| Re: [GMCnet] Alignment-Height Setting [message #78860] |
Thu, 01 April 2010 08:45  |
Gary Casey
Messages: 448
Registered: September 2009
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Senior Member |
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I certainly have to agree with Ken(Burton) in that the process of adjusting all for corners can get confusing, even for the experts. But I disagree with a couple of his methods. The ideal method would be to do all the adjustments with the vehicle on 4 scales. And the objective is certainly to get the coach level side-to-side and at the desired ride height front to rear. But that doesn't require that all wheel loads are the same. Regardless of what happens the total weight of the right side and that of the left side will remain constant, but not the same as each other. The question is how to proportion the side-to-side weight imbalance front to rear. I would like to have all the weight imbalance absorbed by the rears, leaving the fronts equally weighted. The reason is that I would like to give the steering behavior the best shot - a level coach, equal cambers and equal weights means that the directional control should be as good as possible. And
in my case the 400-pound generator is at the extreme left rear and there is no compensating weight on the right side (holding tanks, fuel tanks and water supply tank are all centered). I'd rather let the left rear tires take the load than the left front. How do accomplish that? The ideal, as Ken said, would be to lift the front at the centerline to adjust the rear. Then keep the rears at whatever weights they have and adjust the fronts. If I had scales I would adjust the fronts to have equal loads, but I don't have scales. Where I disagree with Ken(B or H - I'm losing track!) is that I think the fronts CANNOT be adjusted with the rears blocked. You would have no idea how to adjust each torsion bar to achieve equal loading.
Sure, if you adjust the fronts with the required air trapped in the rear bags, adding more load to one front corner will change the pressure in each bag. But, when the fronts are adjusted to level the coach side to side the rears will be back at their original pressure and the fronts will be equally loaded, and that's the objective. What makes the whole thing time-consuming is that you want, I'm told, to adjust the fronts only if the weight is lifted off, reducing the stress on the threads of the adjuster. Then when you lower it back down you have to have some way to let the tires scrub back into place. I do it by rolling the coach back and forth several times, but others have used like a greased plate to do the same thing. Measuring the bag pressures is probably a reasonable way to measure the weight at the rear. But my objective is not to equalize the rear loadings, so I guess I don't care about the pressures.
Here's a crazy, but workable, idea: Loosen the bolts that hold the rear torsion bar crossmember to the frame (is it bolted? - I haven't looked, I'll admit). If the fronts are not equally loaded one end of the crossmember will lift. The "lifted" end will be on the side that has the higher loading. If they are equal the cross member has no load and all the bolts could, in fact, be removed with no affect. So back off the adjustment on the side that lifts until it doesn't. Then adjust the ride height by turning both adjusters equally.
Again, my objective is to level the coach front to rear and side to side, keeping the loading on the FRONT wheels equal to each other. The rears will then NOT have equal loading side to side. What you don't want is to have the vehicle "jacked" with the diagonally opposite wheels loaded more. I think it would be acceptable to have both front and rear to be loaded heavier on one side, but I like the idea of having the fronts equal. The only disadvantage I see in having the rears unequally loaded is that the terminal understeer (just on the verge of sliding) characteristics will be different turning one way than the other. I'll accept that :-)
I apologize for the long post.
Gary
I guess in the end we really need to know is the end objective. Is the objective equal height side to side regardless of weight applied to each wheel, or is it equalizing the weight on each pair of wheels while trying to get the height as near as equal as possible?
One compounding factor in adjusting the fronts or using scales is that the coach will be heavier on one side than the other. So if balancing the weighs side to side is your objective then this offset in side to side weight needs to be taken in to account.
What started all of this discussion with Ken and Alex was my attempt to adjust the ride height using Ken H's methods. I got all of the ride heights within specs, but the weights were way off on opposite corners of the coach. I mean hundreds of pounds off on the left rear and right front. This problem was caused by the automatic action of the rear air system and it's affect on the adjustment of the front.
So it is my contention that the rear heights should be set first. The rears adjust by height only no matter how much weight is applied to them. The automatic air system adjusts the air pressure to compensate for any imbalance in weight side to side. Jacking in the exact center by weight is just about impossible in the front and I see nto way to do it in the rear.
I contend that the rears should be held in place at the correct height on blocks or stands while adjusting the fronts. This keeps the coach rear in perfect balance (height wise) no matter what the weight is applied to the rear suspension while adjusting the fronts.
In the end what I would like to see is a coach set at the proper heights (front and rear) while keeping the weight as close as possible to being balanced side to side. I also contend that if coach is heavy on one side in the rear then it should probably also be slightly heavy on that same side in the front. It definitely should not be heavy on opposite corners which is what I had using Colonel Ken's system.
I think is an interesting idea to measure the bag differential pressure to determine the rear weight balance.
Ken B.
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