Tag axle questions

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Kevin Means

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I did a search on tag-axles and learned quite a bit, but not exactly what I was l looking for. We're "in talks" with an RV dealer on a new 40 foot Class A with a tag axle (a feature I REALLY like), and I have some questions for owners of coaches with tag-axles. There seems to be two types of tag-axle systems:  those that enable the driver to actually lift the rear tires off the ground (which, on the surface, seems best), and those that enable the driver to dump the tag's air pressure, thereby unloading it and enabling it to "skid" a bit while making tight turns. That's the type that's on the coach we're looking at.

My questions are...  over time, does not being able to lift the tires completely off the ground cause excessive wear to the rear tires? I realize that's a subjective question. I'm just trying to find out if people who have such systems on their coaches feel that they are experiencing excessive wear on their tag-axle tires. Secondly, when do you dump the pressure to the tag? While maneuvering on "normal" city streets? While backing? Only during relatively tight turns? Thanks.

Kev
 
I'm not aware of anyone who wears out tag axle tires before they age out, regardless of which system they have. So, I'd call it a non-issue. If someone does, I'm sure they'll speak up.

The tag on my coach has a separate button to push which puts the axle in automatic mode. After the 3 minute wait period it will lift the axle anytime we are driving under about 7 MPH. This includes city streets, driving in rv parks, backing up, etc. The system needs the button pushed each time after starting the engine. I've developed the habit to push the air leveling system into road and the tag axle at the same time after starting the engine.

An added benefit that some may not realize is that this actually reduces the effective wheelbase of the vehicle. On my particular brand of coach the wheelbase of the 43' with tag lifted, is shorter than the 40'.

Ken
 
With our tag-axle raising feature we use it when we think we need it by pushing the toggle switch to the left of the driver.  It is not raised automatically and we do not use it on tight corners while driving.  In fact, it disengages above a certain speed.  We use it when we go in and out of our driveway because of the dip at the gutter.  It prevents the tail end from dragging when turning into the driveway or from digging into the street asphalt when backing out.  It only works at low speed such as when you're maneuvering into a parking site.  We use it when backing into a site so we don't gouge the surface when a lot a weight shifts to that side, for example on an asphalt surface when it's over 100 degrees or on softer grass so you don't dig in and make ruts.

ArdraF
 
As Ken states, don't worry at all about wear on the tags. The little bit they scuff is minor. The plus points for a tag axle coach are many. You will be very happy you went with a tag.
 
I use our tag similar to Ardra's use -- at the lip of the driveway where both the street and driveway angle up. Mine's not automatically lifting, and considering that lifting puts all that weight from the tag axle on the drive wheels, and how little the scuffing matters, I'm reluctant to use the lift more than necessary. Besides, it yells at me when I approach five mph (either direction), and slightly above that automatically goes back down while still yelling.
 
Please pardon me, I'm foggy on this: How does lifting the tag prevent tail dragging? It seems having the tag down would help prevent dragging, as the tag is aft of the drive wheels?

Tag adds 10,000 lbs. to carrying capacity, as far as I've heard, at least in our setup.

 
The coach pivots on the drive axle in a turn and the tag wheels get dragged sideways. That is sometimes called tail-dragging, but that is confusing because tail dragging also refers to literally bottoming out the chassis on a dip in the road. And a tag usually prevents that sort of thing.
 
We had a 1996 gas chassis coach with a tag axle that had absolutely no means to lift or unload the tag wheels (it was a spring suspension). On a dirt or gravel road you could see the marks where the tag tires dragged a bit sideways in a turn, but the overall effect was unnoticeable in terms of steering or tire wear.

In my opinion it is not an issue worth worryng about.
 
Another fairly rare but real nuisance problem with a tag axle is the "bridge" effect that can happen on abrupt dips or bumps in the road. If the tag wheels are on a higher surface than the drive wheels, the drive wheels can lose contact with the road and spin helplessly. If traveling at speed you will coast over the spot, but at very low speeds you may find yourself stranded.  This can happen at unexpected moments, e.g. passing through a curb cut to enter a drive that is a couple feet higher than the road and the road is sloped down to provide drainage at the edge. The coach is moving slowly, the front raises up as you climb the slight grade and the drive wheels pass over the drainage depression while the tag is still on the higher road crown. Another potential situation is crossing a drainage depression in a road (typically a driveway) or a speed bump of a particular shape and height. None of these are common, but they do happen. I saw a coach stranded last summer in Maine as it came down a short but sharp incline (a RR embankment) onto a highway from a side road. As it pulled out into traffic, the coach bridged the transition and the extra weight on the tag forced it down enough to let the rear of the coach drag on the pavement.  It suddenly lost traction, leaving it halfway out into the main traffic lanes, and couldn't move. Tow truck couldn't move it without damaging the rear end where it dragged, so they ended up jacking the coach up and placing wood beams under the drive wheels to get traction and raise the rear enough to move.

