the center of gravity of the vehicle. As an aside, in circle track racing the total weight on the RF and LR tires is called cross weight.
If you set the ride heights equal across the front and equal across the back you will see the weights change as outlined above. The RF and LR will weigh less and the LF and RR will weigh more. I hope I was able to help. Let me know if I can be of further assistance.
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Old 12-20-2015, 10:40 AM   #7
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Dean,

Yes, with the "trailing arm" design (air bag at the back of the trailing arm), ride height materially affects CASTER. Caster certainly affects tracking (or lack there-of).

Brett
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Old 12-20-2015, 12:44 PM   #8
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I consider those differences big.

if the ride is correct, then the adjustable type of links referenced above can make some big differences with small amounts of link change.

On mine the ride height to chassis was correct, but looking head on on flat surface like a scale platform, the drivers side was always lower.

I adjusted the link about 5/8" now the wheel well to ground was almost the same.

I shifted about 600 lbs, the rigs ride and handling was immediately better.

Best of all it looks level.
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Old 01-01-2016, 11:48 AM   #9
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Just had to comment... I had our '07 36FDDS weighed at our local landfill big scale,and the results were very close to yours, with the only major difference being that my pro-pane tank was full. I wasn't able to do the corners, (will find a place to do that later), but here are my weights: Gross wt. 29340, front axle 11,300, rear axle 17860. 620 lbs. seems like too small a diff. between a 36'DS and a 40' TS, eh?








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Old 01-01-2016, 07:26 PM   #10
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At first blush--I am surprised to see the gross wieghts so close. However, in order to get the desired 60/40 split axle weights that WRV was targetting, they had to add a lot of ballast [steel plate] between the frame rails over/near the front axle to compensate for the heavy engine/tranny on the back axle, but spread over a shorter frame. Think the 34 had the most ballast but the 36 wasnt far behind. Hate to think about carting that much ballast around but that is what was needed to build an RV that handles like a sports car.
PS--am still contemplating Brett's advise on "fine-tuning" the bags to distro the weight. Attempted an adjustment and re-weigh but overshot the mark, so returned the bags to spec height but have not re-re-weighed. I was hoping that having the bags within spec would balance the weight but apparently not so. It seems you would need a dedicated scale and some patience to get this right--I dont have either right now [smile].
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Old 01-02-2016, 05:19 AM   #11
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Axle weight--four corners????

If the 07 36ft has smart beds and the 03 40ft does not, than that could explain the weights.

My 08 34 fdds weighs 29300 with smart beds.

My 02 36 fdds weighed 28200 without smart beds.
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Old 01-02-2016, 03:48 PM   #12
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As if I don't have enough to do, now I'm going to have to run out in the driveway and get under the bus and measure the ride height!








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