Gear Strut Conundrum

[Snork]

I've got too much positive camber on my gear wheels, and I am unable to extend the rod end bearings any further to correct the problem. The rod ends are already turned out to their maximum while leaving 5/8 inch of threads into the struts. I believe the recommended distance, center to center on the tires is 70 to 74 inches, which I can't obtain. I'm at 69" center to center with a heavy load. This problem is also making the toe-in worse: as the gear legs splay out, the toe-in problem corrects itself to be acceptable around 72" center to center. But at the current configuration, the toe-in is more exaggerated. (I had to disconnect the tops of the struts while hoisted to get 72", and then measure the toe-in) I have the older streamlined strut design on my BH4, and am looking at other strut options including third party if I can't get this issue corrected. Any ideas out there on why I'm only able to get 69" center to center with stock struts? It seems to me they are just too short, but that doesn't seem to make sense. Others are obviously able to adjust their camber within limits. Thanks, Steve.

[AKKen07]

I apologize if this is obvious, but did you roll the plane around a bunch or put it on grease plates to get the gear to its “happy place” before measuring? Homemade Grease plates worked well for me, but if the gear isn’t splaying out due to tire friction, obviously that would be a problem.

[Snork]

Hey Ken, yes we put it on grease plates. Still only 69”.

[AKKen07]

Shoot, I’d call Mark to talk it over.

[jaredyates]

Are the parts from the factory? Or perhaps a more relevant question, do the measurements match the plans?

[David Swartzendruber]

This may seem obvious, but where are you measuring the center to center distance? Due to the camber, it will be different at the bottom of the tire than it is at the middle in line with the axle.

[Utah-Jay]

So I measured mine today, no wings, but has an engine and all the engine plumbing, seats installed and all avionics. I measured from the front-middle of the tire, I got 68”, I think from previous comments that is where it should be with no wings. Seem correct?

Sorry for the hijack

[jonmiller0]

I also feel like our gear is toed in too much.

Does anyone have the ability to give clear guidance on where we measure this mythical 72"? If it's from center of the tire then clearly this distance is going to be different between wheels - we have goodyears 26". If I had 850s then 72" would result in my gear further splayed out.

[jaredyates]

The gear toe in is a little different than the gear spread. Both are important but they are two separate variables. To check the gear spread, if you are at 72" and your tires are far off from vertical, then measure at the center of the front/back of the tire instead of the top. You want to be at the center of the wheel left-right as well as vertically.
To check gear toe in, you'll clamp straight angles to the brake discs and do lots of measurements per the procedure.
Gear toe in changes as the gear spreads and unspreads.

[jonmiller0]

Hi Jared. I attached a photo. Sorry I was using the wrong nomenclature. Yeah the Toe in is aligned well, but the gear spread measured center of fat tire to center of fat tire has resulted in the gear being (in my opinion) obnoxiously tucked in whether or not I'm flying at gross weight or if it is sitting empty (picture attached). Since we adjusted the gear in by about 3" landings are tougher to grease. This is why my buddy AK2ID and I wonder if we measured the 72" from the wrong spot. If I change the tires to 850s then I'd need to expand the struts to get it back to 72" (as the width of the tire is different). Thanks for the advice. Screenshot 2023-10-27 at 8.57.41 AM.png

[jaredyates]

Just eyeballing, it looks too narrow. You might try measuring at the front of the tire, at the same height above the ground as the center of the axle. I remember hearing Bob say something along the lines of when you are loaded, you don't want the wheel tilted inboard of vertical. You could adjust the shock struts based on the mission if you wanted to go to the trouble.

[svyolo]

I wish I could add to this, but all the info I have read makes me believe this is not sorted. I just reset my gear, wings on, final assembly. With 5/8" min thread engagement, I have what I have. It seems a bit narrower than spec.

This seems like a bit of confusion between the various parties involved in the BH. I am going with min thread engagement, as that is also a limit.

