Periodic Suspension Vibration

[morris9791]

Dear Experts,

Since I am no expert on vibrations, I was hoping that some-one could shed some light on the cause of vibrations in my car.
Unfortunately, I accidentally ran into a kerb with the 2 right side wheels (Low profile tyres). Ever since, the car has been vibrating in the following manner.

1) Accelerating / braking 20Km/hr above and below 100km/hr or constant velocity - The steering wheel shudders.
2) No acceleration or braking at this velocity - no vibrations.
Straight away, this tells me vibrations are a function of possibly damaged drive shafts or brake caliper?.....

3) Beyond 100km/hr, vibrations travel from the steering wheel to the driver seat and back seat.
4) The strange bit – mild periodic vibrations of the whole car approximately every 10 seconds along with normal vibrations when car is running at constant velocity above 100km/hr.

My understanding is that if the alloys are slightly damaged and are rotating constantly then vibrations should be constant, instead I get periodic vibrations. Why is this? What is causing this periodic vibration?

I assume that there is resonance happening somewhere with the suspension system and some other external frequency. Would damage engine mounts cause the frequency of engine to resonate with suspension system? Are there any remedies I can try to resolve this problem?
The 2 front tyres are balanced and to no avail. Would tracking / rotating the wheels have any effect?

One final question, if some component of the suspension system is permanently bent (impact with kerb), does this change the natural frequency of the system?
Sorry for the long winded thread. Any suggestions or comments will be greatly appreciated.

Kind Regards

Eddie
Design Engineer

 

[Twoballcane]

Hmmm, did you get the front end aligned?

Tobalcane
"If you avoid failure, you also avoid success."

 

[BobM3]

The tires could be balanced but still have runout. At 100 kph you are near wheel hop natural frequency, Say it's 10 hertz for your car. If the damaged wheels are generating variable loads and running say 10.1 hertz (at 100 kph) then you will have a modulating frequency of .1 hertz. That may be the periodicity you are experiencing. If one wheel is turning a bit faster than the other then maybe you have a 10 hertz natural frequency, a 10.1 hertz excitation and a 9.9 hertz excitation - so a lot of modulating (low frequency) possibilities.

 

[GregLocock]

4) I'm sorry BobM3, that is incorrect. With very few exceptions (that do not apply here) systems can only vibrate at the forcing frequency, in the steady state.

The phasing is typically because the two wheels run at slightly different speeds. You are hearing them coming into sync and out again, like a WW2 bomber's engines.

You have probably bent the rims. If you persist in driving on them you will explode the shock absorbers.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[morris9791]

Thanks Folks for the replies.

Really, how would the shocks explode? Maybe I should rotate the wheels so and see how that effects the periodic vibrations. Im not prepared to chenge my rims just yet
If i make any progress, I will update this thread.

Thanks again folks

Eddie

 

[GregLocock]

Shockabsorbers absorb energy. If your wheels are excessively out of round, or unbalanced, then they can put a lot of energy into the shocks. The shock will heat up, and fail. It may not explode, it will stop working.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rob45]

I think you're looking for a car repair website...