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Thread: when to raise tag axle?

  1. #11
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    It may be an unnecessary feature, but isn't raising the tag axle a required part of the slide extension protocol? I'm not sure because I don't have Prevost slides. While I probably could back into my garage without raising the tag, doing so makes it a simple 2 point turn instead of a 4 or 6 point. The manual also goes on to say not to lift the coach without the tag raised or you may suffer suspension damage; I'm assuming there's a risk of tag axle over extension if left aired up. Lastly, the warning to come to a complete stop before lowering the tag doesn't really apply to heavy motorhomes does it? In our case the tag is never truly lifted so the tag tires are always spinning eliminating the sudden spin-up when lowering the tag.

    I'm a believer in the reduced turning radius and I often take advantage of that - my wife has a penchant for booking us in old KOA's! I would truly be unhappy if I lost this feature. I don't think the additional load is an issue if speeds are low and durations are short. During cruise speed driving, the dynamic air pressures in our air springs surely exceed 120 psi on a routine basis and the tires and axles can easily handle those loads. I believe design allowables are in the 3 to 5g range in the upward direction.
    Last edited by travelite; 06-02-2017 at 10:37 AM. Reason: spaelling.

  2. #12
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    Yes David, depending on the generation of Prevost slide, the front slide requires the tag to be raised. Many owners probably don't realize this even happens. The tag is unloaded as part of the automatic slide movement operation. This is done to reduce the possibility the chassis is slightly racked. For those with 3 pins in the side of your slide, this is the case. I'm told the slide, when in and pinned is an integral part of the structure. If you have a newer coach with the single pin on the bottom your tag does not unload during slide operations. The single pin is solely to prevent movement and the single pinned slide is not structural.


    Gil and Durlene
    2003 H-3 Hoffman Conversion

  3. #13
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    Dave I'm not real sure that dynamic air pressures at cruising speed regularly exceeds 120.

    A semi with a good set of guages includes suspension air pressure.

    Good Guage fir dump truckers doing excitation where contractors always want to always give u an extra couple buckets.

    Empty suspension pressures r 20 psi loaded around 65 depending.

    I do not recall ever seeing those gauges jumping 60psi loaded on bumpy roads. Closer to 5 or 10 psi bounce and truck suspensions do not couple ping tanks to the bags like prevo, limiting that even more.

    Great input on the slide factor and u defiantly can hyper extend the tag depending how u r raising the bus.

    I am curious as to what Goodyear and Firestone says max pressure wold be for a bag I do not know. I guess about 150.
    Last edited by Joe Camper; 06-02-2017 at 12:53 PM.
    1990 Peterbuilt 377
    3406 B Caterpillar
    13 Speed Roadranger
    No Norgrens


    1 day on paper no machines

  4. #14
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    Thanks Gil,
    Hi Joe,

    This link has data for the Firestone W01-358-9320 which is a typical Class A motorhome air spring. Page 66 of the document shows dynamic pressures of 206 psi at a dynamic load of 2.15g's, or 15392 pounds:

    Screenshot from 2017-06-02 13-29-53.png

    I've also seen these pressures first hand in the Wanderlodge community. A friend has HWH Active Air installed on his late model LXi. Active Air includes an LCD display showing instantaneous air bag pressures. Above 200 psi dynamic pressures weren't unusual. Hitting a square edge chuck hole can easily impose 3g and above loads. Systems out there are handling these kinds of loads. If they weren't, we'd be seeing a lot of broken parts. Could your dash pressure gauges be of the slow speed averaging type, not able to react quickly enough to instantaneous changes?
    Last edited by travelite; 06-02-2017 at 02:29 PM.

  5. #15
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    I don't know why they wouldn't they r plumbed with a T fitting right off the supply to a bag.

    Never heard of slow speed averaging type air gauge what's that??

    Still curious what manufacturers says is max pressure for air bags. Would love to be suprised to learn it is north of 200.

    All that data is from a book. I get my conclusions from the dirt.

    This has developed into quite an informative thread on the tag.

    1 other thing I might be confused on. Static or on very smooth road the drive axle has 20000 lb aprox. On 4 bags. That would be 5000 per bag static, or on smooth road. Right? Where is that 15000lb variable coming from?

