Reply
 
Thread Tools Search this Thread Display Modes
 
Old 12-12-2013, 06:05 PM   #113
4 Rivet Member
 
Livingston , Texas
Join Date: Dec 2007
Posts: 394
Quote:
Originally Posted by Chuck View Post
I think some of this equation is the importance of keeping the center of mass of the trailer forward of its center of gravity. (think: badminton birdie---heavy end goes forward). Its really hard to know where either of those things is, exactly...but you can easily know the weight of the vehicle, and the tongue weight; 10-15% insures a forward center of mass.
(may be totally wrong, but it seems like that might be the source of the rule-of-thumb, here).
No, you are not totally wrong -- IMO, you are totally correct. The location of the center of gravity, relative to the location of the axle(s), is a primary factor in establishing the yaw stability of a TV/TT combination.

Tongue weight percentage simply is a shortcut to expressing the ratio of distance from TT axle(s) to center of gravity divided by the distance from axle(s) to the ball coupler. Instead of saying the distance from axle(s) to CG is 30" and the distance from axle(s) to coupler is 200", we say the tongue weight percentage is 15%. Unfortunately, we then tend to focus on how much vertical load is applied to the ball rather than on how the TT exerts lateral forces on the ball.

A fundamental objective in designing a trailer is to minimize the lateral force which can be exerted on the TV by the TT when the TT is subjected to lateral force due to wind loading, lateral road force on tires, etc. The force imposed on the TV is a function of:
a = distance from axle to CG
b= distance from CG to coupler
M = mass of the trailer
I = polar moment of inertia of the trailer.

Without providing the derivation, the relationship among these four variables is given by:
a*b = I/M
From this equation, if you knew the value for I, you could calculate an "optimum" value for a/(a+b) which is numerically equal to TW%.

Unfortunately, a TT's polar moment of inertia hardly ever is known, so the equation is not of much use. However, we can use a special case to demonstrate where the "rule of thumb" values might come from.

Let's assume the mass of the TT is uniformly distributed front to rear. Then, given that a typical TT is relatively narrow compared to its length (L), we can approximate its polar moment of inertia as:
I = M*L*L/12

If we put this special value of I into the equation for a*b and assume the TT has an A-frame length of 4' and L = 20',
a little algebra would give a = 14', b = 2.38' and TW% = 2.38/(14+2.38) = 14.5%.

However, we know that most real trailers do not have their mass uniformly distributed front to rear. Instead, they tend to have more of the mass located closer to the center near the CG and axles. This reduces the polar moment of inertia. If the value of I for the above example were reduced by 30%, the corresponding "optimum" TW% would be reduced to 10.6%.

Since each element of mass in the trailer contributes to the polar moment of inertia in proportion to the square of its distance from the CG, it's not hard to see that concentrating the mass near the longitudinal center can significantly reduce the inertia value. If you compare the layout of a typical European caravan with a typical US trailer, it should be easy to see why the Europeans can get by with a TW% as low as 7% in most cases and as low as 4% in some.

Unless you have some way of estimating the polar moment of inertia for your trailer, there is no good way of knowing the "optimum" TW%. Some people resort to seat-of-the-pants experimentation with varying TW to try to achieve "optimum" yaw stability.

Ron
__________________

__________________
Ron Gratz is offline   Reply With Quote
Old 12-12-2013, 09:19 PM   #114
Rivet Master
 
ROBERTSUNRUS's Avatar

 
2005 25' Safari
Salem , Oregon
Join Date: Mar 2006
Posts: 7,269
Images: 18
Blog Entries: 51
Quote:
Originally Posted by andreasduess View Post
But keep in mind that this is a drivetrain that has been designed to propel the car up to a speed of 155mph, to 60 in just over seven seconds.

Towing a trailer at 60 should not be an issue.
Hi, but this car was not designed to carry a heavy tongue weight trailer on it's car tires, car wheels, car bearings, car axles, car suspension, Etc Etc Etc.


