Tire brand vs. gear leg shimmy

randylervold · #1 10-08-2007, 12:01 PM

I have 41 hours on my RV-3B now and have reduced the initial gear leg shimmy by reducing tire pressures. 20-22 reduced it to where it will only shimmy on about one in six landings now. It's the typical shimmy, at 10-15 mph something excites it and nothing will stop it other than slowing down as fast as possible. Still, it's annoying and can't be good for all the parts down there.

I balanced my wheel pants when building them so that variable is eliminated, but I'm now wondering whether changing tires would have an effect -- different rubber compounds have different hysterisis characteristics (how 'bouncy' they are). Is this wishful thinking, or has anyone notice the elimination of shimmy by changing to a higher-end tire, say a Michelin or a Goodyear?

pierre smith · #2 10-08-2007, 12:52 PM

Hi Randy,
A few months ago our -6A developed a horrendous shimmy on both takeoffs and landings. I just knew it was the worn down nosewheel but as we landed one afternoon, I looked out the right side and could see the right main shadow shimmying back and forth several inches. Further examination showed it to be severely out of round and I had been convinced in my head that it was the nosewheel. Changed the tire and everything was normal.

Regards,

N908RV · #3 10-08-2007, 01:25 PM

Randy,

On my -8 I switched from the El Cheapo tires that Vans gives you after they wore out - always had a bit of gear leg shimmy from them. I was surprised at that given the stiff gear legs that the -8 has - as opposed to the undrestandable shimmy that is sometimes inherent in the Whitman style rod gear.

After I installed a new set of Michelin's the shimmy went away altogether.

So, yeah, that might be worth a change. Going the step further to actually balance them with lead weights would be even better. I didn't need to do this to achieve no shimmy ground ops.

Best,

Rob

MNRVFlyer · #4 10-08-2007, 01:56 PM

Hi Randy,

I balanced my tires when I put my wheels together. One wheels needed almost 2 oz of lead to get it close to right. I have not flown the wheels yet, but they were out of balance by a fair amount. BTW, these were the cheap ties that Vans gives yo in the kit.

randylervold · #5 10-08-2007, 01:59 PM

Good feedback, thanks. That does it, I'll order some Michelin Airs (lighter than Goodyear Custom II or III) and tubes shortly and give them a try. I'll balance them, to eliminate that variable, and make sure to check for roundness to eliminate that one as well. Can't say I checked the Air Hawks for roundness, maybe that's a causal issue here.

Does anyone know if there's an economical remedy to fix a tire with runout?

Mitch757 · #6 10-08-2007, 02:12 PM

Randy, I think you should be using a higher tire pressure, not because of the shimmy, but to preclude a possible blowout. At the 20 psi or so, the sidewall is flexing quite a bit. I've heard of a few folks who have departed the runway after such a tire blowout.

The fellow I bought my RV4 from warned me about this and recommeded 40 psi.

I sometimes get the shimmy at low speeds and it seems (in my plane) to be coupling with the engine RPM. If I stay just 100 rpm off idle, it doesn't happen.

I'm also going to the Michelins at annual time.

Mitch Garner
RV-4 flying

bjustus · #7 10-08-2007, 02:18 PM

My first RV-4 used to shimmy occasionally -- aero trainer and then air hawk tires. My new -4 has Michelins and has never had the slightest shimmy, can't say for sure if it's the tires or not.

Please report back if this fixes it for you.

pierre smith · #8 10-08-2007, 02:26 PM

BTW,
There is a red dot on the sidewall of my Air Tractor tires but they're much bigger than the 500X5's we use. The red dot should align with the valve stem for proper balance if there is one on the tires. I was led to believe that the better aircraft tires were factory balanced. My AT tires have flat balance weight, sometimes two inches wide glued to the underside of the tread.

Regards,

Ron Lee · #9 10-08-2007, 08:43 PM

It may be a gear leg movement as opposed to left right shimmy in a nose wheel with inadequate breakout force.

I always suggest having a competent person look at the wheels on the ground when it is happening. It is easy to see what it is and I assume first out of balance/out of round.

I helped a guy balance new main tires today and one required five ounces of lead to balance it.

AlexPeterson · #10 10-09-2007, 01:59 PM

I had no shimmy with the Van's tires. I've had two sets of Michelin since, and both sets of those have a shimmy at 23 knots. I now have retreads, and the shimmy is still there at about 22 knots. I believe the mass of the Michelin is a fair bit higher than the Van's tires.

The system is a classic spring, mass and damper system. Change any of those and the natural frequency or amplitude will change.

BTW, I'm sold on the retreads as to their tread life, it appears to be at least twice what the Michelin and Van's delivered.