Yet Another Cycling Forum

Random Musings => DIY => Skip Bike and Bodge It => Topic started by: ElyDave on 17 December, 2019, 03:36:56 pm

Title: Siege engines
Post by: ElyDave on 17 December, 2019, 03:36:56 pm

Having built the trebuchet a couple of years ago, last year I attempted a torsion powered ballista, but didn't get it to work. That one is now on the back-burner while I rethink it.

This year however, a torsion powered mangonel is the job. Found a good bit of 2x4 in the shed, and a couple of narrow bits of ex-shelf of the perfect length and thickness.

I will post pics eventually, but for now I have a substantial ladder frame base about 2ft by 1ft, will be adding uprights and spreader at the top, plus supports for the frame. I've got a 16mm spade bit for the cords to pass through with washers to reinforce. Haven't quite decided on the arm or bucket yet, but have plenty of options.  Will also add a simple trigger.  Daughter off school now, so will try and get her involved as a physics exercise.

Title: Re: Siege engines
Post by: Cudzoziemiec on 17 December, 2019, 03:55:44 pm

We're going to have to see these machines in live action some time. Preferably before you get carted off by a black helicopter!

Title: Re: Siege engines
Post by: ElyDave on 17 December, 2019, 04:41:45 pm

Somewhere on here there is a video of the trebuchet, I'll try and repost it here as well. My daughter's reaction is priceless.

Here it is
https://www.youtube.com/watch?v=hQRpuv0qAkk (https://www.youtube.com/watch?v=hQRpuv0qAkk)

Title: Re: Siege engines
Post by: ElyDave on 20 December, 2019, 02:20:02 pm

photos of current build, since then I've added a block to the top of the frame to stop the arm earlier and give a better release trajectory

(https://i.imgur.com/zaf05PV.jpg)
Basic ladder frame

(https://i.imgur.com/uKsNpw3.jpg)
Preparing the skein of string, I went for about 30 loops

(https://i.imgur.com/3uHGQDn.jpg)
Basic trigger, pivots across the launch arm

(https://i.imgur.com/hT5sIK1.jpg)
finished

Title: Re: Siege engines
Post by: rogerzilla on 29 December, 2019, 05:05:09 pm

Tendon is what you want.  Massive energy storage.

Title: Re: Siege engines
Post by: Canardly on 29 December, 2019, 05:25:47 pm

Having trouble with the neigbours Dave?

Title: Re: Siege engines
Post by: T42 on 30 December, 2019, 10:16:39 am

Have you got enough cordage on there?  I'd have thought about ten times that.

Title: Re: Siege engines
Post by: ElyDave on 30 December, 2019, 04:21:04 pm

Its actually pretty reasonable. The string is wax coated and very strong, no idea where I got it from. The limiting factor actually seems to be the washers, one of which has deformed under the load. Tomorrow's task is to slacken off, add another washer each side and then see where I can get to. 

Release angle seems to be the major variable, too high - 90 degrees and its mostly a downward trajectory, I've worked around 60deg or so.