Need help: Lengthening/shortening trailer tongue to adjust tracking radius

[Robert Bills]

While reviewing a previous thread concerning offroad trailering techniques, I found a comment by Martyn Davies that has me thinking about the length of the tongue on my '46 Bantam T3-C:

As far as tracking goes the AT tracks very close to the vehicles path, it has a lot to do with the distance from the coupler to the axle. . . .

When I designed my coupler, I didn't give much thought to the length of the tongue other than it needed to be long enough for my trailer tub to clear the rear corners of my CJ-7 at 90 degrees. It never dawned on me to determine how the distance between coupler and trailer axle would affect trailer tracking.

The axle width on the Bantam T3-C (also WWII MBT, T3 and later M100) is the same as the MB, GPW and early CJ jeeps. Tire size was the same on jeep and trailer, and these "short coupled" trailers followed the track of the early jeeps quite closely. However, I tow with an '83 CJ-7 with a wider track than my Bantam, and run 35x12.50/15 tires on the jeep and 700/16 military NDCC tires on the trailer (31"). The trailer tracks with a slightly tighter radius in turns, which hasn't been an issue thus far, but may well be in the future.

As you can see from the photo below, I set up my Bantam to interchange couplers and can easily change the distance from coupler to axle by using different length square tube. So my question is, what is the math I need to use to calculate the appropriate distance from coupler to trailer axle so that my Bantam will most closely track my CJ-7?

Thanks in advance for any help, and hopefully the formula won't require the calculus that I haven't used for the last 39 years. :yikes:

 

[ntsqd]

I think an easier way would be to find a flat dirt lot. Tow the trailer in a circle and then look at the tracks. Then you can see where the trailer really is tracking and whether you want/need to adjust it.
I'm thinking that this project may be in conflict with a desirable tongue length.

Strikes me that a pound of flour spread out on a parking lot would do the same thing as the flat dirt area......

 

[Robert Bills]

I think an easier way would be to find a flat dirt lot. Tow the trailer in a circle and then look at the tracks. Then you can see where the trailer really is tracking and whether you want/need to adjust it.
I'm thinking that this project may be in conflict with a desirable tongue length.

Strikes me that a pound of flour spread out on a parking lot would do the same thing as the flat dirt area......

I agree, a "real world" test would provide the same answer, but unfortuantely I live in a hilly area and can't think of any flat dirt lots anywhere close. Also, the ecology activists (and property managers) in my area would take a dim view of spreading any flour (as harmless as it might be). That's why I wanted to do some initial calculations from the comfort of my desk. (My first planned desert run isn't until March due to work commitments, my first opportunity for an actual test.)

I'm curious about your comment that the "calculated" optimum tongue length for true tracking might be in conflict with a desirable tongue length. 'Splain please.

Thanks.

Added on edit: This reminds me of a Boy Scout camporee competition when I was a kid. The assignment was to determine the height of a nearby dam. While the patrol in competition with us were busy calculating with the "handbook approved" stick and shadow method of computing height, and arguing amongst themselves over the correct result, we sat in the shade drinking Kool Aid. We won by turning in the exact height of the dam. Later, when asked how we had determined the height of the dam without doing anything, we revealed our secret: sending the fastest runner in our patrol to read the Army Corp of Engineers sign at the top of the dam, which gave it's height and length.

 

[njtaco]

Not to complicate things, but the distance from the tow vehicle axle to pivot point (ball, coupler, whatever) also affects tracking. The longer this distance (within reason) the closer the trailer will mimic the tow vehicle track by "leading" the trailer around.

Conversely, the closer the coupler is to the CJ's axle, the tighter the trailer will track to the inside of the CJ in a turn.

None of this materially affects how long the tongue needs to be to turn 90 degrees safely. It does affect it, just not much.

 

[Section 8]

What am I missing?

For the trailer wheels to follow in the tracks of the rear wheels:

you have to have the distance from the center of the ball (or prefered couple rotation) the same, to the rear axle of your tow vehicle as the distance from the ball to the trailor axle.

Throw a plumb on your axle and your hitch and find out how long you need to make your tongue to make the distance from the trailor axle to your couple the same as you just measured.

This will likely give you a tongue length that is too short, and won't be practical for doing anything behind your vehicle with the trailer coupled, and if I did it for my Jeep, would probably limit turning even with a tiny trailer.

EDIT: This is assuming identical track between tow vehicle and trailer. If you have a narrower trailer, you get a bonus.

 

[Robert Bills]

For the trailer wheels to follow in the tracks of the rear wheels you have to have the distance from the center of the ball (or prefered couple rotation) . . . to the rear axle of your tow vehicle [the same] as the distance from the ball to the trailor axle.

