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Old 12-26-2013, 02:33 PM   #113
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Another good point. "stiction" causes erroneous measurements. When doing a precision alignment (ride ht is part of alignment) you roll the vehicle back and forth or "jounce" it (as easily as jumping up and down on the sprung mass) to free up stiction in the components that shows up as the parts are moved in the adjustment. With a vehicle as large as a motorhome, only rolling it around will relieve the forces. putting sand under the tires helps too.

Jimbo, if after you adjust front ride height the weights are still way off in the front, try lowering the rear ride height on the opposite side of the heavier front. You will probably not have to move it much. Air bags are kind of weird compared to steel springs, because the rate is adjusting to maintain a given height, as opposed to springs, where the height is adjusted to jack the weight.
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Old 12-26-2013, 03:39 PM   #114
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Info from a Tire Engineer
While I don't have a working knowledge of how all the various air bag self leveling systems work, I can provide some information on why it is important to know the load on your tires.
As posted elsewhere, the actual load is what you need to know so you can establish the proper inflation. Knowing the proper level of inflation is what will help deliver thousands of miles of solid performance, while running less than what you need will at best only result in a loss of fuel economy while at worst can lead to erratic handling or even tire failure with all the associated expenses for tire replacement, service call and possibly even RV body and mechanical repair if a catastrophic failure occurs.

Proper inflation is at least the level shown in the Load/Inflation tables published by the manufacturer of your specific tires.

The inflation shown on your Placard is the level from the RV assembler based their assumption on your load being evenly distributed between the tires.

However if you carry more than what the RV assembler estimates or do not distribute the load uniformly or there are significant after market modifications or even significant variation during vehicle assemble from their original design you may end up overloading your tires.

Damage to tires comes from higher operating temperatures and more flexing than the tire manufacturer designed for. This increased damage is cumulative. Putting more air in your tires after running many miles with low inflation will not repair the damage anymore than putting the burnt hot dogs back in the fridge will repair the burnt parts. With sufficient number of cycles at higher than designed for flexing, failure can occur.

Bottom line.
First you need to inflate the tires to no less than the inflation needed to exceed the actual load on each tire. Second, you should inflate all tires on an axle to the same inflation so they deliver uniform handling and are not fighting each other during cornering. Third the inflation needs to be set with an accurate gauge when the tires are at ambient temperature and have not been driven on for at least 2 hours and not been in the sun for at least two hours.
I recommend you also apply a minimum of 10% extra inflation as a safety margin for various cumulative errors in scale weights, gauge accuracy and variation in actual load and weather changes. This way you are not chasing 1 psi variation which is not really needed.
Finally get and use a TPMS so you will be warned when you have a puncture or valve leak.

You can read posts on my blog if you want more details.
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Old 12-26-2013, 03:56 PM   #115
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Quote:
Originally Posted by VanDiemen23 View Post
Another good point. "stiction" causes erroneous measurements. When doing a precision alignment (ride ht is part of alignment) you roll the vehicle back and forth or "jounce" it (as easily as jumping up and down on the sprung mass) to free up stiction in the components that shows up as the parts are moved in the adjustment. With a vehicle as large as a motorhome, only rolling it around will relieve the forces.
You mean that my turn plates won't work?
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Old 12-26-2013, 06:18 PM   #116
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Yes, your turn plates will need extra steps to work such as bouncing the RV. You large enough to accomplish that. I do not want to pass judgement here but really you are dealing with a different animal.
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Old 12-26-2013, 06:30 PM   #117
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That was a little tongue-in-cheek. I'm pretty certain that 5 or 6,000 lbs of corner-weight would crush my automotive turn plates into worthless scrap metal.
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Old 12-26-2013, 07:40 PM   #118
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Yes, that was a good one. Good sense of humor.
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Old 12-26-2013, 09:19 PM   #119
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I don't use turn plates much but when I do, two pieces of aluminum with de-burred edges and some motor oil in-between works fine - but I've never been able to roll onto them - always have to jack up and put them under the tires - otherwise they just squirt out.

Actually, I can jounce the RV pretty easily!
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Old 12-28-2013, 07:21 AM   #120
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[QUOTE=targaboat;1858264]I have had another thought. 1 inch in 40 feet is 1/40x12 is 0.2%..QUOTE]

This may be correct from a simple math perspective, but may be meaningless. What is not known is how stiff the chassis is. To get a 1 inch difference between right and left sides of an axle may require significant air bag pressure to overcome chassis stiffness. This would then result in big differences in downward force or measured weight left to right. You really need to adjust ride heights to be equal height left to right to eliminate the effects of chassis stiffness.

