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Old 09-26-2011, 10:41 AM   #57
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Appreciate your taking the time to explain some of the things I thought I knew. I think I was either right for the wrong reasons, or wrong for the right reasons??
Also, the rear-wheel steering thing is surely something to ponder--try backing any front-steer vehicle backwards for any length and it can get out of line quickly. Must have something to do with small steering changes causing much more change in side force?
One thing I would think---it should be hard to get a trailer nose-low enough to cause much sway, the suspension travel will limit how close the front wheels can get to the bed. Once at that point, you would have to be lifting the rear wheels off the ground? Nose-high, the tow vehicle hitch would be the limit of how high it could go? But, excessive weight on the trailer tongue would tend to lift the front of the tow vehicle. Either situation, I can see that stability would be much less than a level setup.
Thanks,
Joe
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Old 09-26-2011, 11:39 AM   #58
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3 Point Support

I've had plenty of sway problems with my Utility Trailer. As mentioned above, the Axle was placed close to the center balance point by the Manufacturer. It's a Tilt Bed, and this has something to do with where the [1] Axle was placed. I solve this 'wandering' issue at Freeway speeds by keeping an absconded piece of RR Track safely lashed to the front of the Trailer Bed. I also place my ancient Sears Toolboxes up front IF I'm hauling a light load. As with a Camper Trailer, proper loading saves the day...

My $0.02 is that I like the 'option' of loading a Camper Trailer as suits me, rather than having a Manufacturer put the Wheels far back and 'making' that decision for me. If this means new Trailer Owners and 1/2 Ton Truck/SUV fans have to get smart about loading their Trailer, or choosing Battery/Propane Tank sizes, that's a good thing IMO. Choice = good.

Ignoring the TV for a moment, we've all seen Oldsters using Walkers. 3 point supports are more stable than 2 point supports [i.e. just our 2 Legs]. The third support, and pivot, point for a Trailer is the Hitch WITH downward weight applied within a -/+ weight percentage limit noted in this Thread. Take some weight off the Hitch and you 'degrade' to 2 points of support, allowing Trailer 'wandering' laterally. There's also a factor of cyclical harmonics thrown in, above certain speeds, and this varies with the individual Trailer Hitch-to-Axle length. That is, 'light-Hitch-weight' Trailer 'x' and Trailer 'y' will wander at different cycles/minute, given different distances from Axle to Hitch. I can attest to this 1st Person, despite my Physics knowledge. Case-in-point, the 2 Wheeled 'Segway' Electric Scooters are much hard to stabilize and ride than a 3 Wheeled Electric Scooter.

We Country denizens have all seen 4 Wheeled Hay Trailers or 4 Wheeled Sheepherder Trailers under tow. In these scenarios, Hitch weight is nullified, and the advantage/stability of 3 point support goes out the Window. Tow speeds of these 'wandering' Trailers are always VERY slow; they have to be.

Some old, and some new, Trailers have used an 'extra' Wheel under the Hitch Ball with differing degrees of success.

From the 'Trailer' POV, if Hitch pressure/weight is very high AND that pressure is placed on a single motorized Wheel used at Factories, or on a heavy, short-Wheelbase Forklift, leveled Trailer behavior on 3 support points doesn't change significantly. The reason for caring about 'too' high a Bumper Pull Hitch weight has to do with the TV, and the front end losing downward PSI, and other handling issues.
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Old 09-26-2011, 01:49 PM   #59
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Quote:
Originally Posted by Engineer Guy View Post
There's also a factor of and this varies with the individual Trailer Hitch-to-Axle length. That is, 'light-Hitch-weight' Trailer 'x' and Trailer 'y' will wander at different cycles/minute, given different distances from Axle to Hitch.
First let me thank you all again- the clarity of your explanations is brilliant!

And I'm thrilled that you've thrown harmonics into the discussion, Engineer Guy...now I can ask something I've been wondering about:
Is the "harmonic effect" increased while towing in 5th gear?
My tow vehicle owner's manual advises against it, without explanation.
Naturally I've tried it, just to see what happens, and it does seem to decrease at least the "feel" of stability...