That is simple enough (even for my non-engineering background pea brain).

I always wondered why the military 1/4 ton trailer was designed to be so short-coupled behind the jeep of the day. (Made backing up a real *****, and jacknifing an issue.) But now that you mention it, the distance between the jeep rear axle to pintle was approximately the same as from the lunette to the trailer axle.

Let's complicate things a bit. How (if at all) does a difference in tow rig axle width and trailer axle width affect the result? Military jeeps and the early 1/4 ton trailers shared the same axle width (track), as do the Mutt and the M416. However, the axle widths of my CJ-7 and Bantam are different.

 

[njtaco]

What am I missing?

For the trailer wheels to follow in the tracks of the rear wheels:

you have to have the distance from the center of the ball (or prefered couple rotation) the same, to the rear axle of your tow vehicle as the distance from the ball to the trailor axle.

Makes sense...I've never heard it presented that way.

 

[ntsqd]

It does make sense, but it assumes that the rear axle of the tow rig is pointing at the center of the radius driven. That is certainly true of the trailer's axle, but may not be true of the tow rig's axle due to the steering linkage geometry and whether or not there is something other than an open diff in the tow rig's rear axle.

It was the assumption that Mr. Ackerman as well made in laying out his steering linkage geometry, so it may well be that the effects of the rear locker/LSD/etc. are minimal (not that he considered them, horse drawn wagons didn't have lockers to my knowledge ). What road racers have found is that moving the Ackerman geometry's intersection point forwards in the wheelbase increases turn-in, which leads me to suspect that the rear axle centerline is somewhere 'behind' the curve's center point. If this is correct, then a tongue longer than what you might otherwise expect would be needed to achieve the desired results.

Playing with track widths in my head what I see is a wider tow rig than trailer increases the window for the trailer to stay inside the track of the inside most tow rig track. A longer tongue still won't allow the trailer's inside tire to cross over inside of the inside two rig tire's track.

 

[Robert Bills]

. . . Playing with track widths in my head what I see is a wider tow rig than trailer increases the window for the trailer to stay inside the track of the inside most tow rig track. A longer tongue still won't allow the trailer's inside tire to cross over inside of the inside two rig tire's track.

Makes sense to me, although all the conceptualizing is turning my brain to mush.

 

[ntsqd]

Sometimes I feel like a certain character in the "Princess Bride"........

 

[Section 8]

If the rear tires are not slipping, then what the front is doing doesn't make any difference. What ever the front tires are doing, they are a coupled and will react together because they are in a ridged structure with the frame, causing no independent movement between the frame and rear axle.

Ackerman doesn't change any rear geometry. The bumper and the axle are always aligned (all things being equal), and no matter the steering input, as long as the rear tires are not slipping, the rear tires will follow the frame's lead, which averages any forces from the front wheels to form the resultant.

I would think that wheelbase would have an impact, but a longer wheel base vehicle would just make larger circles, just like a short wheelbase understeering, which is just like making a larger turn... perfect ackerman, or no ackerman. The front could have 4 steering wheels or 1, and the bumper will react the same way to any turning radius... as long as the rear tires are not slipping.

A locker probably would change things, but if the outside tire was tracking true, I would think that it would react like just a wider turn for the most part, because if your arc is disturbed, then the trailer arc is disturbed as well.

 

[ntsqd]

My mentioning of Ackerman was to illustrate that the rear axle centerline may not actually point at the center of the turn radius. If not going straight, a tire on a powered vehicle is scrubbing. How much is a debate that will ultimately lead nowhere useful to this discussion since it's not easily quantified.

That scrub could lead to the rear axle not pointing to the center of the turn, which in turn would cause the simple relationship of the equal distances of the ball from either axle to be skewed. My intuition is that since tire scrub acts to slow down the vehicle that the rear axle centerline would trail the turn radius centerpoint. Which would cause the ball to virtual "axle" distance to increase, and thereby make the ideal tongue longer than one might originally think.

All that being said, if the trailer behaves itself at highway speeds I would not alter the tongue length in pursuit of a marginal gain on the trail. Should the trailer 'cut' the corners a bit you'll adapt in driving line. Better to do that than to sacrifice the high speed stability and safety of the trailer.

 

[jatibb]

i have a yakima 1 wheel bike trailer that follows the path of the bike perfectly. i have gone full speed down twisty trails with it in tow and it does indeed follow the bike. my understatnding of the design is that if you measure from center of wheel on trailer to center of fr. wheel on vehicle, divide in half, that is where the pivot in the middle should be. probably impossible on a truck/trailer combo. i agree, if it tows well at 70 mph. and is close to your turning circle of the vehicle then i wouldnt worry,mess,... it up