Question re alignment of gear legs

lynnem · Post by **lynnem** » Thu Mar 17, 2005 2:17 am

I am currently rebuilding my 170 and am working on wheel alignment. One gear leg appears to be approx. 1/2" behind the other. How critical is this? Where can I get information regarding specs (i.e., vertical wheel alignment and toe-in/toe-out...)?

Any help would be greatly appreciated.

Pat Mitchell.

N170BP · Post by **N170BP** » Thu Mar 17, 2005 3:36 am

Can of worms, meet Pat Mitchell.
Pat Mitchell, can of worms....

Now that you are formally introduced <grins>

Some folks say it's not critical.

I happen to think it is.

Unfortunately, the only adjustments you can make
are at the axles themselves with shims.... so either
the gear-box area is out of whack, or a gearleg is bent.

What I would do:

Verify you have a matching pair of gearlegs (not one off
of a ragwing and the other side from a B or whatever).
Eric Taylor gave us a link a few days ago on how to identify
the various Cessna gear legs via Tailwheel Tommy's (XPMods)
webiste. Once again, the link is:

http://www.xpmods.com/maingear/identification.asp

Zero to 1/16" toe-in seems to be the agreed upon target.

Consult the 100 series service manual which has a section
devoted to this procedure (the 180 procedure will work
for the 170).

Post by **GAHorn** » Thu Mar 17, 2005 4:01 pm

That's funny, Bela. (maybe not so funny to Phil, but it's funny!)

And good advice.
Phil, the gear alignment data is found in the 100 series Cessna Service manual, 1962 and prior. It's also found in the MX Library of this web forum. Zero toe-in/out is preferred at 2,000 lbs wt.
If one wheel seems farther fwd than the other, and that is the only vagary, then you should confirm that by more than just visual observation. You can use a transit, or you can measure from a common point such as a common point on the tailcone to an inner axle attach bolt. Be sure to use the same bolt and reference points on each side. Do not measure from another landing gear such as the tailwheel or any of it's flexible points.
A couple of our members have suggested having the main wheels on a couple of plastic garbage bags to allow the wheels to slide and relieve side loads when making alignment measurements as opposed to the manual recommendation of greased plates. (I apologize for not being able just now to recall and give credit to that person(s). But it's a great idea.
The matter which Bela refers to regarding differing opinions is whether or not it's important that an imaginary line be established longitudinally in alignment with the fuselage...that should be used to determine if the wheels actually point directly forward/aft...or whether it's OK that they be slightly off-alignment with direction of travel. I'm one of those who believes that toe-in/out is only important to the other main wheel. As long as that requirement is met, it's relatively unimportant (in my opinion) as to whether those two main wheels are precisely aligned with direction of travel. (In other words, as an example, as long as they aren't fighting each other, then tire wear and aircraft handling is unaffected by a small degree of off-alignment of the matched pair to direction of travel. The airplane will neatly follow and it will be imperceptible to the pilot.)
If you truly have one wheel significantly in front of the other you probably have structural damage or mis-repair or mis-matched gear legs as Bela suggested. But if the difference is within an inch (not a hard spec) I'd consider it unnecessary to correct as long as the gearlegs are correct/matching part nos. and no structural damage is present.
Keep in mind that before serial no. 25612 the legs were interchangeable and after that sn they weren't. This was the sn that began the so-called "lady legs". So it's important to determine you have a gearleg set that corresponds with each other.

Dave Clark · Post by **Dave Clark** » Thu Mar 17, 2005 4:54 pm

gahorn wrote: A couple of our members have suggested having the main wheels on a couple of plastic garbage bags to allow the wheels to slide and relieve side loads when making alignment measurements as opposed to the manual recommendation of greased plates. (I apologize for not being able just now to recall and give credit to that person(s). But it's a great idea.

