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Jerky Ground Taxi

MVPILOT

Well Known Member
I've owned two RV-12 SLSA a 2015 and a 2020 RV-12iS. I have over 300 hours in factory built RV-12's. Recently I noticed a very jerky response and over response when using brakes to control direction while taxing. It feels like when I touch a brake for steering anywhere from about 1-10 kts, the wheel over reacts and then when I correct using the other brake it repeats almost like a bearing is shot and the wheel is flopping from side to side but when I elevate the nose wheel and test it by hand I don't get this affect. Anyone else noticed. I took it to a mechanic and they looked at it with any disassembly and didn't see the problem...
 
Have you recently completed the nose gear replacement? I have found mine needing increased brake pressure to control, resulting in "jerkiness".
The angle at the lower bend is greater than the original leg so steering geometry is different. With the original leg I had a uniform 3/16" clearance between the front wheel pant and the leg shroud. Now the clearance begins at 3/16" and widens to about 3/8" indicating the bend angle is greater. Just yesterday I unscrewed the gear nut 1 face to see if it helps. Have not flown.
 
The angle at the lower bend is greater than the original leg so steering geometry is different. With the original leg I had a uniform 3/16" clearance between the front wheel pant and the leg shroud. Now the clearance begins at 3/16" and widens to about 3/8" indicating the bend angle is greater. Just yesterday I unscrewed the gear nut 1 face to see if it helps.

I had Vans replace my original SB leg and a friend went through 2 to get a straight one. You will also notice your nose wheel pant is not sitting level. My nose wheel would chatter (not caster properly) at higher speed on roll out with the forward canting leg.
 
I believe my 2020 SLSA came with the new nose and it worked fine for the first 100 hours so I don't think that is the problem.
 
Thank you however I have about 100 hours and it was fine until recently. It seems to be getting worse.
 
Sure sounds like breakout force isn't properly set. Have you or your mechanic checked the breakout force? Takes two minutes with nose lifted so worth a shot.
 
See my post #29 listed above. No disassembly required. Drill small hole - chips will follow drill bit and be extracted during drilling process. Follow with 1/4" drill x 1/4" deep c'bore. Use a small socket to protect the drive-type grease fitting and tap in with hammer. Pump grease until it extrudes from top/bottom of bushings.
 
Thank you Jim. Your solution seems like a good one. I'm not all that handy and might screw it up but I'll check with the mechanic.
 
When comments first arose about different taxi steering response after replacing the nose gear on an RV-12, we invested the time to measure some nose gear legs and find a couple that were at the extreme of the allowable tolerance range for bend angle, and install them for testing on our RV-12iS.

We weren't able to detect a discernable difference in the steering quality with those legs compared to the original.
To assure no customer received a leg that was outside of the specified tolerance range we also implemented additional quality control checks.

The most common cause of poor steering performance (and likely the case for the O.P. for this thread) is improper lubrication and/or break out force adjustment.

As a side note, modifying an SLSA from the certified standard configuration is not allowed without manufacturer approval, and doing so technically makes the aircraft no longer airworthy (so suggesting adding a grease zerk to the nose fork on an SLSA RV-12 is not helpful.

It is our experience (literally many thousands of hrs operating and maintaining airplanes with free castoring nose wheels), that just pumping some grease into the nose fork is never a proper resolution for a stiff nose fork.
We recommend regularly disassembling them, cleaning, checking for rusting on the pivot shaft portion of the nose gear leg (common if it isn't being maintained regularly), then reassembling with all of the parts well lubricated with a water proof grease.
 
When comments first arose about different taxi steering response after replacing the nose gear on an RV-12, we invested the time to measure some nose gear legs and find a couple that were at the extreme of the allowable tolerance range for bend angle, and install them for testing on our RV-12iS.

We weren't able to detect a discernable difference in the steering quality with those legs compared to the original.

I waited 3 months for Vans to test and finally decided my leg was out of spec. I am sure COVID was a factor in the delay. As I recall 1 degree on the bend was allowable but it wasn't clear if that was a total of 2 degrees since there are two bends.

Even if Vans determines it causes no problems with ground handling how is a person to level their nose wheel faring that previously was level on the original leg? My faring required a stack of washers .450" under the back screws to raise the tail of the fairing to level. I don't think a stack of washers is allowable on an S-LSA.
 
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Having just replaced my nose leg I can report there is a difference of XX degrees fairing angle between the old and new. Unfortunately I did not make a measurement with my digital level of the old before doing the change so I have no real data.

The fairing mis-alignment to the airstream is noticeable:
i-3sZvNJ5-L.jpg


And the gap between the leg fairing and the wheel fairing is greater than it was but again I don't have an actual measurement:
i-GqNVXxJ-L.jpg


I have flown the airplane several times since the change and can report that there is no nose wheel shimmy or jerkiness. Have taxied it as safe a high speed I was comfortable with and it feels the same as the old nose leg did. We also did another 12 here and it taxies fine as well. Break out force on both ships set to 18 lbs.

Lately, I have been flying without the wheel pants. I'm doing short local flights less than 100 miles, lots of landings, small airports, and like being able to visually check the brakes, bearings and easily put air in the tires.

I agree with doing regular servicing of the the nose fork. Grit and water can work into the brass bushings and it needs to be disassembled cleaned and relubed with waterproof boat trailer grease, at least on annual CI. It is easy to do.
 
I concur with Tony T's observation that there is a noticeable angular difference between my original nose wheel strut and the new strut. I too have had to add some washers to level the nose wheel pant.
 
Please note that I wasn't meaning to start a debate on whether the new version nose gear leg might have a slightly different bend angle....

