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The mechanics of fishtailing

Post in 'The Gear' started by SolarAndWood, Feb 27, 2012.

  1. Dune

    Dune Minister of Fire

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    Solar, tried to send you a PM. I have a friend who has been looking for a dump trailer for months and is willing to drive up there if you decide to sell.

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  2. SolarAndWood

    SolarAndWood Minister of Fire

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    Hey Dune. It never showed up...I just sent one your way.
  3. ditchrider

    ditchrider Burning Hunk

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    Maybe others have replied, but here is kind of how it works... if you have 15% or so on your trailer hitch, your vehicle has more mass on it's own two axles (it takes the weight off the trailer and bearing the burden), thus being the pulling and stopping force. If you have just enough weight on the hitch to fasten the lock, your trailer has more mass (fi it's fully loaded), thus it tries to PUSH the vehicle it's attached to. Fully loaded your trailer weighs more than your tow vehicle and going downhill, by Newton's Law, it's trying to pass you up. If there is more tongue weight, however, the tow vehicle becomes the greater, and therefore controlling mass.
  4. SolarAndWood

    SolarAndWood Minister of Fire

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    I think I get that. So, on level ground you can get away with less tongue weight because you are pulling not being pushed. On the downhill, you overcome being pushed by having enough tongue weight on the truck and enough weight off the back of the trailer to keep the rear end of the truck from being pushed around? So, when you are fishtailing the ball is actually moving side to side as the trailer swings like a pendulum? Put another way, that trailer loaded that way going down that hill could not have fish tailed behind a triaxle dump truck because it does not have the mass to push the rear end of the bigger truck around?

    Maybe cutting off the back 30" of the trailer and putting some sideboards on it isn't such a bad idea given that I have a 3/4 ton truck. It would move the weight significantly forward and still get me to the towing capacity of the truck. It would make the hydraulics work a little harder to dump but those big dual cylinders never seem to struggle.
  5. ditchrider

    ditchrider Burning Hunk

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    About the 3axel dump truck, yes you will reduce the swing but in that particular case the truck has 14 tires on the ground, a heavier suspension to stabilize the swing from the tongue AND a greater mass than the trailer. However, if the mass of the trailer is balanced 50% behind the centerline of the trailer axles, the trailer may (will) still be inclined to sway, although the mass of the truck will be large enough to resist the force.

    As for cutting of 30" of the trailer, I think that's a lot of work. If it were me I would only make the sideboards for the front 1/3 of the trailer and be cautious not to heap the load in the rear. In that case you would be counterbalancing the rear weight of the loaded trailer with front load. You may still need to take it easy with travel speeds but your 3/4 ton truck will do a better job of pulling the load. I think if you didn't get a sway until 55 mph on the flat you are not too far off balance and you only have a little tweak to do on your load balance. These things alone are the cheap part of the fix.

    Once you've solved the balance issue, an additional fix that may cost a little money (a few hundred, maybe) is an antisway bar setup (load leveler) for your trailer hitch. It requires a special hitch insert and some welding on your trailer. It's hard to explain, but it essentially solidifies the trailer to your truck. The truck still carries the weight but the pivot point is stiffened.

    You should be able to get some answers from a pickup truck accessory store. If you have questions about the antisway bar setup. Although they may suggest you get a 1 ton dually. I hope this has been helpful.
  6. MasterMech

    MasterMech Guest

    If it swayed behind your truck it would have wagged behind a tri-axel too. But it prolly wouldn't affect the truck nearly as much.

    I've towed with trailers that significantly outweigh the tow vehicle without stability issues. That's with gooseneck AND bumper pull trailers. Think a 4500lb trailer with 10 1600lb animals on it (20,500lbs) on a 1 ton reg cab dually. Know any 1 tons that weigh over 10k? Even figuring 25% tongue weight (gooseneck trailer), the truck carrys between 10-11K at that point and the trailer 14K+. And that is close to the max I'd try on a 1 ton dually.

    Bumper pull gets even more interesting. ex. 7 ton equipment or dump trailer (7000lb axles) with a GVWR of 16,100lbs. Even being very generous and saying a 15% tongue load, thats just over 2400lbs tongue weight. _g Figure the same 1 ton truck and the vehicle should carry about 7400-8400lbs with the trailer axles carrying 13,700lb behind it. Keep in mind you'd need a serious hitch to try this kind of load. (2.5" Class V hitches, they make 'em, ::-) ) 10% tongue load would be within range of most 2" Class V hitches but would leave the trailer axles overloaded in this case.

