Thread Tools Search this Thread Display Modes
Old 11-24-2007, 09:31 AM   #1
4 Rivet Member
ljmiii's Avatar
2006 16' International CCD
New York , New York
Join Date: Sep 2006
Posts: 268
A shorter lever - a safer tow vehicle?

I've read a lot on these forums of the importance of a longer wheelbase in preventing loss of control. But as I have read about the value of the Hensley and Pullrite hitches I wonder if another dimension is not equally important - the distance between the rear axle and the hitch.

The 'tail wagging the dog' occurs when the force operating on the hitch ball pushes the trailer lever (of length hitch to rear axle) to overcome the mass of the tow vehicle times length wheelbase. The Hensley and Pullrite hitches do their magic by moving the pivot point forward to the rear axle - and as the lever's length approaches zero the force required to 'wag the dog' approaches infinity.

But when using a 'normal' anti-sway hitch like the Equal-i-zer or Reese dual cam I would think that (length from hitch to rear axle)/wheelbase is a better measure of stability than wheelbase alone. Am I missing something?

ljmiii is offline   Reply With Quote
Old 11-24-2007, 09:46 AM   #2
Rivet Master
JimGolden's Avatar
Vintage Kin Owner
1977 31' Excella 500
Berkeley Springs , West Virginia
Join Date: Sep 2005
Posts: 1,600
Images: 7
You are correct, but both measurements are important.

What you want is the tow vehicle to exert the winning leverage over the trailer, not vice-versa. So the goal is to have a long tow vehicle wheel base combined with a short ball to rear axle dimension. Make it a fraction with the tow vehicle's wheel base over the ball to back axle distance. You want that number to be as large as possible (as in the value of the fraction). \

Example: Vehicle 1 has a wheelbase of 144 inches with distance from ball to back axle = 36 inches. Ratio = 144/36 = 4.0 Vehicle 2 has a wheelbase of 120 inches with a distance from ball to back axle of 48 inches. Ratio = 130/48 = 2.5 Vehicle 1 should be the better tow rig, all else being equal.

I had a Suburban that I used to tow with. It did alright. But it had a fairly large rear overhang. I went to a pickup truck that had less rear over hang. It also has a longer wheelbase. It tows even better.

You could sit down with a spreadsheet and take some measurements and probably come up with the ideal range of ratio's that give you "good" handling, vs. those that give you "so-so", vs. those that are best avoided (say pulling your 30' Stream with a Jeep Wrangler).

But anyway, both numbers matter.

The Hensley does project the turn center ahead, but I would not count on that to fix a basic problem. I'd use it as icing on the cake, but I'd still want the proper sized tow vehicle.

I had an old '75 Mercury Marquis with the 460 V8 and dual exhaust. That old car pulled a 35' Holiday Rambler like it wasn't even back there. It was a fairly long car though.


- Jim
JimGolden is offline   Reply With Quote
Old 11-24-2007, 09:51 AM   #3
Rivet Master
HowieE's Avatar

1991 34' Excella
Princeton , New Jersey
Join Date: Aug 2003
Posts: 6,314
Images: 12
No, you are not.

The principle of the Pull Right hitch is just that. Move the pivit point as close to the center line of the axle as possible. This give min. sway effect to the front axle and thus the rig is less likely to go into oscillation.

That said most people don't want to take a hack saw to the aft end of thier TV.
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles

HowieE is online now   Reply With Quote
Old 11-24-2007, 11:14 AM   #4
Site Team
, Minnesota
Join Date: Aug 2003
Posts: 6,472
Images: 59
The ratio of the hitch to axle length divided by the wheelbase is a commonly used measure for comparing different tow vehicles. I have seen this number used to declare that a shorty Tahoe is as good as a Suburban because it has a similar ratio.

I think a more appropriate comparison would be the ratio of the "hitch to CG" (center of gravity) divided by the "rear axle to CG" measurements. I say this because the levering effect causes rotation around the CG of the tow vehicle, not around the rear axle.
markdoane is offline   Reply With Quote
Old 11-24-2007, 12:50 PM   #5
Rivet Master
1977 31' Sovereign
1963 26' Overlander
1989 34' Excella
Johnsburg , Illinois
Join Date: Jul 2005
Posts: 2,924
The Hensley moves the apparent pivot point ahead a fixed distance established by the design by Hensley. The Pullrite does the same thing. Dependent on the actual overhang distance the effective location of the pivot point might be ahead of or behind the rear wheels but would be an improvement over the Twin cam or Equalizer. If you are using the Pullrite or the Hensely your overhang is not as big a sway problem but the downward weight of the trailer is still out there levering the TV up and down as it goes down the road. The W/D system only stays the same only as long as both the TV and trailer are on level ground. In the real world the trailer tries to lift the front wheels of the TV off the ground everytime the front wheels of the TV goes thru a dip, so the overhang is still important and should be minimized to make the most stable rig. The relative mass and polar moment of inertia of the TV and the trailer are also importsnt. Dynamics is a second year course in Mechanical engineering.
dwightdi is offline   Reply With Quote
Old 11-24-2007, 06:05 PM   #6
Rivet Master
1975 29' Ambassador
Reno , Nevada
Join Date: May 2003
Posts: 1,351
the levering effect causes rotation around the CG of the tow vehicle, not around the rear axle.
This is an insteresting assertion. I can see where it would be true if the TV did not have any forces other than the TT on it but the tires resist.

