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Landing gear shudder RV8

GaryStofer

Active Member
RV8, hobbs 320

I have been chasing a shudder in my left landing gear for at least the last 3 years.

This has been a problem on and off for maybe the last 40 or so hrs. It manifest itself on roll out when slightly on the brakes and crossing a pavement seam or all by itself when either getting on the brakes more aggressively or hitting a seam at higher speed. I surmise that the shudder is a aft/fore movement of the gear leg when under load.

This is driving me crazy! I have gone through the bolts and attachment saddles twice now and can't find any looseness or wear. I had an other RV builder look at the gear leg attachment and he could not see anything wrong either. Everything is tight on the wheel end as well.

Have other people experienced this as well, and if so what was the cause.
Any suggestions ?
Thnaks
 
Brake shudder

Had a similar problem that was caused by a warped/glazed brake disk and sticking caliper pistons (the o-rings had hardened).
You could also check the tires for balance, but if that is the problem then it would probably show up at higher speeds also.
 
Had a similar problem that was caused by a warped/glazed brake disk and sticking caliper pistons (the o-rings had hardened).
You could also check the tires for balance, but if that is the problem then it would probably show up at higher speeds also.

Yeah -- It's not the brakes or tires..
 
Have you checked to make sure the wheels are straight (no toe in or out)? You can check by placing a long peice of angle iron parallel to each wheel and measuring the distance between the two pieces of angle iron at the the front and rear end. If one leg was installed slightly off it can cause that.

If it's not straight you will have to shim the axle until its straight, it's probably not off by very much.
 
Have you installed a camera pointed at the wheels to make sure it's the left one? If so then you might see something that shows when exactly it's doing it and why.
 
RV8, hobbs 320

I have been chasing a shudder in my left landing gear for at least the last 3 years.

This has been a problem on and off for maybe the last 40 or so hrs. It manifest itself on roll out when slightly on the brakes and crossing a pavement seam or all by itself when either getting on the brakes more aggressively or hitting a seam at higher speed. I surmise that the shudder is a aft/fore movement of the gear leg when under load.

This is driving me crazy! I have gone through the bolts and attachment saddles twice now and can't find any looseness or wear. I had an other RV builder look at the gear leg attachment and he could not see anything wrong either. Everything is tight on the wheel end as well.

Have other people experienced this as well, and if so what was the cause.
Any suggestions ?
Thnaks

1. Are you sure the wheel isn't locking up and skidding when this is happening? Does it stop if you let off the brake?

2. Have you you checked to make sure that the gear attachment bolts are tight and properly torqued recently? Is there a proper gap between the outboard gear attachment brackets and wear plates?

Skylor
 
1. Are you sure the wheel isn't locking up and skidding when this is happening? Does it stop if you let off the brake?

2. Have you you checked to make sure that the gear attachment bolts are tight and properly torqued recently? Is there a proper gap between the outboard gear attachment brackets and wear plates?

Skylor

No, not skidding or anything like that. It has happened without brakes just as well. If I have enough speed I can momentarily lift up the left wheel and it immediately goes away. Just like if you have a tail wheel shimmy.
 
3. Check wheel bearing nut for proper adjustment (pre-load)
4. Check wheel pant to spindle bracket for looseness. (Oscillating)

What is the tire wear pattern?
 
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Have you checked to make sure the wheels are straight (no toe in or out)? You can check by placing a long peice of angle iron parallel to each wheel and measuring the distance between the two pieces of angle iron at the the front and rear end. If one leg was installed slightly off it can cause that.

If it's not straight you will have to shim the axle until its straight, it's probably not off by very much.

I have not done that recently It was straight when I built it, but who knows .. -- That's a good idea.. I should be able to clamp the 1/8" AL angle to the brake disk to check.
 
3. Check wheel bearing nut for proper adjustment (pre-load)
4. Check wheel pant to spindle bracket for looseness. (Oscillating)

What is the tire wear pattern?

Done all of the above and thought I cured the problem each time -- Tire wear same on both sides ... I also changed tires thinking it might have something to do with it --- NOT
 
Gear Shimmy

We had the same problem with our RV6 a couple years ago.
Maybe you have done this already-if so ignore this.
1. Remove the wheel pant.
2. Jack the wing until the L or R Gear is off the ground.
3. Remove the tire.
4. Grab ahold of the axel and reef on it; fore & aft, and up & down.
We found play where the gear leg goes into the socket of the engine mount. And we found that the axel bolts were not tight.
We had to go with slightly oversized NAS Bolts & reaming the gear leg & socket On the engine mount.
Replaced the axel shaft to gear leg hardware.
This solved the problem.
 
