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Old 07-15-2007, 09:51 AM   #1
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Have you ever seen a safe-towing chart that shows a relationship between trailer weight, TV weight, and TV wheel base?
Such that a safe ratio of TV weight to trailer weight increases or decreases with changes in wheel base.

Pretty new to egg-towing but keenly interested in the physics of it all.

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Old 07-15-2007, 01:05 PM   #2
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Never have, although there is a formula drifting around the Bulgemobile RV groups with something to do with a maximum trailer length, but many find it lacking (and wouldn't apply to this group except to the very few who tow eggs with lawn tractors).

To my knowledge, there are no specifics, just rough guidelines, likely because there are so many variables, like suspension, tire stiffness, frame stiffness, number of trailer axles, WDH, sway dampening, etc.

Essentially, as I see it, there are a series of levers and pivot points and what we are concerned about is the effect that a sideways movement of the trailer rear will have on the tow vehicle's front wheels.

The pivots are the rear axle of the tow vehicle and the trailer axle. The levers are:

1. Tow vehicle front axle to rear axle (aka wheelbase, longer is better)
2. Tow vehicle rear axle to hitch ball (shorter is betteraka overhang; 5W/gooseneck brings to almost zero)
3. TT coupler to TT axle (longer is better)
4. TT axle to TT rear (shorter is better; actually, it's to the TT's rear-half COG)

I'm ignoring here the front overhang on the tow vehicle, but putting a spare tire, cattle guard bumper, winch or generator on the front of the tow vehicle may also be a factor.

Sometimes inches make a difference. Following a sway incident, I moved my hitch receiver forward about two inches and shortened my ball mount by an additional two inches. The four inch reduction of my truck's overhang made a noticable difference both on the road and when backing up.

Moving some tools to the front seat and the spare tire to the front bumper made still more difference, so it's not just the length of the levers, but the weight distribution along their lengths.
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Old 07-15-2007, 07:34 PM   #3
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Pete's stuff, above, is all good!

For the upper limit, consider that an eight-ton truck can safely tow a 40-ton trailer, with the trailer length more than double the truck wheelbase, if the overhang is kept short, the cargo load is appropriately distributed, and all of the components are designed for the resulting loads (that's roughly a typical highway tractor-trailer rig).
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Old 07-15-2007, 08:15 PM   #4
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So if i understand this correctly.If i was to extend my hitch on Trailer by say a foot it would tow better with my Toyoto Rave 4 pulling it.
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Old 07-16-2007, 07:41 AM   #5
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You've got some good points here Pete. There's a lot of things here that most of us take for granted. We just tow and go 'cause that's the rig we've got and you take the good with the bad. Mabee some of us should rethink the "tow' or the "hitch" to be safer on the road.
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Old 07-16-2007, 11:24 AM   #6
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Quote:
So if i understand this correctly.If i was to extend my hitch on Trailer by say a foot it would tow better with my Toyota Rav 4 pulling it.
I assume that what we're talking about here is extending the tongue of the trailer, not the hitch on the tug.

Yes, I think that it would likely provide better stabiliy by extending the [b]hitch-to-axle distance, but at the same time it would reduce the[b] tongue weight.

While the reduction in hitch load might be a good thing in itself, it's hard to say if this is a change in a single parameter of the towing configuration - both length and load distribution are changing. Just extending the tongue and thus reducing hitch load might be particularly appropriate in this case; some benefit might result from reducing load on the RAV4's rear axle, and decreasing load transfer from its front to rear axle, in addition to any benefit from the actual length change.

What if the tongue were extended, and either the trailer axle moved back (not very practical) or some load shifted so the distribution of weight between trailer axle stayed the same? Then it would be more of a proportional stretching of the trailer - that might be simpler to understand, but not necessarily more desirable.

In either case of extension, if the trailer mass is just as concentrated as before but the distance from hitch to centre of mass is extended, then the tug has a [b]longer lever with which to control the trailer, which is good.

There have been a number of discussions of the [b]structural implications of tongue extensions. Certainly, with no other changes, simply extending the tongue raises the stress on the tongue at the most highly stressed point (adjacent to the front of the body). I think the fact that eggs commonly and unnecessarily use the same material for the tongue as the main frame rails, and continue it in the same size all the way to the coupler, is the only reason that some of these extensions work without failure... there was some margin to work with.

Since tongue extensions are usually done in order to accommodate even more massive stuff between the trailer body and hitch, I think it will be difficult to find an example of the effects of just extending the tongue.
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Old 07-16-2007, 11:34 AM   #7
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I've never seen one other than in photos, but I understand that one of the unusual features of the Trillium 5500 is that it has very long wheel wells, designed to accommodate optional tandem axles. In the normal single-axle form, the wheels are at the front of the available space.

What if we were to move the axle back in a single-axle Trillium 5500, without changing anything else? This would shift more load to the hitch, so it would only be practical for a tug which can handle the additional weight (preferably without a weight-distribution system to confuse the situation).

This configuration would have (compare with Pete's list)
  • an extended coupler-to-axle distance
  • a longer distance from the trailer's centre of mass back to the trailer axle
  • less mass behind the rear axle
  • no change in mass distribution within the trailer (and thus the same polar moment of inertia, for those into physics)
...resulting in
  • a longer lever for the tug to control the trailer mass
  • a longer lever and better lever ratio for the the trailer tires to control the trailer mass
Has anyone tried this? Does my reasoning make sense to anyone?
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Old 07-16-2007, 01:26 PM   #8
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Actually, Brian, an "extension" of that nature was the very first thing I did in response to an early sway incident in a former life in the early '70s.

