What Causes Fishtailing?

[slowzuki]

If you have equalizing shackles, "raking" the trailer to the rear does not load the rear axle more.

 

[284 International]

Reg is right about the dynamics at work in a rear-loaded trailer. In airplanes tail-wheel airplanes are notorious for "ground-looping," where the aircraft rotates around the main landing gear in a spinning fashion. The center of gravity is behind the wheels, and wants to be in front, where the tires are being drug, rather than pushed. Darts are similar, too: if you throw one backwards, the heavy tip will rotate so it leads the resistance of the fins.

If you drive your car backward and steer, the center of gravity follows the steering tires and tends to exaggerate the movement, and will make it keep steering more in that direction. It's a positive feedback cycle, and is why rear-steering things are so squirrely.

The trailer wobbles when loaded to the rear for the same reasons. The center of gravity is behind the resistance of the tires, and has a pivot point at the hitch. It goes sideways, then force builds up in the springs, tires, and frame. If the tires don't skid, eventually those forces will bounce the trailer back the other direction, and the cycle starts over again.

This also explains why accelerating can sometimes stop the wobble, and braking usually makes it worse. Under acceleration the center of gravity will tend to be in line with the direction of pull, or straight. Under braking, the the center of gravity tries really hard to get in front of the resistance to its momentum.

That's oversimplified, but close enough to the principles at work.

 

[Rhon]

Our issue was a tilt trailer. 96 Dodge 3/4 ton extended cab pulling a 16' tilt car hauler with about 3 ton of hay. We were coming down the on ramp to the freeway when it started swaying and by the time everything was done we had half the load blocking both eastbound lanes of I90. That was the last time he used a tilt trailer.

I think my BIL's drivers seat still has a crease down the middle of it.

 

[Reg]

If you have equalizing shackles, "raking" the trailer to the rear does not load the rear axle more.

You would hope so and might even think so.
Unfortunately, as with so many engineered things they're a "not quite" device, e.g. "Amplifiers oscillate and Oscillators don't"

Equalizers don't.

They come close, but they don't quite "equalize".

The "proof" of this of course is our friendly weigh station.
Weigh it axle by axle, then raise the ball an inch (assuming you have a hitch head with multiple positions) weigh it axle by axle again, including the truck.
Further proof, if needed, can be found on the highway if you make the suggested rest stops and circle checks.
The rear tire SHOULD BE detectably warmer than the front one on the same side.
Try it some time as soon as you stop, not after the break.

BTW, the position of the C of G can be easily calculated from the tongue load.
If the tongue load is 10% then the C of G is 90% of the way back from the coupler.
15% it is 85% of the way back, etc.
Run a tape back from the coupler to see where that is, ahead of the front axle is the usual answer.

 

[Reg]

Reg is right about the dynamics at work in a rear-loaded trailer. In airplanes tail-wheel airplanes are notorious for "ground-looping," where the aircraft rotates around the main landing gear in a spinning fashion. The center of gravity is behind the wheels, and wants to be in front, where the tires are being drug, rather than pushed. Darts are similar, too: if you throw one backwards, the heavy tip will rotate so it leads the resistance of the fins.

If you drive your car backward and steer, the center of gravity follows the steering tires and tends to exaggerate the movement, and will make it keep steering more in that direction. It's a positive feedback cycle, and is why rear-steering things are so squirrely.

The trailer wobbles when loaded to the rear for the same reasons. The center of gravity is behind the resistance of the tires, and has a pivot point at the hitch. It goes sideways, then force builds up in the springs, tires, and frame. If the tires don't skid, eventually those forces will bounce the trailer back the other direction, and the cycle starts over again.

This also explains why accelerating can sometimes stop the wobble, and braking usually makes it worse. Under acceleration the center of gravity will tend to be in line with the direction of pull, or straight. Under braking, the the center of gravity tries really hard to get in front of the resistance to its momentum.

That's oversimplified, but close enough to the principles at work.

Funny thing about darts - this is kinda sick, but we were kids.
There are few things more scary than having someone throw a dart at your face and you KNOW it is coming because they threatened to and did the three forearm aiming swings.

It takes a while, but somewhere around mid flight you realize that it is ONLY the flight that has been thrown - and it is DOUBLY SCARY if they throw it backwards, you just get a much bigger and more impressive visual of it as it turns around.

