Brake Disk resurface

[av-mech]

Friends,
Since I started flying my Vari early this year, the one thing that really bothered me was that the gear legs had a shimmy in them.  This would only occur when my speed was something less than 10 mph but it was really pronounced.  This only occured when brakes were applied.

I had come to the conclusion that the brake disks were slightly warped.  I talked with tech support at Matco (I have older Rosehan which became Matco) and they assured me that it was very unlikely.  My disk thickness was .248" when I took them off the airframe.  Tech support said that they are .250" out of the factory.  So I suppose loosing .002" over the span of 30 years and 1000 hours is not too bad!

I gave them to a friend who is a proffesional machinist and he used both a lathe and a surface grinder to true up the disks.  They really came out looking great.  My new thickness is .225", my friend told me that in fact the disks were warped, though just a little bit.

Fast forward to today, I finally got it all back together and went flying.  Brakes are as powerful as you could ask them to be and the shimmy is completly gone!  Fantastic, one more thing checked off the list.

[Radioflyer]

I had a little experience one time with a friend's Cozy having low speed brake shimmy. In fact, I stood by the side of the runway while he applied brakes after a fast taxi and the gear legs could be seen to shimmy when brakes were applied. He worked on this problem for a long while doing things like remounting new tires, tire/wheel balancing, measuring disc thickness at various positions, changing pads, etc. Eventually he simply put an indicator on the disc while the wheel was mounted on the axle and found that the discs were slightly warped. He trued them out and voila, no shimmy. I've heard of other shimmy complaints and it seems to me most often it is due to disc warping. Use an indicator to check for disc warping.

[av-mech]

Radioflyer,

Thanks for the post, I did not use a dial indicator to check for warpage (sp?) but based on general mechanical knowledge, it seem the most likely cause.  I was surprised that the tech rep at Matco bet felt that it was unlikely and he went on to say that in all his years with the company, he had never seen that as a problem. 

[Radioflyer]

Yeah that is interesting about what the factory guy said. Your intuition proved correct. Kudos to you. It may be that our gear legs are flexible or resonant enough to be particularly sensitive to disc warping. Afterall, there aren't too many aircraft types around with purely composite legs.

[av-mech]

Yeah, those legs are kinda long and skinny.