Going by your theory of a "drive train that has been designed to propel the vehicle to a speed of 155mph, to 60 in just over seven seconds." I should be able to tow a 34' Airstream with Honda Gold Wing. WRONG!
__________________

__________________
Bob

2005 Safari 25-B
"Le Petit Chateau Argent"
[ Small Silver Castle ]
2000 Navigator / 2014 F-150 Eco-Boost / Equal-i-zer / P-3
YAMAHA 2400 / AIR #12144
ROBERTSUNRUS is offline   Reply With Quote
Old 12-12-2013, 09:29 PM   #115
Rivet Master
 
ROBERTSUNRUS's Avatar

 
2005 25' Safari
Salem , Oregon
Join Date: Mar 2006
Posts: 7,269
Images: 18
Blog Entries: 51
Quote:
Originally Posted by Andrew T View Post
Hi Steve. You are correct, because the vertical pivot on the Hensley or Propride is still at the ball location you are not using a longer lever from the trailer axle so they do not reduce the effective tounge weight. Since the vertical pivot point is further from the tow vehicle axles it does require more pressure on the torsion bars to transfer the same amount of weight forward so a little more goes to the trailer axle but not much.

The more hitch weight you have the more stable the trailer is however there is a point when more hitch weight is very little extra benefit. The first 25 FB Airstreams were Safari's with one battery, no wrap windows a lighter mattress, an optional spare tire etc. When they became SE and then Flying Cloud models the hitch weight increased quite a bit as the wheels were left in the same position.

Occasionally we do reduce hitch weight a little. We will remove the spare tire and carrier since an Airstream runs quite nicely on three wheels if necessary. We usually change to Michelin tires at the same time. We may also change to aluminum LP tanks and sometimes we will change the batteries to AGM's and install them under the rear dinette seats. I have not seen any real change in handling due to doing this but these units are starting from quite a high hitch weight to begin with.

I don't see any problem with the 300 with the Airstream left in its stock form but you know the options are there if you decide you want to reduce the weight a little.

Andrew T
Hi, the first 25' front bed Airstream came out in 2005; It had a suicide door with a cheap gate hook to hold the door in the open position so it wouldn't hit the awning arm. From 2005 and later, [maybe even before] All 25' Safaris came with 2 group 24 batteries, not one. [unless your Canadian models were cheated out of one of the batteries]
__________________
Bob

2005 Safari 25-B
"Le Petit Chateau Argent"
[ Small Silver Castle ]
2000 Navigator / 2014 F-150 Eco-Boost / Equal-i-zer / P-3
YAMAHA 2400 / AIR #12144
ROBERTSUNRUS is offline   Reply With Quote
Old 12-12-2013, 10:23 PM   #116
Rivet Master
 
dkottum's Avatar
 
2012 25' Flying Cloud
Battle Lake , Minnesota
Join Date: Jun 2009
Posts: 7,716
Good discussion on tongue weight as it relates to the Chrysler 300, a reliable measurement point, optimum percentage, effect of moment inertia, and how it compares to a Honda Gold Wing, and more. I'm not sure we have resolved the questions, but we are learning.

Ron G points out that concentrating weight over the trailer axles reduces tendency to sway. That would seem to be important for all of us to consider when loading, and a strategy that may be particularly useful to the o.p.?
__________________
Doug and Cheryl
2012 FC RB, Michelin 16, ProPride 1400
2016 Ram 1500 Laramie Crew Cab 4X4 Ecodiesel 3.92 axles

The Truth is More Important Than the Facts
dkottum is offline   Reply With Quote
Old 12-12-2013, 10:28 PM   #117
Rivet Master
 
mstephens's Avatar
 
2013 25' Flying Cloud
Cat City , California
Join Date: Sep 2012
Posts: 854
Instead of saying the distance from axle(s) to CG is 30" and the distance from axle(s) to coupler is 200", we say the tongue weight percentage is 15%. Unfortunately, we then tend to focus on how much vertical load is applied to the ball rather than on how the TT exerts lateral forces on the ball.
==================
Thanks, Ron. The idea of TW being a result of another design goal now makes more sense. It would be appear then as a practical matter there's not much to be done to reduce TW by more than say 100 pounds, or even less.
__________________
mstephens is offline   Reply With Quote
Old 12-12-2013, 10:32 PM   #118
Rivet Master
 
mstephens's Avatar
 
2013 25' Flying Cloud
Cat City , California
Join Date: Sep 2012
Posts: 854
Quote:
Originally Posted by dkottum View Post
Good discussion on tongue weight as it relates to the Chrysler 300, a reliable measurement point, optimum percentage, effect of moment inertia, and how it compares to a Honda Gold Wing, and more. I'm not sure we have resolved the questions, but we are learning.