Does it still do it with the wheel pant off? I heard sometimes people have to add lead shot into the nose of the wheelpants to ?balance? them.
 
It is almost impossible to have feet on the pedals without applying some amount of brake. Try taking foot completely of pedal next time and see if it stops.
 
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It is almost impossible to have feet on the pedals without applying some amount of brake. Try taking foot completely of pedal next time and see if it stops.
I know RV-8s are relatively docile, but personally I would never take my foot off the rudder pedals when rolling out in any taildragger, even to troubleshoot a fault.
 
A good wheel & tire balance job might be helpful.

Check you tube for wheel spin up devices. Also dial/or check the OD runout of the tire by jacking the main and rotating slowly.
As others have suggested, there are torque values for the main gear leaf clamping bolts to the fuselage.
 
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You guys need not bother to attain your CFI ratings.

This casual shot doesn't make much sense to me. How many CFIs pursue their ratings in taildraggers? What's the implication here? Are you saying that the skill sets that CFIs possess allow them to safely control a taildragger on rollout without touching the rudder pedals? How many other CFIs reading this thread would be comfortable removing their feet from the rudder pedals in a taildragger on rollout? Being an old lame low-time straggler who seemingly doesn't have the right attitude to be a CFI, I'd really like to know. Go ahead... educate me. I can be taught.
 
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Wheel camber angle for each wheel.

Gotta like a good puzzle

I measured toe/in/out today and camber after moving the plane forth and back 6 times in a straight line. Toe/in is as good as it gets , 0.08 deg when tail high and 0.0 deg when tail low -- Or in other words within the measurement error. Camber on the problem wheel is 89.9 deg. splayed out, on the other side 88.5 deg. splayed out. The bow-through on each gear leg is the same when the gear is loaded, about 1/4", and both legs are straight when jacked up. Oddly enough the RH side is the one with the camber angle not being as I would have expected. However the hangar floor also might not be perfectly level, I will have to measure that tomorrow.

For the other commenters -- No it's clearly not a brake or balance issue of the tires...

When the new U803 and NAS bolts arrive from Van's I will take down the leg and see if I can detect looseness in the NAS bolts holding U803 or in the big center bolt.

Thanks for all the suggestions !!
 
I just repainted the nosewheel on my 150. Looking at it I noticed a flaw in the wheel. Look at the wall thickness around the wheel bearing...

GJB4mWXAxMfmeuDRix6l7-oMFM6SQPsjzyvv3jD3a7twhnQr5kMGA3QVIsnnxg3dwfE3wL4ts7tyIuEJQngQDC-Nqg-EmQCuCDBaFrNkH2xE-cyD0zl4l5_Y71keen0D2Rvv9FoQEasWenJavMzgi1USbPbqLj99M9Ahp_Hop6eMJNpbWNf2BgjnHcD6d2jf1MhtuMlSBEJWzsQ07AqALBFr5KssBjrA01KCYUnLv9CLf_6VdJVEkaQROKOMq24jVyTkMMfV_9AdbVZ_VFtEWOnH21Ahso0pz8Ac7frftlZnbYvyVQh0KnayM8tAHV5_Ie4WfFIW3QHv5XzE-2oMwUuM6tkt3Hq4tOGQRnI5b0OR-X3ULRT0k_nYhUTf7KLW0-5y1zDqTgN2q_jIbFoiMC8BmDf10bIsklYoV-WJszf2TLAyphf4Ay03cjoQVcuX3suSXm-lr5PJ10_peFFm_ji_1O_mh1LrWrOcPfqC6VXsCkPiVrmwE60WAJz3P1cDUwcQ1hnpkqzK_jHnje4XuWua6mU976eoRto6UV5UZvYegFFu7EUB9kEyaw73IaUrA3Feu9OXLYJYhWA-aydUivE0ybfkchVrySUzbL8X-MLQ9lhWwHKxjxVDKhLymll1kTs9aeTdlz19tMzo6tprSNtunOalEil1muCbkSE967WlmvBwgZTfotb1Mhd3YQ=w400
 
You guys need not bother to attain your CFI ratings.

Wow, that was close, almost got my CFI, then you tell me to take my feet off the pedals. No thanks, but thanks for saving me some $$$.

The good news is that if your feet aren?t on the pedals you probably won?t feel any shimmy :D. Problem solved.
 
Shimmy

I have a brief shimmy in my -7 predictably at 21kts on roll out. I don't see it any other times. About three little shakes and it is done. I never really worried about it but will shake the gear leg and look at the tires, etc. the next time I have it apart.

Thanks.

