Prancing Peterbilt.

[showcrop]

Our Fire Department's new 3,000 gal. tanker is on a Peterbilt Chassis. It has a disturbing behavior that we are having problems getting resolved. When under light acceleration at around 35 MPH, if you hit a bump it will start a jumping movement that seems to have a kind of forward backward motion to it. It makes me think of how older loader backhoes would get to pitching forward and back on the road or how a tractor with a heavy drawbar load like a manure tanker could get to "galloping" It has done it to me twice and has done it to others too. You have to hit the brakes and slow right down to stop it. The truck has some electronics that sense motion, and intercede to prevent a crash. If you are rounding a corner too fast you loose power until you slow down a little. Peterbilt did some front to back leveling on it and sent it to the tire store and got the tires checked out, but no resolution yet. They had it for four days this week. Any ideas?

 

[TGP]

Tank have baffles in it?
Maybe not enough?

Chassis wheelbase the correct length for the tank?
Mounting a tank on too short a WB with a overhang in the rear would cause your problem, then the trucks electronics could be
Accelerating it.

Lets us know if it gets corrected.
Tom

 

[shaun wallace]

How many baffles in tank? Or too big holes in them.

 

[Mark-Ia]

Does the 3000 gallon tank have a baffle in it?

 

[grizz02]

its called sloshing ,fill it to the top of the tank problem solved

 

[moresmoke]

What sort of suspension on the rear? Air or spring?

 

[Adirondack case guy]

What does the tank have in it for baffiling. Maybe the water is causing the problem.
Loren

 

[Steve@Advance]

Can the electronic motion/traction control be turned off for a test run?

 

[FBH44]

There's a phenom onboard ship called the Free Surface Effect, where about one of the most dangerous things onboard ship is a wide tank, stretching from port to starboard, half full. A WIDE tank needs to be either almost full or almost empty. In any event, it needs internal baffling to slow things down.

 

[deeretails]

Is the front suspension air? if it is and the tank isn't full or empty there will be a strange ride.

 

[showcrop]

I think that most of you guys are making a pretty strong accusation.

 

[Al Baker(pumpman)]

Sounds like what happens when you haul milk. Al

 

[Rollie NE PA]

Is it a elliptical or square/rectangular tank? I am wondering if the tank sub-frame is mounted too ridged to the truck frame.
WE have a 3800 gal elliptical fiberglass spun wound tank on a 2005 379 Pete. Some of the baffles broke and we replaced the tank but we did not have any driveability issues.

 

[OLWHDA]

Single compartment tanks will give you an interesting ride if half full or so . Liquid slopping around shakes around a lot, just ask a bulk milk
hauler, I hauled bulk milk back in the late 60?s with a single axle BINDER (IHC) kand haven?t forgotten about it.

 

[Rollie NE PA]

Is the motion sensor a factory Peterbilt option or was it installed by the apparatus manufacturer?

 

[Phil 25]

Sounds like the experience on here is hitting the results of free surface effect in a tank.
Your dept has bought a new truck and mounted a used tank on it? all done in house labor wise?
Should be safe to rule out that any frame was not required to shorten or lengthen. If it is a built unit from a emergency equipment supplier, then you have option to return it for service and repair to meet the local DOT licensing requirements?

Nothing wrong per say, just may need to have an "senior" person look at it from a distance for a different point of view?
Good that Peterbuilt is working to find the problem.
Keep at it, a small simple detail is the worst.

 

[showcrop]

(quoted from post at 04:39:44 10/14/18) Sounds like the experience on here is hitting the results of free surface effect in a tank.
Your dept has bought a new truck and mounted a used tank on it? all done in house labor wise?
Should be safe to rule out that any frame was not required to shorten or lengthen. If it is a built unit from a emergency equipment supplier, then you have option to return it for service and repair to meet the local DOT licensing requirements?

Nothing wrong per say, just may need to have an "senior" person look at it from a distance for a different point of view?
Good that Peterbuilt is working to find the problem.
Keep at it, a small simple detail is the worst.

The truck was built by Emergency 1 on the Peterbilt chassis. E-! has been working on the problem as well as Peterbilt. Pretty much everyone involved is confident that when the water is overflowing out of the top of the tank that the tank is full. There seems to be a consensus here on YT that there can be a lot room available down under the water that it is filled with for water to move side to side or front to back. None of us hicks here in NH and Maine are aware of any way for that to happen. The prancing motion is somewhere in the neighborhood of 2X per second.

 

[Jdr315]

May sound dumb but I have found some thing similar with our pumpers. When your driving and hit a bump or sometimes when it shift gears it makes your foot snap the throttle a little and it will get to bouncing. Usually ust let off the throttle for a second and hammer down. All of our pumpers are pierce Sabres with 1000 gallon tanks 450 cummins with allison trans. We just got a new Pierce 3000 gal tender on a freightliner chassis havent drivin it enough to see how it does. The new tender is a tandem axle freightliner with a DD13 and allison trans.

 

[Determined]

I think you are on the right track looking at the suspension and leveling.
Setting the ride height a shade higher or lower might be all it needs.

With the amount of weight on it when the tank full it is possible the suspension ride height system is over compensating and causing the same cycling effect you get on a governor when the droop and gain are out of adjustment.
Over sensitive and slow to respond starts a chain reaction.

Test driving it with less or no water in the tank may help rule out some components.

 

[ss55]

Is the water tank completely full when it gallops? If not, then extra baffles in the water tank could prevent a wave of water from bouncing around inside the tank. When you stop does the truck continue to rock? Also try higher tire air pressure or stiffer springs.

 

[Destroked 450]

As you know fire truck tanks are baffled to help reduce slosh, ours is baffled into 4' x 4' sections. I seriously doubt want your experiencing is caused by water slosh.
I have experienced the bucking motion you described in our old Mack tanker and in my F-450 when pulling my GN trailer over rough roads. What I found was when the truck hits a bump my foot was inadvertently moving the throttle pedal causing the engine to react to the changes in throttle input, each time the truck bucked my foot would move resulting in what I call throttle bounce. to correct it I'd take my foot off of the throttle or mash it to the floor to get the truck to settle down.
Next time you drive it and it starts bucking or prancing as you say let up on the throttle or mash in to the floor and see if it settles down.
If not and you still have to mash on the brakes I'd say some of the motion sensors are fighting each other.