Help Tracking down intermittent shaking

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by RGMrizzo, Sep 18, 2023.

  1. RGMrizzo

    RGMrizzo Bobtail Member

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    Jul 25, 2022
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    Hello everyone.

    I have a 1994 Freightliner FLB COE. rear air ride and front spring ride.

    Starting at around 58 mph I get an intermittent shaking. I say shaking because it doesn't feel like a vibration, more of a shaking. Like an out of balance wheel. Doesn't feel like driveline. Tow low frequency.

    It's most apparent on smooth pavement, and comes and goes. It will stop when I hit a bump. I feel it in the seat but not the steering wheel.

    Things I've done
    Front shocks
    Check front kingpins and bearings - GOOD
    Front tires are not out of balance.

    Based on my experience it doesn't feel like out of balance wheels because those usually feel like a constant shake that gets worse as you speed up and increases in frequency as you speed up.

    This feels more like the tire is dribbling on the ground like a basketball, pointing me to shocks, but they seem to be good, although I haven't replaced them.

    Is there something I should look for within the air ride suspension? Or something in the front axle besides the shocks and kingpins/bearings.

    It's pretty bad on long trips so I'd like to get it sorted before I get back on the road.

    Thanks for your help.
     
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  3. MRMTRANS

    MRMTRANS Light Load Member

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    Apr 28, 2015
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    I've gathered a steer axle tow out causes a vibration starting somewhere around 45-55 mph and goes away about 10 mph faster than when it shows. I've heard steer axle kingpins with worn shims contribute as well. Gathered all of this from MD Alignment. Important to make sure 1/16'" tow in by sipe measure after driving the truck forward before stopping and measuring. And it is also important to jack the steer axle before lubing the kingpins, and then turn the steering wheel both directions as far as you can. Hope this solves your problem some.
     
    BoxCarKidd Thanks this.
  4. cke

    cke Road Train Member

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    Hanger bearing.
     
  5. BoxCarKidd

    BoxCarKidd Road Train Member

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    I do not see how those things would change when you hit a bump?
     
  6. BoxCarKidd

    BoxCarKidd Road Train Member

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    I had a problem that changed with hitting railroad tracks and such. The tires had a powder type balancing compound in them. Moisture from the air caused the compound to clump together and cause vibration. If stopped it would spread out evenly until some hard bumps were encountered.
    Did not even know it was in the tires. Spent a good bit of money for someone else to to teach me that.
     
  7. MRMTRANS

    MRMTRANS Light Load Member

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    It was 10 years ago when I used to listen to Mike Beckett on the radio. If I remember right, the forward rotational force is working against the tow out (there is is always some play-should be 1/32" max at all related parts - tie rod ends, drag link-for a total of 1/16", which is why tow in is that amount). And at some point they reach an equal force, causing a vibration if towed out, until you drive through it until you speed thru it. I used to always have a vibration until I started doing my own alignments and changed kingpins. I'm just guessing hitting bumps might be effecting those competing forces.
     
    uncleal13 Thanks this.
  8. uncleal13

    uncleal13 Road Train Member

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    MRMTRANS is making an excellent point.
    Some string, duct tape, a pen and a measuring tape is all you need to confirm if it’s toed out. Should be towed in 1/16-1/8”
     
  9. Jubal Early Times

    Jubal Early Times Road Train Member

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    If you feel it in the seat and not the steering wheel the problem is going to be in the rear. I would check the torque rod bushings. U bolts, tires for roundnesses.
     
    uncleal13 and BoxCarKidd Thank this.
  10. BoxCarKidd

    BoxCarKidd Road Train Member

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    How long does it stop for after you hot a bump?
     
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