I was pulling a dual axle boat trailer, loaded with an 18' Larson w/4cyl i/o power (so it had a car engine and outdrive sticking out there behind the trailer axles). I shudder to think how much that rig probably weighed (never weighed it), it didn't have brakes and I was pulling it with a '69 Dodge Dart with Slant Six engine, single carb. I only pulled it in Florida, but when I crossed the Sunshine Skyway Bridge, I could use the temperature gauge as an altimeter!

I was crossing a rough patch of road on a two-lane bridge when the trailer started swaying, between the bridge walls. Luckily, no one was coming in the other lane, so I was able to get it under control without incident except the damage to my seat covers...

I thought about it a bit, looked at the trailer and realized the axles were mounted to a piece of angle iron, which was then bolted to the frame and there was a series of adjacent bolt holes.

I dropped the boat and moved the entire axle assembly back a few inches and also moved some of the boat hardware back, but not as far, so the boat was back further on the frame, and the tongue was now longer. I believe the net effect was a slightly higher tongue weight (never weighed it, either before or after) because of the additional frame in front, but am not sure because I also moved the boat. At any rate, the effect on sway was an improvement because the lever was longer, which was my objective.

The single Scamp tongue extension I saw was a 'T', made of frame material and welded under the existing A-frame and tongue. I don't recall if the T went back beyond the weak spot or not, but it wouldn't be hard to do that, which would result in a stronger-than-stock frame.
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Old 07-16-2007, 01:32 PM   #9
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Regarding levers and proportion and tongue weight, it continues to bother me that GB standards for tongue weights are a lower percentage of trailer weight than the US standards.

I'm wondering if the US tongues are typically longer than the GB tongues, thereby needing a greater percentage to come up with the right weight distribution... In fact, I wonder if the tongue weight percentage is a bit high for some eggs, compared to the tongue lengths on some of the Bulgemobiles... I know my Scamp 13' has a longer tongue out in the open than my somewhat similar Jayco 16' did.
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Old 07-16-2007, 03:48 PM   #10
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Electric brakes here vs. surge brakes there is another difference - not sure how that ties in with percentage of tongue weight, if it does at all.

The Euro conventions certainly do lean toward heavier trailer weights with smaller and less powerful tow vehicles and less tongue weight by proportion as compared to what you'd normally see in North America.

They DO get diesels, though, with more torque than gassers, so maybe that helps to offset the power deficiency somewhat.
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Old 07-16-2007, 07:00 PM   #11
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Electric brakes here vs. surge brakes there is another difference - not sure how that ties in with percentage of tongue weight, if it does at all.

The Euro conventions certainly do lean toward heavier trailer weights with smaller and less powerful tow vehicles and less tongue weight by proportion as compared to what you'd normally see in North America.

They DO get diesels, though, with more torque than gassers, so maybe that helps to offset the power deficiency somewhat.

I think I can figure out a general rule... I'll post the engineering results when I am done.

thanks


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Old 07-16-2007, 07:47 PM   #12
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...They DO get diesels, though, with more torque than gassers, so maybe that helps to offset the power deficiency somewhat.
While the current Euro trend is toward diesels, I think the relationship of trailer weight to tug size and the weight distrubtion of the trailers were both established before the current diesel fashion. Euro members, pitch in here and correct me if I'm mistaken.

I'm not convinced that the Europeans really have a power deficiency compared to North Americans - I think they're just working at a smaller scale. Travel trailers here are quite routinely 8 feet wide and 8000 lb loaded, pulled by vehicles with about 300 hp (and 6000 lb of their own weight); is that really any better than narrower trailers at 4000 lb towed by 150-hp sedans? On my most recent trip I found that none of the traditional big box trailer rigs were passing me up hills, and I'm towing a relatively big egg with my minivan.

I think Ron's original post addressed the key issue of proportions, and we might be surprised how big and even powerful our typical tugs are relative to our trailers.
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Old 07-16-2007, 07:56 PM   #13
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Electric brakes here vs. surge brakes there is another difference - not sure how that ties in with percentage of tongue weight, if it does at all.
I don't think the real difference is in the actuation method (electric versus cable or hydraulic), but instead is in the control method (surge versus whatever). In a surge (or overrun, to use the Euro term) braking system, the applied braking force is proportional to the deceleration and the trailer mass... it is inhernently proportional, and automatically tuned to changes in trailer load. The chassis builder can get it right, almost without regard to the body which is later added; our brakes are almost randomly fitted, then "tuned" by amatuers (us). While the same "surge" thing could be done electrically, it's not. I suspect that the surge setup improves the chance that the trailer is doing the right amount of braking.

Getting the right trailer braking should be more important when the low tongue weight means that the trailer isn't contributing much to the tug's available traction, and therefore how much the tug can brake for the trailer.

The right trailer braking also gets more important as the mass of the trailer goes up relative to the tug mass, since the tug becomes less able to handle stopping the trailer without help.
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Old 07-16-2007, 08:03 PM   #14
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When considering control, I keep coming back to the idea that distribution is critical. As Pete and others have repeatedly described, if two trailers have the same overall dimensions and axle-to-hitch weight distribution, whether the mass is concentrated near the middle or stuck out at the ends matters; similarly high versus low relative to track width matters. Any formula which does not consider this - the angular moments of inertia - cannot successfully consider dissimilar designs.

One more Euro note: it seems that the European trailers tend to have nothing but a gas bottle locker (propane tank box) on the tongue, and it is right up against the body. Significant masses do not seem to me mounted at the extreme ends of the trailer, so the coupler is at the end of a useful lever, not just stuck immediately in front of the front-most object (in my case, a pair of propane tanks barely behind the coupler)
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