John
 
Camber

I measured toe/in/out today and camber after moving the plane forth and back 6 times in a straight line. Toe/in is as good as it gets , 0.08 deg when tail high and 0.0 deg when tail low -- Or in other words within the measurement error. Camber on the problem wheel is 89.9 deg. splayed out, on the other side 88.5 deg. splayed out. The bow-through on each gear leg is the same when the gear is loaded, about 1/4", and both legs are straight when jacked up. Oddly enough the RH side is the one with the camber angle not being as I would have expected. However the hangar floor also might not be perfectly level, I will have to measure that tomorrow.

For the other commenters -- No it's clearly not a brake or balance issue of the tires...

When the new U803 and NAS bolts arrive from Van's I will take down the leg and see if I can detect looseness in the NAS bolts holding U803 or in the big center bolt.

Thanks for all the suggestions !!

When you say ?splayed out? do you mean that the bottom of the wheel is slightly further from the aircraft centerline than the top i.e. negative camber? I?m curious because my heavy -8 (1160 lbs empty) with full tanks but otherwise empty and most other -8?s I?ve seen tend to have a bit of positive camber when sitting.

Skylor
 
I measured toe/in/out today and camber after moving the plane forth and back 6 times in a straight line. Toe/in is as good as it gets , 0.08 deg when tail high and 0.0 deg when tail low -- Or in other words within the measurement error. Camber on the problem wheel is 89.9 deg. splayed out, on the other side 88.5 deg. splayed out. The bow-through on each gear leg is the same when the gear is loaded, about 1/4", and both legs are straight when jacked up. Oddly enough the RH side is the one with the camber angle not being as I would have expected. However the hangar floor also might not be perfectly level, I will have to measure that tomorrow.

For the other commenters -- No it's clearly not a brake or balance issue of the tires...

When the new U803 and NAS bolts arrive from Van's I will take down the leg and see if I can detect looseness in the NAS bolts holding U803 or in the big center bolt.

Thanks for all the suggestions !!
Sorry to chime in late here, but based on what you wrote above, it sure sounds like you have an attachment issue. Be sure you've retorqued your attachment fasteners properly. I had to go back and retorque mine twice when new. And I did get a fore/aft oscillation occasionally back in the day. Retorqued, no more issue for 19+ years. (She'll be 20 in two days!)

Other idea. Switch your brake rotors from one side to the other and see what happens.
 
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camber

When you say ?splayed out? do you mean that the bottom of the wheel is slightly further from the aircraft centerline than the top i.e. negative camber? I?m curious because my heavy -8 (1160 lbs empty) with full tanks but otherwise empty and most other -8?s I?ve seen tend to have a bit of positive camber when sitting.

Skylor

yes -- negative camber -- corrected for the level in the floor I see -0.5 on the LH and -1.0 on the RH. I don't think that I ever bounced it hard enough to deform it.
 
Picture of U803

Today I took the U803 off and maybe we are getting somewhere now. in the picture you can see that the aft rear corner of U803 made contact with the wear plate. This even though the front still shows the desired 030 gap. So maybe it was just barely touching and started to rap on the wear plate when some vibrations set in , or the corner was bottoming out giving a false torque indication and left the gear leg without enough clamp. The second picture shows the contact patches where the gear and the saddle meet. Not very uniform !!


I see -- this dumb BBS still cant do pictures right -- here are some links to the picture on Icloud

https://share.icloud.com/photos/0y9PucTP2eCub_mLmAX-vNKjg
https://share.icloud.com/photos/0piK8Rrk9RDCmvVTmYAxaXoIQ


https://share.icloud.com/photos/0y9PucTP2eCub_mLmAX-vNKjg
https://share.icloud.com/photos/0piK8Rrk9RDCmvVTmYAxaXoIQ



0y9PucTP2eCub_mLmAX-vNKjg


0piK8Rrk9RDCmvVTmYAxaXoIQ
 
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If you've got a shimmy, something is not aligned with something else. I've delt with that issue on certificated airplanes for years. Funny how an airplane can fly for 50 years without a single problem of shimmy in the MLG and then all of the sudden, there's a shimmy problem. The pilot never, ever, ever had a hard landing! Never! LoL. It must have been the steel flexing on it's own. Just like that steel bridge that all of the sudden falls apart or the metal machine that all of the sudden just warps. OH MY GOD, THE ALASKA PIPELINE IS GOING TO FALL APART ANY SECOND! Metal just does that all on it's own, the pilot never has a hard landing, never. LoL
 
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Root cause of this shimmy problem !

SO !!

I got the new U803 part today from Van's and it looks substantially different from what I removed. Not only is the bridge on the new part thicker and the curvature symmetrical, the gap between the blocks is 0.012 tighter and right at 3.0" . For those who think it was wear that has affected the part, it is not so. It was manufactured that way.

