Bleven’s Tech Tips

October 2004

Tire Wear and Wheel Shimmy

Tire wear and shimmy certainly can be encountered on our airplanes. Tire wear is a direct reflection of its own cause. A tire that wears only in the center indicates excessive tire pressure. Tire Wear on the two outer edges is an indication of low tire pressure. A single tire that shows wear on one side only is an indication that this tire is not tracking in the same line as the other tires.

Most of our airplanes have three wheels, two of which carry the bulk of the weight and the third, which carries less proportionately. With this form of weight distribution, most misalignment situations will show up on the main gear tire that carries the least weight. The wheel with the lightest load will skid along. A spring and the steering linkage generally cushion the third or steering wheel in our planes. As a result, this wheel will normally just track the main gear tire with the most influence.

If our plane has side-by-side seating, and is normally flown with pilot only, a worn tire on the side opposite the pilot (this would usually be the right side) can be caused by an alignment problem with the left main gear.

Your weight and ground steering habits must always be considered as they too influence tire wear. As with many airplane systems, compromise is often needed. CUPPED or SCALLOPED tires are not the result of wheel balance as often is thought. Tires are designed to carry a specific load at a specific tire air pressure. When this pressure is too low, or the load weight is exceeded, the outer circumference of the tire is compressed. This condition creates a smaller rolling circumference, and now for every revolution of the tire, the surface rubber must be skidded between the tarmac and the new tire circumference. When the tire is scalloped on one edge only, two problems are likely to exist. Tire manufacturing often includes microsiping (small cuts in the tire to help the tire to gradually skid and eliminate some of this cupping)

Wheel shimmy is found in two different planes. Up and down, or horizontal bouncing, is generally not common on our planes. The wing and horizontal stabilizers generally offer up and down resistance to this bouncing, much like a shock absorber does on a car. Also, the small diameter of the wheel does not create a large amount of unbalanced rotational energy. The most frequent and likely shimmy is in a side-by-side plane and is experienced with the steering wheel. As the wheel is turned, the pressure within the tire increases, as do the loads imposed on the gear and springs. When the wheel again becomes straight in tracking alignment, the pressure in the tire goes back down to its original inflated pressure. Hence the result, wheel turns, pressure increases, wheel bounces straight (or generally past straight to the other direction) and it bounces more forcefully in the opposite direction.

Steering springs that allow us to have solid rudder control use spring loading of the tail or nose wheels. These springs can also cause a shimmy frequency to develop, hence two different diameter springs may be used. Sometimes we see a steering shock absorber on nose wheel airplanes and occasionally on tail wheel planes.

A locking tail wheel can do a great job of eliminating tail wheel shimmy, although that is not its primary function. Like a bouncing basketball, when the ball strikes the ground, the pressure within the ball increases. The ball then bounces up with a resulting pressure decrease. You strike the ball with your hand and force it down, repeating the cycle of increasing/decreasing pressure within the ball. The spring would be like your hand, the tire is the basketball, and the tarmac produces the side bounce.

Worth noting here is how important positive caster angle is in eliminating this common cause of wheel shimmy. When the caster angle is negative and the wheel turns to the side, it will actually fall into the turn, which produces an even greater bounce. Positive caster angle actually helps this pressure increase to become absorbed at a horizontal angle, or in the up, down bounce, which will be absorbed through the wing and/or horizontal stabilizer.

Dick Bleven

E.A.A. Tech #4518