Dynamics of tongue weight questions

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Dynamics of tongue weight questions

Postby TD Beej » Sun Oct 11, 2009 10:18 pm

I've been wondering what the purpose of the tongue weight is. Obviously it needs to be heavy enough so that you can set it down on the ball and attach it but beyond just keeping the front of the trailer down what does it do.

I can under stand if you have a high center of gravity wanting some extra tongue weight to keep the tongue down during acceleration and going up hill but hopefully weight would be kept as low as possible to minimize these shifts.

It seems to me that extra tongue weight would make the trailer less prone to follow the TV in a turn. i.e. lower tongue weight would result in easier polar rotation of the trailer and less pushing out to the side on the TV in a turn.

I could see where moving the trailers wheels back would reduce the chance of fishtailing but wouldn't seem to be a real problem except in severe weather conditions.

From what I gathered on some other postings that a higher tongue weight allows for higher speed towing, what is the connection? Could the same result in simply lowering the center of mass of the trailer?

Anyone have any ideas, comments, or expert knowledge they can impart?
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Postby madjack » Sun Oct 11, 2009 10:53 pm

I can only offer anecdotal evidence from many many years of using trailers, both for personal and professional use...too little weight on the tongue will allow it to "float" on the ball...this floating will induce sway/vibration/fishtailing to the axle which can cause the trailer and/or tow vehicle to become uncontrollable(flipping/wrecking/crashing/jacknifing)...too much weight can cause adverse effects to the handling of the tow vehicle because it will reduce weight on the front end...proper weight on the tongue will cause it to "hunker down" on the ball and the trailer will follow along like a nice polite, well behaved little puppy dog............
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Postby bobhenry » Sun Oct 11, 2009 11:22 pm

Need an engineer here............ Help...........

Harmonic resonance.

The slop in the coupler Jack mentioned allows the trailer to shift right and left. That 1/8 to 3/16 of an inch changes the pull angle of the tow vehicle. This is telegraphed thru the hitch to the tow vehicle causing a vey slight wiggle this wiggle is telegraphed back to the trailer and it shifts a minute amount and this occures over and over each time getting worse and worse. In severe cases you will be behind (hopefully way behind) watching a trailer swing into the oncoming lane then the berm of the road like a pendilum of a clock. If the driver of the tow reacts quick enough on the brakes he might get lucky. If not the best he can hope for is the trailer "gets away" during the dance and it is the only casualty if not the tow will be on a wrecker also.

I was having trouble with "Chubby" dancing at 65 MPH so I purchased a 2" drop draw bar. ( was towing with a level in the up position.) He now tows very slightly nose down and was a good boy this weekend at 70 MPH+.

At dead level or slightly higher I can support the tongue with 4 fingers. From on the ground to about 2" nose down I can just lift the tongue bare handed. The higher I lift the lighter the tongue weight gets. This is the reason to NEVER tows nose high. Buy the proper drop of hitch to get level to slightly nose down.
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Postby caseydog » Sun Oct 11, 2009 11:22 pm

I have first hand experience with not enough tongue weight.

I got suckered into pulling a Boy Scout trailer that was home built, with my full-sized pickup. What could possibly go wrong?

Driving down the highway was a white-knuckle nightmare. The axle was way too far forward, and there was very little tongue weight.

Since my truck was heavy, I was never at risk of losing control, but cars around me did NOT want to be next to that trailer. It was swaying all over the interstate.

Going uphill was fine, but downhill was sway city. I stopped to move more cargo to the front of the trailer, but it was no use.

When we got to our destination, I was told that I was chosen to tow the scout trailer, because I had the biggest, heaviest truck. Nice.

So, I can tell you for sure that negative tongue weight is not something you want to experience.

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Postby TD Beej » Mon Oct 12, 2009 12:32 am

Hmm... Thinking out loud here about what I am hearing is:

That as the tongue weight goes down the susceptibility of the trailer to the setup of of a standing wave goes up. Or other words if the wheels are to close to the center of mass the trailer is too easy to turn, lowering its stability and making it more sensitive to environmental inputs.

I am sure the slop at the coupling would contribute to instability but the suspension has a lot more potential for movement, energy storage and release, so good shocks should help a lot more to stabilize then a tighter connection.

Nose up/down I suspect is aerodynamics and/or weight shift. By lowering the nose more weight is placed in front of the wheels. By lowering the front you force more air to go over and less under and create opening wedge causing more of a vacuum under the trailer (side skirts anyone).