The whole thing is a "deer in the headlights" experience.
You KNOW it could take your eye out, you just can NOT cover your face or look away.

Just don't pass this sick trick onto the younger generation (-:

 

[284 International]

Reg, that's a funny story about darts. We were lucky enough to not every play that game. I can only imagine the laughter we'd have had, until mom caught up to us with the belt....:laughing:

Your advice to keep the load on the rear axle but with a forward CG is exactly right, and put into words something I "knew" from experience but never had to articulate. Thanks.

 

[groundcover]

The '11 super duty has a brake controller that automatically activates to stop the 'wag' they had some utube videos,pretty impressive .

Talk about too much weight on the rear, ( no not the 'ol lady) I was with a guy one time and we were unloading a full size TLB on a gravel road that was quite steep. Guess we were in too much of a hurry to chock the wheels, boy did it get exciting when the truck tires came off the ground ! He kept with it and got off the trailer so everything got stopped before any injury or death !

 

[Reg]

Reg, that's a funny story about darts. We were lucky enough to not every play that game. I can only imagine the laughter we'd have had, until mom caught up to us with the belt....:laughing:

Your advice to keep the load on the rear axle but with a forward CG is exactly right, and put into words something I "knew" from experience but never had to articulate. Thanks.

Here's the "short" version;
Tongue load represents where the C of G is relative to the axles and hitch ball.
With very little tongue load the C of G is so far back that the trailer represents a teeter-totter, one that could go either way (unstable).
With increasing tongue load the C of G is farther forwards and is supported by the axles and hitch more like a bridge (more stable).
By maintaining proper ball height the "farther back bridge support" makes that bridge about 3ft longer.

Ya kinda hafta draw it out (-:

 

[slowzuki]

Just to play devils advocate, when on the scale, raise it to 6 inches then lower it back to 1 inch. You'll find trailer springs and shackles have a lot of friction in them which is why you don't need shocks. This friction will fool you into thinking you've loaded the rear axle more when you measure it your way.

To be fair, many equalizers are improperly installed so they will bias, or have very short shackles which will cause some bias when raked. These tend to cause problems like shackle flipping, binding, and rough ride too.

You would hope so and might even think so.
Unfortunately, as with so many engineered things they're a "not quite" device, e.g. "Amplifiers oscillate and Oscillators don't"

Equalizers don't.

They come close, but they don't quite "equalize".

The "proof" of this of course is our friendly weigh station.
Weigh it axle by axle, then raise the ball an inch (assuming you have a hitch head with multiple positions) weigh it axle by axle again, including the truck.
Further proof, if needed, can be found on the highway if you make the suggested rest stops and circle checks.
The rear tire SHOULD BE detectably warmer than the front one on the same side.
Try it some time as soon as you stop, not after the break.

BTW, the position of the C of G can be easily calculated from the tongue load.
If the tongue load is 10% then the C of G is 90% of the way back from the coupler.
15% it is 85% of the way back, etc.
Run a tape back from the coupler to see where that is, ahead of the front axle is the usual answer.

 

[Reg]

Just to play devils advocate, when on the scale, raise it to 6 inches then lower it back to 1 inch. You'll find trailer springs and shackles have a lot of friction in them which is why you don't need shocks. This friction will fool you into thinking you've loaded the rear axle more when you measure it your way.

To be fair, many equalizers are improperly installed so they will bias, or have very short shackles which will cause some bias when raked. These tend to cause problems like shackle flipping, binding, and rough ride too.

I agree, that confusion would be likely if you were to do it statically while still on the scale - it is a mechanical analogue of hysteresis.
{Although at one time (the days of core memories) hysteresis was sometimes explained in similar mechanical terms (-:}

When doing this I have always had to "GET THE H311 OFF THE SCALE TO DO THAT !"
Then come back and roll on again for an axle by axle weighing.
Just turning around in the yard takes up the "sticktion" (sticking friction) that you refer to.
The slightly warmer rear tires are the "running proof" when on the road.
I stop every 3 hours and touch every tire.
Overly fussy ? Maybe, but I do know when a slow leak has started, a wheel bearing is unusually warm, a brake is dragging, or there is something else wrong in that area.
i.e. I have a very good idea of what is "normal" (for MY rig) at the wheel level.