BTW, I find it strange that a coach has both left and right ride height adjustments on the steer axle. Most don't so that a leak on either side affects both approximately the same so steering isn't impacted.
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Old 12-28-2013, 08:11 AM   #121
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[QUOTE=ImagineIF;1860367]
Quote:
Originally Posted by targaboat View Post
I have had another thought. 1 inch in 40 feet is 1/40x12 is 0.2%..QUOTE]

This may be correct from a simple math perspective, but may be meaningless. What is not known is how stiff the chassis is. To get a 1 inch difference between right and left sides of an axle may require significant air bag pressure to overcome chassis stiffness. This would then result in big differences in downward force or measured weight left to right. You really need to adjust ride heights to be equal height left to right to eliminate the effects of chassis stiffness.

BTW, I find it strange that a coach has both left and right ride height adjustments on the steer axle. Most don't so that a leak on either side affects both approximately the same so steering isn't impacted.
I think they figured that out later? There's that reason, and higher potential for torquing the body to the point windshields start to have issues pulling into/out of gas station driveways at an angle for instance?

Or maybe having more side resistance to wind gusts with 2 placed on the axle with the most weight on it?

IMHO, that's why later coaches have the 2 sensors in the rear, 1 in the front layout. 2 in the front doesn't make a lot of sense when you start thinking about it....
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Old 12-28-2013, 09:36 AM   #122
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Our 99 Bounder diesel on a Freightliner chassis had two ride height valves on the front. I had to replace one that had a slow leak. I don't know what was in the rear. Our 43' Allegro Bus is on a Power Glide chassis and it has 4 ride height valves. It makes it very easy to fine tune the corner to corner loads. It 1/4" change can transfer 4-500# to another area. That's what the manufacturers have in mind when they allow a range of adjustment.

Edit: I don't think the OP was talking about a one inch difference from side to side. That much could possibly indicate a twisted frame.
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Old 12-28-2013, 09:42 AM   #123
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To get a 1 inch difference between right and left sides of an axle may require significant air bag pressure to overcome chassis stiffness. This would then result in big differences in downward force or measured weight left to right. You really need to adjust ride heights to be equal height left to right to eliminate the effects of chassis stiffness.

BTW, I find it strange that a coach has both left and right ride height adjustments on the steer axle. Most don't so that a leak on either side affects both approximately the same so steering isn't impacted.
You have it backwards. it takes large changes in rate to overcome chassis FLEX. the stiffer the chassis, the less change it takes because the opposite corner responds immediately to the change.

My Beaver has two on the front and one on the back. With only one on the back, when you can't really effect much diagonal change to the front. I can see where having one sensor on the front makes it easier to make the weight jack forward, but it will also make it more susceptible to roll - especially in long corners or tilted pavement (the "sway bar" works on the high frequency side of the problem) since the single sensor can only adapt to the average ride height.

For all you guys that keep worrying about chassis flex - consider that if it were as flexible as you all imagine, the normal dynamics of driving would pop out windows all the time. Chassis flex really comes in to play when using a four post leveling system.

The Beav has a three post system and I've never had a flex problem as long as I get the front suspension nearly extended (no air dump) before working the rears so that I can't jack a lot of weight onto a front corner as a result of big lifts on one rear. It just levels out perfectly with no stress every time.
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Old 12-28-2013, 12:53 PM   #124
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I thought Hicks was going to take a nap.

Jimbo, hope you are not discouraged with all of the ideas. They represent different ideas from people with different backgrounds. Some are very good and some you will want to ignore. My background is work vehicles, however, I am always learning new things. For example, I have no idea if an inch differential corner deflection is significant to creating stress in the frame. However, the mass of the frame on RVs are really light weight as compared to a construction vehicle. Therefore, I suspect that an inch is not a big deal. You really need the ability to be able to watch the scale while you tweet the height adjustment which is probably not possible. Not knowing fully the results of what the effects of what you are doing we used to refer to as throwing darts. This issue has a large influence on my thoughts that since your couch has lived over ten years that not tweaking necessary. If something has obviously changed or is broken than that condition requires attention.

However, with all of this, I see nothing wrong with bringing the suspension height to the manufacturer specs. I like things that go down the road straight and level. That has to be good thing.
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Old 12-28-2013, 01:44 PM   #125
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I'm blown away by the 124 responses to this post! Why not a bunch of you experts meet with the OP, empty his coach of everything, adjust the ride height to specs, and pay for a 4 corner weighing. Then start with your micro managing.
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Old 12-28-2013, 01:58 PM   #126
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I think the old saying about picking the fly s$&t out of the pepper applies here. I have to laugh at some of posts while others are very informative. I think I've gained some useful knowledge here.
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