A trailer doesn't react to a tractor's backing up until 10 feet of travel

[DaveC426913]

...the trailer will react to the driver's turning the steering wheel (during straight line backing) IMMEDIATELY. Maybe the trailer's reacting to the driver's turning the steering wheel might be too subtle to detect until the tractor has backed ten feet, but it seems to me that the trailer's reacting to the driver's turning the steering wheel should happen when the tractor has moved just one foot.
...
Are the instructors at my truck driving school and at my trucking company correct that the trailer will not react to the driver's turning the streering wheel until the tractor has backed 10 feet?

It is quite likely that the instructors are talking in practical terms. i.e. what is the minium a driver needs to know to take the correct action.Afterall, there's a lot to learn.

If drivers were told that - technically - it reacts right away, that could very likely lead them to expect a visible change right way, and - if they don't see it - they may over-correct.

 

[EnumaElish]

The 10 foot rule is bogus. The trailer will react to the driver's turning the steering wheel instantly when the tractor backs up.

Everyone I've consulted on this agree it's bogus. Railroad locomotives back up while pushing RR cars. Sometimes they do that on a straight patch, and sometimes to push the car(s) through a switch, at an angle. If this rule were true, there would have been RR lore about attached car(s) not reacting before the engine has backed up 10 feet. But AFAIK there's no such belief among the railroad folk. Not one that google can find anyway. So why do people keep repeating it? Could the 10-ft rule meant to be the distance before the trailer will jackknife (unless the driver corrects), rather than the distance for the trailer to react? E. g. a correction rule? Perhaps that's how it started, but over time its meaning and intent was obfuscated?

 

[sophiecentaur]

Railroad locomotives back up while pushing RR cars.

Once you involve rails in the situation, everything is totally different. The path followed is totally governed by the rails and there is no 'steering' involved. There are, of course, minimum radii for railroad curves but that is a different matter, I think.

 

[EnumaElish]

Once you involve rails in the situation, everything is totally different. The path followed is totally governed by the rails and there is no 'steering' involved. There are, of course, minimum radii for railroad curves but that is a different matter, I think.

I agree with you on steering. But I do not see why the action-reaction lag should be any different. Actually your observation is very helpful to nudge this thread in the right direction. If the 10-ft rule does not apply to RR engines and RR cars then the only explanation for it to apply to tractors and trailers ("TT") must be because of steering. If steering cannot explain the rule for TT then the rule must be bogus.

 

[sophiecentaur]

I agree with you on steering. But I do not see why the action-reaction lag should be any different. Actually your observation is very helpful to nudge this thread in the right direction. If the 10-ft rule does not apply to RR engines and RR cars then the only explanation for it to apply to tractors and trailers ("TT") must be because of steering. If steering cannot explain the rule for TT then the rule must be bogus.

As far as I can see, the rule is followed by most TT combinations because the other dimensions are much the same, not surprisingly because they are probably near optimal. Overhang, wheelbases and positions of axles (and even the limits of the tractor steering, probably) will be what governs the locus of all the wheels.

 

[256bits]

As far as I can see, the rule is followed by most TT combinations because the other dimensions are much the same, not surprisingly because they are probably near optimal. Overhang, wheelbases and positions of axles (and even the limits of the tractor steering, probably) will be what governs the locus of all the wheels.

Three types of tractors, each with its own version of the rule guidline ( for new drivers )
Sleeper - the longest tractor wheelbase - most dificult to back up with.
Regular - for day trips - medium whellbase - what most one will see on the road.
Shuttle - shortest wheelbase - easy connect diconnect - open view - used around yards to manipulate the trailers around - excessively productive compared to the other types.

 

[256bits]

Three types of tractors, each with its own version of the rule guidline ( for new drivers )
Sleeper - the longest tractor wheelbase - most dificult to back up with.
Regular - for day trips - medium whellbase - what most one will see on the road.
Shuttle - shortest wheelbase - easy connect diconnect - open view - used around yards to manipulate the trailers around - excessively productive compared to the other types.

PS. trailers. 53 foot.
Rear three axle and two axle.
Rear axle can be on a dolly that can be moved ( just do the release and push the trailer box back or forth on the locked rear axles ) from most rear to about 1/3 from the rear, depending upon weight distribution.
Dolley position will affect the "ten foot rule" and driver will have to adjust.

 

[EnumaElish]

As far as I can see, the rule is followed by most TT combinations

Dolley position will affect the "ten foot rule" and driver will have to adjust.

Are you all referring to a rule in the sense of a physics theorem, or as folklore regardless of what physics says about it?

 

[sophiecentaur]

a rule in the sense of a physics theorem,

This isn't 'Physics'. It's Maths (Geometry), at least to a first approximation. You would get similar results if you treat everything as 'idea' and with no slippage or drag. The patterns will be derived from combinations of cycloid and tractrix type curves.

 

[EnumaElish]

This isn't 'Physics'. It's Maths (Geometry), at least to a first approximation. You would get similar results if you treat everything as 'idea' and with no slippage or drag. The patterns will be derived from combinations of cycloid and tractrix type curves.

Great. Now read the OP. Either point to a proof (sketchy is okay) or abandon all pretense.

I'll go first. My railroad analogy was posted under the false presumption that the OP was about backing up. I've gone back and reviewed it. I realize that it's about a change of direction while backing up. So my RR analogy totally misses the point. Thanks for pointing that out.

 

[jim hardy]

The angle between tractor's axis and trailer's axis drives itself toward zero when moving forward
and away from zero when moving backward.

