Percy and the bad drivers

The question of tolerance continues to interest me. With those loose connecting rods, how tolerant is Percy of misalignment of the wheels, or uneven crankpin eccentricity? It turns out, Percy is not only tolerant, he’s indifferent!

I didn’t have a front axle with the original crankpins, P87 wheels and the gear printed on, and so I used the one with the slightly thicker crankpins. I reasoned that this should actually make Percy less tolerant of misalignment in the crankpins. I used a spreadsheet to determine the maximum allowable misalignment in the two dimensions – quarter and eccentricity – and printed wheelsets that I thought should just work. These worked so well, that I printed four more wheelsets with increasingly poor alignment.

Along the way, the ridiculous alligator clips through the windows finally broke Percy’s feeder wires. I created new wipers between the axles with some leftover phosphor bronze (PB) sheet and some very fine PB wire. They’re not the neatest work, but a reasonable experimental setup.

For quarter, the maximum that one wheel should be able to wander from 90 degrees is about 7 degrees. I printed a wheelset at 96 degrees, and tried it out, expecting it to work, but to exhibit a tight spot or two per revolution. It worked fine – no apparent tightness. I printed two more at 98 and 100 degrees, and found that at 98 degrees there was a slight thumping tightness, and at 100 degrees, each revolution thumped noticeably, and at slow speeds the mechanism bound.

10 degrees out of quarter is a surprising amount of badness. While 6 degrees is just barely perceptible, I can easily tell that the wheels are 10 degrees out of quarter by looking through the spokes.

For eccentricity, my spreadsheet indicates that I should be able to move a crankpin .020″ without binding. The first wheelset had one crankpin .015″ closer to the wheel centre than the others. This wheelset indeed ran fine, and I tried two more at .022″ and .03″. The .022″ wheelset exhibited a very slight binding (less than the 100 degree quartering), but the .03″ wheelset bound right up.

I would expect that the two variables compound. That is, I probably can’t have both a crankpin that is .015″ closer to the wheel centre and 6 degrees out of quarter. Despite the fact that the thick crankpin on the front wheel does not appear to have an impact, I also suspect that the tighter crankpins should make a big difference. However, so far these experiments have yielded too many surprises to claim foreknowledge.