Wednesday, Oct 20, 2021 at 23:04
Chris, Perhaps Mal too.
I didn't know you have broken two axle housings or is it one housing in two
places?
You mention a bigger weld on the new axle, does it spread the stress over a wider area but same tube size?
The axle tube does crack as a result of metal fatigue, but that is caused by localized flexing at that point. No flex or not much flex means less likely to fatigue.
Undoubtedly the heavy loading is a contributing factor but the axle doesn't simply crack with the load on it, only when flexing with that load “past” a certain degree does a problem arise.
I agree with the numbered points you have shown.
Weight on axle yes,
Spring stiffness is a difficult thing to quantify, a stiff spring may not deflect much and have less initial curvature and no appreciable ride height. Others may be more compliant and begin with more ride height and are far more compliant. That I like!
Shock absorbers may resist initial movement very abruptly, not so good, others are slower to resist and therefore kinder to
suspension. Only a
test graph from a manufacturer would reveal that. Again hard to test/gauge by the average user.
Bump stops. You may be surprised how much the OE bump stop squashes when impacted and they don't have much ability. Most folk rubbish them. An airbag, in my opinion has VERY LITTLE internal bump stop and so little ability to finally absorb shock. Unfortunately, the airbags reduce the overall travel simply because they have a top plate and a bottom plate which all takes up travel room. I used some on my cruiser and was acutely aware of the LESS travel they caused. They had practically NO bump rubber action at all if bottomed. See how far you an compress them with no air inside. You may be shocked. Sales hype is the opposite!
Have you actually seen the internal bump stops in an airbag. Best to make sure you really know their limitations, other wise the same problem is still present.
With the available dimensions of the whole setup, ie ride height/travel UP and down what percentage of up travel do you have when loaded? When fully loaded for travel, 1/3 down and 2/3 up travel of whole distance would be desirable so as to begin to be able to “absorb” without undue stresses. If there is isn't around 2/3 of total for upwards travel then the same situation remains as before. For my cruiser I made sure I had that specifically, so bottoming of the axle didn't happen and shocks could catch movement during compression. I did it because the bags had NO detectable bump stops although I was assured by the Air Bag Man they did.
With a raise of 1cm which is only 10mm and airbag dimensions taking some of the up travel, it would worry me.
Isuzu are strange in a way.
Mine said 80mm clearance to bump stop when new, As new, no load
mine was 52mm clearance. I changed it to 120mm before load. Then I had up travel to use.
5,6,7,8, are all manageable situations and vary.
With regard to axle movement, most bumps are felt at an angle to the axle, ie, a line leaning backwards, no direct vertical unless stopped. Any impact is resisted by the suddenness of the shocks and compliance or lack of of the springing. There is also the mass of the whole diff housing and internals. All causes an axle stress load before the axle actually moves and so localised stress at the weld area. Some vehicles have large dia front eye bushes which absorb some suddenness. My 2011 front eyes are small and simply transfer much of the shock to the body. If felt there, the axle tube must be feeling it too. What size front bushing is in the later models? I haven't checked them to see.
I simply place importance on springing and shocks and travel height with sufficient upward movement to actually work. In the Isuzu you can't change the axle much, apart from gusset bracing in vital areas, so other component action and dynamic operation become important considerations.
If the axle tube is “light on”for the task then only strengthening it will overcome the flex and failure issue. The above assists the life expectancy.
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