When backing , the rate of angle increase per foot of travel depends on the angle. At zero angle it's zero. At any other angle it increases , proportional to magnitude of angle.
Anything whose rate of change is proportional to its magnitude is suffering exponential growth.

When moving forward , the rate of angle decrease per foot of travel depends on the angle. At zero angle it's zero. At any other angle it decreases, proportional to magnitude of angle. That's exponential decay instead of growth.

A perfectly aligned tractor and trailer with angle = 0 would be able to back clear across the Seven Mile Bridge were it not for that dogleg.
.

Exponential growth begins imperceptibly.
So while it might take something like ten feet for the rate of change of angle to become perceptible, rest assured it started much earlier.
Forward motion is the opposite - angle approaches zero asymptotically.


I recently moved the axle of my 12 foot long utility trailer eight inches rearward for two reasons.
1) make it easier to back up. It was overly prone to jacknife , as all short trailers are. Look at the geometry above.
2) move axle behind center of gravity to stop divergent side to side oscillations on the highway.
That dramatically improved its "user friendliness" on both counts.

old jim

 

[sophiecentaur]

Great. Now read the OP. Either point to a proof (sketchy is okay) or abandon all pretense.

I am not after a "proof" because that would certainly be very complicated but the basic behaviour of back and front wheels on a car will follow the same sort of argument as is used in the derivation of the tractrix curve. (with modifications that make it more or less impossible for me).
The following is "sketchy" but I think it heads on the right direction. The behaviour of the tractor when backing up and turning is, I reckon, based on the tractrix curve - see this link and the animation half way down (pushing part). That curve shows what would happen if the front wheels were steered to follow a straight path backwards but when there has been a slight deviation (i.e. it chooses between right or left) It will produce a jackknife, eventually and the curvature increases rapidly towards that condition. But the driver will prevent this condition by chasing a better path. This involves keeping to the same point on a newly positioned tractrix with a different pair of exes. It seems that for all steered vehicles, the tractrix curves are dependent only on the wheelbase and the direction of the steering motion relative to the axis. he shorter the vehicle, the sooner it will turn by a specified angle. The long trailer must move back further than the tractor, to achieve a given angle and, of course, the pivot will always be on the opposite side of the axis from the tractor steered wheels. So the reasoning in the OP would seem to be correct when it says that the reverse steering for the trailer, is always in the other direction but much smaller with a long trailer. Looking at the animation, the deviation from straight is very little until the movement of the driven end is something like the length of the string / bar / wheelbase. At this point the trailer will not deviate by much (tractrix applies here too, I think) - just enough to be noticed by the driver. The lateral movement of the pivot divided by the trailer length would give you the angle
The ten foot 'rule' is obviously very crude but it was probably thought up when rigs became all of fairly similar proportions (related to a Standard Container, these days) and the resulting error is actually not very great.
I know it would never apply to the situation of a saloon car and a short, general purpose trailer that's used for trips to the local dump or for camping because the trailer length is about half the wheelbase of the vehicle. It will jackknife in about one metre of bad reversing.
Edit: The 10 ft rule is only approximate but it must relate to the wheelbase of the tractor (and they will all be pretty similar). Using a specific number of turn of the wheels may be a better rule because (I imagine) the bigger tractors will also have bigger wheels (?) so there will be some compensating factor doing it this way.

 

[EnumaElish]

I think the rule can be seen in the attached video, mainly between 1:46 and 1:48 and up to 1:52. At 1:46 the trailer is halfway jackknifed. As the tractor backs up, the trailer pretty much pivots at the same spot. At 1:49 it starts to move back. So my thinking is, an actual tractor would have moved about 10 feet between 1:46 and 1:49. Hence the "10-foot rule."

http://www.loveyourrv.com/back-fifth-wheel-trailer/

P.S. Had the trailer fully jackknifed with a 90-degree angle, no amount of backing up could have pushed it back; it would merely pivot on its wheels. I suppose in that case the applicable rule would be the "no good rule" instead of the "ten foot rule." But then, a 90-degree angle is probably not part of an optimal trajectory when backing up a trailer.

P.P.S. Just realized @OCR had already explained this. And with a real-life video. Minus above hilarity.

 

[jim hardy]

Can a Tesla do this ?

http://www.usatoday.com/story/money/cars/2015/05/21/ford-f-150-backup-assist/27705471/

 

[OCR]

But then, a 90-degree angle is probably not part of an optimal trajectory when backing up a trailer.

In certain circumstances a 90°, or even more, angle can be useful to get the tractor in a better position for making pull-ups, for instance... I've used the technique when forced to make a u-turn on narrow roads.

I have even unloaded our dozer, and "dozed" the back end of our lowboy into a better position... that might not work too good in an RV park, though... lol

Just a couple pictures of dozer on lowboy... I was in the Big Horns, heading home from a fire.

 

[sophiecentaur]

Shame there's only a 'like' available for the above post. It would be nice to put in a 'heart'.

 

[DaveC426913]

New Series: The Littlest Dozer. (spin off from The Littlest Hobo*)

Goes across country, town to town, doing good deeds and helpin' folk outta trouble.

​

* Shoot it's a Canadian show. You prolly won't get the reference.

 

[jim hardy]

"The difference between men and boys is the size of their toys."

I just got a case of dozer envy.

 

[OCR]

* Shoot it's a Canadian show. You prolly won't get the reference.

Lol... well, I did have to look that up...

But it's not a German Shepherd ...

It's a Deere ... you know ?

"Nothing Runs Like a Deere"​

... better get back on track (s) ... lol