Main Landing Gear Wheel Alignment

The main gear axles should be checked for proper alignment with the plane in the three point position, and with the tread at 72 inches, measured from the center of one tire to the other. Check that both axles (referencing off the brake discs) are square with a centerline dropped with a plumb bob off the engine crankshaft center (or nose of spinner) and the tail post or tail spring bolt center. TOE IN should be 0 to ½ degree. Bearhawk Main Landing Gear AlignmentTo get the tire tread at 72 inches, you may need to roll the plane forward or backward. Rolling the plane backward spreads the gear out, and rolling the plane forward moves the wheels closer together. There are a few ways to measure the alignment. There will be first a description of how to check your toe in/toe out on a project with the brakes mounted and also the weight of the engine and wings on the gear. Then there will be guidance on how to check your landing gear if you don’t have the brakes mounted yet or the engine/wings on. It is interesting to see how the tread measurement changes as you roll the plane forward or backward. Start by rolling the plane 10 feet forward using NO brakes to stop it. Touching brakes will interfere with getting good measurements. Then repeat the same measurements after rolling the plane backward for 10 ft. You will see a difference in the measurement of tire center to tire center or the distance between the two brake discs if you do this. This will give you an indication of whether you have toe IN or toe OUT. The goal is to have around 2 inches of difference between these measurements, from toe-in. That means the measurement after backing up should be 2 inches less than the measurement after going forward. You can purchase two 4 ft straight steel angles. Clamp them to the brake discs as shown in the picture. In the picture, the steel angles are clamped to the bottom part of the brake discs with the brake calipers loose. You can just as well clamp the angles to the top of the disc. Just make sure you get two clamps on the steel angle with enough distance between the two clamps so you get the angle true to the brake disc. Different brake calipers mounted in different positions might require a different placement of the angles. But you get the idea. Adjust the angles until they are parallel with the ground to get the most accurate measurement. Do not use the tires because they are not flat and the measurements off them will be inaccurate. Main wheel alignment You can then measure from one brake disc to the other to get a “baseline” measurement for the spread from one side to the other. This measurement can be at the aft edge of the brake disc or the forward edge. Then measure the separation of the steel angles out 40 inches in front where you took the first measurement. If the measurement at 40 inches forward is less than the measurement you took of the brake discs themselves – you have toe in. If the measure is bigger at 40 inches in front, you have toe out. NOTE: these measurements so far give you the total toe in/toe out but not the position of each wheel/tire in relation to the fuselage centerline. Drop a plumb bob off the crankshaft centerline or the spinner tip. Do the same off the tailpost or the bolt center that holds your tailsprings. Pop a chalk line or mark on the floor this centerline. Then measure from the steel angles clamped to your brake discs to this centerline to determine if the toe in/toe out for the left and right are the same. Measuring tolerances are about ¼ degree because of all the moving parts that can make the measurement hard to get completely accurate. At 40 inches in front of where you took the first measurement, ½ degree toe in would show a measurement .350 inches closer to the centerline than at the disc 40 inches aft. This is for each gear to the centerline. Total for both would be twice that. Again, what you should end up with is Toe IN of 0 degree to ½ degree. To measure the toe in/toe out if you don’t have brakes, tires, engine, or wings mounted yet – you have to use a different method. Level the fuselage by using the lower longeron as shown on page #16 of the plans (Datum, between station C & D). You will have the front of the fuselage on a saw horse and the tail up in the air on something. Attach your gear legs. Make a mark on each axle that is 3 3/8” outboard of “ring” that your brake calipers mount to. This mark is more or less the center of where your tire will be on the axle after the wheels/tires are on. Then spread the gear legs until the marks on your axles are 72” apart. Make sure the distance between each mark and the centerline of the fuselage are equal. Using a long straight edge determine if your axles are in line with each other (zero toe in/ toe out) or at most have ½ degree toe out per side while measuring with your fuselage in this position. Refer to drawing #23. Camber should be zero degrees with +/- two degrees.

Articles with the green titles are part of a kit builder manual section that is still under development. If you are the owner of a quick-build kit for a 4-Place Bearhawk and would like to participate in beta testing, please let us know!

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