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!
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.
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.
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.
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?
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!
Holler at Vinnie. I believe he got his out of a Crown Vic and had it shortened.
http://www.dennysdriveshaft.com/ or go to ebay and search custom aluminum driveshaft
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.
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?
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.
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
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!
You work in a Ford dealership. Ford dealership service departments are required by warranty to have a tire balancer that measures roadforce. Measure roadforce.
If you have another set of wheels, ideally heavy ones. It should shift the vibration down in speed.
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.
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/technical-101/vibrational-issues/
In industry, we use a torsional damper that you place part way down the length of the shaft. It basically shifts the resonance out of the speed range at the cost of added rotating mass. You couldn’t do that in a garage obviously without a lucky guess or trial and error.
Years ago we explored whether material really matters (as much as people believe) as stiffness is easier to alter. A steel shaft at 3.5” and the wall thickness of the AL will have quite the resonant frequency for instance.
shop manual troubleshooting step for either confirming, or determining if the DS is the culpurt.
mark the DS near where the weights are at the 12, 3, 6 & 9 o'clock positions.
install a hose clamp around the ds while positioning the buckle to the 12 o'clock location.
drive car and observe symtom got worse or better.
rotate the hose clamp to the remaining 3 locations.
rotate accordingly as desired the hose clamp to the location that now removes or reduces the vibration.
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Apparently, the shop manual implies that if a remedy is reached, the hose clamp is then tightened down and kept in place, customer work order is completed.
This is easy and you should be able to have this small test wrapped up in one round trip test hit
not all hose clamps are designed equally so you can manipulate the weight felt on the DS by selecting one with a different amount of steel or bolt size to retest just in case you think the first hose clamp may have been too heavyl
Good suggestion as that can be overlooked, but we have road force balanced my tires and rims. Roadforce is well below spec so no problem there. Thanks!
I guess I should try this method with a hose clamp before spending a lot of money on a custom made DS as it won't cost a lot other than my time spent. It's just I've seen so many on this forum say they've done the same 3.73 mod that I have done and they've sent their DS out for balancing and this doesn't fix their vibration problem. I've probably seen at least a half dozen on this forum who have had no luck balancing the original steel shaft so I guess this put me off spending time and effort and money into trying to fix something that probably isn't fixable. Doesn't hurt to try though, so I think I'll go through the motions. :D
Had car up on hoist after work today and noticed a small dent in my DS, about the size of a dime and about 1/8" deep. Would this affect the balance of said DS?
if you have a magnetic base dial indicator .............
-locate dent
-position magnetic base dial indicator to the floor pan of the car
-clean off until smooth with emory cloth the drive shaft all the way around "next to the dent.
-position / set up dial indicator to where its touching the ds.
-zero out the dial indicator guage.
-i think you can turn one of your wheels to get the ds to rotate
-document the amount of thousands of an inch out of round.
-wash/rense/repeat in the dead center of the ds.
-with the two values you can on grid paper document the arch of bend "if" the ds is bent and where the arch is.
-obviously a bend in the middle would be absolute worse case vibration symtom but... the closer to the rear diff the worse as well from what i am told.
If you dont have a magnetic dial indicator.......
-locate dent
-tie thread to one ujoint retainer clip to one end of the DS.
-scotch brite the length of the ds you plan to pull the string across.. any debris will screw up your measurement
-pull string to an 'equal plane" location on the other end of the ds then wrap string and sinch tight.
-using a machinist ruler, measure the ds depth from the ds surface to the string.
maybe we should do a CSI investigation into if the DS for a 3.73 is a different weight set / design than those cars with other rear ratios?
considering it would take a really good hard hit to make that dent,.........
you may have the ds slightly bent.
if the ds is not bent but the metal is bent inwards,, it represents a value of ounces.
its relocating these ounces closer to the center line of the ds which is centrifically tied to the exterior oem tac welded counter weights.
I would speculate that if simply dented but your ds is not bent, the problem would be more on the minor side, hard to see on a balancer and somewhat moot.
if the dent has bent the ds.... welp,, you just found your problem.
go find yourself a chunk of 1/8'' thick steel.
lay it down on a rubber mud flap or something that gives,, maybe cork board (simulating the air gap inside your ds.
hit it the 1/8'' s steel with a ball pin hammer as hard as you can swing.
you would be hard pressed to with one swing move that amount of metal 1/8'' deep!!!!!! Im being serious
thats what happened to your ds at some point
within the location of this dent, can you detect any of your floor pan areas showing damage ?
someone mentioned clocking the ds by re-connecting it to the rear diff 90deg or 180deg from where it is now.
clocking ds's that telescope and have a knuckle and more than two ujoints like in my bronco or those with a carrier bearing , this is improtant. Normally no clocking needed on a two ujoint single pipe DS like on our cars.
our ds's are three piece. Ive worked on equipment with 8 pieces to make up the DS assembly and usually there are arrows welded or stamped or some sort of indicator to assist the tech in getting the telescoping spline setup to the orientation of the remaining two ends for ujoint and yokes at either end. Still yet the final front and rear connections better have been marked before the DS assembly came down or its going to be trial and error getting rid of the vibration.
by the way everyone................
inside the tail shaft housing,, inside the transmission of an AOD.. isnt that big hunk of metal clamped on the output shaft a counter weight?
Save yourself a shiznit load of time and frustration and message Vinnie so he can explain to you what he went through and finally just put an aluminum DS under his car and solved his issues. It is a known phenomenon and either a carbon fiber ($$$) or aluminum DS will solve the issue. Vinnie's solution was by far the best one I have seen.
http://www.foxtbirdcougarforums.com/showthread.php?31374-Aerostar-aluminum-drive-shaft&highlight=crown+victoria
http://www.foxtbirdcougarforums.com/showthread.php?39413-high-speed-vibration&highlight=aluminum+drive+shaft
http://www.foxtbirdcougarforums.com/showthread.php?34196-Finding-An-Aluminum-Driveshaft&highlight=aluminum+drive+shaft