[Fatiger53]

I had the exact same problem on my 4 place plans built with IO360. I used 1" 4130 steel and machined a male to female spacer to extend the strut. I first made them 4 1/2" net length then ended up at 3 1/2" by trial and error

suggest you make a non airworthy set first to dial in the length but you will need ALL the weight in place

1" x 3 1/2" oal steel
tap both ends 1/2-20 RH thread
insert a 1/2-20 stud cut from a grade 8 bolt
when finished loctite for security

[Fatiger53]

[BradW1062]

I had this same concern on my A-model QB kit, where the gear was "tucked-in" looking. Of course, I still have not put any weight on it, so it's hard to tell how much it is going to spread. Looking at the pictures of other completed airplanes, like the one Jon Miller posted earlier in this thread, I was concerned that there is a problem somewhere. And there were other posts of people having this same problem of not being able to get the 72" spread.

The tire size really has nothing to do with it - if the tires are properly straight up and down and centered on the axle with standard 6.00-6 wheels, then you should get the 72" tread as indicated on the plans. The plans do not indicate any camber at all, so that's what tells me these cambered ones are not turning out correctly. If you have a larger/different wheel that is offset outboard to handle a larger balloon tire or something, then there would be more width, but certainly not less, and no reason to have camber because of tire size or type. I don't know anything about those balloon tires, so I can't say much there, just my speculation. But changing tires on the same wheel won't change the measurement, only load will do that - and toe-in if there is any. If the axles aren't perpendicular to the track, then tire diameter would slightly change the tread measurement at the front of the tire - that's one reason there was an engineering note to check the toe in and adjust it to zero.

Because of all this, I spent some time measuring everything and scaling the drawings and doing calculations to understand what the strut length should be. Here's what I figured out:

For my airplane (and I assume all 4-place Bearhawks), the key is on plans drawing 23, which has a diagram in the top left that indicates 'Tread 72 with shock strut extended 1.38'. It also shows 26" reference height on the axle to the datum line (bottom longeron of the fuselage). On the bottom right of drawing 23, the distance along the axle from the center of the wheel (I have Grove 60-1A wheels that are 3.25" bearing to bearing) to the strut bolt on the axle is 6.625". A 36" half of track distance minus this wheel offset from the strut bolt leaves 29.375" from this bolt horizontally to the center-line of the fuselage. So the right triangle formed by this and the 26" reference height gives a long side of 39.23", which matches the scaled measurement on the diagram for the strut from the axle bolt to the center of the fuselage. But, here is the *key* point: as the drawing indicates, the center strut attachment is at "C", and the gear leg attaches at "B" (and at C for the rear of the leg). Those are fuselage stations on the lower longeron; B is at the front door post and C is at the structure below the seats where the struts (including the wing struts) and the rear of the gear leg attach. These two stations are 13.5" apart. Also, the axle is 2" forward of the station at B according to the side-view diagram to the right of the diagram above, resulting in a total rearward sweep of 15.5" for the top of the strut. This sweep of the strut to the rear means that it is not 39.23" long, it is 42.18" from the gear bolt to the center of the fuselage tube #19 at C.

With the tab on the fuselage at the center where the strut attaches, the bolt hole is 2" away from the center of station C (T19). This means the strut length is reduced to 42.18" - 2.00" = 40.18". And, as the plans say 'with strut extended 1.38', the un-extended length is 40.18" - 1.38" = 38.80". That's your reference measurement for an unloaded strut. Further, since the spherical rod end (XAM-7) is indicated to be 2" from the top of the strut cylinder body to the center of the eye on drawing 25, the strut body length comes out to 38.80" - 2" = 36.8" from the top of the strut cylinder to the bolt at the axle. When I measured the factory struts that came with my kit, they are 36.625" long here. Close enough for me - Yipee!!

