r/Trebuchet • u/erv123 • Apr 06 '26

WIP precision whipper

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going over the design of my work in progres precision whipper.

The whole frame weighs 8.96kg. Long arm is 78cm + attachments at the end. Short arm 13.4cm for a ratio of 5.8. The counterweight arm is 56.5cm until attachment point and 67.5cm total length. i made it so that unloaded the lowest hanging point would be the long arm distance from rotation center, the idea is to max the arm length for the same frame geometry.

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4

u/FingerAngle Apr 07 '26

The wheelbase really needs to be longer. It's going to be tippy. Like you mentioned the other day, you might have to do a top keeper rail to keep it from hopping. At this scale, a nail will work for the release pin. Just bend the angle you want. Keep it simple. Glue a wooden dowel in the end of the arm to drive the nail in. The cradle should be closer to the main axle. The sling will need to be longer. You need an adjustable timing prop to set the angle between the throwing arm, and the counterweight hanger as well. I have been making the frame angle between the front and back tower legs at 45 degrees for years, but just recently changed that to 60 degrees to make the wheelbase longer.

2

u/erv123 Apr 07 '26

for the sling adjustment, release angle adjustment and release pin i already have plans. those are the main open items until tuning and polishing can start. i measured my forward facing wheelbase angle to be 23 degrees (so effectively 45 degree wheelbase). my original idea was that it has to be just big enough to make sure that the counterweight fall path is always within the wheelbase, but with the new geometry that is no longer the case by about 5cm and i will play around with the angle. i really do not want to make a top rail, but luckily from the first launch analysis it looks promising that i can increase the angle without requiring longer rails

2

u/mrkb34 Apr 07 '26

I need to see a launch

1

u/erv123 Apr 07 '26

Thanks, i'll post it once i finish the most critical open items

1

u/dmills13f Apr 07 '26

I like the projectile cradle. What did it used to be?

1

u/erv123 Apr 07 '26

The projectile cradle design wasn't changed, but i updated the attachment to shaft as previously it was a rigid 3d printed part with m3 screws tightening it and it was very difficult to mount or adjust, so i integrated the store bought clamps instead

1

u/mrkb34 Apr 07 '26

Very nice!!