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'11-Up General Questions General questions related to 2011-Up Super Duty trucks. If it doesn't fit the other categories, post it here. Gas engine discussion that pertains to all models is allowed. Specific gas engine questions should use the Gas Engines forum.

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Old 11-12-2010, 07:19 PM   #1 (permalink)
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Rear end bounces @ 40-45 MPH

I know some have already read this on FTE but some haven't.

Ever since I picked up my truck (350 6.7, c/c, srw, lb) I get a "bounce" in the back of the truck right at 40 mph and it stops right at 45 mph. My first thought was that I have a bad tire. I have 18" Continental A/S tires on it. The bounce gets progressively worse if I stay between 40-45 mph but clears right up if I slow down or speed up out of that speed range.
This week I took the truck in because of the change oil light came on with only ~1200 miles since oil change. I finally remembered to the the service department about the bounce. They had my truck for 3 full days and all they have managed to do is confirm my complaint, not fix it. They road pressure balanced the tires 3 times and they still had the bounce. They called the Ford hotline and were told to take the tires off of a brand new lot truck and see how they were. The bounce is still there with the brand new tires.
The bounce is unaffected by rate of excelleration or gear the tranny is in... it just happens at 40-45 mph no matter if I drive like my granny or a jack rabbit.
I took my truck back for the weekend but the tech is waiting on another reply from the hotline as to what to do next. I heard the comments like "its just the nature of the beast", but I don't think so.
Since the new tires didn't help my next guess is the drive shaft or rear end since it is definately speed related.
I just realized that I don't think I notice it when I am hauling my 5'er but I am not 100% sure about that.
Anyone have any thoughts?
Thanks,
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Panama City, Fl area

2011 F350 4x4 Ultimate Lariat crew cab SRW, LWB, 6.7 PSD, 5th wheel prep, factory 5'er and gooseneck hitches, 331 E-locker, factory bedliner. Duraflap mud flaps. Viair onboard air compressor. Firestone airshocks. Weathertech window vent shades.
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1996 F-250 super cap XLT 7.3 PSD 200,000 miles.
-----------------------------------------------
1953 F-100 Deluxe cab, 50th anaversery in midst of frame up restoration.

Last edited by Thom12; 11-12-2010 at 07:22 PM. Reason: typo
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Old 11-12-2010, 08:11 PM   #2 (permalink)
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Well, it sounds like at those speeds, you are hitting a natural harmonic of your suspension, but shocks should damp that out. Did they try replacing the rear shocks? What shocks did your truck come with? I have the same truck except in a 250 and no air bags, I DON'T have the bounce. After reading your sig, I'm wondering if the air bags are part of the problem. If you are getting the bounce unloaded, does varying the pressure in the air bags have any effect? How much air do you carry in them when you are unloaded? I can't help but think that a few psi in an empty truck is causing a cyclic oscillation that builds on itself and the shocks aren't handling it. Did the truck always do this or has it appeared after the air bag install?
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Also have a 1997 F450 7.3 XLT with 14' box, Hypertech programmer
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Old 11-12-2010, 08:33 PM   #3 (permalink)
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The signature line is slightly misleading because the airbags are still in a box in the back seat, wanted to get this cleared up before installing them so fingers couldn't be pointed at a truck mod.
You discribed it exactly the way I have always thought of it when you said "a natural harmonic of your suspension" and if I stay at that speed it does build in intensity slightly.
No, the shocks have not been changed yet, I will have to look and see what kind I have and let you know.
You use terms that make me believe you have experiance as a helo mechanic (same as me, 24 years navy helos).
Thanks for your input!
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Panama City, Fl area

2011 F350 4x4 Ultimate Lariat crew cab SRW, LWB, 6.7 PSD, 5th wheel prep, factory 5'er and gooseneck hitches, 331 E-locker, factory bedliner. Duraflap mud flaps. Viair onboard air compressor. Firestone airshocks. Weathertech window vent shades.
-----------------------------------------------
1996 F-250 super cap XLT 7.3 PSD 200,000 miles.
-----------------------------------------------
1953 F-100 Deluxe cab, 50th anaversery in midst of frame up restoration.
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Old 11-12-2010, 09:26 PM   #4 (permalink)
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While I do not think it is normal. All vehicles have a speed they do not "like". I have been able to find it in every vehicle I have owned. Although it should not be nearly as bad are you are describing. I am assuming that it happens on different roads?

There are a couple of asphalt roads around here that make empty trucks bounce and ride funny. Once I get off of the them things go back to normal.

