Well, since there are no 450/550 axle discussions this week, I might as well expound upon my brief answer above and blather about another chassis components of interest: shock absorbers, and specifically, the difference between the Rancho 9000 and the 9000xl.
Note that there is also a Rancho 9000x that was sold for a five year period of time inbetween the original RS9000, introduced almost a quarter century ago and discontinued in the summer of 2002, and the current RS9000xl, first announced five years ago at the beginning of 2007.
The 9000x introduced more incrementation in the user adjustability of the 9000 shocks. Whereas the 9000 had 5 positions, the 9000x had 9 positions. Notice that I said more increments, not more range. The amplitude of range in adjustability is about the same. The 9000x merely increased the number of indents on the adjuster dial.
Some natural questions come up...
1. Who needs more incremental indentations? Isn't 5 enough?
Apparantly no. Unlike the user reports of "pro comp" brand shocks, the adjustments to the Rancho 9000 series are so perceptible that one could feel when 3, for example, was too soft but 4 was too hard, and there was a desire for a goldilocks compromise in between.
2. Why not just set the 5 position dial inbetween 3 and 4?
Because the detents are critical for the adjustment setting to stay in position. The adjustment knob is like a worm gear acting on a spring. The spring supplies a constant opposing force, so if the knob is inbetween detents, the spring will be able to push against the worm gear until it stops in the next detent. So an inbetween adjustment will eventually revert foward or backward to a resting place (a detent) that the spring is not powerful enough to overcome.
3. If the in cab remote adjuster is used, isn't the adjustablity infinite within the range?
Yes. With any version of the 9000, 9000x, or 9000xl shock, the in cab remote adjuster removes the knob altogether, and uses an air pressure system to supply constant pressure against the spring. The variation in pressure is attenuable in much greater variation than the 5 position or 9 position detents. But, that pressure must be constantly maintained. The numerous air lines, fittings, and the compressor itself all add to the complexity under the rig, creating more exposure to maintenance and things gone wrong. So there was value in increasing the number of detents from 5 in the 9000 to 9 in the 9000x and 9000xl.
The 9000x also introduced a change in Rancho's "triple" tube design. Keep in mind, all three iterations of the 9000, the 9000x, and the 9000xl use triple tubes, as follows:
- Tube 1 = External reservoir tube that stores the reserve of oil and the cellular foam sleeve in the 9000 and 9000x (the 9000xl is gas charged)
- Tube 2 = Intermediate down flow path tube that routes the working oil in a mono directional path downward toward the adjustable valve, regardless of whether the piston is acting up or down in jounce or rebound directions. This is the unique feature of all 3 models of Rancho 9000 shocks over typical "twin tube" shock absorber designs
- Tube 3 = Internal pressure tube where the oil is worked and acted upon by the piston
The change in the triple tube design released in 2002, particularly in the intermediate flow path, necessitated a reduction in the piston diameter. Did you ever notice that Rancho stopped advertising the piston diameter after this change? Ranch currently makes a huge deal about advertising the BODY diameter of the new 9000XL, which does make for a larger reservoir of oil. However, Rancho no longer makes any mention whatsoever of the piston diameter, which is actually smaller in the new version of the 9000 than it used to be in the earliest version.
I guess it doesn't help to sell the new model shocks when you go from a former 1 3/8" diameter piston down to a 1 1/4" diameter piston (32 mm, to be precise).
However, to be fair to Rancho, the reduction in piston diamter is somewhat immaterial due to the manner in which fluid flows through the absorber, as well as the increase in diameter of the rod that moves the piston, which adds to the total displacement of fluid from the pressure cylinder.
As an illustrative example, suppose you had two empty 5 gallon painter's buckets, and one bathtub full of water. Take one 5 gallon bucket, cut the bottom off, and throw the rest of the bucket away (I mean recycle of course). To this freshly cut bottom affix a nice D shaped handle so that you can grasp the disc, like a small Viking shield.
Leave the second bucket as it is, but bring it as well as your homemade viking shield to the bathtub full of water. First grasp the D handle to the shield (that has the cut off bottom to the 5 gallon bucket) and push it straight down into the bathtub of water. Some water will be displaced and spill out over the sides until the water in the tub subsumes the flat disc and surrounds your relatively skinnier arm as you press your shield to the bottom of the tub.
Remove the shield, refill the tub, and grab your bucket. Now press the completely in tact 5 gallon bucket into the bathtub. Notice how much more water spills out over the side of the tub, because the barrel circumference of the 5 gallon bucket is so much greater than your arm, so the total volume of displaced fluid is greater.
Same with the newest Rancho 9000xl shocks. Although the piston diamter is 1/4" smaller, the ROD diamter (your arm vs the sides of the bucket) is somewhat greater for some models of 9000xl, up to 18mm in diamter, versus the 16mm diameter of the 9000x, and the 5/8" diameter of the 9000.
There are several other features in the original Rancho 9000 series shocks that have changed in the current Rancho 9000xl version:
The original 9000, and the successor 9000x, had Cellular Foam Sleeves in the outer tube. This foam expanded when the shock oil heated up from being worked. The expansion of the foam took up volume that applied pressure to the heated oil that would otherwise cavitate.
The new 9000xl uses nitrogen gas pre charge for this function.
And, this is the very spring board of much debate... between cell foam and gas charged shocks. I'm surprised no one has chimed in.
Come on now, don't let me keep the fire warm here all by myself!