Highway speed vibration, worse on deceleration

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Highway speed vibration, worse on deceleration

October 04, 2017, 04:20:31 PM

Hey everyone,

Starting to get depressed with my Kat. It has a highway speed (65 mph and up) vibration from the rear of the car. I don't really feel it in the seat but I can hear it pretty good. If I take foot off of the gas it gets worse. It's been into the dealership where I work 3 times on this problem with someone who's been with Ford for over 20 years. First time was bent axle shaft so those have been changed, but did not fix. Second time we changed the upper control arms to CHE adjustable to correct the pinion angle (car is lowered), did not fix. Third time we changed the tranny mount as it was collapsed but this did not fix either. Car has 3.73's and problem seems like it got worse after gear install. U-joints and all rear axle bearings, shafts, gear set are new (Ford Racing, Moser). Wheels are balanced. Driveshaft hasn't been balanced but looks clean and original weight is still there.

Any ideas? Help with this is appreciated!

Highway speed vibration, worse on deceleration

Reply #1 – October 04, 2017, 04:44:29 PM

I had three weights on my '86 when I got it, and there was a noticeable, repeatable vibration between 25-45 mph. Had a new driveshaft made and that solved the problem. A good driveshaft should either balance out with no weights, or have one weight. Don't know how mine got 3 but it sure looked factory.

You could try removing the counterweight ("dogbone") from the front of the axle housing, that may help a bit. Sounds like everything else has been fixed or checks out alright though.

Highway speed vibration, worse on deceleration

Reply #2 – October 04, 2017, 05:14:38 PM

Tried removing the "dogbone" too. Didn't help, but made it slightly worse at about 60 mph, so back on it went.

Thought I had all the bases covered but still no answer.

Highway speed vibration, worse on deceleration

Reply #3 – October 04, 2017, 05:30:29 PM

3.73 gears and the stock driveshaft? Yeah it's the stock driveshaft. I had the stock one in my car balanced 3 times trying to fix the vibration issue after 3.73 gears. I gave up and had an aluminum driveshaft made. Fixed the issue instantly.

Highway speed vibration, worse on deceleration

Reply #4 – October 05, 2017, 10:12:26 AM

Try unbolting the driveshaft and turn it 180*. I reused my driveshaft from my 2.79s, then on my 3.73s, and finally on my 4,10s without any issues.
What about the trany bushing?

Highway speed vibration, worse on deceleration

Reply #5 – October 05, 2017, 11:08:24 AM

Hmmm, don't think we've checked the tranny bushing. Problem seems to be coming from the rear end but I guess the sound might be traveling down the driveshaft. Worth a look.

Still using the stock DS. DS has been rotated/turned 180* - didn't fix either.

I wouldn't mind spending money to get an aluminum shaft made up if I knew this would fix the problem but I've already spent over $1500 or more getting rear end done, upper adjustable control arms and tranny mount etc. It's getting expensive with no results. I'll have to see if that's the path to try next.

Does anyone recommend a good place to get an aluminum shaft made up in Southern Ontario (KW area preferably)?

Thanks!

Highway speed vibration, worse on deceleration

Reply #6 – October 05, 2017, 03:58:08 PM

Holler at Vinnie. I believe he got his out of a Crown Vic and had it shortened.

Highway speed vibration, worse on deceleration

Reply #7 – October 05, 2017, 05:40:33 PM

Quote from: AT8 Cat;463065

Hmmm, don't think we've checked the tranny bushing. Problem seems to be coming from the rear end but I guess the sound might be traveling down the driveshaft. Worth a look.

Still using the stock DS. DS has been rotated/turned 180* - didn't fix either.

I wouldn't mind spending money to get an aluminum shaft made up if I knew this would fix the problem but I've already spent over $1500 or more getting rear end done, upper adjustable control arms and tranny mount etc. It's getting expensive with no results. I'll have to see if that's the path to try next.

Does anyone recommend a good place to get an aluminum shaft made up in Southern Ontario (KW area preferably)?

Thanks!

http://www.dennysdriveshaft.com/ or go to ebay and search custom aluminum driveshaft

Highway speed vibration, worse on deceleration

Reply #8 – October 05, 2017, 10:45:40 PM

If it's a buzz and you see it in the rearview mirror it's more than likely the driveshaft. Unless the gear install was fubar but, in that case, you would feel it in the seats.

Highway speed vibration, worse on deceleration

Reply #9 – October 06, 2017, 09:08:04 AM

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/showthread.php?33113-vibration-thread-aluminum-driveshaft-cures-it!

Highway speed vibration, worse on deceleration

Reply #10 – October 10, 2017, 03:24:05 PM

Quote from: Aerocoupe;463088

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/showthread.php?33113-vibration-thread-aluminum-driveshaft-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?

Highway speed vibration, worse on deceleration

Reply #11 – October 10, 2017, 03:44:48 PM

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.

Highway speed vibration, worse on deceleration

Reply #12 – October 10, 2017, 07:34:52 PM

Quote from: AT8 Cat;463174

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

Highway speed vibration, worse on deceleration

Reply #13 – October 10, 2017, 08:37:14 PM

Some light reading:

http://www.markwilliams.com/driveshafttech.aspx

https://www.driveshaftshop.com/driveshafts-101

http://pstds.com/critical-speed-chart/

Highway speed vibration, worse on deceleration

Reply #14 – October 11, 2017, 09:04:58 PM

Quote from: Aerocoupe;463177

Some light reading:

http://www.markwilliams.com/driveshafttech.aspx

https://www.driveshaftshop.com/driveshafts-101

http://pstds.com/critical-speed-chart/

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!