when to raise tag axle?

[jbrhodes76]

I have been rising my tag when I'm on flat ground making a 90 degree turn. Is it ok to raise tag while in turning motion or am I supposed to come to a complete stop first and then raise tag? This could be a problem by breaking the flow of traffic however I don't want to cause damage to my bus. Any advice?

[Gil_J]

Raising the tag, or not, can be like discussing the merits of a political candidate.

Ideally, the tag will be unloaded during all low speed extreme turns. The reality is there's probably little value when compared to hassle for any turn not performed during parking or multi point turns. In these situations you should consider unloading the tag. The one caveat is unloading the tag will reduce the rear ground clearance of the coach. There will be situations where ground clearance is critical. I unload my tag when parking into a camp site or parking space after making sure ground clearance is not an issue. As soon as I begin to straighten the steering wheel I load the tag.

There is tremendous side force on the tag tires and hubs when making a slow extreme turn. This is probably not good on the tire's sidewall or hub. It also results in tire tread scuffing and a slight increase in turn radius.

So, unload when making slow speed extreme turns. This would not include normal right turns at an intersection.

[dale farley]

One time, I raised my tag while making a 90 degree turn off a street into a parking lot without thinking about the dip as I went into the lot. I really scraped the trailer hitch because the dip was more than I expected and the bus was lower since the tag was raised. Be cautious when entering parking lots with dips.

[TheGadgetGuru]

Coming soon to The Gadget Guru, the answer to the burning question: If and when you should lift the tag axle.

So, stay tuned!

[Joe Camper]

A quite unnessessary accessory.

As u worry about side force and scuffing on a tag in a turn how about all those other undesirable factors transfered to the drive axle like 35000lb+ on 1 axle just for 1 example.

I say u will age those tag tires out long before u wear them out weather u raise the tag or not. Isn't there enough other things to concentrate on behind the wheel without having to dig for that switch.

Lift axles serve a way more useful purpose on trucks when they r loaded and then empty back and forth on a regular basis.

I never used mine the switch was in a real rediculas place. It never caused any adverse result I could gauge.

Be safe.

[Joe Camper]

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Here is a gauge on one of the drive axle bags tag down.

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Here is where the pressure gets to after u lift the tag. Clowdy picture, gauge needle pointed straight down that reads 120 psi if it were calibrated that high, and that's with 4 bags on the job.

[Gil_J]

Joe,

Great data point.

[Joe Camper]

The bus I tested in the pictures was an H3-40 no slides. Not even a heavy bus.

So in conclusion, when your tag is up whatever pressure u r seeing on your dash air gauge, that's the pressure in the drive bags.

Unnessessary accessory is a bit harsh I apologize.

Even if the tires will age out first....

If u use your bus alot and have become very comfortable and proficient behind the wheel, u put plenty of miles on it and u have trained your left hand to know where the switch is, and u use it on most tighter corners..... sure u will get to age out on the tires with more tread no doubt about that.

If u don't use the bus alot the switch is in a tough spot and distracting from real driving. Or u can snooker yourself like Dale posted.

Be careful that's all.

Also if u don't exercise that tag lift those norgren controlling it all, in the event u do use it after long periods of not, it could go bad, or get stuck,or leak afterwards and give u a lean.

If there is any one axle in the universe I would want for a heavy load a prevo drive axle would be right up there at the top IMHO.

[Gil_J]

Just a couple of tidbits.

At some point (2013 or earlier) Prevost's X-3 chassis was factory equipped with an automatic feature that reduces the load on the tag axle by 70%. This is done without any indication given to the driver it's being done. The driver can also fully unload the tag manually. I'm not sure why the manual full unload feature is still available. Maybe, it just hasn't been deleted yet.

There was a time when Prevost had an optional system that automatically lifted the tag. I believe that may have been in the mid 90s. I'm pretty sure parts for this system are had to find. So, many, if not most, have removed this feature.

[Donnie Myers]

I have to side with Joe on this issue. Tag down normal driving.

[travelite]

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.

[Gil_J]

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.

[Joe Camper]

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.

[travelite]

Thanks Gil,
Hi Joe,

This link (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjOjZvr1Z_UAhVFPiYKHeY-BkgQFghAMAA&url=http%3A%2F%2Ffirestoneip.com%2F-%2Fmedia%2Fwww%2Ffsip%2Ffiles%2FCommTruckTrailer%2 FAftermarket%2FAftcat_smweb.pdf&usg=AFQjCNExcynJ4PpDthipolqsUOuPcWonEw) 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:

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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?

[Joe Camper]

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?

[travelite]

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.



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.



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.



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.



This has developed into quite an informative thread on the tag.
Yes it has. :)



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.



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.

[Joe Camper]

Well there u go.

[TheGadgetGuru]

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


https://www.youtube.com/watch?v=hHANE7nI7JA

[travelite]

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

[dale farley]

Another excellent video Andy. Thanks.

[TheGadgetGuru]

Thanks David!

Be prepared for something different on the next video!

[TheGadgetGuru]

Thanks Dale!

[Fratto]

During our recent stay are Prevost Winter Garden, we had the pleasure of a long post flush test drive with Eugene. Eugene drove Prevost buses for 25 years or so then went to work for Prevost.

For what its worth, Eugene always raised the tag coming into any turn 90 degrees or more and put it down as soon as he exited the turn and started going straight.

[PrevostNewbie]

I am new to having a Tag but in my brief time driving my bus so far, (under 1000 miles) i did notice on my first hard 90 degree turn in a parking lot the tag tires were scuffing pretty bad. I raised the tag and the issue went away. At that point I have been raising the tag on tight maneuvering situations every time. I don't think it hurts anything. and it's easy to do.

[georgiapeachinc]

Nice video Andy. Like Joe Camper I have lived in a trucking world for 40 years.
I was reminded once that my Prevost is not a truck. Very true but many drivetrain & suspension components share similar characteristics. All of the comments I read here and Andy's video make for an educated decision about raising tag axles.

I tend to agree with Joe Camper. During a normal course of turning on any road you usually don't notice any significant or noticeable difference with the axle up or down. Noticeable river wear on the tag axle tires most likely would be unnoticeable being so close to the drive axle anyway. If you are an experienced tri-Axle dump truck driver or in our business using spread axle trailers, constantly flicking the dump valve on every 90 degree turn to raise the an axle is second nature and defiantly adds to decreased tire wear and a tighter turning radius. I raise my tag axle during low speed situations where turning into tight places is critical. If you own an older bus as we do, Joe is right. Fumbling around for the switch behind you is distracting. My first thought when I saw where the tag axle valve switch was located on our coach was " why didn't Prevost put it in a place that's easier to get to if it is so important?". Just sayin!

[Coltonh]

I whole Agree with that