Tag Axle vs Single Axle

[Gary RVRoamer]

Wow, all these scientists explaining why more rubber against asphalt doesn't increase stopping ability. Next they'll explain why drag racers do just as well with narrow tires on the back as they do with wide ones because it doesn't matter how much rubber is on the asphalt, it depends if the car is balanced properly front to back.......

Only if "all other things are equal". I think its obvious if you had a single axle set-up and then added a tag, you would gain both rubber contact and brake surface area. But if instead you design two chassis for a given GVWR (load), one that does the job with a single axle and one that has two, you would have to design the single axle with larger brakes and tires to carry that same amount of weight. Splitting the load between two axles doesn't automatically give more of anything. Nobody designs a chassis with 2x 10k rear axles instead of a single 20k cause it makes no sense, engineering or economical. It doesn't stop faster or go faster or do anything except cost more.

The point is that the axle(s) are designed to carry and brake a given load. A single axle is good up to about 24,000 lbs. There is very little value in using a second (tag) axle if the load can be carried and braked with one. If the GVWR is such that the rear end is going to exceed 24k, then changing the design to use two smaller axles makes sense. Some coaches are even big enough to require a 24k drive plus a tag to add another 10k-16k.

[dandandan]

Originally Posted by Gary RVRoamer

The pros and cons are largely academic anyway. It's not an option. You are going to get a tag axle if the GVWR of the coach you choose requires an extra axle to carry its weight and not otherwise. That's simply because there isn't enough extra value in a tag to justify the extra cost except for increased GVWR.


I suppose there are a few buyers who would choose a larger/heavier and more expensive coach than they really wanted just so they could brag about having a tag axle. If that's your thing, go for it.

I think many buyers assume that the biggest and more expensive MH they are shown is the best. Especially with sales men saying things like more brakes is safer which is simply not true.


What percentage of these buyers weigh their MHs and proportion the axle weights close to 2/3, 1/3.


One person already posted here that it is not necessary. Most do not understand the physics and will just drive it how ever it is delivered to them.



Freightliner, Spartan, etc do not test their chassis with a MH on top. They add weight intelligently and then pass DOT braking tests. The weight distribution of the test is not required to be reflective of the weight distribution of the MH delivered to buyers.

[jcussen]

I am thinking that building any 4 slide 42 ft coach on a 24000 lb rear axle and a front 20000 lb axle is going to involve a lot of compromises.

[CampDaven]

I’m thinking that folks buy what they want, like a tag unit, and enjoy it without any problems or consequences or safety issues from braking, load, etc.
For me, this discussion is moot.

[Crasher]

Originally Posted by CampDaven

I’m thinking that folks buy what they want and enjoy it without any problems from braking, load, etc.
For me, this discussion is moot.

You're absolutely correct. As far off as my coach and many others on the Powerglide, Freightliner or Spartan chassis were/are, most do not exceed the tire or axle ratings. Most owners have no idea what their weight distribution is which points out how unimportant it is on a motorhome. My wife says I'm anal about the weights and other things related to tires, oil, service, etc and she's probably right. I just feel better when I know our rides are safe and setup correctly.

[Ivylog]

In over 140K miles of DP driving I’ve only once (in my Dynasty) had to push on the brake pedal with everything I had and I’m not a small/weak person. The antilock did not activate so no skidding or black marks as I changed lanes plus I run 130 psi in the air system. Changed lanes because I was not sure I’d get stopped in time...I did but not by much. The tag axle helped with the lane change and the extra braking from 2 more tires definitely helped.

Like Crasher, I’m careful about my weights and pressures and I have my doubts about anything that’s a nice round number, fraction, or %.

[dandandan]

Originally Posted by Ivylog

In over 140K miles of DP driving I’ve only once (in my Dynasty) had to push on the brake pedal with everything I had and I’m not a small/weak person. The antilock did not activate so no skidding or black marks as I changed lanes plus I run 130 psi in the air system. Changed lanes because I was not sure I’d get stopped in time...I did but not by much. The tag axle helped with the lane change and the extra braking from 2 more tires definitely helped.

Like Crasher, I’m careful about my weights and pressures and I have my doubts about anything that’s a nice round number, fraction, or %.

Stability from the tag would have helped you change lanes.



"Extra" braking from the two "extra" brakes is a lack of understanding of how distributing braking from one axle to two axles works when adequate braking is supplied in both cases.


Having doubts about "nice round number, fraction, or %" does not specifically state the 2/3, 1/3 weighting but can not imagine what else it refers to. 2/3 for 4 drive tires and 1/3 for 2 tag tires is simply trying to get EQUAL weighting on each tire. As you deviate from equal weighting the sooner a low weight tire will skid or activate anti lock braking. Sorry the math works out to 1/3 and 2/3 so that it seem not trust worthy but that is just how it works.

[Ivylog]

You assume the tires are all the same size.

[dandandan]

Originally Posted by Ivylog

You assume the tires are all the same size.

Yes


People already do not understand a simple model with as few variables as possible. Needless complication of additional variables will not help make understanding better.


The "extra" brakes do not add any stopping ability and as the normal force deviates from equal load it leads to skidding or anti lock braking kicking in early and poorer stopping. The "extra" brakes simply distribute the braking to different tires they do not add braking ability.

[ScoobyDoo]

Originally Posted by dandandan

Yes


People already do not understand a simple model with as few variables as possible. Needless complication of additional variables will not help make understanding better.


The "extra" brakes do not add any stopping ability and as the normal force deviates from equal load it leads to skidding or anti lock braking kicking in early and poorer stopping. The "extra" brakes simply distribute the braking to different tires they do not add braking ability.

I don't know about RVs, my tag experience is with trucks. I had a old twin screw Pete that I had worked a couple of years. Over a weekend I added a tag. We where paving city streets, and the city would no allow loads over 20 tons net. Friday, my GVW would run about 61,000. Monday, GVW was about 67,000 with the same load. Sure felt like it would stop better. Once I got the pressure dialed in she stopped 82,000 gross about same as the 61,000 did without it. Payday looked better.

[CampDaven]

Originally Posted by dandandan

Yes


People already do not understand a simple model with as few variables as possible. Needless complication of additional variables will not help make understanding better.


The "extra" brakes do not add any stopping ability and as the normal force deviates from equal load it leads to skidding or anti lock braking kicking in early and poorer stopping. The "extra" brakes simply distribute the braking to different tires they do not add braking ability.

Okay.

Can you please explain what dangers arise from my not being OCD about these issues? And what may I gain from compliance?

[palehorse89]

Originally Posted by ScoobyDoo

I don't know about RVs, my tag experience is with trucks. I had a old twin screw Pete that I had worked a couple of years. Over a weekend I added a tag. We where paving city streets, and the city would no allow loads over 20 tons net. Friday, my GVW would run about 61,000. Monday, GVW was about 67,000 with the same load. Sure felt like it would stop better. Once I got the pressure dialed in she stopped 82,000 gross about same as the 61,000 did without it. Payday looked better.

Wondering how your GVW increased 6k and had the same load on it? Tag axle weight that you added?

[Dutch Star Don]

If you don't feed them....they won't return!

[jcussen]

Originally Posted by Dutch Star Don

If you don't feed them....they won't return!