I have a 2008 F-450, 40K miles. It never had a goose neck hitch until I bought it 3 years ago at 32K miles, so it was unlikely to have had much prior abuse.

I use it to haul a flat bed and a dump trailer, both of which get the total package load up to 30K lbs. Under a light load, I never have breaking problems. Under a heavy load, the truck will often start bucking when I hard break, bouncing the truck up and down about once / second or two. It bucks so hard that the only thing that keeps me from hitting the roof is my seat belt. The only way to stop the bucking is to let off the breaks, but I'm hard breaking for a reason at that moment and it is VERY scary and dangerous to stop breaking. Also, with the bucking, the breaks are not working even close to full effectiveness.

Two trailer mechanics and several other people have told me to dial the trailer breaks down. It has not helped. I even tried putting the trailer breaks at the lowest setting and I could still reproduce the problem.

When it starts bucking, I let off the breaks and gently reapply. This helps. Sometimes the bucking stops, sometimes it restarts.

Anyone experienced this? Any advice other than to park the rig until I get it figured out?

 

It does this with both trailers when braking hard, but not otherwise correct? Your brake controller is factory OEM or aftermarket?

Steve

 

Factory controller. Problem is with both trailers. Although it happens more often with the dump trailer. The worst time was with the flat bed and I was over loaded at about 35k lbs.

 

I was trying to figure out if there was an auto-reset breaker anywhere in the circuit that was cycling the trailer brakes. I don't know why that would be with the factory set-up though. Do you have any sense the trailer brakes are cycling off and on?

 

I dont think it is in the trailer but rather the truck. But I'm no mechanic and could be wrong. Maybe I should just unplug the trailer breaks and see if I can reproduce the problem.

 

That would be a reasonable approach, but do it in a safe place. It is an interesting problem. It sounds like the brakes are rapidly grabbing and releasing and I have been trying to think of what could cause that. I don't know what else could upset the truck suspension to such a degree.

If you had an emulator box that plugs into the truck and mimics the trailer brakes, you could actually see if anything strange is happening there.

 

There is another thread around here about a shimmy when braking with a trailer. You might want to look that up. Unfortunately I don't remember which subforum it was in.

Can you feel the bucking in the steering wheel? If so, then it probably is warped front rotors. But it could also be worn steering components.

If there is no bucking in the steering wheel then it could be the rear brakes.

What happens if you manually engage the trailer brakes without pressing the truck's brakes? If it is smooth then the problem seems to point to the truck.

I'm rather **** about my brakes. I've never had any of my vehicles get to the point where the brakes were squealing due to worn pads. I replace them long before that usually because my wife has cooked them.

 

Have you ever serviced the slide pins in the calipers, they should slide with no resistance. If they stick the brakes don't release all the way causing heat buildup warping the rotors, when they cool off because the are so thick they will go back to normal. I had this happen to my 2000 F350 DRW and it felt like it was going to tear the truck apart.

Denny

 

I think the first step here would be to determine whether the trailers are at fault. If this problem can not be duplicated unless towing under hard braking, it seems the trailer or the brake feed to the trailer is at fault.

Steve

 

thanks for the responses and assistance.

No shimmy thru the steering when this problem occurs. I doubt it is an over heating problem because the worst time ever was when I was a couple of tons over loaded and hadn't gotten more than a few miles from the pick up point. But that does make me think of a possible issue. I had too much tongue weight that day. The rear of the truck was sagging a bit and it takes a lot to make that sag. That was when I experienced the worst bucking.

I had a fairly light load on today, maybe total package at about 25K lbs of which about 8K lbs was horse manure. I really had to work at slamming the breaks to get any bucking and I didn't get much. So the heavier the load, the worse the problem.

The dump trailer has very poor breaks. I dialed them to zero and then to max, each time I applied trailer breaks only and got no response as you would expect at zero and smooth breaking at 10. I disconnected the trailer electric, tried a hard break, and experienced no bucking problem.

I've had the truck breaks checked, but I'll do so again, armed with this additional info. I have to find the cause because it has gotten worse and when I have to load my 19K lb backhoe, 36K total, it could be a real problem.

When I was almost home today, driving a big gold truck with a big rust red triple axle dumper in tow, a motorcycle pulled out in front of me. He was on a cross street on my right and turning left across my lane. When he realized his mistake, he stopped half way into my lane. Fortunately there was no oncoming traffic so I dodged into the opposing lane. I laid on the horn, locked the breaks briefly till I was sure he would stop, and I swerved around him while he presumably shat his pants. If he had gone under my truck and trailer, that is a lot of mass and tires to grind a human into the pavement.

 

So this is a suspension issue then. Sounds like that heavy load is setting up a harmonic. How are your shocks?

Steve

 

I'm guessing that there may be a sweet spot on your brake controller that the bucking won't happen on. But like Steve said above, something else is FUBAR.

 

I'm having the breaks and shocks checked today.

Question: On a goose neck hitch, where should the ball be in relation to the rear axle, directly over, slightly to the rear, or slightly to the front? My backyard mechanic friend is wondering if it was installed wrong and I have never noticed a problem except under heavy loads.

 

Goosenecks and 5th wheels should be ahead of the rear axle. On the trailers you are pulling, are they leaf springs or torsion suspensions?

With leaf springs, it might be time for new bolts and bushings.

Check tire pressure too.

 

Got my truck back from the mechanic. Brakes on truck and trailer are good. No issues with suspension. Dump trailer has had everything rebuilt in the last couple of months including brakes, springs, and all the wear pieces that I can't remember the names of. Truck had everything from shocks to springs checked. Gooseneck ball is slightly ahead of rear axle.

As RV tech originally suspected, it is the brake controller. It works fine when operated by thumb, but when you press the brake pedal, it is not sending a reliable signal to the trailer brakes.

New factory controller is $450 + $100 to program it (don't know what that means but the Ford dealer said it was necessary).

Thanks for everyone's help.