Thanks!

Francesca
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Old 09-27-2011, 12:37 AM   #60
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Engineer Guy

I tend agree with what you said. I donít agree that the Manufactures put out a product that is safe to use and we just have to figure how to load it right. A case in point: I bought a 17í TT many years ago when I was still as dumb as a box of rocks about these things. On our maiden trip for a overnighter when I got up to 45 the TT was all over the road so it was 40 until we got home. I donít remember how I decided it was the tires but that is what I focused on. The TT tires were 5:40X14 4ply trailer tires with 2600lbs cap at max pressure. The dry weight of the TT was 2300. I wanted to have a tire that would be interchangeable with the PU (Ď67 1/2T Ford with 700/15LT 6 ply tires). That couldnít be done but I found that Ford car wheels fit the TT so put the PU size tires on the TT. End of problems. In fact I towed it as high as 70. I have felt for years that if RVs had more tire cap a lot RV handling problems would disappear.
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Old 09-27-2011, 10:30 AM   #61
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Francesca,

The TV gear itself has no bearing on trailer stability, the sometimes mfg warning about towing in overdrive has to do with overloading the engine's torque capability to maintain speed without seeking a lower gear. This has become mostly a moot point in new vehicles, particularly in diesels--they have all the torque needed to tow in overdrive.

This has certainly been an interesting discussion...

Joe
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Old 09-27-2011, 10:56 AM   #62
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The Devil's In The Details...

I guess there's a need to clarify/reiterate exactly what I wrote, and that I meant no more than what's in this quote:

'The Manufacturers you're looking at likely have many years in the Business, and have no vested interest in churning out Trailers that whipsaw around, or behave/brake erratically. So, question your assumptions, and then question them again'.

I'm not saying Manufacturers do not ever churn out Products with ill-advised trade offs. I'm precisely saying only that they have no vested interest in doing so. This thought is in response to the idea that one has to 'toss out the Baby with the Bath Water' and not even consider Trailers with non-sensical Tongue Weights. Those weights, and Tire sizes, and 20 lb vs. 30 lb Propane Tanks and other loads, all can - and should be - examined and tweaked by the informed Trailer Owner. Indeed, my POV is quite the opposite of assuming one can hook up and drive away since Manufacturers have produced a Product intrinsically safe, stable and needing no adjustments. Further, this is why I go on to suggest some tweaks I've made, and assumptions those tweaks were based on.

My lil, used TT had what I consider to be idiotic, undersized Safety Chains. So, I replaced them. It had no Gas Detectors nor a Smoke Alarm And so on, and so on... Read here - and elsewhere - the Forum Posts to become a believer in Market realities of Manufacturers meeting minimum specs, and then take corrective action with the Trailer of choice.

'Wingnut60' nailed the answer just now I'd composed herein off-line. IME, unstable Trailers prone to do so 'break into' oscillation above unique speed 'x'. So, it could be that reaching a speed that even allows using 5th Gear is the true variable at work: speed, not Gear selection. Towing slightly uphill - typically not compatible with 5th Gear - also tends to stabilize a Trailer by 'tying it' to the TV in pull mode. The opposite tends to be true re: stability while going downhill, so I lock my Auto Tranny into a lower Gear and let the World go by.

I absolutely agree that Tire diameter; Sidewall 'stoutness'; Tread style; Inflation; and running temperature are each very important factors.

This famous Video showing the Engineering mishap in designing a Bridge of a length resonant to the Gorge spanned is classic Harmonic Oscillation vividly illustrated.

Famous Bridge Collapse Video
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Old 09-27-2011, 12:02 PM   #63
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Yes!

Good old "Galloping Gertie"!

She's in my neck of the woods- or should I say, she USED to be...

Funny you'd bring Gertie up, Engineer Guy-

Until this discussion, the only practical thing I knew about harmonics had to do with bridges.
One year we watched a parade crossing Portland's Steel Bridge. I asked my Dad why the bands "didn't march right" when they got to the bridge, and he said that unless they were made to "break step" while crossing, a Marching Band could bring a bridge down.

That's harmonics!

Francesca
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