Trailer whip for nerds

[jss06]

I've been trying to picture this formula. When you consider an 18 wheeler as an example, it doesn't work since the trailer is so much longer than the drive section. Also the CG will shift further forward as the cargo is unloaded, yet they are still stable enough to take onto the highway.

Because the Cabs carry the trailer load over the rear wheels instead of hanging off the back of the chasis the forces react entirely differently and is inherenlty more stable.

The same basic principle applies to goosnecks and 5th wheel trailers for pickups.

This also allows a much higher tongue weight then a bumper pull. My truck is rated at 3300 lbs. of tongue weight for a goose/5th wheel. This is limited by the physical capacity of the truck itself. Idealy you want to carry 15% t0 25% of the trailer weight on the tongue on these trailers.

All that aside, If you properly load and balance the trailer then you should never experience whipping except in severe side winds.

[swissarmygirl]

ugh...my head hurts...

funny...I was just thinking the same thing.

[brian_bp]

I spent some time this summer with some really great guys that happened to be engineers...

As Andrew suggested, "engineer" is too broad a term; there are disciplines within engineering, and as well a lot of people who use the title who are not engineers at all. A qualified engineer should not present this rule-of-thumb even if vehicle dynamics were outside of his specialty, because it would not be in the best interest of the public... there's a professional code of ethics for engineering.

I don't doubt that the intentions of Gregg's friends were good, but the effect of bad advice is still bad. Associating bad advice with "engineers" gives it credence which it should not have, perhaps causing people to fail to validate the information the way they should. When someone pulls a thought out the air and passes it on, they shouldn't label it as professional advice; just like a doctor guessing wildly at some medical situation or a lawyer tossing out a possible legal interpretation, they should include a "I really don't know but here's a possibility" sort of qualification.

...I thought I'd share it just because it is a little different than the seat of the pants way I usually work!

Actually, I would agree with the comments about matching what is seen on the road, and say that this rule-of-thumb is entirely "seat of the pants", rather than being based on any solid understanding of the principles.

Like any "urban myth" or "old wives tale", this one has some little grains of truth behind it: a longer overall tow vehicle length probably means a longer wheelbase (which is relevant), and if the C of G is further forward in the trailer the tongue weight will be higher (all else being constant), causing more load transfer from the tug's front axle to the rear, which can cause handling problems when overdone.

The overall effect - obtained by ignoring the underlying phyics - is complete bunk (in my opinion, of course). The most unfortunate part is that following this rule-of-thumb would encourage people with short tugs to keep the C of G close to the axle, which is the opposite of what is really desirable. As bobhenry pointed out, the length from the ball to the trailer axle is important to stability, and is entirely ignored in this formula.

As is often the case, taking the proposal to an extreme shows that it is unworkable: in this case, balancing the trailer such that the centre of mass (or gravity) is exactly over the axle leads to zero hitch/tongue weight and guaranteed instability, yet the formula says that a tug of even zero length would be adequate.

I think an underlying (and invalid) assumption is that all trailers are suitably designed and stable (with the right tug), so the formula is to be used to pick the minimum tug size, not to adjust the trailer design.

[brian_bp]

Here's another way to spot pseudo-technical bunk: wrong units of measure.

The distance from the hitch ball to the front of the tow vehicle should be equal to, or greater than, the square of the distance from the trailer center of gravity (CG) to the center of the trailer axel (CA).

The problem here is that the physical measurements are distances, and the formula compares a distance (units of length) to the square of a distance (units of length squared, or of area).

As an example, if the distance from the trailer center of gravity to the center of the trailer axel is 12 inches, the distance from the hitch ball to the front of the tow vehicle should be 12 feet (12"x12" 144" or 12 feet).

Really? In this example, the CG-CA distance is one foot, and the square of one is one, so the length of the tow vehicle only needs to be one foot! If changing the size of the units, such as between feet and inches or between feet/inches and metric units (metres), makes the formula not work anymore, then it was wrong in the first place.

Any equation describing a physical situation, so it has units of measure (feet, pounds, whatever...) associated with the numbers will balance. If you combine all of the units on the left side of the equals sign, they will be equivalent to all of the units on the right side of the equals sign. That's equivalent, not identical... a volt times an amp is equivalent to watt, but a foot times a foot is not equivalent to just a foot. Sometimes that includes some constants, which have units of their own. If the two sides don't match, then something was missed, and there is an error in the equation.

[greg755]

OK my head has exploded....

Why don't you engineers come up with a tongue that when traveling down the road at 20 miles plus, it is rigid and cant swivel on the ball, like it is an actual extension of the tow vehicle frame, then when you need to back it up or turn, you flip a switch, it cuts the locking mechanism loose, and the trailer can then swivel.

You should be able to work this out for small trailers.

There that should keep ya busy for awhile, now go play.

[brian_bp]

... a tongue that when traveling down the road at 20 miles plus, it is rigid and cant swivel on the ball, like it is an actual extension of the tow vehicle frame, then when you need to back it up or turn, you flip a switch, it cuts the locking mechanism loose, and the trailer can then swivel...

I don't think that a fixed transition speed would work: what happens when I go around a tight freeway ramp or highway curve at only 30 mph, and the trailer can't follow the tug properly?

A more workable solution is to realize that the tug-trailer connection is a joint which needs to be controlled, and rather than abruptly locking it solid, just add damping to control excessive motion. That's what the friction bar devices do poorly, and it could be done well with a hydraulic shock absorber, like a steering damper. That would cost a few dollars more than the friction devices, and trailer stuff is generally cheap, so it is not done.

[Steve_Cox]

I spent some time this summer with some really great guys that happened to be engineers.

Greg,

Hang in there

[Frog]

Two Grand:

A few things might want to try before lengthening the tow bar on your vehicle could save some welding. Forgive me if you've already tried these.

(1) Before doing anything else, I'd add about 100 lbs or so to the front of the trailer inside then try it on the road. As you probably know a lot of trailer sway is caused by inadequate hitch weight.

(2) If the hitch is elevated when hooked to the tow vehicle instead of level it shifts the center of gravity away from the hitch. It will lighten the tongue weight which could cause a sway problem. I found this out when I was loading a small utility trailer with a 750 Honda and weighing the tongue. When the tongue was down on the ground on top of the scale it weighed more than when it was level. When it was elevated a foot or more above level it weighed even less on the scale.

(3) Check that the tires are of equal pressure and are wearing evenly. A defective tire could also cause a high speed sway problem.

(4) Make certain the axle is square to the trailer frame and not bent.

(5) Make certain that there is not a lot of side play at the hitch.

It's even possible that the areodynamics between the tow vehicle and the trailer change the loading on the front and/or rear of the trailer decreasing tongue weight.

Just some thoughts. Good luck on solving the problem.