REFLECTOR: wheel shimmy

[David Ullman]

I know the wheel shimmy topic comes up often.  I am now making the front
wheel pant for my SEFG and began to think about it (my velocity is still 2
years from flying).  In gradual school I took lots of dynamics courses and
looking closely at the tire/strut system on my plane, I appear to be
building a shimmy factory.  With the tire that came on the plane
(3.40/3.00-5 slick with a flat profile 3" wide), it cant help but shimmy. 

To have shimmy you must excite it with an off center contact point between
the tire and the surface.  Two ways to get this is to have the strut
misaligned side-to-side (I carefully aligned mine, but the first hard
landing will take care of that) or have a tire that can put the contact
point off to the side - like my tire which can have contact 1.5" off to
either side. Exasperating shimmy is the flexibility of the front strut and
the tire (rated at 50psi, but rim says only 30psi).  Like I said a shimmy
factory. 

I looked on the web and there are lots of different 3.40/3.00-5 tire
profiles.  So why did my plane come with a 3" wide foot print. I could see
this if I was going to do soft field landings and had a heavy nose load, but
I don't have either.  Seems that I am going to find a tire with contact in
the center, as peaked as possible, before I finish making my wheel pants. 

Any thoughts on front tire profiles?

David G. Ullman PhD ME
Robust Decisions
ullman at robustdecisions.com
541-754-3609
 

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