So things that can be done to increase stability are:
1. Move wheels further behind the center of mass to reduce sensitivity (decrease leverage on TV).
2. Well tuned suspension with lots of dampening (remove energy system).
3. Good aerodynamics (suck the trailer down at speed minimize lift).
4. And of course lower center of mass (shorten the "pendulum").

More?

Thanks,
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Postby TD Beej » Mon Oct 12, 2009 12:56 am

One more thing occurs to me about lowering the hitch height, it would reduce the inputs to the suspension of the TV which is probably a greater factor then the weight shift forward or the change in aerodynamics. It would mostly take energy storage of the TV's springs out of the system but not elimnate any inherent instability of the trailer.

So:
5. Lower hitch height (reduces side to side inputs to TV suspension).
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Postby clarkbre » Mon Oct 12, 2009 12:56 am

I think hitch height should be determined by if the trailer is travelling level behind the TV. A properly placed axle and correctly loaded trailer will keep the trailer tracking correctly.

I was taught that it's like a tail wagging a dog. With not enough tongue weight, the weight in the rear of the trailer will push the rear end around and sway back and forth. If you have at least 10% tongue weight then the trailer is forced to track straight behind the TV.

A lot of proper weight distribution is how you load the trailer. The camp trailer I'm currently building will need all of the heavy stuff up front and the lighter stuff in the rear. I've used the trailer balance spreadsheet on this site and it has been absolutely great. From that spreadsheet, I figured out that I should put my water, cooler, and awning up front and load my lighter weight stuff in the rear. With the axle 42% from the back of the trailer, I need more weight in the front to keep it from swaying.
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Postby angib » Mon Oct 12, 2009 10:20 am

Here in Europe, as we generally have smaller tow vehicles, it's usual to tow with lower hitch weights (and much higher trailer weights) than in the US.

So trailer sway is a serious issue here and you might like to visit this website where they present some stability studies and have a simulator where you can try out your ideas:
http://www.towingstabilitystudies.co.uk/

As others have said, positive trailer hitch weight reduces trailer sway. The physics of why this is true is complex and I've never seen it presented in a single formula, for example. I'm not aware that free play in the ball-coupler has any significant bearing on trailer sway - that sounds like an old wives' tale to me.

One detail of trailer sway that seems to be little known is how speed reduces stability. One important (but not the only) reason for this is that almost all trailers generate aero lift at the front and this reduces hitch weight and stability. A "stable trailer" isn't always stable - it's just stable up to a higher speed than a less stable trailer.

A small teardrop travelling inside the wake of a bigger tow vehicle will be in confused airflow and so won't generate the same aero lift - which may explain how teardrops can be towed at speeds high enough to cause sway in many trailers.

I was recently following a big truck (semi-sized) pulling a tiny generator that was used to power the temporary traffic lights at roadworks and this had the most extreme sway I've ever seen. The truck was so big that I don't think they even knew the trailer was swaying, but it was moving far enough and sharply enough to get trailer wheels off the ground and I could hear from my car the shriek from the trailer tyres as they landed back on the ground! Amazingly it seemed able to keep on doing this - I was following some distance behind as I was fairly sure the trailer was going to snap its tongue.

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Postby Arne » Mon Oct 12, 2009 11:49 am

Forgetting about trailering, per se, the trailer without tongue weight will try to rip off the clamp on the hitch because the trailer will bob back and forth as you hit bumps. The taller the trailer, the more force is exerted.. having weight on the ball will keep that force to a minimum...

The hitch can take a lot of weight, but the clamp can not. The clamp will bend or the bolt will break. Then you will have a lot of problems as the trailer falls off the ball.
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Postby caseydog » Mon Oct 12, 2009 1:03 pm

TD Beej wrote:Hmm... Thinking out loud here about what I am hearing is:

That as the tongue weight goes down the susceptibility of the trailer to the setup of of a standing wave goes up. Or other words if the wheels are to close to the center of mass the trailer is too easy to turn, lowering its stability and making it more sensitive to environmental inputs.

I am sure the slop at the coupling would contribute to instability but the suspension has a lot more potential for movement, energy storage and release, so good shocks should help a lot more to stabilize then a tighter connection.

Nose up/down I suspect is aerodynamics and/or weight shift. By lowering the nose more weight is placed in front of the wheels. By lowering the front you force more air to go over and less under and create opening wedge causing more of a vacuum under the trailer (side skirts anyone).

