2004 Dakota Rear axle or not, the problem of vibration?

lilian75

Hi.
My name is Lilian.
I run 3.7 Dakota 2004 sport. I have the problem with vibration witch start at 40-50ml/h. Mechanic says that is the rear axle bearing because is shiftting. But its not a noise is vibration what looks a tyres vibration. Tyres cheked twise. is ok.

Could be vibration caused by rear tyres bearings or axel bearings?

Mr_Shiftright

That doesn't sound like the most plausible diagnosis but of course without a physical inspection it would be hard to definitely say "no".

But a "speed sensitive" vibration like this is more often related to a balance issue, either with the tire, the wheel itself or even the truck's driveshaft.

lilian75

Thanks for the answer. 
I checked the wheels and tyres and its ok.  I changed the oil and we put the camera inside of axle and we found a big shaft of the bearings inside. But i dont have a noise just vibration even if i put in neutral on the speed 40-45 milles. Its looks like wrong balancing of wheels. 

lilian75

all this its happens on the long way right after I changed the axle seals in the same day. Its possible that if to fix the driveshaft back in not right in the same bolt hol  on the axle as its was coud create this problem?

Mr_Shiftright

Yes, on many vehicles you can re-position the driveshaft incorrectly and cause a balance problem. No harm in rotating it 90 or 180 degrees. You may have to experiment to see how much rotation works best.

lilian75

Thanks. Will try 

lilian75

I changed the position of driveshaft on 180 degrees but nothing helps. Changed also the oil in transfer case because the chek lamp switched on. Still have vibration. Any suggestions please!

thecardoc3

What frequency is the vibration occurring at?

While that might sound like a tough question to answer, it is information that a tech absolutely needs in order to try and figure this out. Now normally we would drive the truck and feel the vibration in person and while doing that we would do things like change gears for different engine speeds we could start to rule in/out what the source of the sensation is.

Here is an Android APP that can measure vibrations using your cell phone's sensors. https://play.google.com/store/apps/details?id=com.gamma.vibrationmeter&hl=en

I need to know what the frequency if the vibration displayed is using this app along with the engine speed and vehicle speed plus the size of the tires.

capriracer

First, a tire/wheel vibration generally occurs in the 50 mph to 70 mph range. That's because suspensions have a resonant frequency in this range where vibrations are not damped out.

Plus, a tire/wheel vibration usually occurs at one corner only. Rotating the tires/wheels should make the vibration move from the steering wheel to the seat back (or vice versa). If the vibration doesn't move, then the vibration is coming from somewhere else - and I think that is the case here.

thecardoc3

capriracer said:

First, a tire/wheel vibration generally occurs in the 50 mph to 70 mph range. That's because suspensions have a resonant frequency in this range where vibrations are not damped out.

I'm no expert in that, but that range would be the speed that the average vehicle spends the greatest amount of time traveling in so if that is actually true wouldn't it make sense to design the resonant frequency to occur in a different range?

This article tends to support my perception. https://www.rqriley.com/suspensn.htm
Here is another. http://www.eng-tips.com/viewthread.cfm?qid=344263

Anyway, a wheel/tire vibration at 60mph would be right around 12hz. A driveshaft somewhere close to three to four times faster at 36-48hz. The engine (flexplate, torque convertor) running at 2000rpm would produce a vibration of 33hz different accessories would be some multiple of that based on the turn ratio of their drive pulley to the damper.

capriracer

thecardoc3 said:

......... I'm no expert in that, but that range would be the speed that the average vehicle spends the greatest amount of time traveling in so if that is actually true wouldn't it make sense to design the resonant frequency to occur in a different range? .......

It has been a long time since I went through the math, but in the past, everyone I talked to who dealt with vehicle suspensions always pointed to the combination of spring rate, damping rate, and vehicle mass as falling within that speed range. My experience in dealing with tire/wheel vibrations seems to confirm this.

thecardoc3

If the resonant frequency was designed to be hit in the 60mph range, then that would mean it would be set to approximately 12hz. So any vibration in that range would amplify instead of getting filtered out. Ref. Gallopin Gertie. https://www.youtube.com/watch?v=j-zczJXSxnw

If you want to try and figure out the resonant frequency of your car's suspension try this.

At 5mph, stop aggressively and you will notice the front end dip and recover. It may go up and down two to three times. Measure the time from when you come to a complete stop until the body movement is negligible (about the third cycle). Take the time and divide it by the number of up and down cycles. Say three rebounds in two seconds. 3/2= 1.5 hz

When you are able to drive down the highway and from the vehicle motion determine not only that a car has a bad shock or strut but exactly which one is bad, what you are actually noticing is the different resonant frequency of that corner of the car compared to the rest. If three corners of the car resonate at 1.5hz and one is anywhere from 3 to 5hz it makes for a very unpleasant ride.

thecardoc3

Speaking of unpleasant rides etc. Did you ever hit a stretch of highway that started making your car start porpoising?

Watch this black Camaro on the launch. The bouncing that you can see is occurring at the suspension's resonant frequency.

https://www.youtube.com/watch?v=QcKiR-hK8us

capriracer

I don't think I've explained this well.

It isn't that the suspension resonant frequency is designed to vibrate at the frequency that equates to a 50 to 70 mph range - It's that the result of designing a suspension to do what it is supposed to do, results in a natural resonant frequency in the 50 to 70 mph - and in order to get that resonant frequency out of that range, you have to do something that results in unwanted properties.

It's been over 25 years since I did the math but it goes something like this: Take the corner weight of the vehicle. That dictates a certain range of spring rates - too soft and you get too much suspension travel - too hard and the ride is jarring.

That spring rate dictates a certain range of damping in the shocks. Too soft and the suspension doesn't damp out the input fast enough (sort of like what you see in the Camaro video) - too hard and the ride is jarring.

When you've done with that - and factoring what is typical for tire diameter - you wind up with a suspension whose resonant frequency is in the 50 to 70 mph range. To get a different frequency range and not screw up the way a suspension is supposed to work, requires a much larger tire diameter - and that is just not what normally is done.

lilian75

thanks a lot