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Old 05-06-2021, 12:35 PM   #1
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Back of ball ratio and trailer stability - 21NE?

Been reading a few of the posts regarding sway accidents with the 19 and it brought back memories of my Avion T-27 dynamics and a not often discussed, that I have seen, aspect of trailer towing physics.

IE - the farther back the axle(s) are located on a trailer, the more inherently stable they will tow. "Back of ball ratio" is the term I think.

Measure from the ball to the rear of the trailer, then measure from the ball to the middle of the axle(s). Divide the axle measurement into the overall measurement. What I have read is that a 70% ratio is "stable", while 60% or less is inherently less stable. Makes sense. Think of the stability of a 5th wheel.

In a quest for manageable tongue weights, and maybe better maneuverability, many manufactures place the axles more forward than you might think for a stable pull.

Have to balance all the limitations and make choices though.........Not pointing any fingers.

Granted, proper loading, tongue weight (and ratio to total), a properly adjusted WD hitch, sway control mechanism, angle of trailer to level, and TV characteristics all come into play, but with an inherently less stable (term likely over stating) you are just attempting to offset an existing characteristic.

My Avion was <60% BOB ratio and nothing short of a Pro Pride Projection hitch (fantastic hardware but REALLY heavy and expensive) cured the propensity for that trailer to want to "wag the dog".

As I am in (a long!) line for a 21NE, can someone make the above measurements?

Appears just from looking that the 21 will have a higher BOB ratio than a 19 though the 19 appears (eyeballed) to be greater than 65ish% ?

I'm sure I will be inundated with replies countering my above.......so be it.
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Old 05-06-2021, 04:37 PM   #2
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21ne

I just went out to our 21NE and have some numbers



So I have 2 measurements: ball - center or axles, and ball - center of space between tires.


ball - center or axles: 158in
ball - center of space between tires: 168in


ball - rear bumper 255in
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Old 05-06-2021, 05:58 PM   #3
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Hmmmmmm.......................?


I would think from the ball to the center "of" the 2 axles would be the same as to the center of the 2 tires? (straight line from ball back) or I'm missing something....(likely!)



Or, maybe the 158" was to the front axle?






Using the 168 / 255 value looks pretty good @ 66%


Thought it would be a bit higher based on just looking.



Thanks!




BTW - beautiful area you are in around there - 20 years ago I spent a few years working in Cullman and "Muntgummery" ...............Cullman won!


Mmmmmmmmmmm..........orange rolls!
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Old 05-06-2021, 06:34 PM   #4
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I concur with the 66%. I used Escape's spec of 21'4" coupler to rear bumper and laid my tape so I had 256" at the end of the bumper, and the dumb end looked to be close enough to the front of the coupler. My eyes saw 170" as the center between the axles, and 170/256*100=66.4%

I will say that percentage of tongue weight seems to be more important than the axle / overall length ratio, although that ratio has a large effect on the tongue weight percentage. Anyway, the tongue weight on my 21NE usually runs around 12%, which is in the sweet spot of the 10% - 15% I so commonly see. The largest effect on my tongue weight is the amount of water in my fresh water tank. If I travel with a full tank I make sure that I load more things inside towards the front. I haven't had any evidence of sway, and I do not use any sway control on my hitch, although I do use WD.
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Old 05-06-2021, 06:59 PM   #5
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I should clarify that my 170" measurement was between the wheels. Spindle to axle measurement is almost 6", so if you use that the ratio drops a little to 164/256*100=64%. I'm not smart enough to figure out which measurement is more meaningful.
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Old 05-06-2021, 07:02 PM   #6
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Quote:
Originally Posted by sherminator View Post
.........snip............. I haven't had any evidence of sway, and I do not use any sway control on my hitch, although I do use WD.

Great to hear!


