Why two axles???? - Fiberglass RV


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Old 01-24-2013, 04:27 PM   #1
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Why two axles????

I've looked all over the place for an answer to this question without any luck at all, so I thought I'd put it out here:

What is the rationale for putting tandem axles on any of our relatively small trailers instead of a single heavier axle??? Can't be a "redundancy" thing, since the individual axles used in such applications aren't sized to bear all the load on their own, nor are the tires mounted thereon.


It can't be a suspension thing, either (torsion vs. leaf-spring), since I just got finished combing through Dexter's application guidelines (link), and big torsion axles such as the 7,000 pounder can be used singly with properly load-ranged tires.

This question is based on my own aversion to tire maintenance/towing issues related to the use of tandem axles, not the least of which is simply the doubling of tire replacement expense. I'm not criticizing any trailer maker, I'm just curious as to what if any practical "design/performance" reasons exist for their use.


So what's the deal with tandem axle trailers, especially in our "small compared to many trailers" world???

Thanks!

Francesca
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Old 01-24-2013, 04:35 PM   #2
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Pros and cons at: RV.Net Open Roads Forum: Travel Trailers: Tandem axle vs single axle
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Old 01-24-2013, 04:45 PM   #3
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Yes, I saw that thread..., and others like it. Belief in " providing redundancy" seems to be a big theme, and since neither tandem axles nor the tires on them are sized to do that it, it can't be a design parameter.

Any ideas in that department would be greatly appreciated!

Francesca
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Old 01-24-2013, 04:46 PM   #4
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The dual axle tows easier and there is less bounce, sway, movement of interior items. They also add a level of safety while on the road to blow outs. That said, they add about 400# of weight and tolls will be triple what a single axle are. Personal preference.
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Old 01-24-2013, 04:54 PM   #5
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I'm just glad they do put 2 axles on trailers. I have 2 on my 25' Airstream. It's heavy enough to make sense of 2 axles, but the big advantage for me has been safety. If you have a flat or blowout or broken axle while towing, you still have control of the trailer because the other tire/axle supports the trailer, allowing you to pull over safely without damaging the trailer. Then, you don't even need a jack...just raise the trailer by driving the good tire on the disabled side, onto blocks used for leveling, change out the tire, and you're on your way. I'm a little nervous towing my single axle Scamp for that reason.
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Old 01-24-2013, 07:12 PM   #6
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I always thought the advantage was that it tracked better. But I have absolutely no experience. The only trailer I have ever towed was a Trillium, (not the 5500).
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Old 01-24-2013, 08:10 PM   #7
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I know that on horse trailers its a safety issue - imagine pulling a single axle on the freeway and having a tire blow out with a horse in the trailer. Giddy up big time!
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Old 01-24-2013, 08:20 PM   #8
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Merle Lilly supplied a write-up for our Document Center on his dual-axle Scamp 5er. After the modification, the blowouts stopped: Fiberglass RV - Document Center - TandemAxleSetup
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Old 01-24-2013, 09:14 PM   #9
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PRO: Tandem axles can result in less tongue weight as a percentage of trailer weight.
Slightly less prone to sway.

Con: A tandem axle must be towed level enough to evenly distribute the weight on the axles. While that sounds elementary, you can seldom drive a hundred miles on any major highway without seeing an obvious violation of this principle. Tongue too high, loads the rear axle...tongue too low, loads the front axle.

Properly loaded, the tires still scuff on turns.
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Old 01-24-2013, 09:43 PM   #10
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Quote:
Originally Posted by Francesca Knowles View Post
Belief in " providing redundancy" seems to be a big theme, and since neither tandem axles nor the tires on them are sized to do that it, it can't be a design parameter.
I disagree.
I have experienced tire failure with my tandem axle Fiber Stream. With 75% of my tires unaffected, the only way I even knew I had a tire fail was a subtle sound that went tic-tic-tic-tic-tic-tic-tic-tic-tic-tic-tic for about 1/2 mile. There was no change in handling characteristics at all. My passenger and I were in conversation when I stopped and said,"Something doesn't sound right." I pulled over and looked; the trailer's attitude had not changed- it was still level. I got out and looked at each tire; the 4th one counter-clockwise from from my driving position (forward axle, curb side) was blown on the inside side-wall. Very easy to change to the spare and continue on our way.
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Old 01-24-2013, 09:50 PM   #11
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Interesting...aside from a few per handling differences, most responses so far seem to be consistent with what I read on other user forums: that the main purpose of the second axle is to provide "protection" from- what-deficient tires???

Is it thought that this is a design parameter for trailer makers, and if so, why are there any single axle trailers on the road at all???

