Vibration!!!!!!!!!

i have been getting a little vibration at 40 MPH at the rear of my car, as soon as the needle gets off 40 it stops... i have 4 new tires on it but i didnt get the back tires balanced, at first it wasnt but now its really annoying me. could it be something with the drive shaft? do i need to get tires balanced on the back? like always any suggestions would be helpful

 

You need to have the tires balanced.

 

.... did you have the vibration before the new tires?

 

when I got the car the.tires where dry rotted, so after we got.the.engine running I just put four new tires on..... but it always since I took my first drive vibrated at 40

 

start with the tires...

 

Always run balanced tires, and that would be the starting point. Just an ounce of imbalance can make a tire hop off the pavement on each rev at 60 mph. Not only does the misbalance throw the entire weight of the wheel/tire/half-of-the-axle, but it is also compressing the spring! Making sure they are balanced will allow you to chase down other details. It also keeps you from wearing bearings, etc. out.

Realistically, a wheel imbalance should get worse with speed. If balancing the tires does not make it go away, and this one sound like it does go away above 40, I would look at the u-joints and driveshaft. I had u-joints go bad in a Torino ... very bad shake, that came on suddenly, just at 45 mph. Bearings in them crumbled when they were taken apart. The shake was very speed-specific.

As a sidenote ... if you have a Mav or other Ford from the era and the front shakes at around 45 mph ... look at the idler arm .... very common problem.

 

Where do you feel the vibration, do you feel it in the seat? steering wheel? floorboards?

 

New tires.......and you didnt balance them????????? DUH!!!

 

Here is a relatively cheap way to remove one possibility. First, pick up some new u-bolts for your rear driveshaft u-joint to pinion, and Second rotate the driveshaft 180 degrees than remount... may not seem like much but I had the same problem you are having and unbolting the driveshaft from the differential then rotating it 180 than remounting it stopped the vibration.

 

This is why people should mark the driveshaft before removing it so they can install it back in the same spot where it came from.

 

......

when you get a...new, balanced...driveshaft it doesn't come with an orientation as to what way it is to be installed. if you send your stock driveshaft off to be balanced it neither comes with an orientation... stock driveshafts weren't balanced on the car from the factory so why should it matter...

 

I've wondered about that too... the best justification I have come up with is that if you mark it, at least you are certain that you are not introducing one more problem into the equation, and you can be certain that you are definitely back at square one when you are trouble-shooting. You are dealing with very small amounts of weight imbalance generating possible very bad vibration problems. If the pinion flange isn't perfect and the shaft is bolted up 180, you may be introducing more confusion, and you will never get it sorted out.

Case in point ... I have been chasing down a 75mph vibration problem in my DD Explorer. It doesn't get driven on the highway at that speed much at all, or I would have this solved already. One tweak that helped ... loosen the motor and trans mounts and cycle it gently between forward and reverse to make sure that wasn't in a bind, placing the motor/trans at an angle that is at odds with the driveshaft (as per a factory service bulletin for this model, unlikely it would be something needed on another model). Since doing this, I have a shake that definitely seems to come from the rear axle where I wasn't as certain before, so it is likely had two sources at work prior to this.

Had a friend observe the back tires at speed ... no wobble, but the LR has a bit of a hop, RR has less so. Going to have the balance re-checked, if no problem there, I am inclined to say it needs u-joints. Thinking the rotation of the driveshaft is somehow affecting the LR more, but I would expect the opposite, (i.e lift on the RR) so we'll see. Going to be looking at trans angle vs pinion angle in more detail too.

 

When you get a new or rebuilt driveshaft it is up to the installer to get the driveshaft installed to produce the minimum vibration. Ford and all the other car manufactures in the world didn't put paint marks on the driveshaft and the rear yoke for the fun of it. They "Mass Balanced" things and marked it with paint.

 

I thought that to but for the few minutes it takes to unbolt rotate and rebolt I figured why not and like I said it stopped the vibration I was having at the time which was a speed specific one. It's a project of elimination, eliminate the cheapest/easiest probable solutions before moving onto more costly/difficult ones.

 

Wonder if something like this will help? http://www.mustangforums.com/mustan...HILE-IN-4TH-OR-5TH-GEAR-COBRA-MODEL-ONLY.html