I'm not trying to scare you away from a tag axle, but it is something to be aware of if you own one.
 
Thanks, Gary, for that lucid explanation.

In the case of the drive wheels being stranded, the remedy would be to lift the tag, thus lowering the drive wheels back down to pavement?

If lifting the tag isn't really necessary to prevent skidding the tires in a tight turn, then there's no reason ever to lift the tag except for the above drive-wheel issue?
 
In some cases, lifting the tag might not be enough to avoid this -- depends on the slope, etc. -- since they're not lifted very far, at least on mine, and even with lifting I still get some sideways skidding, as evidenced by the skid marks on my driveway, but it's better than not lifting.
 
Pierat said:
Thanks, Gary, for that lucid explanation.

In the case of the drive wheels being stranded, the remedy would be to lift the tag, thus lowering the drive wheels back down to pavement?

If lifting the tag isn't really necessary to prevent skidding the tires in a tight turn, then there's no reason ever to lift the tag except for the above drive-wheel issue?

Since we're all engineers on here expressing our opinions..... I'd be hesitant to say that the coach manufacturers spent the time and money to engineer, build and install the lifting mechanisms if there's "no reason ever to lift the tag except for the above drive-wheel issue". My coach tends to turn sharper easier when the tag is up. There is some type of drag as the coach turns that goes away when the tag is up. The tags don't shift as much as a set of tandem drive axles do on a large truck but there is some give.

Ken
 
I understand why a tag axle would be desirable/needed for lengths of say 43-45ft.  If a person were considering a 40ft, what are the advantages/disadvantages of having a tag?  How much more weight can you carry?  Impact on ride?  Impact on mileage?  Better for towing?
 
The only reason to have a tag axle is to carry any rear axle weight over the maximum allowed 20,000#.  It's not a function of the length of the coach, but its weight.  If the manufacturer didn't have to add a tag axle to carry the weight, they wouldn't do it.  It just adds cost to the vehicle for them.
 
On my rig the tag axle adds 10,000 pounds of capacity. (front axle 14,600, dual axle 20,000, tag 10,000) Many 40' MH's are overweight on at least one corner. Some are overweight on every corner. Look at the articles for new 40' coaches and see what the available CCC (cargo carrying capacity) is. Some new rigs after filling with fuel and water with empty waste tanks have 1,000 to 1,200 pounds of CCC left. Think about just the weight of the people you plan to carry, then figure in food and clothes, a tool or two, and magically you're over the limit.

My coach rides very well. I do not get the buffetting from semi's that the 36' I previously drove did. Mileage is so subjective as to driving style and total weight of the rig that I don't think you can get a real firm comparison. Pulling 2 extra tires must have some impact on mileage, how much is just someones guess. More tires does mean less sway so I would say that it helps with towing. I tow a Chevy Malibu which you can't tell is back there without looking in the camera. Would I have better off the line acceleration without the extra 3400 pounds? Of course, but handling-wise, it's a non-issue.

Ken
 
Looking around the internet produced a couple more points about a tag (if true): Add more stability on the road (passing trucks, side winds, etc.); and lifting the tag to add more weight to the drive and the steer wheels for increased traction. Some say lifting the tag enables tighter turns.
 
Ned said:
The only reason to have a tag axle is to carry any rear axle weight over the maximum allowed 20,000#.  It's not a function of the length of the coach, but its weight.  If the manufacturer didn't have to add a tag axle to carry the weight, they wouldn't do it.  It just adds cost to the vehicle for them.

Now that its legal (at least on Federal Rds) to have a 23,000lb rear axle you will probably see fewer 40ft tag axle coaches. Some manufacturers have been using 23K rears even before it was legal.


 
If lifting the tag isn't really necessary to prevent skidding the tires in a tight turn, then there's no reason ever to lift the tag except for the above drive-wheel issue?

I only said that tag skidding isn't an issue that needs a lot of concern. Having a fully liftable tag is better than a non-lifting one or a partially liftable (unloaded) one.
 
Ken & Sheila said:
Now that its legal (at least on Federal Rds) to have a 23,000lb rear axle you will probably see fewer 40ft tag axle coaches. Some manufacturers have been using 23K rears even before it was legal.

Has Ohio Turnpike raised their axle weight limit too?  Does the 23K apply to all federal highways, just the interstates, or all roads unless otherwise posted?
 
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