landing gear alignment

russfarris · Post by **russfarris** » Mon Nov 22, 2004 6:06 am

Hi guys, I've had some abnormal tire wear on 43A, so I decided to check the alignment and re-do it as required to bring it into specs.

The right main, with one rotation with the left at about 120 hours, (now 250) has excessive wear on the outboard half of the tire. (So does the left, but not quite as bad.) I used the Cessna method shown in the 100 series, 1962 and prior manual to check toe in/out. I used plastic garbarge bags instead of greased plates to unload the gear; worked great!

Both mains measured 3/16 toe-in; three times the book value of zero degrees to 1/16 toe-in. Well, that explains the tread scubbing off the outside of the tire! I removed the right axle, with the two shims and re-installed them to reduce the toe-in. It's still out of limits at 1/8 inch toe-in.

First question. I'm doing this with the airplane weighing only about 1,400
pounds. I'm well aware of the fact that the book says to use about 2,000 pounds doing this alignment. It's easy to see how weight affects camber, but a Cessna SB from 1948 (yes, I know) states camber has little or no effect on tire wear, compared to toe-in/out. I do plan on filling the tanks and adding water ballast in five gallon jugs to bring it up to my average flying weight of 1,700 pounds, and checking toe-in again. To the point: does weight affect toe-in or out? If it does, I would think achieving zero camber/zero toe/out at my average flying weight, which certainly isn't 2,000 pounds would be the correct procedure.

Second question. The shims have no parts numbers. If I need to reduce the toe-in further after bringing the weight up, how do I know which ones to order? The parts book gives the degrees each shim can change camber/toe, but no dimensions. And they are pricey new. If I get them from the junkyard, what do I tell them I need? Two local 170s have washers installed to get the correct alignment, but that doesn't seem right, or legal for that matter. Think stress riser. Also, the wheel fairing mount was mounted to the gear strut, then the shims and axle. This is not the way the parts manual shows, which is shims, fairing mount then axle. Correctly assembled the mount is awfully close the the brake disc, but it should be OK, at least on the right side.

Incidently, I ordered two of those "Classic" retro tires from Dressler like George did, with the diamond tread. They look exactly like the pictures in my 52's Owners Manual, which sent this originality buff into low earth-orbit!

Any ideas or thoughts out there would be greatly appreciated...Russ Farris

Post by **GAHorn** » Mon Nov 22, 2004 10:24 am

Aircraft Spruce's shims (using the same PN's as Cessna) are specified as to the amount of camber/toe-in adjustment provided in inches. They are usually very helpful in going to the shelf and making measurements when I've called them about other items.
According to their catalog PN 0441139-5 ($66) provides 0.12" of toe-in adjustment (and 1/2" of camber). 0441139-6 ($30) provides 0.20-0.25" toe-in (1" of camber), and 0541111-2 ($121) provides 0.10-0.25" toe-in (and 2.5" of camber!)

russfarris · Post by **russfarris** » Mon Nov 22, 2004 3:54 pm

Thanks GA - I'll report back on any toe-in changes once I run the weight up to 1,700 pounds or so. Russ

blueldr · Post by **blueldr** » Tue Nov 23, 2004 6:23 am

I suggest caution when using plastic garbage bags instead of grease plates. I found they worked with a light airplane with low pressure in the tires, but with high tire pressure in a heavy airplane the footprint pressure was too great on too small an area and the bags would not slide freely enough . The footprint pressure is spread out by the metal grease plates and allows the gear to completely unload its pressures.

Dave Clark · Post by **Dave Clark** » Tue Nov 23, 2004 2:51 pm

I've used the plastic garbage bags whilre doing my Cessna 195 which has a much hugher loading PSI than the 170. You can tell how wellit's working by a sloght push side to side. It should slide real easy.

zero.one.victor · Post by **zero.one.victor** » Wed Nov 24, 2004 1:21 am

Russ, I'd have to dispute anyone who sez camber has no effect on tire wear. My wheels have positive camber -- tops out-- and the inboard side of the tire gets little to no wear. After a couple hundred hours, I rotate (reverse) the tire on the wheel as to start putting the wear on the fresh side of the tire. It's easy to see that the tire wear on the outboard side is from the camber--after all, the tire can't wear where it doesn't touch the ground. I kinda like the weay the camber's set-- I get twice the mileage outa my tires.
I'm not saying that this is necesarily the case with yours, but it may be a factor.
Also, it seems like a little slippery stuff (oil?) inside the garbage bags might help them slip around with a load on, if high tire pressure seems to be preventing that.