    I think we've proven that the trailer can outweigh the tow vehicle significantly and not even be outside of mfg. ratings.

    Solar, I wouldn't go cutting up your trailer just yet. ;)
    ewdudley likes this.
  7. MasterMech

    MasterMech Guest

    They call those weight-distributing hitches and most have dual-cam sway control built in these days. Expensive but worth it if you have high tongue loads or tow big broad-sided trailers (big RV's) .

    Solar, the WD hitch would help you utilize the most you possibly could out of the 3/4 ton 'burb but new it'd prolly cost you as much as the trailer did. They pop up used from time to time and I think Bluedogz mentioned he had one for sale if your interested.
  8. ditchrider

    ditchrider Burning Hunk

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    Well said. So how can you put to better explanation how the majority of weight behind the centerline of the trailer axles causes the trailer to steer itself rather than BECOME steered by the tow vehicle. I was thinking your thoughts while trying to explain, but the simplest road I could put to words was the one I traveled. Moral of the story is a minimum of tongue load percentage (load balance), NO?

    I'd be willing to lead a horse to water, but I darned sure don't want to stand behind it and push.:eek:
  9. MasterMech

    MasterMech Guest

    It makes sense in my head but I just can't seem to get it out my fingertips. Maybe somebody better versed in the physics of this can explain it better?

    Moral of the story is play by the rules. Stay within your truck, hitch, and trailer ratings and you will be fine. 10% - 15% is the rule of thumb for bumper pull tongue weights so long as you do not exceed your hitch capacity.
  10. StihlHead

    StihlHead Guest

    No no no no no no noooooooooooooooo...

    I am an engineer, and I know the answer to all this. Its all so simple really. Just get a turbo entabulator transmission, and you are set.

    ditchrider likes this.
  11. SolarAndWood

    SolarAndWood Minister of Fire

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    That works pretty well for firewood but it is hard to convince stone, sand, grain, etc from staying off the back of the trailer. The hinges are 30" from the rear of the bed. So, it would literally mean just cutting off the back with no other modification required.
  12. SolarAndWood

    SolarAndWood Minister of Fire

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    I looked into them and came to the same conclusion. I've started shopping for a newer used southern 3/4 ton burn as the salt is starting to have its way with this one. I'm thinking that once I find the right replacement, I'll get it set up right to maximize towing. As of last night, the burb is no longer the daily driver so the next one should last a lot longer than the 4 years/100,000 miles this one did after I brought it north of the salt line.
  13. ewdudley

    ewdudley Minister of Fire

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    There's a pretty good thread over on tractorbynet:

    http://www.tractorbynet.com/forums/trailers-transportation/202117-what-causes-fishtailing.html

    Slowzuki himself had a credible explanation:


    Tow-out hasn't been mentioned yet in this thread, and this can be a big aggravating factor. The OP's trailer looks like it would be easy enough to check with a tape measure. You don't have to be all the way up to the middle of the tire to get a good idea of what the toe is.

    Also later in the thread they discuss weight distribution between front and rear tandem axles, the goal being to bias weight towards the rear for stability. An interesting point was that redistributing weight to the tongue onto the rear of a vehicle that is not up to the task can cause the hitch point to lower and shift more weight to the forward tandem axle of the trailer.

    A fishtailing load is pretty scary. But once you commit to the devil's bargain of accelerating down the hill come what may, it gets to be fun after a while once you get the knack of cresting each rise slowly enough. On the level it helps to grit your teeth and work the steering wheel constantly to apply feed-forward corrections with gentle expertise, adding speed as your skill-set horizon expands.
  14. SolarAndWood

    SolarAndWood Minister of Fire

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    lol EW, I was doing 50 as I crested that ridge between Lafayette and Nedrow on I-81 and started that long decent. Came to the conclusion as I rounded the bend at the bottom that the devil was going to eventually win that bargain if I didn't change the game. Never even noticed that bend until that night.

    Thanks for the link, I'll check it out. I barely sag into the truck suspension with the trailer fully loaded evenly. The last few loads I have heavily biased forward with much better results.
  15. ewdudley

    ewdudley Minister of Fire

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    Funny you should mention that hill, it's the exact same hill I was thinking of. I was flat-towing a salvaged diesel F-250 with a borrowed Winnebago type self-propelled camper. Had a loosey-goosey clamp-on bumper hitch that had a fair amount of slop in it, plus the bumper had slop where it attached to the frame rails. I was getting pretty cocky at that point having made it all the way from Great Bend/New Milford, but soon realized I had over-played my hand.

    --ewd

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