eddieseve

Well Known Member
Having recently repaired a flat tyre and then having changed tyre pressures to 40 psi, I found that I had managed to produce the dreaded wheel shimmy, the best solution to fixing this initially was to reduce tyre pressure. I have found that around 34 psi worked well for me, however there where still situations where shimmy would still be evident as I was rolling out on landing and if I got a little fast during taxi.

Also for those of us flying with the little wheel at the correct end, I was also facing the dilemma of having to pull off the perfect 3 pointer so as not to get 3-4 landings for the price of 1.

Having heard about gear leg stiffeners from the VANs instruction manual and also the Vans Air Force Forum I did wonder how well this device might actually work.i

When the opportunity arose to purchase some and have them included in Rob Lawrie?s FWF shipment, I thought, why not.

A guy called Roger (woodmanrog in the VAF forums) in the US makes these specifically for the RVs and at $50ish they are not overly expensive in the scheme of things.

Last weekend I installed them on VH-EWS my RV7.

They are designed to install onto the rear side of the gear leg and come pre-routed so that you pretty much just hold them against the gear leg at the correct angle (a little bit of fiddling with tape and installing removing the fairings is required to determine the correct position) and you?re ready to attach them.

My only hitch was that I run my brake lines up the rear of the gear leg which required me to route a channel into the stiffeners to accommodate the brake lines, not a big deal unless you?re at Wedderburn (my airport in Sydney's) and have no power and don?t own a router.

It took me 45 minutes wondering hangar to hangar to find a kind soul who had both, and then 15 minutes to route the channel in both stiffeners before I could start my install.

Installation then only required that I shorten the stiffeners so that they did not interfere with the top and bottom of my gear leg fairings.

Once you have everything positioned the instructions ask you to bond the stiffeners to your gear legs with fiberglass, from a maintenance and rust perspective this did not appeal so I chose to strap them into place using 3M Gaffer Tape. Using the 3M tape, the bond that you achieve is incredibly strong, and I am sure that I could do a nicer job of the wrapping if I did it again, but?.it is serviceable.

So do they work?

You bet, I went out and shot some touch and goes on what could only be described as a hot bumpy day (2 pm last Sunday) with a 5-7knot cross wind and they worked a treat, there is a vastly reduced tendency to relaunch you back into the air if you drop on slightly in the flair.

I was also able to easily get my left wheel to run down the runway and hold it there while the right was still in the air and counter the cross wind until speed diminished and the right wheel and tail wheel came down.

Rolling out there is no tendency to shimmy at all, so I am sold.

I would highly recommend this solution to anyone that is experiencing the dreaded bunny hop or the dreaded wheel shimmy or both.

Cheers
 
I have shop made wooden dampers on both my RV and My Rocket. I agonized over the complexity and weight in both cases, so it's nice to get a pirep that shows their effectiveness.

For the record, they are not really stiffeners... A piece of wood bonded to a steel or titanium gear leg does not add a lot of stiffness. They are more correctly called dampers... They absorb energy to prevent oscillation (shimmy). It's like the oil in an oleo. You would think that the type of wood matters for this... I prefer shade-grown, fair-trade old-growth Douglas fir, cut from the coastal rainforest of Haida Gwaii by free-range organic loggers and seasoned in a non-GM organic cedar-wood fired kiln. It may cost more, but at least I know the tree had a happy life before it was chainsawed.
 
Tape longevity

Once you have everything positioned the instructions ask you to bond the stiffeners to your gear legs with fiberglass, from a maintenance and rust perspective this did not appeal so I chose to strap them into place using 3M Gaffer Tape.Cheers

Eddie, I used tape on my RV-4 legs in '88 and they are still holding today.
 
I just worked through some numbers and the stiffness in the fore-aft direction increases by around 75%, based on some back of the envelope calcs.

As for dampening, unless they get pretty warm they aren't absorbing much energy.

Dave
 
Thanks Jim

Eddie, I used tape on my RV-4 legs in '88 and they are still holding today.

Thats really good to know, the only other thing I an thinking of doing is pulling them off and sealing the bare timber with some epoxy resin just to try and keen oil and moisture out of them, not sure weather that is worth while or not, but will give it a go regardless.

Cheers
 
Hi Vern

I agree they are dampeners and they do a fine job of dampening shimmy and rebound :)

I have shop made wooden dampers on both my RV and My Rocket. I agonized over the complexity and weight in both cases, so it's nice to get a pirep that shows their effectiveness.

For the record, they are not really stiffeners... A piece of wood bonded to a steel or titanium gear leg does not add a lot of stiffness. They are more correctly called dampers... They absorb energy to prevent oscillation (shimmy). It's like the oil in an oleo. You would think that the type of wood matters for this... I prefer shade-grown, fair-trade old-growth Douglas fir, cut from the coastal rainforest of Haida Gwaii by free-range organic loggers and seasoned in a non-GM organic cedar-wood fired kiln. It may cost more, but at least I know the tree had a happy life before it was chainsawed.

Cheers
 
I just worked through some numbers and the stiffness in the fore-aft direction increases by around 75%, based on some back of the envelope calcs.

As for dampening, unless they get pretty warm they aren't absorbing much energy.

Dave

Yeah - -probably not much damping, but they sure do change the natural frequency in one axis, and keep the gear more stable (happy gear). Woodmanrog has a youtube video of before and after.
 
baseboard and zip ties...

I used some baseboard to fabricate dampeners... sanded back to back with a back cut edge then finished to shape with a dowel the same size as a gear leg.

8ibZ1N9s.jpg


High strength zip ties to hold it in position for the past 700 hours.

xgVMS0Ce.jpg
 
I just worked through some numbers and the stiffness in the fore-aft direction increases by around 75%, based on some back of the envelope calcs.

As for dampening, unless they get pretty warm they aren't absorbing much energy.

Dave

Hmmm. Is the shimmy longitudinal or transverse... Or rotational? I can see extra stiffness longitudinal, but the transverse direction has very little. Empirically it seems to work nevertheless.

If we are after just stiffness, why use wood? Aluminum angle bonded to the gear leg would do that. Maybe wood is cheaper.
 
wrapped with tape

I used woodman's stiffeners on my 9A. I needed to sand them to fit my install to the backside of the leg. I wrapped mine with 3M reinforced tape.
 
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I don't know what motion the shimmy has, but it seems to be a coupling between the spanwise and the fore/aft directions. There's probably some rotation in there too. The increased stiffness in the fore/aft direction decouples those two bending modes of vibration, so it works.

It's the fairly large shape of the wood piece that gives sufficient stiffness since wood has a modulus of elasticity of one million psi or a bit more, and the steel has a modulus of 29 million psi. If you're going to make an improvement to it, either glue some uni carbon to the edge of the wood that's farthest from the steel gear leg or some aluminum. But from what people write, it seems to work as-is. You'd have to run the numbers to assess whether an aluminum angle would be sufficient. I have no feel for that.

Dave