    That chart also starts at 100psi static that's 40psi more than where the drive bags r static. Could that be another variable that needs adjustent for a correct conclusion?
    Last edited by Joe Camper; 06-02-2017 at 06:25 PM.
    1990 Peterbuilt 377
    3406 B Caterpillar
    13 Speed Roadranger
    No Norgrens


    1 day on paper no machines

  6. #16
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    Quote Originally Posted by Joe Camper View Post
    I don't know why they wouldn't they r plumbed with a T fitting right off the supply to a bag.
    An event like hitting a square edge chuck hole sends the suspension upward in tens of milliseconds. This requires a much higher bandwidth sensor to capture. A piezoelectric pressure transducer placed at or in the air spring. This is what HWH Active Air uses to sense pressures and act on them.

    Quote Originally Posted by Joe Camper View Post
    Never heard of slow speed averaging type air gauge what's that??
    The long narrow diameter pipe and the T-fitting and the analog gauge all act to attenuate any pressure signal from the air spring. This setup might be okay to see 0.1s variations but it filters shorter lived disturbances; hence, it averages.

    Quote Originally Posted by Joe Camper View Post
    Still curious what manufacturers says is max pressure for air bags. Would love to be suprised to learn it is north of 200.
    All I've ever seen for design pressures say 100 psi static max, but dynamic pressures can be much higher because they're short lived.

    Quote Originally Posted by Joe Camper View Post
    All that data is from a book. I get my conclusions from the dirt.
    I don't have any data on Prevost drive axle air springs, but I do have a chart for a Prevost IFS air spring. I'll dig it out and post it. It has a pressure of 100 psi at static design height, so any road input is going to send the pressure well above 100 psi. True the ping tanks lower the systems resonant frequency and therefore the spring rate is softer and the pressures should be lower, but the ping tank's effect is frequency dependent. For instance, for a given ping tank size, hose length, and hose diameter the ride will be softer below some frequency, say 10 Hz, but harsher above. Hitting a chuck hole is a high bandwidth event, so I'd still expect internal air spring pressures to spike.

    Quote Originally Posted by Joe Camper View Post
    This has developed into quite an informative thread on the tag.
    Yes it has.

    Quote Originally Posted by Joe Camper View Post
    1 other thing I might be confused on. Static or on very smooth road the drive axle has 20000 lb aprox. On 4 bags. That would be 5000 per bag static, or on smooth road. Right? Where is that 15000lb variable coming from?
    Just from a quick example that I came up with of a truck spring that has a max static design pressure of 100 psi but can handle a 2G dynamic load at 200 psi.

    Quote Originally Posted by Joe Camper View Post
    That chart also starts at 100psi static that's 40psi more than where the drive bags r static. Could that be another variable that needs adjustent for a correct conclusion?
    So, static design pressures for the drive axle are around 60 psi. I don't know what the max static pressure is because I don't have spec's for a particular Prevost air spring, but I suspect it can handle 100 psi static because that's the most common. What it can handle in dynamic loading I don't know. All I know is that my friends Wanderlodge LXi with four drive axle springs and the same 60 psi (static) saw 150 psi (dynamic) or more on the HWH LCD display.

    I guess we headed down this path because I said the added loads on the drive due to raising the tag shouldn't be an issue. With a system able to handle 3g's in the upward direction without breaking I believe it can certainly handle the extra load of a raised tag and there should be no issue with the air spring for the short durations involved. Oh well, that's my opinion.
    Last edited by travelite; 06-02-2017 at 10:59 PM. Reason: spaelling, :)

  7. #17
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    Well there u go.
    1990 Peterbuilt 377
    3406 B Caterpillar
    13 Speed Roadranger
    No Norgrens


    1 day on paper no machines

  8. #18
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    Here's an interview with Robert Hitt, the Service Training Manager for Prevost and the topic is: When To Raise The Tag Axle.


  9. #19
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    Thanks Andy, I enjoyed that. I really appreciate you donating your time and your skills to the group. Great video!

  10. #20
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    Another excellent video Andy. Thanks.
    Dale & Paulette

    "God Loves you and has a plan for your life!

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