Ron G points out that concentrating weight over the trailer axles reduces tendency to sway. That would seem to be important for all of us to consider when loading, and a strategy that may be particularly useful to the o.p.?
Yes. One think I will try immediately is moving our "gear" such as it is, from the TV to the center of the TT. Perhaps gaining a 100 pounds or something on that order. If nothing else, I have learned something VERY specific to replace old rules of thumb, guesses, estimates, black magic, and other unuseful information. That's super. For me, worth having the discussion.
__________________
mstephens is offline   Reply With Quote
Old 12-12-2013, 10:38 PM   #119
Rivet Master
 
ROBERTSUNRUS's Avatar

 
2005 25' Safari
Salem , Oregon
Join Date: Mar 2006
Posts: 7,269
Images: 18
Blog Entries: 51
Quote:
Originally Posted by dkottum View Post
Ron G points out that concentrating weight over the trailer axles reduces tendency to sway. That would seem to be important for all of us to consider when loading, and a strategy that may be particularly useful to the o.p.?
Hi, on my trailer, filling the fresh water tank does exactly that. Mine is mounted between my axles.
__________________
Bob

2005 Safari 25-B
"Le Petit Chateau Argent"
[ Small Silver Castle ]
2000 Navigator / 2014 F-150 Eco-Boost / Equal-i-zer / P-3
YAMAHA 2400 / AIR #12144
ROBERTSUNRUS is offline   Reply With Quote
Old 12-12-2013, 10:52 PM   #120
Rivet Master
 
dkottum's Avatar
 
2012 25' Flying Cloud
Battle Lake , Minnesota
Join Date: Jun 2009
Posts: 7,716
Quote:
Originally Posted by ROBERTSUNRUS View Post
Hi, on my trailer, filling the fresh water tank does exactly that. Mine is mounted between my axles.
Good point. Mine is that way as well, maybe that's the reason Airstream placed it there. Long-time Airstream professional Inland Andy has suggested filling the fresh water tank for stability many times.

I believe that is explained in detail by Ron Gratz and something I never fully understood before. I thought is was simply to lower the center of gravity.
__________________
Doug and Cheryl
2012 FC RB, Michelin 16, ProPride 1400
2016 Ram 1500 Laramie Crew Cab 4X4 Ecodiesel 3.92 axles

The Truth is More Important Than the Facts
dkottum is offline   Reply With Quote
Old 12-13-2013, 12:28 AM   #121
Rivet Master
 
mstephens's Avatar
 
2013 25' Flying Cloud
Cat City , California
Join Date: Sep 2012
Posts: 854
We never leave home without a full fresh water tank. Maybe one reason I rarely experience any form of sway.
__________________
mstephens is offline   Reply With Quote
Old 12-13-2013, 05:20 AM   #122
CLOUDSPLITTER "Tahawas"
 
ROBERT CROSS's Avatar

 
2003 25' Classic
Zanadude Nebula , WNY
Join Date: Jun 2007
Posts: 11,153
Images: 1
Thumbs up

^
X3



Bob
__________________
PFC.....

“After all these years the reason I continue to love Thanksgiving.....I still sit at the kids table.”
RLC

Sandra wanted to go to Cleveland on vacation,
but I’m the Husband, so we went to Cleveland.
RLC
ROBERT CROSS is offline   Reply With Quote
Old 12-13-2013, 07:42 AM   #123
Rivet Master
 
SteveH's Avatar
 
2005 39' Land Yacht 390 XL 396
Common Sense , Texas
Join Date: Sep 2007
Posts: 5,311
Paper About Sway Resulting From Improper CG (good info)

This paper contains a lot of analytical math (most of which I do not understand), but the text is easy to understand, and it analyzes the effects of proper/improper loading of trailers (CG), or tongue weight, and it's resulting sway.

