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Elite Veteran
 
Location: Victoria, BC. Canada | Hey,
Just started the investigation into my air-spring install. I went through both Service Manuals I own and went through what's on the FMC site and can find no reference to the usable (available) travel of the trailing arms.
Anyone know this spec, I'd be much obliged if you'd share it?
I know the shocks also figure in the equation, but me thinks the working stroke of the air-spring has to be secondary to the limits of the trailing arm stroke.
My airbags have a usable 9" stroke with a 3.8" min compression and a 12.8" max extension. So if the travel of the trailing arms dictates what my air-springs can do then I need to know before the trailing arm extensions for the airbags get built. There also appears to be some flexibility (a few inches) as to how far the bags can be mounted behind the rear wheels. Further back means more weight transferred forward as well as a greater stroke, so knowing the travel limitations of the trailing arms would really help.
After some deliberation my cock-eyed notion that I might design and install the mounts for the airbags 'before' the engine & trans goes in has evaporated. First, I'll need to measure at what height the coach sits with all the new baggage (engine/trans etc) I'm throwing at it, then figure what the air-springs need to be set at.
Any and all advice really appreciated.
Thanks,
Terry
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Elite Veteran
Location: Oceanside, CA | I couldn't find it so I disconnected the shock and let it droop completely, I found that this exceeded the axle shaft joints angle and actually caused them to bind so I jacked the trailing arm back up to allow clearance. I cut off my upper shock mount and extended my shock fully and welded the upper mount in at that (new higher) point. So that now my shock at full travel allows all the droop I can safely have. You can then attach your airbag to an upper mount that you fabricate, measure 12.8" down(or temp install the bag and fully inflate it) and that's were your lower mount will go(tack weld a piece of scrap to that spot). To double check you are safe at full compression jack the swing arm full up until you bottom out your shock (or timbren) and make sure you still have more than 3.8" left in that same spot. |
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Elite Veteran
Location: Victoria, BC. Canada | Thanks Len!
One of the things I was hoping these airbags would bring to the table was the ability to raise the coach a couple of inches while traveling on rougher forest-service roads etc. But I, too, was wondering just how high the prop-shafts would allow you to raise things off the ground? So I understand that the bump-stop (or Timbren as you mentioned) will limit the upward travel of the entire trailing-arm and wheel assembly, but what stops the whole works from tearing the prop-shaft apart as it comes crashing downward when it gets airborne as you fly over a big bump - at 80' while getting chased by the cops etc?
Seems a lot to ask of a shock-absorber. Guess I need to bone-up on shocks, too, as mine are pretty shot I figure.
I'd like to do the air-bag and trailing arm extension work while the engine and trans are out as there's currently a crap-ton of room to work under there. But as I mentioned earlier my concerns about attempting this work without the known elevation-effect on the coach with the Cummins & trans installed, wouldn't I be smarter to wait until the engine/trans are in?
Also, I've got the coach sitting up on its wheels on blocks - thanks for the idea Rico! - and I'm rather loathing the idea of getting the coach's arse-end back up blocked on the frame or torsion bar blocks etc. again to free up the trailing arms for measurements etc.
http://www.fmcowners.com/mbbs22/photos/show-album.asp?albumid=191&p...
So, my question is, do you think I ought to go ahead and do this as you described 'before' the additional weight of the engine/trans is brought to the party?
AND, if so, would I be wise to measure the current position/stance of the trailing-arm without the engine/trans - say, from the center of the spindle to the lower face of the chassis frame member above it - then subtract another inch (or two), in anticipation of and to estimate roughly what the extra weight (1400-1500 lbs) will do, then use that measurement to establish the trailing-arms neutral point or center of travel when the coach is loaded and sitting at rest on level ground?
(I won't even pretend that I understand what I just wrote; hopefully you do.)
I think the air-springs I'm going to try to use are on the larger side for this application, but I would like to get them set up so there's a more or less equal amount of travel available in both directions.
Feedback much appreciated.
Thanks,
Terry
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Elite Veteran
Location: Oceanside, CA | I did all my airbag work with the engine out specifically because the access was so much better, I never had a single concern for it, nor have I had a single problem with my install.
actually I think I did half of it with it out...ran out of material and had to finish up later...regardless, I had the coach up on blocks the entire time and never worried about what the coach stance was with the new engine setting on the stock suspension..
Edited by LCAC_Man 2019-10-08 8:16 AM
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Elite Veteran
Location: Victoria, BC. Canada | Okay thanks again.