The unsymmetrical machining of the saddle curvature put uneven pressure through the clamp bolts which lead to the elimination of the gap on one side and bottoming out. On one of the bolts the threads bottomed out because of the saddle bridge being machined too thin.

I have a low serial number, so if this part has seen an ECO and I was not told about it that's not good, especially since I inquired about this problem multiple times. As a matter of fact the support from Van's was not great, borderline insulting at times...


Of course it could also be that this is just a goofed up part from the machining and that the other side is just fine. Will remove and see which one of the scenarios I have.

I'm sure this new part will solve the problem!

https://share.icloud.com/photos/0nadzgTEQmSn-rR7rMdjjmA9w

0nadzgTEQmSn-rR7rMdjjmA9w
 
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U-Bracket gap

SO !!

I got the new U803 part today from Van's and it looks substantially different from what I removed. Not only is the bridge on the new part thicker and the curvature symmetrical, the gap between the blocks is 0.012 tighter and right at 3.0" . For those who think it was wear that has affected the part, it is not so. It was manufactured that way.

The unsymmetrical machining of the saddle curvature put uneven pressure through the clamp bolts which lead to the elimination of the gap on one side and bottoming out. On one of the bolts the threads bottomed out because of the saddle bridge being machined too thin.

I have a low serial number, so if this part has seen an ECO and I was not told about it that's not good, especially since I inquired about this problem multiple times. As a matter of fact the support from Van's was not great, borderline insulting at times...


Of course it could also be that this is just a goofed up part from the machining and that the other side is just fine. Will remove and see which one of the scenarios I have.

I'm sure this new part will solve the problem!

https://share.icloud.com/photos/0nadzgTEQmSn-rR7rMdjjmA9w

0nadzgTEQmSn-rR7rMdjjmA9w

Gary,

The -8 plans require a minimum of 1/32” gap between the outboard U brackets and the wear plates when torqued. Depending on how the 3/8” bolt holes we’re drilled and where the u-brackets sit against the gear leg in relation to the bend, they might not end up perfectly parallel to the wear plates, therefore the gaps needs to be checked all the way around around and the brackets must be ground accordingly to ensure that they meet the 1/32” minimum gap requirement. The gap is needed to ensure that the gear leg is fully clamped, especially during different levels of flex and braking loads.

Skylor
 
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Gary,

The -8 plans require a minimum of 1/32? gap between the outboard U brackets and the wear plates when torqued. Depending on how the 3/8? bolt holes we?re drilled and where the u-brackets sit against the gear leg in relation to the bend, they might not end up perfectly parallel to the wear plates, therefore the gaps needs to be checked all the way around around and the brackets must be ground accordingly to ensure that they meet the 1/32? minimum gap requirement. The gap is needed to ensure that the gear leg is fully clamped, especially during different levels of flex and braking loads.

Skylor

Thanks Skyler -- I just installed and torqued the new bracket and can see that with this U803 the inside clearance is disappearing before I have the full 20 ft/lbs on the bolts. Even on the other side I have nearly no gap on the inside while I have the desired gap from the outside. I can give it a cut on the mill no problem.

Still very puzzled why the original bracket did not have a symmetrically rounded profile on the clamping surface and was significantly thinner on one side versus the other.
 
Thanks Skyler -- I just installed and torqued the new bracket and can see that with this U803 the inside clearance is disappearing before I have the full 20 ft/lbs on the bolts. Even on the other side I have nearly no gap on the inside while I have the desired gap from the outside. I can give it a cut on the mill no problem.

Still very puzzled why the original bracket did not have a symmetrically rounded profile on the clamping surface and was significantly thinner on one side versus the other.

Are you the original builder of the plane?

Skylor
 
Are you the original builder of the plane?

Skylor

Yes original builder.

So on U803 I machined 025 off at a 1.5 deg angle to have clearance all around.

Went for a few TG's. The loud/sharp rapping is gone, however I can still provoke the shimmy/shudder when getting on the brakes aggressively as soon as I have good contact with the pavement like in a short field landing scenario. Maybe between 55 and 35 mph.

Camera mounted on the wing looking at the wheel was useless -- Got Jello just from the motor running. I might try again with a different mount or with a camera inside the cabin to record the speeds more precisely.
 
Camera mounted on the wing looking at the wheel was useless -- Got Jello just from the motor running. I might try again with a different mount or with a camera inside the cabin to record the speeds more precisely.

I think you need a better mount. I use a stick on mount and it?s solid. I used to use one that had a 1? long mast and it wobbled so much it looked like my plane was vibrating all through the flight.
 
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