So things that can be done to increase stability are:
1. Move wheels further behind the center of mass to reduce sensitivity (decrease leverage on TV).
2. Well tuned suspension with lots of dampening (remove energy system).
3. Good aerodynamics (suck the trailer down at speed minimize lift).
4. And of course lower center of mass (shorten the "pendulum").

More?

Thanks,
Beej


To simplify all this, I would just say that you need to make sure that there is positive tongue weight on you trailer hitch. It needs to be positive not only standing still, but at highway speeds.

So, you may have positive tongue weight of 50 pounds sitting still, but depending on aerodynamics, it's possible for you to go negative when you hit a high enough speed.

Getting your axle in the right place is the most important step of those you mentioned. You can fine tune the balance of a TD when traveling by simply moving gear around inside, but getting the axle placement right is the big step.

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Postby GeoDrop » Mon Oct 12, 2009 1:48 pm

caseydog wrote:To simplify all this, I would just say that you need to make sure that there is positive tongue weight on you trailer hitch. It needs to be positive not only standing still, but at highway speeds.

So, you may have positive tongue weight of 50 pounds sitting still, but depending on aerodynamics, it's possible for you to go negative when you hit a high enough speed.

Getting your axle in the right place is the most important step of those you mentioned. You can fine tune the balance of a TD when traveling by simply moving gear around inside, but getting the axle placement right is the big step.

CD


I'm planning on just 'tack welding' the axle on to the frame during construction and then relocate it (if necessary) once I'm complete. So the question is:

Given the ability to move the axle to any position, what is the ideal position? Should the toungue weight be X% of the gross weight?

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Postby madjack » Mon Oct 12, 2009 2:03 pm

Mathew, 10-12% would be considered ideal and upto 20% OK if not too much for tow vehicle...problems start pretty quick when dropping under 10%...as far as axle location goes, my rule of thumb on a standard design tear would be 36"s for an 8'er adding around 3" for every foot of increased length...39-40 for a 9'er and 42-43 for a 10'er...these numbers are not exact but will get you within loading range of "just right" to begin with......
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p.s. keep in mind, it all depends on how you load...if you have a front box and where batteries/propane/dutch ovens and other heavy items are placed/located.....mj
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Postby TD Beej » Mon Oct 12, 2009 3:11 pm

Thanks for the link Andrew, it also let me to this little device: http://www.al-ko.co.uk/alko-Trailer.htm. I was playing with the idea of trying to adapt a ABS/DSC unit from a car and even though it isn't quite that this ATC unit is a step that way.

I am gather that:

the further back behind the center of mass the wheels are the more stable the trailer is. Basically the trailers wheels and suspension will have more leverage on the mass. Sort of like a rocket having the center of pressure behind the center of mass.

Lower hitch height reduces leverage on the TVs suspension.

Lower center of mass controls leaning.

Air pushing back on the top of the trailer has the result of unloading the downward pressure on the hitch.

Lift in back has the effect of lighting the weight on the suspension of the trailer reducing its ability to dampen.

To much down pressure in back will result in unloading at the hitch.

Ground effects will load up the suspension causing the trailer to settle down.

Excess tongue weight will increase the the the pushing on the trailer and lighten the front suspension affecting handling of the TV. Shouldn't be much of an issue with a light trailer on a heavy TV but a light TV and a heavy trailer will have problems.
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Postby GeoDrop » Mon Oct 12, 2009 3:28 pm

madjack wrote:Mathew, 10-12% would be considered ideal and upto 20% OK if not too much for tow vehicle...problems start pretty quick when dropping under 10%...as far as axle location goes, my rule of thumb on a standard design tear would be 36"s for an 8'er adding around 3" for every foot of increased length...39-40 for a 9'er and 42-43 for a 10'er...these numbers are not exact but will get you within loading range of "just right" to begin with......
madjack 8)

p.s. keep in mind, it all depends on how you load...if you have a front box and where batteries/propane/dutch ovens and other heavy items are placed/located.....mj


Thanks... I will have a 10ft long "body" but the trail itself will be a foot shorter or so. The design is a Grumman 2 profile stretched vertically to make it taller on the inside. TV will be a Dodge Magnum, so trying to keep the tounge as light as possible but stay within the safe zone.

Thanks!
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Postby TD Beej » Mon Oct 12, 2009 4:58 pm

Here is an interesting video demonstration about how weight distribution relative to center of mass on stability: http://www.towingstabilitystudies.co.uk/stability_simulator.htm
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