Just stating that testing has been done and a different bend angle that is within the specified production tolerance wasn't found to cause any noticeable difference in ground handling / steering response.
 
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And the gap between the leg fairing and the wheel fairing is greater than it was but again I don't have an actual measurement:
i-GqNVXxJ-L.jpg

Tony, based on this photo, I would say that the angle of the wheel fairing hasn't changed at all.
If you look close at the gap to the nose leg fairing, it looks like it is constant.
That would indicate that the entire wheel fairing moved slightly lower relative to the nose gear leg.
If properly aligned, the angle of the wheel fairing should parallel the angle of the cockpit side rail when sitting on the ground. This will make the wheel fairing aligned with relative wind, between the nose and tail of the fairing in flight (I.E., the cockpit side rail is pretty close to level in cruise flight).
 
Just state that testing has been done and a different bend angle that is within the specified production tolerance wasn't found to cause any noticeable difference in ground handling / steering response.

Can you confirm what the production tolerance is. I thought I remember it being 1 degree but don’t know if that is per bend = 2 degrees total or 1 degree overall.

The one I installed that had the shimmy was 3 1/2 degrees forward. The wheel pant was down in the rear about 1 1/2”.
 
RV-12 iS SLSA KBVS. Ooops... my bad. I didn't see you have S-LSA. I doubt your mechanical will add grease fitting. Modifications to S-SLA ares prohibited.
 
Sure sounds like breakout force isn't properly set. Have you or your mechanic checked the breakout force? Takes two minutes with nose lifted so worth a shot.

No the mechanic looked at it with the nose wheel elevated but asked me to learn what the breakout force is.
 
When comments first arose about different taxi steering response after replacing the nose gear on an RV-12, we invested the time to measure some nose gear legs and find a couple that were at the extreme of the allowable tolerance range for bend angle, and install them for testing on our RV-12iS.

We weren't able to detect a discernable difference in the steering quality with those legs compared to the original.
To assure no customer received a leg that was outside of the specified tolerance range we also implemented additional quality control checks.

The most common cause of poor steering performance (and likely the case for the O.P. for this thread) is improper lubrication and/or break out force adjustment.

As a side note, modifying an SLSA from the certified standard configuration is not allowed without manufacturer approval, and doing so technically makes the aircraft no longer airworthy (so suggesting adding a grease zerk to the nose fork on an SLSA RV-12 is not helpful.

It is our experience (literally many thousands of hrs operating and maintaining airplanes with free castoring nose wheels), that just pumping some grease into the nose fork is never a proper resolution for a stiff nose fork.
We recommend regularly disassembling them, cleaning, checking for rusting on the pivot shaft portion of the nose gear leg (common if it isn't being maintained regularly), then reassembling with all of the parts well lubricated with a water proof grease.

Thank you for your sharing your experience. The mechanic asked me to learn what the proper break-out force is for my 2020 RV-12 S-LSA Where can I get this information?
 
Well - I'm guessing he didn't look very hard if he wasn't aware of what the breakout force should be set at. Tough to 'guesstimate' :)

But, the answer is 18-20lbs as referenced in Section 35-07 of the Kit Assembly Instructions:
"Tighten the MS21025-20 Nut
until a force of 18-20 lbs is measured as the
Nose Fork Assembly pivots around the
spindle. U"​

Thanks!
John
 
RV-12 iS SLSA KBVS. Ooops... my bad. I didn't see you have S-LSA. I doubt your mechanical will add grease fitting. Modifications to S-SLA ares prohibited.

Thank you. At this point I'm looking for the published break-out force for my 2020 RV-12iS S-LSA. After reading some posts here it sounds like some lubrication might solve the problem.
 
Well - I'm guessing he didn't look very hard if he wasn't aware of what the breakout force should be set at. Tough to 'guesstimate' :)

But, the answer is 18-20lbs as referenced in Section 35-07 of the Kit Assembly Instructions:
"Tighten the MS21025-20 Nut
until a force of 18-20 lbs is measured as the
Nose Fork Assembly pivots around the
spindle. U"​

Thanks!
John

Thank you.
 
Thank you for your sharing your experience. The mechanic asked me to learn what the proper break-out force is for my 2020 RV-12 S-LSA Where can I get this information?

Claude,
For an SLSA RV-12 to me maintained in a legal manner, it must have the maintenance manual available to any mechanic doing work on it.
SLSA maintenance manuals are aircraft specific (it gets identified with the aircrafts N# / Serial #). The reason for this, is that any modification or design change that gets incorporated into the airplane is required to have the related documentation added to the back of the manual and is then considered a permanent part of the aircrafts maint. records.

You should have gotten one with the aircraft. If you do not have one, I suggest you contact Van's and get one so you can reestablish meeting this regulatory requirement.
In the maintenance manual, you will find that it specifies that the KAI (Kit Assembly Instructions) are considered an integral part of the maintenance manual. The maint. manual refers to them for a lot of maint. and service data (including the proper adjustment of the nose fork break-out force).
If you do not have a copy of the KAI, I recommend you get that from Van's as well (make sure they know which version of the airplane you need it for... 12ULS or 12iS)
It is these two documents that your mechanic should be using to reference service info, not you asking for him on an internet forum (can be very hit and miss whether you will get the proper information or not.....).

I see in another post that this is an RV-12iS. If you are the original purchaser, when it was delivered to you, the airplane would have come with a maint. manual and a copy of the KAI on a USB thumb drive.
The later post where someone gave you the proper adjustment value, is correct, but unless your mechanic knows how to properly measure that value, that information is worthless. I suggest referencing the maint. documentation.
 
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