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Worn CAB

Posted by wodcutr 
January 21, 2009 03:06PM
Can anyone tell me how a worn control arm bushing behaves like? I am suspecting one of my CAB were not installed right. What would the bushing feel like if the installer did not put the load on the bushing during installation before the lube dried out and set up? I have my suspicions, but do not want to put a new lollipop on if that is not it. That's a lot of work to do not knowing for sure. Thanks!
January 21, 2009 09:05PM
If that is your only concern I don't think you would ever feel it. I think the bushing might feel a little stiff or soft for the first mile or so and then the bushing would just twist the last little bit on the shaft and seat itself where it wants to be. Normally when bushings are bad they are really soft and give sort of a pulsing feel when braking. You may notice some other things like noise over bumps if they are really bad or the ball joints are bad that faint pulsing is a dead giveaway.
January 21, 2009 09:26PM
Could it cause the wheel to feel out of balance at certain speeds? I get that kind of a feel from my right front tire usually around 65 mph and then it goes away once you hit 70. The other day I was flying down the freeway and hit 105 mph and got a more pronounced similar feel. Needless to say I slowed down! My theory is that if the bushing was not pre loaded when installed then it would act like an amplifier not dampening certain vibrations. The thing that is weird is that it always does this at the same speed.
rkj
January 21, 2009 10:23PM
Yes, a worn control arm bushing feels like something is loose up front, usually you'll pick this up going over a speed bump or after backing up with the wheels turned and braking. If you look at them you usually can see them starting to come apart but not always, how many miles on them?

Mileage is not always the tell all though, you can pop one from abuse/potholes and the like.

Take a few close ups and post them.

Rick
January 21, 2009 11:23PM
Quote
wodcutr
Could it cause the wheel to feel out of balance at certain speeds? I get that kind of a feel from my right front tire usually around 65 mph and then it goes away once you hit 70. The other day I was flying down the freeway and hit 105 mph and got a more pronounced similar feel. Needless to say I slowed down! My theory is that if the bushing was not pre loaded when installed then it would act like an amplifier not dampening certain vibrations. The thing that is weird is that it always does this at the same speed.

I have never heard of that from a control arm not being seated correctly. It sounds like you might have a wheel out of balance. Have you hit a pot hole or anything that might have bent the wheel? Otherwise, it is possible to "throw a weight" which will cause the wheel to go out of balance. Finally, as dumb as it sounds, mud, or more likely this time of year, ice on the wheels can throw them out of balance. I actually just had that happen in my jeep the other day...gave it a bath and it drives fine now...
January 22, 2009 12:19AM
I second the mud or dirt on the wheels. Remove them and clean the back sides.

Bob in Everett
January 22, 2009 12:53PM
Here are pictures of my control arm bushings I could not get the camera to focus that well on the bushing itself but hopefully you can see what you need to. The bushings only have 20k miles on them and were changed 3.5 years ago. As far as dirt and snow on the inside of the rims I can not imagine that this is the case as I never drive the car in the snow and the roads are always dry. This car has never even had mud on it! I guess that a wheel weight could have come off so my next move is to have the tire rebalanced just to be sure. At the time the bushings were replaced so were the shocks, control arms, tie rods, and everything else up front and the rotors and pads are almost new. I guess it could be a wheel bearing, but the car only has 133k on it and is pretty tight with no noises.
rkj
January 22, 2009 04:34PM
The centering of the shaft of the control arm looks a little sloppy but that is probably just the angle of the picture, These bushings do have a certain way they are supposed to be set in there. They look very intact. When they're loose you can really feel it in the steering and brakes. Maybe your ball joints need attention? or other parts up front??? They don't look twisted as they might be if they were installed badly, was the cars weight on the wheels when you took those pix?

Are you having any specific issues and if so try to be as detailed as possible.

Rick
January 22, 2009 08:29PM
Quote
rkj
The centering of the shaft of the control arm looks a little sloppy but that is probably just the angle of the picture, These bushings do have a certain way they are supposed to be set in there. They look very intact. When they're loose you can really feel it in the steering and brakes. Maybe your ball joints need attention? or other parts up front??? They don't look twisted as they might be if they were installed badly, was the cars weight on the wheels when you took those pix?

Are you having any specific issues and if so try to be as detailed as possible.

