Is this the drive line lash?

Discussion in 'Transmission & Drivetrain' started by Horse, Mar 14, 2016.

  1. Horse

    Horse Well-Known Member

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    Was reading the `thud' thread and saw someone mentioned drive line lash there. Thought I'd made a thread about it, as I suspect I have it.

    Here it is: when I am moving, say in 3rd gear. (I have a manual V6 if this matters.) While in 3rd gear at reasonable speed/rpm, whenever I push the gas pedal deeper in a somehow abrupt manner, I can hear some kind of noise (like a subdued clunk?). I can reproduce this noise almost whenever I want -- just let the car cruise a bit and them quickly step on the gas pedal.

    Maybe this is normal for S550. Just would like to see how many of you are having it and how you all are dealing with it.
     
  2. GT Pony

    GT Pony Well-Known Member

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    Jack the rear wheels off the ground, put the transmission in 1st gear and then rock the wheels back and forth to see how much slop is in the drive line. I would think the tires shouldn't rock back and forth less than +/- an inch or so at the circumference. I haven't measured this on my GT, but have on other cars that had quiet drive trains. You can also determine the slop in just the differential if you do the same thing but watch how much the tire moves before the drive shaft starts to move.
     
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  3. Catax

    Catax Well-Known Member

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    Are you saying some cars may have more/less slop than others? Mine must be huge.
     
  4. GT Pony

    GT Pony Well-Known Member

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    Yes, it's possible some cars have more than others. Depends how the differential gears were setup, and other tolerances that could be different, like in the half shaft splines, etc.

    Doing the test like I described above gives you an indication if the slop is huge or not.
     
  5. jasonstang

    jasonstang Well-Known Member

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    this actually does not fully exposed the issue because transmission also has back lash. all gears have back lash. the best way to find out would be transmission in neutral and hand brake on. twist the drive axle.
     
  6. GT Pony

    GT Pony Well-Known Member

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    There are all kinds of ways to do checks on the drive train slop. Yes, if you just want to see the slop in the half shafts + differential, then you could put the emergency brake on and have the transmission in neutral and rotate the drive shaft. But that would actually also include the transmission gear backlash side of the drive shaft.

    A better way would be to monitor just the half shaft + differential slop is to see how far the wheels rotate back and forth before the drive shaft moves at all. It will seem like a lot since you are looking at the outer circumference of the tire. Typically it's about 2 inches total on the circumference as seen on other vehicles I've had. I also plan on measuring this on the GT when I get a chance.
     
  7. OP
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    Horse

    Horse Well-Known Member

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    Thanks.

    As I don't have proper equipment to check drive line slack, I have to go by hearing and butt feeling.

    If this is normal then I can certainly live with it. (One of the reasons I was asking is that my last 150 HP manual car did not do this.)
     
  8. EXP Jawa

    EXP Jawa Well-Known Member

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    Ford measures backlash in the rear drivetrain (before installation in the car) by holding the rear hubs from turning, and then putting torque of the pinion flange. The closest thing would be to set the brake and turn the prop shaft with the gearbox in neutral, or unbolt the shaft from the rear joint. I would expect upwards of 5 degrees or so of total rotation, but that depends on how much torque you apply. I think that Ford uses something like 100 lb-ft to make the measurement.

    Doing that, however, only tells you one part of a bigger system. If you wanted to feel transmission lash in that, you'd have to try to move the rear wheels while in gear/park. If you had, say, 4 degrees at the pinion (by measuring the previous method), plus 2 degrees from the transmission, and 3.55:1 final drive, you'd have about 1.7 degrees of rotation at the wheels - if you rolled them together (like you might a preloaded Traction Lok. That should upwards of a half inch of movement on a 27" tire. But if you held one wheel and just rotated the other, you'll get double that due to the nature of differential gearing. Those are just off-the-cuff numbers, obviously it could be a bit higher than that.
     
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  9. jasonstang

    jasonstang Well-Known Member

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    RWD or 4WD cars tend to have more because of more complex drive train.
    FWD cars have less.
    A lot of this thud feel can be solves with clever throttle pedal programming but the mustang's are not designed that way even on my auto.
    My Toyota FJ is cream smooth. They did it by momentary unlocking the torque converter when I lift off the gas and reapply it. The Mustang does not do that and you can feel the jerky motion.
     
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  10. yellowragtop

    yellowragtop Well-Known Member

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    There is a little toyota/subaru rear drive sports car, and when they talk about the "lash" in their car it sounds 10 times higher than what the mustang has. So this issue indeed occurs with everyone else, even Toyota.
     
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  11. OP
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    Horse

    Horse Well-Known Member

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    Thanks. Good to know. (And indeed, the 150 HP car I mentioned was fwd.)
     
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