Then the paper goes into studying the effectiveness of symmetrical vs asymmetrical braking (active sway control), and concludes the active sway control is much better.

"3. Results of the experiments performed on the road
conform to those learned through analytical modeling
and simulation.
4.
The trailer’s center of gravity location relative to its

resultant axle load location is a critical factor in trailer
stability.
a.
Small variations in CG location fore or aft of the

resultant tire normal load have significant effects
on achievable speed and stability levels.
b.
In the cases shown here, trailer stability appears

to be more dependent on CG location than on
trailer tire cornering stiffness.
5.
Control strategy involving direct yaw moment via

asymmetric braking of towing vehicle is more
effective in stabilizing the snaking oscillations than
symmetric braking.
a.
Direct yaw moment control can damp oscillations

without significant reduction in speed
b.
Symmetric braking can also stabilize the system,

but it has negative second-order effects on
system stability and requires large reduction in
vehicle speed, significantly below the critical
speed, to utilize natural damping in the system."


http://delphi.com/pdf/techpapers/2008-01-1228.pdf
__________________
Regards,
Steve
SteveH is offline   Reply With Quote
Old 12-13-2013, 08:38 AM   #124
Rivet Master
 
dkottum's Avatar
 
2012 25' Flying Cloud
Battle Lake , Minnesota
Join Date: Jun 2009
Posts: 7,716
Thanks Steve, a mention of the significance of the automatic trailer stability systems now common in late model vehicles, which we often disregard in tow discussions.

Does the Chrysler 300 have this system?
__________________
Doug and Cheryl
2012 FC RB, Michelin 16, ProPride 1400
2016 Ram 1500 Laramie Crew Cab 4X4 Ecodiesel 3.92 axles

The Truth is More Important Than the Facts
dkottum is offline   Reply With Quote
Old 12-13-2013, 08:42 AM   #125
Rivet Master
 
SteveH's Avatar
 
2005 39' Land Yacht 390 XL 396
Common Sense , Texas
Join Date: Sep 2007
Posts: 5,311
Quote:
Originally Posted by dkottum View Post
Thanks Steve, a mention of the significance of the automatic trailer stability systems now common in late model vehicles, which we often disregard in tow discussions.

Does the Chrysler 300 have this system?
I don't know, but with a towing rating of 1000 lbs., I seriously doubt it.
__________________
Regards,
Steve
SteveH is offline   Reply With Quote
Old 12-13-2013, 09:25 AM   #126
Rivet Master
 
mstephens's Avatar
 
2013 25' Flying Cloud
Cat City , California
Join Date: Sep 2012
Posts: 854
Thanks for posting that Steve. I found this summation of the parameters for stability to be quite concise and helpful.

Instability increases as:

QUOTE
1)the mass of the trailer (relative to the vehicle’s mass)
increases,
2) the center of gravity of the trailer moves rearward,
3) the moment of inertia of the trailer increases,
4) cornering stiffness of trailer tires decreases,
5) cornering stiffness of the vehicle’s rear tires decreases
6) the distance from the vehicle rear axle to the hitch point increases,
7) vehicle wheelbase decreases.

Even for a specific vehicle-trailer combination, most of these parameters can be greatly altered by changing parameters that are in control of the user, such as weight distribution or tire type and tire pressure.
END QUOTE

I had not given much thought to #4 in the past. I will see what I can find out about my Michelins. As to #5 I am wondering if the TV suspension itself is also a factor. e.g. Limiting side to side slop.
__________________

__________________
mstephens is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



Virginia Campgrounds

Reviews provided by




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 06:38 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.

Airstream is a registered trademark of Airstream Inc. All rights reserved. Airstream trademark used under license to Social Knowledge LLC.