On Sunday, May 3, 2020 at 6:17:40 AM UTC-7, wrote:

Scrubbing is what happens when you turn a tire on a radius.

If the wing wheel is biased forward, so that it is inline with the main gear axle, there will be no scrubbing as the geometry is correct for any radius turn.

Ignoring who said and did what, I just learned a useful geometry lesson.

Assuming 'scrubbing' is when the rubber on a tire rubs instead of rolls over the pavement.

Another symptom is 'hopping' where the wheel tracks wrong and rolls out of position flexing the mount. Eventually, there is enough force from the flex to make the wheel hop back into position. Definitely some 'scrubbing' involved as well. Definitely a worrying situation because the flex is putting weird loads on the flying parts.

If the wing wheel is vertical and in line with the line of travel, why would there be scrubbing when you are going straight?

If in addition, the axle of the wing wheel points to a line directly above the main wheel, why would there be scrubbing when the glider makes a turn?

Maybe in the limit, neither one of these can be aligned perfectly, but if you put the wing wheel in the right place it ought to work pretty good.

That explains why my old wing wheel still hopped on curves even after I adjusted it so it points straight ahead. Also why the new MM one doesn't.

Guess a swivel might be another out of the hole, but not the only way.

To be correct geometry for a specific radius turn, the wing axle must point towards center of the turn radius. The axle lines of the main and wing wheel intersect at this center. If the main wheel is the center, then the wing axle must point towards it. A swiveling wheel will align itself to this geometry. For a non swiveling wheel to be correct geometry for *any* radius, the axles have to be in line (more specifically their projection to the ground plane has to be on a line), guaranteeing that both axle lines intersect at the circle center regardless of radius. The wing wheel will then not scrub, even if it is the center of the turn. It will turn about its vertical axis, just as the main gear (or indeed the swiveling wheel) must, in other circumstances. That is not "scrubbing" as the term is usually applied.

And now, back to the popcorn....