Tag Axle

[JIM CHALOUPKA]

Just having one of those DAH! revalation moments. Recently looking at a bus and I became aware of the capability to raise the tag axle. Is this common on all buses? I have never heard anyone discuss this topic. Is it so basic that it just isn't mentioned. It seems to me that as simple as the principle it has the potential to be a safety issue if it should malfunction when cruising. How does one know that it is carying it's share of the load ? I assume this is for better maneuverability, tire scrub elimination. What are the trouble aspects of this mechanical system? Is this always, never, or sometimes by Prevost? Is it air or hydraulic actuation? Is there an alarm on the dash to warn against over the road drieving in the raised position? JIM :eek:

[Ray Davis]

Jim, I can't answer all the detail questions. But I've never seen a bus without the tag axle lift. Generally there is an alarm when the tag is lifed, both audibly AND a red light or warning light on the dashboard.

I say generally, because at one point, my tag axle warning stopped working. What I discovered is that at a recent service at Prevost, the tech had removed the little speaker/relay while working on something. It's a unit that plugs in in your electrical compartment, right under the driver. It plugs into a standard relay slot, but this gadget has the speaker built in there. Plugging it back in restored my audible alarm.

[Joe Cannarozzi]

Jim, the newer busses automaticly dump tag air, and lift, while simotainiously putting air to air over hydraulic shocks on the drives(to keep it from instantly squatting from the extra load allowing it greater quicker lift) and it all happens automaticly by simply turning the wheel,I believe, 1 complete turn or something like that.
The older busses have a manual dump switch in the cockpit. Not as creative but a lot less to go wrong.

[Jon Wehrenberg]

The tag axle is just intended to carry a little weight. It is not driven. The tag axles can be lifted, and in later coaches (45') the air was dumped from the tag axle air bags to make turning a little easier and to minimize the sidways scuffing when the steering wheels were turned to the extreme.

To shorten turns drivers will lift the tag axle. That has the effect of moving the theoretical rear axle centerline forward, shortening the turn radius. A second use for picking up a tag axle is to increase starting traction on slippery surface. It will increase the weight on the drive axle by slightly more than the weight carried by the tag axle if the tag is lifted.

Tag axles should not be lifted at speeds much faster than walking speeds.

[Jon Wehrenberg]

Joe,

On coaches with the automatic tag axle air bag dumping I don't think the tags are lifted.

The steering arm in the steering compartment has an actuator that moves a switch lever on the coaches to dump air and actuate the air over hydraulic system.

[Joe Cannarozzi]

Well I stand corrected, thank you Jon.

I was close:rolleyes: And what is it, 1 turn of the wheel or all the way to one side or the other?

[Orren Zook]

I thought that dump/toggle for the tag was to redesign your exhaust pipe into a more aerodynamic shape whenever you wanted.... who'd a thought it was to lift that axle

[Joe Cannarozzi]

Dont use it too much any more hu:rolleyes:

[Jon Wehrenberg]

Joe,

The steer axle wheels have to be cut fairly sharp before the tag dumps, and that in itself limits speed. I don't know how many degrees or turns of the steering wheel, but it is based on travel of the steering arm in the steering compartment.

I'm not sure lifting the tag axle by means of the little lever brings the air over hydraulic cylinders into play. I don't think it does.

[JIM CHALOUPKA]

Thanks my friends for the dialogue! This all helps to increase my knowledge base for a proper coach purchase. :D

[dreadnought]

A lot of buses, including mine, are so heavy that the
a**end just about drags the ground if you dump the tag. I found this out the hard way:eek:

[JIM CHALOUPKA]

Ya, I can believe it. That in part is why I asked the question. I had visions of the Looney Toones going down the road in a bus, having a malfunction. The tag goes up and the front wheels go up with a loss of steering, a rear drive tire blowing and the bus swerving across center into uncoming traffic. Ha Ha Ha What keeps it down?? A mechanical lock after positioning, perhaps. JIM

[Jon Wehrenberg]

Prevost never intended for the coach to be driven at highway speeds with the tag axle lifted. Given enough time and pressure the tag can be raised from the road, which disconnects the tag axle brakes reducing the coach's ability to stop, and as was stated previously it does reduce the weight on the fron axle slightly, which in some conditions could compromise steering.

The drive axle has four air bags and they are capable of carrying the added weight, but may be over inflated and the proper height may not be attained. Our air bags generally have a max pressure rating of 100 PSI, but I know some exceed that.

[garyde]

Here's what my manual says about my tag;
."...the axle will be raised,by the switch, by air pressure..."
"The tag axle service breaks oprate only when tag axle is in the WHEELS DOWN position."
["B]Caution[/B], In order to prevent damage to the suspension, always raise the tag axle before lifting vehicle"
"Caution: Never lower the tag axle while vehicle is moving"[/B]

I'm not sure what the first caution is trying to say, I do remember my previous Country Coach would automatically lower the tag if I forgot after raising it.

[Jeff Bayley]

I asked Preovst techs when you should or should not need to lift the tag and mixed answers. I got in the habit of liting in almost every turn I make in traffic except a wider turns. It I'm making a 90 degree from on street to another I use it unless I'm on a wide outside lane. I personally saw my tires wear out really fast when I pretty much ingored lifting them. I think moving the mechanisim around is healtier to keep it working likes it's supposed to but that's just a huntch.

[Jon Wehrenberg]

If your coach is 45' the tag axle air is dumped automatically during a sharp turn.

If you manually lift the tag via the lever I am not sure your rear end does not sag.

This is my opinion, but I usually do not ask aircraft mechanics for tips on aviating, and I avoid asking bus mechanics for tips on driving our buses.

[MangoMike]

You always ask me for tips on eating. ;)

mm

[mike kerley]

When I dump the air to the tag on our 93, the rear end does drop, but given a few minutes (with the engine running at high idle) it will come back up to almost level. It just takes time to put extra air in the bags over the drive axle. I'm told this is normal operation.

Our manual states to never lift or drop the tag unless stopped. I've tried it and the rear sinks, rubber does not scrub off the tires quite as fast and slowly the rear end crawls back to normal height.

It does improve the turning radius substantially.

Mike K

[Jon Wehrenberg]

"You always ask me for tips on eating. "

An aviation mechanic usually doesn't fly, and a bus mechanic usually doesn't own or drive a bus, but you do eat don't you?:D