Thud and Crabbing Under Braking

Discussion in 'Ford F-250 and F-350 Suspension' started by Rosher18, Jan 31, 2019.

  1. Rosher18

    Rosher18 Junior Member

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    Brand new to the forum, but not to mechanic-ing. (Last summer I replaced the #5 piston in the farm truck after it burned through due to pinging.)

    I noticed today that when loaded (not sure of the exact poundage but 6 3/4" thick horse stall mats is enough) the truck will thud and crab right side first if I hit the brakes moderately hard. As I accelerate from stopped after this happens, I feel a thud in the rear end. What the hell is happening?

    I think the thrust angle is changing under braking but I can't find anything that looks like it's been sliding front to back on the rear leaf springs, mounts, bushings, hangers, or axle.

    Other details: the previous owner installed air bags between the frame and axle. When the bags are full (60 psi) the baking thud is roughly the same and the acceleration thud is less pronounced and delayed by a second or two.
    Truck is 2WD standard cab long bed with 249k miles

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  2. Roush PSD Interceptor

    Roush PSD Interceptor Straight Piped Diesel Staff Member

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    @Rosher18
    Could the right rear wheel bearing be loose causing this ?
     
  3. Rosher18

    Rosher18 Junior Member

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    It could well possibly be. I had Les Schwab do the rear bearings about 2 years and 6k miles ago, right after I bought it. My brother in law and I did the front wheel bearings around that same time.

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  4. Roush PSD Interceptor

    Roush PSD Interceptor Straight Piped Diesel Staff Member

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    I wouldn't think bed mount being loose would cause the crabbing issue,
    it has to be some thing that can't be seen by visual inspection.

    IMO it would be worth pulling the right rear wheel off to have a look see.
     
  5. Rosher18

    Rosher18 Junior Member

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    I didn't have anyone available at my house to run the truck for me, so I did the next best thing. I chained my truck to the tractor and pulled back, then took the first 4 pictures. (first picture in each pair is on the passenger side of the truck, then driver's side). Pics 3 and 4 are of the passenger and driver front spring bushings.
    Pics 5 and 6 were shot after releasing the backward pull and swapping the tractor to the front for a forward pull.

    I measured 1/16" difference in passenger side front-to-back axle position from backward pull to forward pull, but I want to re-measure that today after I'm done transporting horses for some 4H kids. [​IMG][​IMG][​IMG][​IMG][​IMG][​IMG]

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