Thanks.
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Old 05-06-2021, 10:04 PM   #7
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The simple length ratio isn't a great measure by itself, because it doesn't reflect the mass distribution with the trailer. The tongue weight fraction is probably a better measure of the same thing.
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Old 05-06-2021, 10:08 PM   #8
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Originally Posted by sherminator View Post
I should clarify that my 170" measurement was between the wheels. Spindle to axle measurement is almost 6", so if you use that the ratio drops a little to 164/256*100=64%. I'm not smart enough to figure out which measurement is more meaningful.
If by "axle" you mean the Torflex crossmember (the square steel tube)... ignore the position of that, as its location has nothing to do with trailer stability - it's the tire centre location (or midway between the two in the case of a tandem) that matters. If you're trying to assess the axle location, that means the spindles for this purpose.
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Old 05-06-2021, 10:36 PM   #9
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Quote:
Originally Posted by Brian B-P View Post
The simple length ratio isn't a great measure by itself, because it doesn't reflect the mass distribution with the trailer. The tongue weight fraction is probably a better measure of the same thing.

"Simple"..............


Didn't say it was a "great measure by itself".................but think about your statement a bit. Both of those metrics are interdependent.


Care to explain how you can have a "normal" (say 12.5%) tongue weight with a very low BOB% and stay below any realistic GVW limits? Leverage always wins..............




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Old 05-06-2021, 10:37 PM   #10
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Quote:
Originally Posted by Brian B-P View Post
If by "axle" you mean the Torflex crossmember (the square steel tube)... ignore the position of that, as its location has nothing to do with trailer stability - it's the tire centre location (or midway between the two in the case of a tandem) that matters. If you're trying to assess the axle location, that means the spindles for this purpose.



Agreed.
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Old 05-06-2021, 10:51 PM   #11
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OK, Try this:


Take 2 exact loaded identical trailers.


Except, one has a BOB% of 50%, the other 70% (due to axle placement only).


Which will be more stable? And, which will have a higher tongue weight %






No treadmills involved.
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Old 05-06-2021, 10:56 PM   #12
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Quote:
Originally Posted by Brian B-P View Post
The simple length ratio isn't a great measure by itself, because it doesn't reflect the mass distribution with the trailer. The tongue weight fraction is probably a better measure of the same thing.
I agree with this.

I recall during my parent's long-ago Air-Streaming days there was a bit of a kerfuffle about two floorplans of a particular model produced by Airstream ... identical chassis and body dimensions and empty weight: one with the dry-bath across the rear/twin beds each side of the aisle immediate forward of that; one with twin beds in the rear, the dry-bath split across the aisle immediately ahead of that; both identical layouts forward of that. The under-floor black and grey tanks 'followed the bath' in the different arrangements. I don't recall the exact length but these were ~30ft-range trailers invariably towed with a WDH.

The rear-bath version was invariably lighter on the tongue in spite of most efforts to redistribute loaded cargo sensibly, and notoriously less stable than the rear-bed version as observed by the owner community. The 'BOB" measurements did nothing to explain or predict that situation, they were identical; the tongue-weight fraction (representative of center-of-mass) was the 'telling' indicator / predictor.

This situation was also a good demonstration (IMO) of the fact that use of a WDH cannot 'cure' an inherently poor center-of-mass issue.

Yes, extreme 'BOB' ratios may give a hint of likely center-of-mass issues, but it's rare, IMO, to see truly extreme BOB's on travel trailers. Floorplans with rear-biased mass concentration (or storage provisions which encourage same), on the other hand ....
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Old 05-07-2021, 10:37 AM   #13
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Quote:
Originally Posted by Brian B-P View Post
The simple length ratio isn't a great measure by itself, because it doesn't reflect the mass distribution with the trailer. The tongue weight fraction is probably a better measure of the same thing.
Case in point. The 2003-2007 Bigfoot 21-FB's were recalled due to light tongue weight when towed dry. Fix was to reinforce the tongue and add ballast (275# of weight plates). The "back of ball ratio" being discussed looks quite high based on the placement of the axles in the attached picture. To Brian's point this doesn't account for the mass distribution of the trailer which made it a very rear-heavy trailer. Major sway issues ensued. Of course if the axles were further forward the situation would have been even worse.

https://www.escapeforum.org/forums/f...tml#post314081
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File Type: jpg 2005 bigfoot 21FB layout.jpg (142.6 KB, 7 views)
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Old 05-07-2021, 11:27 AM   #14
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Appreciate all the replies, especially the ones focused on actual measurements as I requested in the original post.

