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Old 11-24-2007, 09:31 AM   #1
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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?
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Old 11-24-2007, 09:46 AM   #2
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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.

Cheers,
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Old 11-24-2007, 09:51 AM   #3
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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.
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Old 11-24-2007, 11:14 AM   #4
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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.
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Old 11-24-2007, 12:50 PM   #5
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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.
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Old 11-24-2007, 06:05 PM   #6
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Quote:
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.
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Old 11-24-2007, 08:09 PM   #7
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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)
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Old 11-24-2007, 09:00 PM   #8
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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.
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