Maybe you have a bad axle or bearing. A couple of more things to check after shocks, driveshaft, u-joints. Also I believe the long beds have a split driveshaft, maybe the carrier bearing is bad? I have driven at least a dozen 2011's and none have done this. I would check under my truck to see if I could figure something else out, but I don't have it yet!
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Old 11-12-2010, 09:26 PM   #5 (permalink)
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OK, good to know the air bags are out of the loop so that narrows it down to the stock suspension. My truck is the FX4 with the Rancho shocks. I still wonder if it isn't a shock issue. My 97 F450 had a similar problem but at lower speeds, like 30-35. It would not start bouncing until I hit a series of uneven spots on my local road, however once it started, the amplitude of the bounce would continue to build until I let of the throttle. I think it would get to the point I would loose control if I didn't back off. I installed a set of heavy duty shocks and that seemed to stop the problem.

BTW, not a helo mech. but had a grandfather who (among other things) built and balanced rotor assemblies for Sikorski and Paisecke(sp) back in the 60s. I picked up most of my mechanical knowledge from him. On a side note he had a long carear with Lycoming later AVCO dating from WWII. One of the things he was most proud of was being the chief test engineer on the 36cly radial engine. There were only two ever built and the only one remaining is at the Smithsonian. Until a few years ago when he died (at 103) he still had the waterpump and other odd parts from the other engine which was scrapped after the war. I think how proud he would be to see "his engine" as he called it in a national museum.
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2012 F550XL 19,500gvw, Oxford White/Steel interior, 6.7psd, Stainless simuators, Sync, 16Ft Morgan body, Waltco tuckaway 2500 lift gate.

2011 F250 CC Lariat Ultimate, Royal Red/Pale Adobe, 18" wheels, 3.31 locker, 172"WB factory Nav/Sync,etc.

Also have a 1997 F450 7.3 XLT with 14' box, Hypertech programmer
1987 F350 6.9 Banks Sidewinder, 12' box
Still going at 200K+
Traded:08 F250 CC 4x4 Lariat, 172"WB
Traded:01 F350 CC 4x4 Lariat, 172"WB
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Old 11-14-2010, 03:52 AM   #6 (permalink)
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My 2011 Superduty did the same thing when new untill I hauled some heavy loads in the bed and that sort of sprung the springs.
When my truck was new the back end sat lower than the front untill I hauled some heavy loads, now the back end sits higher and the bounce has gone away, it rides smoother now than when new, I kinda think your rear leaf springs need broken in.
also check the tire air pressue, mine calls for 65 psi and it only had 40 psi in it and i still don't understand why my low pressure sensor didn't warn me.
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Old 11-14-2010, 07:54 AM   #7 (permalink)
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Put the rear axle up on jacks, remove the rear wheels, run to critical speed and see if you have the vibration. No vibration, put the wheels back on and try again.
I suspect driveshaft or rear axle critical speed issue.
I find it hard to believe it is the suspension only as it happens over any terrain, correct?
It is probably a critical speed of the rear axle and the suspension is not dampening it out without weight on the rear.
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Old 11-14-2010, 07:11 PM   #8 (permalink)
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Blackout,
First you are correct, it happens on any number of roads, even one that has recently been repaved and the logging trucks have not yet made their ruts in.

Second, I also agree that the chances of it being suspention (springs) are pretty slim because I have about 3,000 miles towing my 5'er. If those springs aren't broke in yet I don't think they ever will. Now shocks might be a possiblity. Can't see a brand name on them, just plain old factory black colored shocks.

I will put my truck on the hoist this comming weekend and see what she does that way. That way I will get a good look at everything (shocks, shaft, carrier bearing and yes even the springs) that everyone has sugested. Thanks to all for your inputs! I will let you know what we find.
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Panama City, Fl area

2011 F350 4x4 Ultimate Lariat crew cab SRW, LWB, 6.7 PSD, 5th wheel prep, factory 5'er and gooseneck hitches, 331 E-locker, factory bedliner. Duraflap mud flaps. Viair onboard air compressor. Firestone airshocks. Weathertech window vent shades.
-----------------------------------------------
1996 F-250 super cap XLT 7.3 PSD 200,000 miles.
-----------------------------------------------
1953 F-100 Deluxe cab, 50th anaversery in midst of frame up restoration.
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Old 11-22-2010, 06:33 PM   #9 (permalink)
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Just an update for those with the same problem...
My truck has been in the shop since Wednesday and I got a call Saturday saying it was ready for pickup. I got there Saturday afternooon and my regular service advisor was gone for the day but he left all the paperwork with someone else. I read the paperwork and it said the bounce must be normal because even though they can duplicate it, they can't find anything wrong. Boy was I hot! I actually felt sorry for the poor guy that had my keys and paperwork because he took the abuse of the messenger getting shot with both barrels and he kept a very cool head and he actually agreed with me that he didn't think it was normal.
I left the truck sitting right there that day. This morning (Monday) I called the Service Manager on his cell even before he got into work and told him I did not agree that it was normal and my truck is sitting at his shop waiting for him to test drive and let me know if he thought it was normal.
30 minutes latter he called me and asked if I meant the bounce at 40 mph and I said exactly. He said he will get to the bottom of it.
He called at 3 this afternoon and said they have been in constant contact with the Ford engineers all day and are going through everything the engineers can think of but still haven't found the cause....
They still have the truck but atleast I feel now I have them actually working it.