The original post was also to bring up another aspect of safe towing -the BOB ratio which does not get much press.

It is a foundational aspect of how a trailer will tow, and how it will be affected by other forces.

Just another piece of the puzzle to consider.........

I acknowledged many of those other conditions, such as tongue weight / %, in my original post.


Meanwhile, back to the long wait and option choices!
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Old 05-07-2021, 12:00 PM   #15
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You cannot come to Cullman and not try the Orange rolls!!!


The reason I sent the 2 measurements is because the torsion axles and the center of the wheels are not at the same place. So I gave both to let you choose which one you needed. (See picture)



We tow with a Toyota Sequioa, and use the E2 Fastway installed by Escape. We have had zero issues on 4 trips. It tows like a dream.







Quote:
Originally Posted by 7Gentex View Post
Hmmmmmm.......................?


I would think from the ball to the center "of" the 2 axles would be the same as to the center of the 2 tires? (straight line from ball back) or I'm missing something....(likely!)



Or, maybe the 158" was to the front axle?






Using the 168 / 255 value looks pretty good @ 66%


Thought it would be a bit higher based on just looking.



Thanks!




BTW - beautiful area you are in around there - 20 years ago I spent a few years working in Cullman and "Muntgummery" ...............Cullman won!


Mmmmmmmmmmm..........orange rolls!
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Old 05-07-2021, 12:22 PM   #16
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Originally Posted by Amel View Post
You cannot come to Cullman and not try the Orange rolls!!!

Thanks!



I hoped the 21NE was a good towing experience, just got a bit spooked by the 19 sway events I read about.



I'm tongue weight limited on my current TV, so a Pro Pride or Hensley Cub would not work (too heavy) if sway was an item. BTW - Those hitches are amazing..........Appears it is not an item though........good deal.


Yes, the orange rolls at All Steak were amazing............ate there about once a month for 3 years.


Was also introduced to N.Al. "white sauce" for BBQ chicken...

Weird BBQ approach for a Texan but pretty good.


Also, The Bright Star in Bessemer - breaded veal steak with brown gravy or snapper almondine, YUM!



Heck of a sporting goods / hunting supply / gun store down the street there I think as well........


Many moons ago................
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Old 05-07-2021, 12:55 PM   #17
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Originally Posted by 7Gentex View Post
Thanks!

I hoped the 21NE was a good towing experience, just got a bit spooked by the 19 sway events I read about.

Where did you read those reports? I've been on this forum since 2008 and I don't recall any.
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Old 05-07-2021, 02:05 PM   #18
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Example:



https://www.escapeforum.org/forums/f...egg-18893.html
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Old 05-07-2021, 02:16 PM   #19
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I think you need to read the entire thread before drawing conclusion that the 19 has a sway problem.
"The week before we had driven on the freeway in the same configuration but with 2 road bikes on the back and did not have a whisper of a problem. We have towed on far worse roads in far worse conditions of wind, rain, traffic and so forth and have never felt a wiggle".
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Old 05-07-2021, 02:33 PM   #20
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I think you need to read the entire thread before drawing conclusion that the 19 has a sway problem.
"The week before we had driven on the freeway in the same configuration but with 2 road bikes on the back and did not have a whisper of a problem. We have towed on far worse roads in far worse conditions of wind, rain, traffic and so forth and have never felt a wiggle".

I never drew a such a conclusion.............I referenced "sway events".


3 events were referenced.
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