It appears to me that tail wagging by a TT is going to shove the rear axle one way and the front axle the other as it tries to rotate around its CG. As long as the tires don't slip, that means the rear axle is a fulcrum and the front tires the load. The front tires as the load is why sway is often driver aggravated as the driver gets unusual steering feedback causing oversteer. Without driver input, the front tires tend to go with the push rather than resist it.

The Pullrite and the haha and 5th wheel hitches and semi's gain handling advantage by moving the lateral force at the hitch closer to the rear axle. That means that nearly all of the force is handled by the rear axle tires and very little (ratio of lengths in the lever) by the front end. Smaller forces handled by the steering axle means less impact on steering and that means better handling.

The Airstream is an interesting example related to the wheelbase to overhang ratio. One of the features of the Airstream is its long ball to axle ratio. An extreme case like this is the semi. That puts the TT CG in front of the TT axles which allows the TT tires to assist in resisting rotations around the TT CG inhibiting sway.

The assertion about CG does raise the issue of the importance of various terms in the sway equation. That discussion usually centers on mass ratios between the TT and the TV. Sometimes it gets into 3d models rather than 2d as in tipover situations (especially with 5th wheel vs Airstream). From what I can tell, the wheelbase to overhang ratio is a strong contender for most significant term in most cases.
bryanl is offline   Reply With Quote
Old 11-24-2007, 08:09 PM   #7
HiHoAgRV's Avatar

1991 34' Excella
1963 26' Overlander
Central , Mississippi
Join Date: Nov 2006
Posts: 5,006
Images: 29
Blog Entries: 49
Top 10 thread . I'm 100% with ya'll that the pivot location relative to to the rear axle is probably the key factor in handling. Pullrite and Hensely move this forward and mimick a semi or 5th wheel. Dynamically this will keep 'sway' as is most commonly described, to a minimum. (with enough side force the rear axle will slide sideways) Pitching is a seperate issue and our units still have to deal with that but I view that as a comfort issue. Polar moment of inertia and height of CG are both evil, and unfortunately, design dependant (unless we want to tow with water tanks full)
Hi Ho Silver RV! Vernon, Sarah, Mac the Border Collie -
A honkin' long 34' named AlumaTherapy
and a 26' '63 Overlander, Dolly
HiHoAgRV is offline   Reply With Quote
Old 11-24-2007, 09:00 PM   #8
3 Rivet Member
GO Bob's Avatar
1982 34' Limited
1975 31' Sovereign
Grovetown , Georgia
Join Date: May 2006
Posts: 105
I went to the towing siminar at International. An enginer studied this problem and presented it in laymans terms with pros and cons. From memory

Pullrite best with shortest tail, but lose spare tire, add turn radius, fairly costly.

Hensley next, heavy, complicated, very expensive.

Dual cam does help it track straight, cost

Friction only works after sway starts.

If you have sway Grip wheel tight with left hand, hit gas, and with right hand tap the brake controler while praying.

"Humans get a lot done,not because we're smart, but because we have thumbs so we can make coffee." Flash Rosenburg

"Decaffinated coffee is the devil's blend."
GO Bob is offline   Reply With Quote

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

Similar Threads
Thread Thread Starter Forum Replies Last Post
Get the right tow vehicle woolygums Airstream Motorhome Forums 0 09-19-2004 06:45 AM
Airstream Safer Than Home in Hurricane? Pick Our Community 17 09-02-2004 08:05 PM
new tow vehicle whistler On The Road... 10 04-14-2004 07:07 PM
Tow Vehicle hrnicho Tow Vehicles 22 10-23-2002 12:20 PM

Virginia Campgrounds

Reviews provided by

Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities

Copyright 2002-2015 Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 09:26 PM.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, Jelsoft Enterprises Ltd.

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