I get to take credit for that

And it does work great.

zero.one.victor · Post by **zero.one.victor** » Thu Mar 17, 2005 6:42 pm

Using a transit seems like overkill to measure symmetry of the MLG legs, plus I don't know how you'd do it- but then, I'm not a surveyor either. It's easy to triangulate from a point aft- a bolt, seam, or rivet back near the tail. You can even use the front of the tailwheel for this measurement, as long as someone isn't bouncing the airplane between taking readings.
BTW, I have heard of a 180 that had 2 different model gear legs installed (early and late) with considerable difference in rake, and it allegedly handled just fne (per Mike re: Marlin's airplane, Bela). I don't see how it coulda handled any way other than strangely, but there you are.....
An easy way to measure castor (toe-in/out), besides laying a straightedge across the front of the tires & squaring the wheels or rotors off that, is to snap a centerline on the hangar floor,using marks plumbed down from the tailcone & the firewall. Then clamp a piece of angle or something to the brake rotors, plumb down marks at measured distances fore & aft of the axle, and measure over to your centerline. Like George sez, zero toe-in is spec'd at 2,000# airplane weight, and don't forget to float the tires by using the greased plates or Dave's garbage bags.

Eric

blueldr · Post by **blueldr** » Sun Mar 20, 2005 4:47 am

I tried the garbage bags the last time I was doing an allignment and found that it wouldn't work for me, especially at a weight of 2000 pounds. Too much friction on a relatively smooth concrete floor. ---even using three or four double layers of the plastic. Maybe my bags were the wrong brand.
I reverted to the grease plates and had no further problems.

ak2711c · Post by **ak2711c** » Sun Mar 20, 2005 8:44 am

I had the same problem with the bags. My plane was empty too. Oh well I had grease plates near by and was just trying to avoid the mess.
Shawn

Post by **GAHorn** » Sun Mar 20, 2005 2:41 pm

Aerosol cooking spray-oils (like brand-name "PAM") sprayed on a plastic sheet, then with another plastic sheet layed upon it, to roll the airplane onto..... I wonder if that'd improve the action any? It'd still be an easy clean-up for the casual mechanic.

Dave Clark · Post by **Dave Clark** » Sun Mar 20, 2005 3:06 pm

Funny the bags worked well on the 195 and it's, well, a little bit heavier. I used two bags under each tire.Maybe I used the heavier millage bags but I like Georges Pam idea.

AR Dave · Post by **AR Dave** » Sun Mar 20, 2005 11:30 pm

Wouldn't putting Pam in the bag do the same. (No Joe!) You could put greese or anything in the bag (give you plastic on top and bottom). No mess. Two bags sounds good to me. Where is a good place to find scales? Kyle mentioned race track.

blueldr · Post by **blueldr** » Mon Mar 21, 2005 5:09 am

Re: The use of plastic bags---- I wish now that I had tried some of my girlfriends cold cream!!!

Curtis Brown · Post by **Curtis Brown** » Fri Sep 08, 2006 3:53 pm

It is annual for me. I need to check my wheel alignment. I get very uneven wear on the tires and they do not last long.
My question is from the IA who has ask me to find shimms.
Where do I find them? What will we need?

N2865C · Post by **N2865C** » Fri Sep 08, 2006 4:26 pm

It is never a bad idea to check your alignment, but in my experience with Cessna Spring gear tire wear will always be uneven (maybe I should check my alignment

). If you look closely when a 170 takes off, when the weight comes off the gear the gear "springs" inward a considerable amount. This rotates the tire inward and results in the outside of the tire getting a lot of abuse on touchdown. I just rotate my tires at annual and it all evens out pretty well. Check your IPC for the shim part #'s. iwantcessnaparts.com has em...$$$

Post by **GAHorn** » Fri Sep 08, 2006 9:29 pm

Shims are available from any Cessna distributor...and of course, Aircraft Spruce. http://www.cessnaparts.com http://www.aircraftspruce.com

russfarris · Post by **russfarris** » Sat Sep 09, 2006 4:35 am

I went through this gear alignment business a few years ago, some of you may recall. The plastic garbage bags worked fine for me, BTW.

I know that conventional wisdom has it that spring steel gear Cessnas
wear down the inside of the tire quickly, but Cessna says that toe-in or out is much more of a factor, and my recent experience bears this out.
Before I corrected it, excessive toe-in was wearing out the the inside tread
of my tires in less than 100 hours. With the gear at zero now, at 175 hours both tires look like I put them on last week. I seldom fly off grass!

The improvement in ground handling and tire wear with the correct alignment was dramatic. I highly recommend checking the gear out...Russ Farris