Percy’s wide chassis

For Daylight Savings Hour —- that magical extra hour of modelling time you get once a year when all the clocks go back — I designed and printed frame spacers for Percy that give the chassis dimensions similar to #622.

For the first time I also tried pushing the chassis on glass, which turns out to be a great litmus test for chassis smoothness. With a wide chassis like this, it pays to ensure it rolls smoothly on glass before installing the rods, and again after.

Having now lost two wedges to the ether, I’ve started to secure them with blobs of reusable adhesive. This enables me to turn the chassis upside down without hearing the tiny tinkle of parts hitting the floor.

Once I’d assembled everything, Percy ran happily up and down the track a number of times. Then (get this!) it jammed solid. I’ll have to disassemble and reassemble it to find out why, but perhaps I am finally going to get to the bottom of this mystery.

8 thoughts on “Percy’s wide chassis

  1. Does the latest version represents another design change? The width of the frames? That seems almost incomprehensibly unimportant to me.

    At some point, it just won’t be Percy anymore. You’ll have to decide when that is . . .

    Will be awaiting the story of disassembly!

  2. I’m assuming you know that your track is perfectly level from side to side on the layout and correctly in gauge in your testing area? I’m wondering if that somehow is factoring into the issue some how. Or are you testing via the workbench? If the chassis is rolling just fine on the glass and not jamming then that leads me to suspect that the issue is track related. Does/did #622 roll fine on the workbench prior to the motor install?


    1. Here’s my thoughts behind the track gauge being an issue. Don’t know how true it might be. A tight gauge would cause a wheel/flange to raise above the railhead, but if that wheel has compensation, then the wheel instead of riding up the rail instead flexes slightly in the frame (which we would expect, right?) But because the connecting rod is rigid to the wheel and even motor that vertical movement is just enough to cause binding momentarily. Then as the wheel drops, that tension is removed, and suddenly it’s not binding anymore. Bind it long enough, and then it’s forced to get out of quarter.

      I have no clue if this is actually a viable discussion but would it be worth building a small 6- 12″ section of track on the workbench that is tight and loose gauge? You could almost even print ties/shims of various heights to see if the cross level of the track makes a difference as well.

      If everything rolls fine on glass my logical guess would be the rail interface.

      Can you run Percy via motor leads with the 3d printed wheels so this theory could be tested on glass.

      1. Interesting theory, Craig. Maybe there is a case of hammering the joint by repeatedly raising and lowering a wheel…

        Of course the rods would have to be a very close fit to begin with as the vertical movement is less than a mm, and that doesn’t make a much longer distance between crank pins.

  3. Did a wheel slip on the end of the axle, changing the quartering and leading to the jamming? As someone who is building a 1/3 scale, 7.25″ gauge steam locomotive I can testify that a chassis jamming is never a good thing!

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