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Old 08-05-2009, 01:06 AM   #1
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Adjusting WD Hitch to Air Suspension

I am pulling a Safari 27FB with a Mercedes ML diesel equipped with air suspension and an Equal-i-zer hitch. Because the suspension levels the vehicle whatever the load, I can't adjust the hitch by making measurements through the axles. I took the rig to a scale today for a weigh-in: the whole rig, the trailer alone, both tow vehicle axles (hitched), and the tow vehicle front axle (hitched). With the hitch dialed in tight, my weight distribution is 46%F, 54%R. I didn't weigh the tongue but the difference between the TV weight hitched and unhitched is 960 pounds. That's a lot more than AS has advertised, and well beyond the 600 lb. rating of my hitch.

As it is now set up, the rig handles very well. My questions - using a WD hitch, is the TV receiver relieved of tongue weight strain when some weight is transferred forward by the trunnion bars? If not, can a hitch be reinforced to accept the excess weight? Is my 46/54% weight distribution OK?
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Old 08-05-2009, 02:53 AM   #2
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My TV has auto-air suspension as well. There are 2 known values in this equation. First is the height of the TV / receiver, second is the height of the trailer tongue when level. You will need to get an adjustable height tow-bar and adjust the height of the ball so that it is at the level of trailer tongue ( this must be done when the TV is running to be able to get the proper height ). Once this is acheived and your trunion bars are tightened up, the trunion bars will absorb and deflect and sudden changes in height change. The reason it's done like this is so the air suspension is not continuously attempting to cope with height adjustment. Also when you have the trailer hooked up and stop for fuel or whatever reason the jack must come down so the air suspension is not carrying the load. This has worked fairly well for me.
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Old 08-05-2009, 05:26 AM   #3
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Disable the air suspension, then set the trailer up properly, then turn the air on. zz
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Old 08-05-2009, 10:42 AM   #4
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Adjusting WD Hitch to Air Suspension

Greetings dpulling!

Quote:
Originally Posted by dpulling View Post
I am pulling a Safari 27FB with a Mercedes ML diesel equipped with air suspension and an Equal-i-zer hitch. Because the suspension levels the vehicle whatever the load, I can't adjust the hitch by making measurements through the axles.
Have you checked your Mercedes owner's manual for instructions on adjusting your weight distributing hitch? I have had three different tow vehicles with auto-leveling suspension systems and each had a very specific set of directions for adjusting weight distribution setup. Generally, the vehicle had to have its ignition off during the entire hitching procedure, but the actual sequence of events varied.

Quote:
Originally Posted by dpulling View Post
I didn't weigh the tongue but the difference between the TV weight hitched and unhitched is 960 pounds. That's a lot more than AS has advertised, and well beyond the 600 lb. rating of my hitch.
Even before adding variable weights, the hitch weight of your 27FB was probably well in excess of your Mercede's 600 pound rating. Based on the specifications for the similar 2009 coach, the empty or dry hitch weight of the 27FB was 775 pounds.

I would also be concerned about the load pulling capacity of the hitch as well. I know that European manufacturers rate their hitches a bit differently than the U.S. manufacturers, but if the relationship is similar -- a hitch rated for tongue weight of 600 pounds would have a pulling maximum of 6,000 pounds. The dry weight of your coach is likely in excess of 5,500 pounds.

Quote:
Originally Posted by dpulling View Post
As it is now set up, the rig handles very well. My questions - using a WD hitch, is the TV receiver relieved of tongue weight strain when some weight is transferred forward by the trunnion bars? If not, can a hitch be reinforced to accept the excess weight? Is my 46/54% weight distribution OK?
Your weight distributing hitch should carry a specification for both "load carrying" -- no weight distribution in use as well as a capacity when weight distribution is in use. Finding a welder willing to make changes to the factory setup will be difficult as most won't want to deal with the potential for liability should there be a problem down the road.

Kevin
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Old 08-06-2009, 11:49 PM   #5
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Thanks for your inputs. The bottom line: my TV GVW and distribution (46/54) is within the limits of the air suspension specs in the owner's manual (which outlines a routine for hitching with the air suspension, but not with a WD hitch). However, the load on the hitch (900+ pounds static plus torque from weight distributing hitch) is well beyond its tongue weight capacity. Yesterday I discovered what I feared, fissures in the welds at the hitch receiver. Today I found a welder who will repair and reinforce the factory receiver and hitch to increase its capacity by 500%+ (the stock setup is very wimpy). Finally, I will replace the Equal-i-zer WD hitch, which I'm told is overkill for my rig, with a Eaz-Lift. Photos of the factory recall weld failures: MobileMe Gallery
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