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Wheel alignment really matters!

scsmith

Well Known Member
When I built the RV8, I used the wheel alignment shims that Vans sells to get my wheels as closely aligned as I could. They only go down to 1/2 degree, so you can only get within a 1/4 degree of perfect. I ended up with just a very slight amount of toe-out on one wheel (something like 0.1 degrees), and the other one essentially perfect.

Fast-forward 13 years, and a silly mishap this winter when I got stuck in very soft mud while taxiing across a dirt field. The airplane didn't go all the way onto its nose, but the prop did slice into the soft mud, so -- new prop blades and an engine inspection (dial indicated crank flange, magnetic-particle inspection of crank accessory drive gear, new bolt and pin, even used "food-grade anti-seize" as specified in the AD--we are still laughing about that).

After that misadventure, I noticed right away that the airplane was a lot more skittish directionally. I just told myself I was just rusty. But I even noticed that it was harder to make it go straight on take-off, and landings were more of an adventure than they used to be, for sure. I also noticed that I was starting to see tire wear on the inside edges, whereas my tires have always worn pretty even before.

So, I checked my wheel alignment again and, AH HAH, I had a 1/2 degree of toe-out. Seems hard to believe that a gear strut would have twisted, and all the sheet metal looks normal, so my hunch is that the gear struts just got nudged aft within the bolt clearances. Measuring to the aircraft centerline, almost all of the toe-out was on one side (the side that used to be about 0.1 degree toe-out was now 0.4 degrees)
Anyway, I switched out a 1-degree shim for a 1/2-degree shim, so now I have just a very slight amount of toe-in, about 0.1 degrees.

Now, my RV8 is a pussycat! What a difference!

This isn't to re-hash the debate of toe-in vs toe-out (I believe a slight amount of toe-in is better than a slight amount of toe-out) but rather to say that you should try to get the wheels as close to straight as you can.
 
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Very interesting point - I had not thought about gear legs moving during an "event". I guess even a really hard landing could do that.

If I recall correctly, I set wheel alignment before I put the tires on. Just wondering if the wheel alignment could be done accurately enough with the tires on...or is there a better way?

Seems with all the cheap lasers available, a good tutorial on wheel alignment would be helpful.
 
Steve, just curious....did you do the alignment with the airplane level or taxi attitude? My experience has shown a change in alignment when lowering the tail from level to taxi.

And yes, I have scratched my noggin over this little discovery
 
Steve, just curious....did you do the alignment with the airplane level or taxi attitude? My experience has shown a change in alignment when lowering the tail from level to taxi.

And yes, I have scratched my noggin over this little discovery

If you have any camber (tire in or out at the top) then you will have slightly different alignment When level compared to 3-point. I check both. I originally did the alignment with airplane level, reasoning that there would be more weight on the wheels in a wheel landing than a 3-point landing, and that might be the dominant influence on landing characteristics. I don't really know.

As I have it now after a slight camber adjustment shim on one side, and fixing the toe-in on the other side, I have essentially the same alignment both ways.

I do my alignment checks on the tires. First, I always roll the airplane forward for a short distance to make sure the tires are in a relaxed position for foward motion. I have a pair of aluminum 3/4" square tubes 3 feet long that I rest on the axle nut and press against the tire with a cinder block (CMU). I center the tubes over the axle (fore and aft) and measure at the front and back ends. One source of error is the raised lettering on the tires. When you are trying to get toe-in to within better than 1/8" over 3 ft. a small error from different letter height on the tire sidewall can be significant. I have just been repeating the measurement several times. But I am contemplating an improvement.

I could drill thru the square tubing in three places. Thru the center hole, a 1/4" bolt could go into the threaded hole (plate nut) in the axle nut. The other two holes would be located so that long bolts would bear against the wheel rim. Adjusting the length of those two bolts with a caliper so they are exactly the same length, and just long enough so that the tube just barely touches the end of the axle nut, would provide a much better alignment tool. To install, you just put the center bolt thru and hand-tighten.
 
So you are strictly talking about toe-in/out and not camber correct? And are the shims used only to correct toe-in/out or can you turn the shims 90 deg and correct for camber?

My rt wheel wears evenly and is tilted in (top of tire leans toward the fuselage) at 0.7 deg and the left wheel is tilted out 2.0 deg. So I assume for the left wheel to wear evenly I would have to adjust the left wheel a total of 2.7 deg.
 
When doing these measurements with weight on wheels, skid plates can remove some sources of inaccuracy. Under each tyre, place two thin, flat sheet metal plates, with grease between them. Having wheeled the aircraft backwards/forwards on to the plates, leave it to sit for a while so that the plates will move to take out transverse loads.
 
I use the brake disk as a reference. Cleco clamped 10mm aluminum extrusion sections to the disks and used a tape measure across them. It was repeatable to 0.1 degrees or so. I used 50 cm long sections which are longer than the ones in the picture.
 

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So you are strictly talking about toe-in/out and not camber correct? And are the shims used only to correct toe-in/out or can you turn the shims 90 deg and correct for camber?

My rt wheel wears evenly and is tilted in (top of tire leans toward the fuselage) at 0.7 deg and the left wheel is tilted out 2.0 deg. So I assume for the left wheel to wear evenly I would have to adjust the left wheel a total of 2.7 deg.

