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Old 07-08-2017, 07:23 AM   #1
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1987 34.5' Airstream 345
Menomonee Falls , Wisconsin
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Steerable tag axle on a 325

Hey,
I got this thought rolling around, about putting a front axle from a p30 stepvan, and using it as my tag axle. My concern is this, I can make all the bracketry, air bag mounts, etc etc, but, a steerable axle brings in the picture a whole new set of peramitors, when it comes to frame strength flex. But what it doesn't have is the drag factor a solid axle brings. Every time you go around a corner that tag is scrubbing sideways, a steerable axle does not create the scenerio. Take a look at dump trucks that front tag has to make the tightest turn of all the axles. My other concern is backing up. It may get quite funky there too. I could make a dump valve for the bags to take the strain off the tag, that's doable. It's totally align able given one gas the expertise to do same. And I got the alignment machine for the job. So it looks like a doable thing. Suggestions?
And I got a complete axle with disc brakes (same calipers as the 325) all I would need to do there us get 19.5 wheels with the small 8 bolt lug pattern.
6 wheel disc brakes a way stronger axle, I don t see a downside. DJ
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Old 07-08-2017, 07:44 AM   #2
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1983 31' Airstream310
Hillsburgh , Ontario
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Ummm, I don't mean to step on your dream; but exactly how are you planing to steer the wheels on the tag? I know that GM had a computer controlled rear steering on their trucks, along with some Nissan and Toyota models; some heavy equipment also has rear steering, but exactly how are you going to control the amount of steering angle?

If your plan was to let them free wheel back there; very, very, very bad idea.

Cheers
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Old 07-08-2017, 08:06 AM   #3
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High end motorhomes with tag axles have a system where the tag axle is raised automatically below 15 mph. That solves the tight turning issue, and at highway speeds the extra axle is not scrubbing in turns, there's enough flex in the sidewalls.
At work, we had 53' semi's with the rear axles separated by 10' or so. When the drivers had to park them in close quarters, the tires dragged sideways so hard I thought the lug nuts would snap off.
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Old 07-08-2017, 02:23 PM   #4
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1987 34.5' Airstream 345
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Hey,
The steering part is not the problem.ifo you look at dump trucks cement trucks ,and even busses, you will see two shock absorbers with coil springs over the shocks. These mount on the axle and connect to the cross tie rod. They adjust the tension on the coil overs to center the axle,so when running down the road, the axle tracks straight. They don't run a lot of caster in this axle, so you don't have to worry about high caster shaking. The coil overs also dampen this movement.
As for scrubbing sideways, that setup on your 53 ft trailors, is called a spreader axle. It is a dual or a super single tire/wheel assembly,and yes they scrub sideways big time. This won't be the case in this application. Axles are close and the rear one will turn.
My concern is mainly frame strength. I can set ride height separately for this axle, have a dump for the bags when backing up. I may not even need that, being the axles are close.
I thank you for your thought though. Info is how I make a plan even better. Any info is good, so thanks. DJ
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Old 07-08-2017, 04:31 PM   #5
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I've been out of the business for awhile but all the drop axles I have been around have enough caster that they must be raised to back up. Just like towing 4 wheels down, you don't back up. They do carry load and they do provide braking. The other problem is they do tend to chew up tires.

They may have gotten better but those dump trucks and cement trucks used the drop axles to carry heavy loads for relatively short distances. They were rarely used for long distances on the highway.
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Old 07-09-2017, 12:21 PM   #6
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Castering/Steerable Axle

Having driven a lot of big trucks with castering lift axles and pulled trailers with the same kind of axle the biggest issue I have with them is when you get close to the right shoulder and maybe drop off the edge. You know when you drop off the steering wheel wants to pull out of your hands and it takes a little effort to steer it back on the pavement? Well...with these castering axles when you start to drop off they automatically steer to the right just like the front end does only there is nothing to stop them from steering on their own pulling the whole rig toward the ditch!!! This, I believe, is why passenger buses with the extra axle behind the drive axle is a solid axle. Granted, castering axles have dampeners and springs to slow this action down but is doesn't help much. A solid axle will be more stable. If you back up a bit with a solid axle - no problem! A castering one will immediately turn to the stops and bind everything up if not lifted. A solid axle may scrub a bit on tight corners but if everything is mounted solid it is not going to hurt anything. Just my thoughts! Happy Airstreaming!!!
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Old 07-09-2017, 12:49 PM   #7
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Why do you think you should have a tag axle??
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Old 07-10-2017, 12:37 AM   #8
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On the 32.5 the weight exceeds the capacity of the rear axle so an additional axle is need as well as additional brakes.
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Old 07-10-2017, 06:23 AM   #9
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1987 34.5' Airstream 345
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Hey,
Thanks guys
Milky, very good point about the tag going off the road. Hadn't even though of that. This nixes the thought rolling around up there. Not worth the hassle. For the amount of weight to scrub on a turn, I'll live with it . Anyway,,,, now I can concentrate on more important things, like polishing striping, and I think I found a window channel to replace our gray top and bottom pieces. I went to a marine site. Marine window channels. They had one , available in 8 ft lengths. It may just work DJ
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