Tag Lift Question

[Mike Canter]

If we are all in agreement that the drive axle is the pivot point then the radius of the turn will not change with the tag up or down. That is simple geometry. The only that happens when the tag is down in a tight turn is you scrub the tires and put an increased lateral pressure on all the tag axle components

[Adam Hunter]

Hey Mike,

I like your posts and find them really constructive and I say that with alot of courtesy!

As the OP I have enjoyed everyones comments on this topic. Its great to see what all the other users are doing in the real world.
I'm really only continuing this discussion for the heck of it - not to agitate you.

I agree fully that the tandem drive axle is the primary pivot point, and that as users we should lift our tags for tight turns.........in this case the wheelbase would be centre of the steer axle to the center of the drive axle.


Someone with more skillset than myself doing trigonometry on a calculator plugging numbers into a formula might look at this differently. Lets just suggest that a user leaves their tag down for the same turn, I would then suggest they use the distance between the steer axle and the open space between the drive axle and the tag axle hub. (strictly for a formula standpoint) as a new value.
Look at it this way, just say for example there were multiple tags behind the drive axle on a coach, the turning radius would be bigger than a unit with a single tag. And as you have already mentioned a great deal of the resistance to turn would be caused by the friction of the scrubbing tag tires. It would be like me saying I could complete the same 180* turn at 55 mph that I could complete at 5 mph.

And just to make you get a real laugh out of all of this I have included this link for the next time everyone following this thread is really, really bored;

http://www.idsc.ethz.ch/Courses/vehi...ing_Theroy.pdf

[Gordon Dewald]

I only have an opinion on the action of the tag and the drive axles. I think that with the tag down you have effectively moved the length of the wheel base behind the drive axle. How much would be a exercise in proportioning weight, tire contact area and other factors. I expect the distance may be small but it is there.

Then you would have to factor in the radius of the turn. On a long radius turn there is likely enough play in the suspension so the radius may be almost in between the drive and tag axle. On a tight turn you would have to consider the amount of skid in the tag and drive. I do not believe for a moment there is no skid on the drive axle, logic (IMO) says there will some based on skid resistance, weight, tire contact area and other factors.

I also believe there will be torque on the frame transferred to the front axle if the tag is not lifted. Why because the resistance to the turn at the back has to go somewhere and the only place is the front. Might be some slip there - probably not much but there will be resistance to the turn and the front has to deal with it.

Recall the old days driving on ice with a limited slip rear end. When the box was empty it was not too difficult to make a turn but load the box and making 90 degree corners was sometimes interesting.

I will be experiencing the tag axle issue first hand in a month or so when we get our new coach. Here is an excerpt from the Owners Manual.

TAG AXLE SUSPENSION
SYSTEM
Freightliner® Chassis
Your coach may be equipped with a Tag Axle
Suspension System, which is designed to
increase the Gross Vehicle Weight Rating
(GVWR). This system is to be used when the rear
suspension/axle will be loaded to a weight
greater than 20,000 lbs. When certain conditions
arise, air can be exhausted from the air springs to
improve maneuverability or traction on the rear
axle by operating the Tag Axle switch (located on
the driver side trim panel).

[MRUSA14]

I am no fan of Goodyear tires, but that doesn't look like the rivering that I had on mine. My river wear showed an irregular scalloped pattern. I think yours may just be regular wear from turning and scuffing.

[Adam Hunter]

Thanks Marc

I stopped by the shop where I bought the new Michelins for the steer axle of the coach 2 days ago. I trust this place.
They thought the same as you, not a Goodyear "induced" problem such as rivering but just a wear issue due to the nature of where those tires are located - in the tag position.

Thanks Adam.

[96 Wideglide]

The way I see it, if your making a 90* right turn on a city / town street, unless you have a red light, or stop sign. I cant see the tags being clear of the ground between the time your slowed to 10mph, and the time it takes to complete the turn.
It looks to me like it takes 20 - 30sec for the tag to unload, then lift. I dont use it around town.

[bgbranch]

Originally Posted by Adam Hunter

Relax Gentleman,

No one is disputing the fact that the drive axle is a pivot point. We were discussing the "rate" of a turn.

If you look at commercial trucks with "tag" axles and "pusher" axles (those that lift ahead of the drive axle) there is more than one reason why a manufacturer makes them so they can be lifted.

If you are saying your turning radius increases with the tag down, then your are saying the coach is not pivoting on the drive axle but is somehow now pivoting on the tag. That does not happen.

[super_rep]

Originally Posted by Mike Canter

If we are all in agreement that the drive axle is the pivot point then the radius of the turn will not change with the tag up or down. That is simple geometry. The only that happens when the tag is down in a tight turn is you scrub the tires and put an increased lateral pressure on all the tag axle components

How's this "fly"?

I look at making a turn with the tag down kind of like making a cross wind landing in a plane. The tag is fighting to keep you going straight, but turing the front wheels is trying to make you turn. If the tag by scrubbing the tires, is resisting the turn, then it must be increasing the turning radius by some amount. If a tag can scrub hard enough to change the track of the tag tires by 3-5" with sidewall distortion (I've seen this in real life when behind a vehicle with a tag) there has to be some effect on the turning radius.

BTW, Doesn't Newell claim that their steerable tag gives them a decreased turning radius?

[Adam Hunter]

Originally Posted by bgbranch

If you are saying your turning radius increases with the tag down, then your are saying the coach is not pivoting on the drive axle but is somehow now pivoting on the tag. That does not happen.

Ron,

I think "pivot" is not the best choice of words, the only machine that can pivot on a point is a vehicle equipped with say castors. Everything else is a arc. The radius of the arc will be complicated by the side forces of any axles behind the drive axle. And I feel that that radius starts at a point either located at the center of the drive axle when the tag is up or at a shared point "somewhere" between the centre of the tag and the centre of the drive axle if your turning with the tag down.

Try this; next time you back into a grass or gravel site (somewhere you won't hurt your tires or wreck the ground) back in doing a 90* reversal with the tag down. Then pull ahead and do that same backing 90* turn with the tag up and watch how fast the nose of the coach swings straight. There is a big difference!

I don't think there is a big dispute here, in fact we are all on the same page. Simply put - when able lift the tag. Thats the consenus I'm getting from everyone.

[Jeff753]

Does he want 6 more two year old G399s with 6,000 miles on them

Originally Posted by Adam Hunter

Too funny!!





Actually the local truck shop said he would take the Goodyears in on trade for 6 new XZA2's. I guess they would fit on some profiles of road trailers.



(We spent a bunch of cash on the inside of the coach this winter - so tires were not in the budget for this season. Still on the fence though about it, I would like to try to get 2 more seasons out of the back rubber)





A

Jeff

[Adam Hunter]

You never know Jeff he might!? I guess everyone is on a different budget, I guess he might even sell them to local farmers for ag stuff.

Last year on a trip south to Destin we picked up a bolt in the tag on our coach, of course it was Sunday. I payed through the teeth for a tire!!
This past winter we set up our F150 to flat tow. Now call this overkill if you like, I carry 2 unmounted spares with me in the back on any trips of great distance. A Goodyear for the back 6 and a Michelin for the steers.

We pay through the teeth here in Canada for tires and the FMCA tire discount doesn't help Canadian members so..... That 315 XZA2 unmounted cash and carry was $902 after our provincial taxes!

ouch.