I will keep ya'll posted as the saga continues.


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Panama City, Fl area

2011 F350 4x4 Ultimate Lariat crew cab SRW, LWB, 6.7 PSD, 5th wheel prep, factory 5'er and gooseneck hitches, 331 E-locker, factory bedliner. Duraflap mud flaps. Viair onboard air compressor. Firestone airshocks. Weathertech window vent shades.
-----------------------------------------------
1996 F-250 super cap XLT 7.3 PSD 200,000 miles.
-----------------------------------------------
1953 F-100 Deluxe cab, 50th anaversery in midst of frame up restoration.
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Old 11-22-2010, 07:07 PM   #10 (permalink)
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Thom, definitely let us know what you find out. I've got the exact same issue with my truck -- 06 350 crew cab long box -- at the same speeds. I installed Ride Rite airbags for towing. They help a bit, don't eliminate the problem.
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Old 11-22-2010, 07:26 PM   #11 (permalink)
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Mine is a '10 F350 Dually 4x4 with the same issue at the same speed. At 7k miles now and about 1k pulling 14k 5er. I had originally thought this was just an issue with my truck. Kind of glad it's not, but now more concerned.
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Old 11-22-2010, 08:37 PM   #12 (permalink)
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golfdoc,
do you notice any differance when the 5'er is hooked up?
I haven't had a chance to verify it since my OP but I thought it was better with my 13K 5'er hooked up.
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Panama City, Fl area

2011 F350 4x4 Ultimate Lariat crew cab SRW, LWB, 6.7 PSD, 5th wheel prep, factory 5'er and gooseneck hitches, 331 E-locker, factory bedliner. Duraflap mud flaps. Viair onboard air compressor. Firestone airshocks. Weathertech window vent shades.
-----------------------------------------------
1996 F-250 super cap XLT 7.3 PSD 200,000 miles.
-----------------------------------------------
1953 F-100 Deluxe cab, 50th anaversery in midst of frame up restoration.
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Old 11-22-2010, 11:06 PM   #13 (permalink)
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I have not noticed it with my trailer. Just this past weekend, after unhooking and leaving the campground it did it. The only thing I feel with the trailer is the axle wrap upon acceleration which is common on these trucks.
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Old 11-23-2010, 06:43 PM   #14 (permalink)
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Todays updates:
1) Took 4 tires/wheels of new truck (second time) didn't fix it. contacted engineers again.
2) Replaced both drive shaft sections and carrier bear. didn't fix it again. contacted engineers again.
3) Installed vibration monitoring equipment and captured the vibes. 7.4 hz, 14.8 hz, and 81.4 all recorded with ~.05g's. These are definately some sort of harmonics...just need to figure out what is causing them. Sent data to engineers and awaiting response.

Everything they have changed only leaves two areas in the rear of the truck... shocks/springs and rear end/axles.
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Thom

Panama City, Fl area

2011 F350 4x4 Ultimate Lariat crew cab SRW, LWB, 6.7 PSD, 5th wheel prep, factory 5'er and gooseneck hitches, 331 E-locker, factory bedliner. Duraflap mud flaps. Viair onboard air compressor. Firestone airshocks. Weathertech window vent shades.
-----------------------------------------------
1996 F-250 super cap XLT 7.3 PSD 200,000 miles.
-----------------------------------------------
1953 F-100 Deluxe cab, 50th anaversery in midst of frame up restoration.

Last edited by Thom12; 11-23-2010 at 07:46 PM. Reason: corrected the "g's" from .5 to .05... big differance!
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Old 11-23-2010, 07:00 PM   #15 (permalink)
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All harmonics and subharmonics of that initial 7.4Hz vibration. Interesting that the the third frequency skips from 14.8 to 81.4Hz. that goes from second to eleventh. I still can't believe they haven't swapped out the shocks yet. All four tires but not shocks. They are so simple and easy to do what are they waiting for? Can't remember if you have the heavy duty Rancho shocks or not. My truck is identical to yours but has Ranchos and DOES NOT have the bounce. If shocks don't do it, perhaps having the springs re-arched would help, or add a leaf to the spring pack. I can't see what could be wrong with an axle short of a bent shaft, but that should be obvious to any tech with a dial indicator.
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2012 F550XL 19,500gvw, Oxford White/Steel interior, 6.7psd, Stainless simuators, Sync, 16Ft Morgan body, Waltco tuckaway 2500 lift gate.

2011 F250 CC Lariat Ultimate, Royal Red/Pale Adobe, 18" wheels, 3.31 locker, 172"WB factory Nav/Sync,etc.

Also have a 1997 F450 7.3 XLT with 14' box, Hypertech programmer
1987 F350 6.9 Banks Sidewinder, 12' box
Still going at 200K+
Traded:08 F250 CC 4x4 Lariat, 172"WB
Traded:01 F350 CC 4x4 Lariat, 172"WB
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