For me, this means I should be able to get the required adjustment range on the rod end to to get a loaded length of about 42.2". Like I said, I haven't actually put any load on the gear yet, I don't even have my engine mount at this point, but I am now fairly confident that the struts have enough range to make it work. The other fact is - I have found no confirmation anywhere on the plans what the specific weight or load is that results in the "extended1.38" note. I have always assumed it is at design gross weight, 2500#, as that would be the normal design methodology. I don't know for sure, but I assume the 1.38" stretch would be nearing the maximum compression on the spring, with some headroom left. Most of these extra-heavy-load die springs (3000# on drawing 25) reach "solid stack" full compression at 25%, which is 2" for an 8" spring. It maybe 15-20% max for a 3000# spring, but I don't have the specs on these particular ones. A 1.38" compression is 17% of 8" free-length, and for a 3000# spring that has a 25% maximum compression, that is about 2000# of tension on the strut (1.38 x 1500#/inch). This translates to about 1200# at the wheel, which would work; that's 2400# on the mains and 100# on the tail, confirming that the 1.38" extended measurement is at gross weight. There is, of course, a bit of "tolerance" in these numbers since I don't have exact spring specs and such. These calculations addressed my concerns, so I thought I would share them to help others.

Bob's prototype N33RB weighed 1270#, and had 600# on each main wheel, according to the weight and balance he published as a reference. This translates to about 975# on the spring, and that wouldn't get you to 1.38" compression on a 3000# spring, but only about 5/8". Or a bolt-to-bolt strut length of about 38.80" + 0.625" = 39.425" at empty weight, depending on the airplane. This gives a track width at empty weight of about 70". So the "design" range on the track is 70" to 72" from empty to gross. There really isn't that much travel in this gear, maybe 10-12" total, in and out, a lot less up and down - but that's normal. On my airplane, sitting on the gear with an unfinished fuselage, basically little to no weight on the gear, I measure about a 63.5" track. Using the 38.8" un-extended strut length I calculated above, I should have 64.2", so that turns out to be pretty close since I am measuring 38-5/8" overall strut length (less than a 1/4" short). It's within adjustment on the rod ends for sure (and my calculation tolerances), but I haven't bothered messing with it to see what the eye-to-cylinder length would be if I set it to that 64.2" calculation. I will just wait for the loads, and should see about 6" more with finished empty weight.

For those who are plans building, the strut measurement from the gear bolt to the fuselage attachment could probably be taken with the airplane lifted/supported and the gear in the proper position to get the 72" spread. But for those of us with QBs we are stuck looking at the "tucked-in" gear until we finish the airplane. You could do the same measurement on a QB kit to see what the strut length should be without the struts in place, and then compare it to 38.8" to see if it is close.

For the OP Snork, I think maybe you should measure your strut length and compare it to what I have calculated. It sounds like you may have something going on somewhere that is a different length than what I have come up with, but at least this would give you some plans reference to go by (based on my interpretations and measurements, naturally).

Your friendly hyper-analytical engineering freak ,
Brad

[BradW1062]

A Slight Correction: I said above that the toe-in was to be adjusted to zero, but the engineering note gives a tolerance of zero to 0.5 degree for the toe-in. And the note also says the camber should be 0 +/-2 degrees. Or within +/- 2 degrees of vertical.

[Battson]

The tire size really has nothing to do with it - if the tires are properly straight up and down and centered on the axle with standard 6.00-6 wheels, then you should get the 72" tread as indicated on the plans. The plans do not indicate any camber at all, so that's what tells me these cambered ones are not turning out correctly. If you have a larger/different wheel that is offset outboard to handle a larger balloon tire or something, then there would be more width, but certainly not less, and no reason to have camber because of tire size or type.
Brad

Your opening statement is right. Unless you use spacers to move the centerline of the wheel outward, then changing tires does NOT change the 72" measurement.

Those commenting above are also correct. The wheels should be vertical in normal operations, not cambered inward at the bottom or the top, and not spreading over 72" measured at the centre lines of the wheel / tire. The exception is if the aircraft has just been turned on the spot, which causes the gear to splay out.

If the wheels / tires are cambered inwards significantly like the picture above, then when the aircraft is sitting in the three-point attitude, the aircraft is likely to have unwanted toe inward when sitting in that attitude. This can make the aircraft harder to control on the ground.

If your aircraft looks like the picture above, then it would be wise to consider making a change to get the wheels parallel - noting the small allowance for recommended toe-in (0.5 degrees from memory?).