Eric

russfarris · Post by **russfarris** » Wed Nov 24, 2004 3:33 am

Thanks for the replies on my questions on gear alignment. Here's what I think about some of the answers I got from the 170 brain trust.

Dick: Thanks for the information on using the garbage bags. I had both mains at the recommended tire pressure, 24 PSI. Instead of 2,000 pounds
as the owners manual suggest, I loaded her up to about 1,800 - my average flying weight. The gear easily unloaded when pushed back and forth across the bags, so I guess I lucked out!

Eric: That little tidbit about camber having little affect on tire wear came from a 1948 Cessna service bulletin I found on the Internet (and it's also in the IC170 books, I forget which one.) They state that toe-in or out has a much greater affect on tire wear than camber, and I can see why. With toe-in or out, the rolling tire is at a slight angle from the direction of travel; it's literally scrubbing the tire tread off. If the camber is off, more weight is placed on say, the outboard section of the tire resulting in greater wear in my opinion, but I still think it's not as critical as toe-in - the forces at work are not as abrasive. In 1948 Cessna had only two years experience with the Whitman gear, albeit with thousands of hours of field experience with the 120/140, 190/195 and the new 170. On reflection, I've always wondered have much outboard tread wear was due to the fact the gear in-flight and at touchdown hangs down well into positive camber; it's just the nature of the beast.

GA: Thanks for the info on the shims from Aircraft Spruce, but it looks like I won't need them.

Now on to the alignment. I loaded 43A up to about 1,800 with full fuel and water ballast in five gallon jugs. Removed the left axle and re-oriented the shim to the recommeded Cessna position - thickest part up and forward (don't ask where it was!) re-assembled correctly, this time putting the fairing mount between the shim and the axle...no wonder my wheel pants have been so hard to put on!

Rolled it across the bags, got out the square and ten foot hat section straight edge...and the stars and planets must be in perfect alignment,
because I now have ZERO toe-in/out both sides, just like the book says.
I must have measured it five times to make sure I wasn't dreaming.

Still some positive camber, but not much. I'll bet at 2,000 pounds it would be zero, just like Cessna intended using the stock shim 0541111-2 properly installed.

Of course, the real test will be when I take it out for a test hop. If it wants to head for the boondocks on take-off, it's back to the drawing board!

To answer my own question, the weight change from 1,400 to 1,800 did change the toe-in from 1/8 to zero on the right main (before I corrected the left .)

Thanks for the input men, hopefully my new retro tires will last longer than 250 hours. Being an airplane, 43A showed me the next project - both pistons have a little leakage, so next I'm off to brake caliper rebuilding land...Russ Farris

mvivion · Post by **mvivion** » Wed Nov 24, 2004 4:37 am

Russ,

Thanks for sharing your experiences with working on proper gear alignment. This is the single issue that I've seen that turns people off on Cessna tailwheel airplanes. They get an otherwise really nice airplane, with poorly aligned gear, go fly it, and get spooked that it is overwhelmingly difficult to fly. Too bad.

All it takes is someone like yourself, willing to take the time to research and experiment and make the gear just right, and you'll find that the aircraft is, indeed, a very nice plane to fly.

Thanks again,

Mike Vivion

Post by **GAHorn** » Wed Nov 24, 2004 4:59 am

By the way Russ, ...since the PN's of hte Spruce shims are identical to the Cessna shims...I'll bet dollars to doughnuts the figures regarding their effect are identical as well. (That was my suggested intent anyway.)

russfarris · Post by **russfarris** » Wed Nov 24, 2004 5:27 am

Gosh, Mike thanks for the kind words...I'm not being entirely altrusitic here, I just want to keep my tires from a premature death. If my experiences help anyone, I'd be very gratified.

