Rear End Question

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Texas Tornado 1962, May 15, 2021.

  1. Texas Tornado 1962

    Texas Tornado 1962 Bobtail Member

    May 15, 2021
    Rear Ends have to be the same ratio correct? I have a major vibration, bad enough it is actually dismantling itself. If I leave my ink pens lying on the counter, or table, or the bed at the end of my day they gave unscrewed and are in pieces. Help. I have narrowed it down to having 373 in front 358 in back. It feels like the differential is engaged even though it is not.
    baha Thanks this.
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  3. Lazer

    Lazer Medium Load Member

    Jan 22, 2017
    I am no ‘Mr Goodwrench’ but I would think the ratios wouldn’t matter, until you engaged the power divider.
    Speaking of that, years ago drove a company truck with the same symptoms, put the truck OOS many times for the issue, mechanics, and mgt. cancelled the work every time claiming it was ‘ all in my head, you just don’t like that truck’.
    Well one day, a city driver got in the truck, put it in gear, and ‘BANG!’, the power crapped out.
    Week later got truck back from the shop, and lo, and behold, the vibration was completely gone.
  4. jamespmack

    jamespmack Road Train Member

    Mar 25, 2014
    You absolutely must have same ratio front to rear. Just because diff is not locked, power can and will be transmitted through it. They are fighting eachother. Not a good deal. You need to stop operating it asap. One of them is going to leave the group discussion.
    Hulld, Smellfunny, w9l and 1 other person Thank this.
  5. lester

    lester Road Train Member

    Jan 2, 2012
    NW, Iowa
    If you truly have different ratios they aren't going to last long. First of all if they are different and you engaged the power divider in anything but pure mud or ice bad things will happen. Obviously the front and rear axle will be trying to turn at different speeds and they aren't going to like that.

    Secondly it doesn't matter if the power divider is NOT ENGAGED. Power flows through both axles. There isn't one axle or the other that always gets the power. Cruising down the highway power basically flows to all 4 wheel ends assuming all things are equal. When ANY of the 4 wheel ends looses traction that wheel will spin. Could be any of the 4, which everyone has the least traction.
    If you have new tires on one axle and worn out tires on the other they aren't the same diameter and you can run like that without problems but it is harder on things.

    Basically what I'm saying is I highly doubt you have different ratios
  6. AModelCat

    AModelCat Road Train Member

    Jul 7, 2015
    Poor power divider is getting the crap beat out of it.

    Power divider and axle spider gears are meant to compensate for minor speed variations between the wheels ends (ie turning corners, slightly mismatched tire heights). They aren't designed to compensate for that much variance and on a constant basis either.
    w9l and Tb0n3 Thank this.
  7. Tb0n3

    Tb0n3 Road Train Member

    Oct 5, 2012
    Yeah. I was going to say that. The ring gear and pinion are made for the majority of movement and the smaller gears work best not moving and just transferring load. I was recently looking at power dividers to figure mine out and I believe that's a planetary gearset too.

    Scratch that, just a regular spider gear setup and a pass through shaft through the front of one of the gears into the rear, with a collar to lock the two together. I knew it was something like that, just didn't remember the specifics because it's a bit weird.
  8. kemosabi49

    kemosabi49 Trucker Forum STAFF Staff Member

    Jan 13, 2013
    SW Arkansas
    Has this been happening the whole time you have had the truck? Many things can cause this. Alignment, bearing end play, air bag height incorrect causing bad drive line angle etc.
    Tb0n3 and baha Thank this.
  9. baha

    baha Road Train Member

    Jul 25, 2013
    A way of ck. to be sure the rears are diff. are the same, is to jack 1 side of tires off the ground and mark a spot on both tires and on side of driveshaft on ft. side of rear end, turn the tires 1 round while looking at mark one driveshaft to see how many times the mark on driveshaft turns with the last time in quarter of turns write it down and turn the other tire and mark it down on the paper and you will find if the marks on drive shaft land in same place you have same rear gears, a driveshaft out of time may be the problem, look if all the end caps on both driveshafts are lined up a single spline off will make it vibrant. like crazy
    Last edited: May 15, 2021
    Brettj3876 Thanks this.
  10. Tb0n3

    Tb0n3 Road Train Member

    Oct 5, 2012
    Here's a example of how bad drive line angles can cause vibration.
    God prefers Diesels and lester Thank this.
  11. Brettj3876

    Brettj3876 Road Train Member

    Nov 18, 2014
    Canceling cancel culture/CCC
    Would that even make it more than a few miles if it had those 2 different ratios?
    Tb0n3 Thanks this.
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