You can use the shims for both camber and toe-in. But this pretty much only applies to the RV-8. I don't know how you make adjustments to the RV-4,6,7,9,etc.
 
I use the brake disk as a reference. Cleco clamped 10mm aluminum extrusion sections to the disks and used a tape measure across them. It was repeatable to 0.1 degrees or so. I used 50 cm long sections which are longer than the ones in the picture.

That is a really good idea if you have 500x5 tires and you can clamp to the brake disc. Much more solid and repeatable. With 380-150 x 5, I don't think I can get anything into the gap between the brake disc and the tire to clamp to the disc.

50cm is a bit short to get good resolution - use pieces that are as long as you can get. A meter would be great.
 
One can jack the plane up and spin the tire and scribe a line on the tire then take the measurements. In my racing days, I used to spray paint the tire prior to scribing simply to make the scribe mark more visible. It's best to relieve any binding by either rolling the plane a bit or use the greased plates as was previously mentioned before taking the measurement.
 
I should have told the rest of my story. I installed the gear legs as per the plans. When I got the axles on I found both wheels toed-in very close to 1/2 deg on both sides. I bought 1/2 degree shims to correct it. Fast forward few months until the plane was flying and I found the shims still in the bag…Put them in. Couldn’t tell any difference in ground handling.
 
I should have told the rest of my story. I installed the gear legs as per the plans. When I got the axles on I found both wheels toed-in very close to 1/2 deg on both sides. I bought 1/2 degree shims to correct it. Fast forward few months until the plane was flying and I found the shims still in the bag…Put them in. Couldn’t tell any difference in ground handling.

Your experience reinforces my opinion that a little toe-in is not a problem, whereas my experience shows that a little toe-out is a big deal.
 
Pazmany wrote a book called Landing Gear Design for Light Aircraft. It is extraordinarily detailed and packed full of all kinds of data for the designer on tires, struts, you name it. At some point he addresses the question of toe in vs toe out for tailwheel aircraft. It is less than a page as I recall. He says, and I’m paraphrasing, that in the course of putting together the book he communicated with every expert in the field that he could find. He concluded that it was impossible to find any consensus on the question. He found just as many proponents of toe in as for tow out and many simply were not sure. So he basically gave up. I don’t know of any more detailed compilation of design data pertaining to light aircraft landing gear design. And I certainly have no clue. So if he can’t answer the question in a general way applicable to all airplanes then it probably can’t be answered. If you guys have tried toe out on the 8 and given it a thumbs down then so be it. Good enough for me. But as for the debate in general I think it will remain right up there with the origins of the universe.
 
In retrospect I should have stayed out of this one. I don’t have enough experience to say what’s good or bad. I’m all over the place with the shims in or out!
 
Wheel alignment really matters

I find it a bit disconcerting to hear an argument here that this is a question up for grabs. I think it clearly it is not. First, the experience from the automotive world is that toe in is clearly highly desirable. It is directly related to stability. While one might argue that somehow things are different for airplanes, I have recently had an experience confirming the importance of toe in. I started noticing significant stability/directional hunting issues on landings, to the extent of gear oscillations that got scary. Some measurements revealed that my gear had acquired about 1.5 degrees of toe out. The original build setting was about 1 degree toe in, which most likely due to some hard landings was lost. With toe in restored the problems went away. The amount of toe in required to provide directional stability is not very much and indeed should be limited to avoid excessive tire wear. Finally, just looking at the geometry associated with toe in v.s. toe out should make it clear that toe in promotes directional stability and toe out is divergent.
 
Just a point............

All tailwheel RVs incorporating the Whitman style gear has toe-out by design.

Van has you set the gear at 0° with the gear in the air. Once you "load" that gear, it has a small amount of toe-out.

I used Pazmany's book to design the conventional gear on my Moni Motorglider in the mid 70s. I initially set it up with small amount of toe-in. It had horrible ground handling characteristics. I changed it to a very small amount of toe-out and it immediately became a pussycat. Coincidentally, my Moni gear design has been used on quite a few airplanes in the U.S. and overseas.
 
Pazmany wrote a book called Landing Gear Design for Light Aircraft. It is extraordinarily detailed and packed full of all kinds of data for the designer on tires, struts, you name it. At some point he addresses the question of toe in vs toe out for tailwheel aircraft. It is less than a page as I recall. He says, and I’m paraphrasing, that in the course of putting together the book he communicated with every expert in the field that he could find. He concluded that it was impossible to find any consensus on the question. He found just as many proponents of toe in as for tow out and many simply were not sure. So he basically gave up. I don’t know of any more detailed compilation of design data pertaining to light aircraft landing gear design. And I certainly have no clue. So if he can’t answer the question in a general way applicable to all airplanes then it probably can’t be answered. If you guys have tried toe out on the 8 and given it a thumbs down then so be it. Good enough for me. But as for the debate in general I think it will remain right up there with the origins of the universe.

Having worked a fair bit in vehicle stability, I think I can make credible arguments to support both points of view. That is why I used the word "believe" in my original post. It may be rather configuration dependent as far as wheel base to track width, and/or gear strut stiffness, and/or ???
On balance, I would say to try to get the wheels as close to zero angle as possible, with weight on the wheels.
 
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