Gonna' giver' and see what comes of it. Got a bunch of 1/4" angle iron and U-channel I scored from a friend I'm hoping to use. Since the only reference I have for this is the work and pics that you and Frank have shared, that will be my starting point.
BTW: Have you, or anyone else out there, actually done any adjustment or playing around with the torsion bar adjustment? (see: Group 6 - pg. 6-2, parts #21-22)
I'd like to know/learn more about the adjustment of the torsion bar set up.
There's nothing out there that I've seen that actually tells you what the adjustment does and how to get there. I read that the torsion bar is made of spring steel, so, it will twist. I used to make cutting-bars for chainsaws (Cannon Machine Works, Burnaby BC) for a few years so I have a good grip on the qualities of spring-steel. I also have a set of FMC/MCC torsion bars in my parts collection (thanks Jay!) and I find it hard to imagine those things, made of spring-steel or otherwise, actually allowing enough longitudinal twisting action to act as a spring.
Is it as simple as that?
If that's the case, then the adjustment should permit you to raise/lower the rear stance of the coach some. (?)
I will also do a rear toe-in alignment-check (with string) before I start the air-spring install.
'A string don't lie'.
Thanks,
Terry
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Expert
Location: Cottage Grove, OR | Steve Centracchio has adjusted his torsion bars so that they have very little of the coach weight. It is virtually all on the air bags. |
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Veteran
Location: Houston, Texas | with our 12V Cummins & Allison trans, when we adjusted the rear with the torsion bolts,
the front would came up just as much and the front would feel much lighter...
We are still trying to find the right air pressure for our rear air bags while driving it,
maybe between 20 - 30 psi …
Rico & Carmen
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12 V Cummins & 6 Speed Allison |
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Elite Veteran
Location: Victoria, BC. Canada | Thanks you guys.
Any chance you could post a few pics of your air-spring/trailing-arm set up, Rico? Any additional perspective on this mod would be useful to me and any future coach owners doing the 'bag-thing'.
Hey, that could be a song - "Bag-thing... do-do, do-do-do, you make my thing sing..."
I digress.
Hopefully Steve see's this and can add to it as well.
Terry
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Elite Veteran
Location: Oceanside, CA | I've never messed with the torsion adjustment, if I'm running with nothing on the hitch (which is pretty much never) then 30-50psi is good, with 500lbs of tongue weight (my standard load) I run them at 80psi. I rarely run more than 25psi up front and that is mostly for improved sway control in windy conditions.
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Veteran
Location: Oslo Norway |
andy1canada - 2019-10-08 3:52 I used to make cutting-bars for chainsaws (Cannon Machine Works, Burnaby BC) for a few years so I have a good grip on the qualities of spring-steel. I also have a set of FMC/MCC torsion bars in my parts collection (thanks Jay!) and I find it hard to imagine those things, made of spring-steel or otherwise, actually allowing enough longitudinal twisting action to act as a spring. Is it as simple as that? 😝😝 So with your great grip on “spring steel” Sir, you’re basically saying that you find it hard to believe that Volkswagen, GM trucks, FMC M113 APC’s and who knows how many different types and brands of other vehicles out there today does not have a sprung suspensions system that works? And while we’re at it, what relevant suspension systems have the spring tension as a preferred, recommended or even allowed way of adjusting the ride height? What would be your best quote on those MCC (non functional) torsion bars? I might be Interested even if they are useless after all. Stay cool Kjetil |
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Elite Veteran
Location: Oceanside, CA | rear air bag install for reference http://www.fmcowners.com/mbbs22/photos/show-album.asp?albumid=202&p... |
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Elite Veteran
Location: Victoria, BC. Canada | Thanks Len for being so generous in the sharing of your work on your coach. Will help a lot of people.
Now, if I can just get Rico to share a couple of pics of his air-spring install...
Terry
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Elite Veteran
Location: Oceanside, CA | there's no "on right way" to do it. Just get some scrap wood and start templating up some stuff, see what you think you can pull off then transfer it to metal, I always build my metal projects in wood first (usually buy a couple bundles of 1"x2" furring strips) and staple it together, its fast to work with and your mistakes cost less than a $1.
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Elite Veteran
Location: Victoria, BC. Canada | Try a 'hot-melt' glue gun instead of staples. We use it to glue 2" strips of mahogany door skin together to make stair-tread templates for example and it works like a hot damn! Fast & cheap, like me.
Terry
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Elite Veteran
Location: Oceanside, CA | okay...show us.
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Torsion bar & trailing arm.
