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Thread: Highway speed vibration, worse on deceleration

  1. #11
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    Quote Originally Posted by Aerocoupe View Post
    If you put the trans in neutral with a manual gearbox and you still have the vibration then it isolates it to the rotating assemblies from the output shaft of the trans to the tires. This includes bushings, bearings, shafts, etc. With a slushbox I do not know because I have never educated myself on how they work so I don't know if this holds true with putting one of them in neutral.

    So the other issue is also something that has been talked about for years over in the Mustang community with regards to 3.73 and steeper gears which is called critical speed.

    http://www.foxtbirdcougarforums.com/...shaft-cures-it!

    Just read all 10 pages of this. :-O

    Sounds like getting an aluminum shaft is going to be the next step but.............critical speed?? I've heard of guys getting their factory steel shaft balanced but that still didn't fix the problem. Do you think the factory steel DS could be balanced differently if the driveline shop knows that you've changed gearing and therefore have affected the "critical speed" of the stock shaft, or is our factory shafts just not meant to turn those speeds?
    88 XR7 5.0L low mileage Florida Cat. AOD, 8.8, headers with x-pipe and flowmaster cat-back, 3.73's, lowered 1". 1 of 421 according to Marti Report.


  2. #12
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    i think it has more to do with having one rotating mass of one material (steel) with its characteristics coupled to another mass of similiar material but different characteristics (weight, shape, balance, etc.). So if this is done several times (vehicle powertrain) then hitting a critical speed in one component may then be amplified by another. By introducing a rotating mass of a different material like aluminum or carbon fiber the amplification is removed and hopefully it acts as a dampener.

    Anyhow that is about as simply as I can break it down. You will see that an aluminum DS will grow in diameter to compensate for torque / hp. The aluminum DS in my Bird is a 4" which made fitting the 3" exhaust a pain but it fits.

  3. #13
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    Quote Originally Posted by AT8 Cat View Post
    Just read all 10 pages of this. :-O

    Sounds like getting an aluminum shaft is going to be the next step but.............critical speed?? I've heard of guys getting their factory steel shaft balanced but that still didn't fix the problem. Do you think the factory steel DS could be balanced differently if the driveline shop knows that you've changed gearing and therefore have affected the "critical speed" of the stock shaft, or is our factory shafts just not meant to turn those speeds?
    it has something to do with the harmonics of aluminum vs steel. steel tends to start doing something like a whipping motion at certain speeds

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  5. #15
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    Great reading! Makes a lot more sense having read that. Thanks for posting!

    Looks like I need to start planning for a new DS. I'm thinking after all the mods I've made to this car (lowering and 3.73's) and also all the new parts and repairs that have been made that the DS has to be the culprit. It makes total sense. I'm exceeding critical speed for stock driveshaft with the 3.73's by roughly 20%. I will be looking for a driveline shop about this to remedy my problem.

    Thanks to all for your help and advice. Hopefully this spring I'll be vibration free!

    Cheers!
    88 XR7 5.0L low mileage Florida Cat. AOD, 8.8, headers with x-pipe and flowmaster cat-back, 3.73's, lowered 1". 1 of 421 according to Marti Report.


  6. #16
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    You work in a Ford dealership. Ford dealership service departments are required by warranty to have a tire balancer that measures roadforce. Measure roadforce.
    1988 Thunderbird 5.0
    GT40P, TFS-1, KB domes, 30#, MAF conversion, Explorer GT40 Upper/Lower, Smog delete, Wide Ratio AOD Mod, Unlocked Speedo, 3G Charging System Upgrade, Hi-Torque Mini Starter, 3.73 LSD, BBK 1 5/8 unequal headers, H-Pipe.

    88 Thunderbird/Cougar EVTM
    (Thanks to Trinom for hosting)

  7. #17
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    If you have another set of wheels, ideally heavy ones. It should shift the vibration down in speed.

  8. #18
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    If the vibration is due to a tire or wheel issue.

  9. #19
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    Quote Originally Posted by Aerocoupe View Post
    If the vibration is due to a tire or wheel issue.
    No, if you change the total rotating inertia of the rotating system that he resonant frequency will shift accordingly.

    I am not saying imbalance, I am saying the system. You treat it like a series of rotating masses on a shaft, and the system resonances respond accordingly to the excitation all other things being equal.

  10. #20
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    I saw your reply in the email on my phone and was confused but when I logged in I saw my mistake. What I wrote was supposed to be a question not a statement (? not a .) so thanks for the clarification on that and I do agree with what you are saying.

    I will say that the only things that affect the critical speed of a driveshaft are length, material(s) it is made of, and diameter. To fix a critical speed issue with the DS the only way is to alter one or more of those three things.

    I also found another pretty good read here:

    http://www.machineservice.com/techni...tional-issues/

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