Some thoughts on toe-in/out on taildraggers. I've flown R/C model airplanes for 30 years, and the consensus is to have a little toe-in on taildraggers (obviously wear on a 3.00 dollar tire is a lttle different than
a 75.00 one. Flying off grass makes a big difference, both in wear and handling, for airplanes big and small.)

The theory is with toe-in a change in direction will increase the drag on the tire opposite the direction of the swerve, i.e. to the right places the left side in a higher cornering force, increasing the drag and turning the model...er, airplane back toward the left. Toe-out would have a DECREASED drag on the the left, depending on the amount, which would make the airplane head even more to the right. That's the idea, anyway.

Zero toe-in/out is a compromise between tire wear and directional stability. In the Cessna specs, zero to 1/8 toe-in is acceptable on the 120/140, 190/195...and 1/16 on the 170- but no toe-out!

It would be interesting to see what my airplane is at 2,200 pounds in regards to toe-in/out since it did change from 1/8 toe-in to zero from 1,400 to 1,800 pounds.

Stay tuned, Russ Farris

zero.one.victor · Post by **zero.one.victor** » Wed Nov 24, 2004 5:30 am

Russ, from your comment about the 10-foot straight-edge, I assume that you laid the straight-edge across the front of the tires and squared off that to check the toe-in/out. This is great, assuming that: 1) the tires are the same width (rim-to-tread), a reasonable assumption; and 2) the main gear legs are aligned fore-and-aft with each other, maybe not so reasonable an assumption. I have heard of several airplanes where this was not the case.
You can check this second item by measuring from a given point at the aft end of the airplane ( I used a bolt on the bottom centerline of the tailwheel bracket) and another given point at each wheel- I used the lower, aft axle bolt.
If the LH/RH gear legs are off from each other fore & aft, the straight-edge across the front of the tires is NOT square to the airplane, and all bets are off.
Some might call this nit-picking, but as long as you're making all the effort to properly align your wheels, I'd double-check the gear leg alignment.

Eric

N170BP · Post by **N170BP** » Wed Nov 24, 2004 5:34 am

I've heard that excessive toe-out can actually rip the gear
out of the airplane (dunno if it's true or not, have no way
of substantiating that claim/comment).

Mine is currently set up with a very slight toe-in on one side
and about 3/8" toe-in on the other (as measured at the horizontal
stab leading edge)

I'd like to get my hands on a "grab-bag" of axle shims to
see if I can make it any better.

russfarris · Post by **russfarris** » Wed Nov 24, 2004 5:38 am

That's an excellent point, Eric. After a take-off crash in 1954 and a mid-air in 1956 who knows how true old 43A is...I'll measure it with your method tommorow to make sure the Cessna procedure is accurate - See Ya, Russ Farris

zero.one.victor · Post by **zero.one.victor** » Wed Nov 24, 2004 5:43 am

Bela's post brings up another interesting aspect of the alignment question. Where do you measure the toe-in/out? It sounds like he measured by extending a line from the wheel ( perhaps by holding a straight-edge against the brake disc) back to the horizontal. It wouldn't take much toe-in/out at the wheel to indicate 3/8" way back there.
Or do you measure at the wheel itself? A sixteenth measured at the wheel would equate to an inch or more measured at the horizontal.

Eric

zero.one.victor · Post by **zero.one.victor** » Wed Nov 24, 2004 5:53 am

From everything I've heard & read (in the 170 Book & the SRAM book), zero to 1/16" toe-in is what you're looking for with a 170 or 180. But it doesn't say why.
Some friends of mine, who are very taildragger-savvy, say just the opposite--just a little toe-out is the ticket. They claim that toe-in will result in increasing a swerve. For example,if the airplane swerves to the right, it puts more weight on the LH main, which--being toed in-- will want to continue the swerve to the right. They say if the LH main was toed out, it would correct the swerve in the above example, instead of making it worse.
Sounds logical to me. Who can tell me why toe-in is supposed to make for better handling? I would think that ZERO toe-in/out (neutral castor) would be best.

Eric