Francesca
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Old 01-24-2013, 10:38 PM   #12
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My former 23' Rockwood had tandem torsion axles, and stuff bounced all over the place; I had to do all sorts of tricks to keep drawers and cabinet doors closed. My current trailer has a single spring axle, and it rides so well the stuff inside stays put. Go figure.
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Old 01-24-2013, 10:55 PM   #13
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The Casita I own had lousy tires on it from the factory. I had two blow outs and it was scary. After that I put better tires on and went up a size. That took care of the problem. But the factory kept putting on those crummy Goodyear tires and everyone back in those years complained of blow outs. Now the factory puts on 15 inch tires and it works great as it gives the trailer more ground clearance for off road use. Don't know if this helps about tandem axles though?
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Old 01-24-2013, 11:03 PM   #14
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Tandem axles double your maintenance. More tires, brakes, wheel bearings. They also result in more weight and more drag ( tires on surface ).
As for safety, perhaps, but there are reports of drivers beating their wheel wells to death because they weren't aware that they had a flat ( maybe more so with stick built? ).
So why tandem axles? Because people believe four wheels are better than two and that's what the market demands. It's why people use Classic Coke to clean their toilets and remove silicone.
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Old 01-25-2013, 07:33 AM   #15
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Replacing a pair of tandem axles with one would require a pretty big wheel - using two smaller wheels makes for a lower chassis and the possibility of a flat floor over the wheels.
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Old 01-25-2013, 07:37 AM   #16
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Originally Posted by Francesca Knowles View Post
. . . . Is it thought that this is a design parameter for trailer makers, and if so, why are there any single axle trailers on the road at all???

Francesca
Design parameter; single axle under 3500lb.; double over 3500. Two design parameteers for two sizes and two price points and two sorts of customer. The single-axle guys have heard us crying in the wilderness. Also why we have limited headroom, short beds. Let us give thanksl that less is less.

jack
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Old 01-25-2013, 09:32 AM   #17
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Replacing a pair of tandem axles with one would require a pretty big wheel - using two smaller wheels makes for a lower chassis and the possibility of a flat floor over the wheels.
Yes, in theory, but
  • none of our trailers (including the tandems) have that flat floor (because even with little 12" wheels that's still a floor a couple feet off the ground),
  • raising these trailers is the most common chassis modification in this forum (so the low height is not valued), and
  • "pretty big" is a tire only an inch or two larger in overall diameter for the smaller tandems (some members here are fitting tires to their single-axle 17' trailers which have more than enough capacity to be used on a single axle version of the tandem Escape 19').

I'll stay out of this round of this debate, except to point out this deviation between logical reasoning and current product reality.
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Old 01-25-2013, 09:32 AM   #18
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I have use lots of single, tandem an tri axles over the years, and the towing characteristics of a tandem axle in many ways is preferable to a single. For starters there is less bounce, as when hitting a pothole or bump, there is another tire to maintain the even attitude better.

As well, dual axles trail much nicer too, and are less prone to sway.

The safety factor is there in the case of a blowout, but not really a bit issue for me.

As far as rolling goes, dual rolls every bit as easily as a single axle, as there is half the weight on each set of tires. This is especially true if you keep the tire pressure at, or near, maximum as I have always done. On level pavement I can move my 3,500+ lb trailer by myself with no problem.

There is twice as many components to maintain, though with 4 sets of brakes, there is less wear there. For me, this cost amortized over the length of use is a small price to pay for the advantages I find.

Plus, I can now use the BAL X-chocks, which add an incredible amount of stability to the trailer when set up.

Tight turns, especially when reversing, do cause a bit of sideways skid on the tires with tandem.

All this said, there is absolutely nothing wrong with towing with a single axle designed to carry the load. For some it may seem like overkill, but how many of us do that with other setups for our trailers?

Like many things, there is really no right answer, just a matter of personal preferences.
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Old 01-25-2013, 09:41 AM   #19
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Originally Posted by Francesca Knowles View Post
Yes, I saw that thread..., and others like it. Belief in " providing redundancy" seems to be a big theme, and since neither tandem axles nor the tires on them are sized to do that it, it can't be a design parameter.

Any ideas in that department would be greatly appreciated!

Francesca
They don't need to be sized for the whole load. It's incredibly rare to blow one tire, then have the second one on that side blow, even if you were already at max weight for both tires. Been there, done that. Trailer tires usually don't go slowly, they just flat out let go (at least in my experience). I've had them blow a few minutes after checking the pressure at a gas station.

The tandem axle just gives you a tire to ride on with no sway, no dragging trailer, brakes still work, etc.

Tandem axles are also very nice for having dual brakes in hills.
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Old 01-25-2013, 05:29 PM   #20
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Interesting food for thought...I think I'll next take a closer look at what ever it is about European "Caravans" that makes single axle use so much more the norm for "small-to-medjum" sizes over there.

Thanks for the inputs, Folks!

Francesca
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