REFLECTOR: Main LG vibration on braking ...

[Jim Sower]

I agree.  Basically, it seems that a cantilevered flexible strut with a 
mass on the end of it is resonant system just *begging* for a little 
excitation.  Just about anything can accomplish that.  Seems further 
that the heavier the wheel or the lower the tire pressure, the lower the 
resonant frequency.  Bad news is that it's kind of impracticle to make 
it heavy enough or soft enough to move the resonant frequency out of the 
landing rollout envelope.

Get carefully machined rotors, hard tires and pray for rain ... Jim S.

Ron Brown wrote:

> THANKS, Pat.
>
> Excellent data points for the vibration and I believe you have nailed 
> it - disks that are not perfectly straight and flat.  I pretty much 
> tried many of the things you did except for the resurfacing.
>
> I think it is a resonance problem that apparently is excited by a non 
> perfect set of disks.  I measured mine and found less than .001, but 
> apparently, that is enough to set up a bit of chatter that excites the 
> gear.
>
> Ronnie
>
>
> ----- Original Message ----- From: "Pat Shea" <xl340hp at yahoo.com>
> To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
> Sent: Tuesday, February 01, 2005 7:21 PM
> Subject: Re: REFLECTOR: Main LG vibration on braking ...
>
>
>> Jim,
>>
>> I battled main gear vibration while braking in the
>> 45-25 speed range on my XLRG for a while. I worked w/
>> George at Matco trying all sorts things. These all had
>> little or no affect:
>>
>> -Adjusting toe in/out
>> -different pad material
>> -Different calipers (upgraded to XT)
>> -Different discs (different material)
>> -Line restrictors/dampeners
>> -Different tire pressures
>> -Balancing the wheels/tires
>> -Shimmed away any disc run-out
>>
>> I finally did what Scott Swing said to do early on,
>> which was have the surfaces of the discs ground flat.
>> I found a grinding shop (versus machine shop) that
>> charged me $20 to grind both discs. My discs were only
>> about .005 out and slightly bowl shaped (again, about
>> .005 worth). This solved the problem. I didn't do this
>> initially because I had sent my brakes to Matco for
>> inspection and specifically asked them to verify that
>> the discs were true - they said they were fine...
>>
>> Pat
>>
>> --- Jim Sower <canarder at frontiernet.net> wrote:
>>
>>> All,
>>> There is a pretty extended discussion on the Cozy
>>> list about Main Gear
>>> vibration.  Seems as if the gear legs start
>>> vibrating / oscillating
>>> during braking from about 45 kts down to 30 kts or
>>> thereabouts.  If you
>>> let off the brakes at 45, coast down to 30 and
>>> re-apply the brakes there
>>> is no vibration.
>>>
>>> I was wondering if this might be a known  / well
>>> known / common
>>> phenomenon in the Velocity / RV communities.  Seems
>>> as if it's fairly
>>> common in all of the direct derivatives of Rutan
>>> airplanes (EZ, Cozy,
>>> Defiant).
>>>
>>> Inquiring minds need to know ... Jim S.
>>
>>
>>>
>>
>>
>>
>>
>>
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