Rick

Rick - balls joints were replaced with everything else and they look good. Yes the car was fully on the ground when the pictures were taken. I really don't think that the bushings are bad and that is why I hesitate replacing them. The car drives so well except when you hit 65 mph and then I just feel a slight vibration and it feels like it is coming from the right front. I also notice that my rear view mirror vibrates a little too so that it is hard to get a good image from it. If I reach up and grab it to stop the movement then I can see fine. I wonder if I might have a ujoint going bad on the drive line or the guibo is bad, but that does not explain the vibration up front. I could be reading it wrong or something, but I do not like going 105 and getting that oh shit feeling if you know what I mean. Certainly I rarely go that fast, but when I do I want to know the car is capable of handling it...isn't the car supposed to be able to go 125 or 130? My stupid truck will only go 100 and then it cuts out. I have been 100+ on my snowmobile, surely the BMW should be able to keep up!
January 22, 2009 09:15PM
Its definitely not normal and yes, they will do an indicated 135mph and handle it very comfortably. I would check the balance and true of that wheel and make sure all the lug bolts are torqued to spec. first. If all checks out then start looking at other things more closely, but those seem like the most common causes of your symptoms.
rkj
January 22, 2009 11:22PM
Quote
Andy 90 325i
Its definitely not normal and yes, they will do an indicated 135mph and handle it very comfortably. I would check the balance and true of that wheel and make sure all the lug bolts are torqued to spec. first. If all checks out then start looking at other things more closely, but those seem like the most common causes of your symptoms.

Also, give the front wheels a shake with your hands 12 and 6 o'clock for wheel bearings and then get the wheels checked for trueness and balance and look the tires over, make sure there are no bumps or flat spots like Andy sez.
January 25, 2009 12:21AM
The 65 mph is exactly the resonance of the suspension. Mine does that when I have a bent wheel on the front. Had it straightened and problem solved. So, it sounds like tire out of balance or bent rim.

Bob in "Everett
January 25, 2009 09:54AM
Quote
Bob in Everett
The 65 mph is exactly the resonance of the suspension. Mine does that when I have a bent wheel on the front. Had it straightened and problem solved. So, it sounds like tire out of balance or bent rim.

Bob in "Everett

What exactly is suspension resonance and how do you know that 65 mph is what it is?
January 30, 2009 12:23AM
If you have and out of balance tire or bent wheel, at some speed the slight force generated by the rotating mass being off center. At some speed it will match the natural frequency of the spring/mass system that is the car and suspension system. You can demonstrate this by using a rubber band and hanging a weight on it. Just bounce it at a slow rate and the weight follows your hand pretty closely, but as you speed up the oscillations, you get to a frequency that only takes a very small movement to make the weight bounce with huge displacement. If you speed up past that frequency, the oscillations will diminish. The car is a spring mass system with two masses, a large one that has the motor in it and a smaller one with lug nuts on it. The spring between them and the springiness of the tire make a spring mass system that has two natural frequencies related to the two springs. The 65 mph one is related to the tire bouncing like a basket ball. The lower frequency, maybe one to two per second is more obvious when going over speed bumps. At just the "wrong" speed the car will almost jump off the ground going over and bottom the springs on the other side. The shocks are there to damp out both these so you have a nice smooth ride.

At the natural frequency, a small force exciting the system will result in destructive displacements unless damped. You know where it is by where the vibration shows up. If you drive faster the vibration will diminish.

Bob in Everett
January 30, 2009 10:05AM
Thanks Bob, that was well put and an excellent explanation. I am definitely having my tire rebalanced. I might even need to check out my rim too. I hate anything to be wrong with my car and this has been bugging me so I am going to fix it for sure!
February 05, 2009 04:13PM
Update - I went and had all of my tires rebalanced today and it seems to have helped as I did not get a resonating vibration at 65 mph. I have yet to test @ 105, but will do when I have an appropriate stretch of highway and the police are eating doughnuts.
February 05, 2009 10:02PM
Quote
wodcutr
Update - I went and had all of my tires rebalanced today and it seems to have helped as I did not get a resonating vibration at 65 mph. I have yet to test @ 105, but will do when I have an appropriate stretch of highway and the police are eating doughnuts.

They're usually at my local Tim Horton's Donut shop between 10 and 11 am Sunday mornings winking smiley
rkj
February 05, 2009 10:21PM
Quote
Bob in Everett
If you have and out of balance tire or bent wheel, at some speed the slight force generated by the rotating mass being off center. At some speed it will match the natural frequency of the spring/mass system that is the car and suspension system. You can demonstrate this by using a rubber band and hanging a weight on it. Just bounce it at a slow rate and the weight follows your hand pretty closely, but as you speed up the oscillations, you get to a frequency that only takes a very small movement to make the weight bounce with huge displacement. If you speed up past that frequency, the oscillations will diminish. The car is a spring mass system with two masses, a large one that has the motor in it and a smaller one with lug nuts on it. The spring between them and the springiness of the tire make a spring mass system that has two natural frequencies related to the two springs. The 65 mph one is related to the tire bouncing like a basket ball. The lower frequency, maybe one to two per second is more obvious when going over speed bumps. At just the "wrong" speed the car will almost jump off the ground going over and bottom the springs on the other side. The shocks are there to damp out both these so you have a nice smooth ride.

At the natural frequency, a small force exciting the system will result in destructive displacements unless damped. You know where it is by where the vibration shows up. If you drive faster the vibration will diminish.

Bob in Everett

Wow, nicely explained. I'm impressed smileys with beer
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