Toe Bar

Home shop machinist.

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Uneven tire wear is usually an indication of a problem with wheel alignment. Most of the time, the issue will be with toe, which is the cant of the wheel, from front to back, as when viewed from above. This month’s project will show you how to make and use a gauge to measure the toe on your airplane.

Camber is tilt of the wheel when viewed from the front. The sketch on the left depicts both wheels with an equal amount of positive camber. This would typically be the stance of the gear in flight or on the ground when lightly loaded. The center sketch is what you don’t want to ever see: one wheel with positive camber and one wheel with negative camber. The sketch on the right depicts both wheels with negative camber. This might be perfectly normal if the plane is loaded to the maximum takeoff weight.
This set of sketches illustrates the basic conditions of toe. Toe-in means the front of each wheel points inward (left). Zero toe is when both wheels are parallel (middle). Toe-out is when the front of each wheel points outward (right).
This set of sketches shows the importance of measuring toe using a reference centerline. Measuring front to back without a centerline merely indicates an overall toe. One wheel could have toe-in and the other zero toe (left). Or, if the wheels are skewed, the toe measurement might show zero toe (center) or may appear to have toe (right), but the wheels will be out of alignment with the fuselage.

Camber, which is the angle of the wheel to the ground when viewed from the front, could also be an issue. So before we jump into making the toe-gauge, a quick look at camber is in order.

With the toe-gauge set behind the tires and lined up to the centerline projected by the laser (left), the pointers are adjusted to the middle of the second groove in the tire tread (right) and locked in place with the thumbscrews.

Camber is relatively easy to check by using an angle finder. Perfect would be the wheels perpendicular to the ground with zero camber. But since most homebuilt airplanes have spring-steel or fiberglass landing gear, as the gear flexes, the camber will vary depending on the loading. The manufacturer will usually specify the camber angle based on a particular loading condition, such as full-fuel and no passengers, etc. If you encounter a situation where the camber is different from the left main to the right, such as both wheels cambered parallel, you should take steps to figure out why and correct it.

Move the gauge to the front and re-center to the laser line. Toe-in, toe-out or no toe will be indicated by the pointer relative to the groove in the tire. The pointer in this example shows the starboard side wheel to be in a toe-out condition. The amount was just over half the width of the groove, or about 0.10 inch (right). A bit of trigonometry suggests that a 0.5° wedge is needed to straighten the wheel. Note how the pointer is attached to the trammel via spot welds (right).

Back to toe. Critical to measuring toe is to have a good centerline reference. You can use a string or snap a chalk line along the ground to represent the centerline (front to back) of the airplane. What I use is a carpenter’s laser level to cast a beam on the floor of the hangar along the center of the fuselage.

The shop-made toe gauge consists of the crossbar (painted red with a centerline mark) and a set of locking pointers (left). The length of the pointers must match the radius of the tires being measured. An inexpensive laser pointer sourced at the local home center completes the set (right).

With the centerline reference in place, position the toe gauge behind the tire, center it to the laser line and then set the pointers to the tire reference line. With the pointers set, move the gauge in front of the tires and line up the centerline. The pointers will then indicate the amount of toe-in, toe-out or zero toe for each wheel.

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To center the nut on the trammel for welding, lathe-turn a shoulder on the nut (left) to fit a matching hole in the trammel (right).

This is a simple gauge to use and almost as simple to make. It’s really nothing more than a trammel with long pointers. The material needed is a length of 5/8 x 5/8 x 0.065 square tubing. This should be at least a few inches longer than the stance of your landing gear. You will also need some 16-gauge steel sheet to make the pointers and about four inches of 3/4 x 3/4 x 0.065 square tubing to make two sliding trammels. To receive the thumbscrews that clamp the trammels in place, each trammel has a 1/4-20 nut tack-welded opposite the pointer.

The turned shoulder makes tack-welding the nut in place a much easier job.

If you find the toe on one or both wheels needs to be adjusted, the usual method is to add a tapered shim between the gear and the axle stub. It takes a little trigonometry to calculate what angle shim you need, but it’s worth doing because having the wheels aligned means your tires will last longer, and there will be less stress on the gear from the wheels trying to steer off in different directions.

Safety tip: Use only “raw” plain steel (left) fasteners for welding. Fasteners treated with shiny or colorful coatings such as zinc (center) or cadmium yellow-chromate (right) will give off toxic fumes when welded.

You can buy tapered shims to correct camber and toe from Aircraft Spruce. They are listed under “realignment” shims. Of course, you can make your own! If you’ve got back issues of KITPLANES® or are a subscriber with access to back issues online, look up the “Home Shop Machinist” in the June 2016 issue.

That’s it for now, time to get back in the shop and make some more chips!

1 COMMENT

  1. Muriatic Acid, at pool “service” concentration, can be used to “dissolve” most commercial forms of plating – Cadmium being the most widely used. Removing the plating does two things for the weld & welder. For the weld, it removes the contaminants that interfere with weld quality & integrity. For the welder it removes the toxic fumes developed when the plating is heated from the weld process. The only caveat in using the acid to remove the plating is the fume of the acid itself – a little bit upwind and a little separation will remove most of the exposure, and the acid, at that concentration, isn’t very aggressive on skin from accidental contact. Just put the washers, nuts or ? in a open container, pour the acid over (a little foaming) them until covered – when the bubbles stop (effervescing completed), return the acid to its container and rinse the parts with water to stop the process.

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