by madmattd » Sat Dec 02, 2023 7:23 pm
ChrisBSR,
Yes, throwing arm ratio is typically a design choice and not something you tune with. Decide you want to change the ratio later? Build a new arm. 4:1 and 5:1 are the right range usually, unless you are running well under a 100:1 counterweight:projectile ratio. There's an old rule of thumb that says arm ratio should be 1/20th of the expected mass ratio, and that works OK if you're in the 75-110:1 ratio range. Above that, I'd stick with 5-5.5:1. Below, find more counterweight!
Pin angle, sling length, counterweight ratio are your usual tuning options for a standard HCW, yep.
Counterweight ratio: eventually, you want to run at max weight and not change it. Unless you are running some very extreme ratios (in which case you're competing in World Championship events and shooting 2000' plus), it's unlikely you'll get better performance with less weight. Projectile weight of course affects things, but most of us are shooting a narrow range of projectile weights (baseballs, 8-10lb pumpkins, 4lb ice balls, etc).
Sling length on a standard HCW is highly unlikely to be equal to the long arm length. 80% is a good starting point though.
Pin angle doesn't have a "typical" range as it can vary heavily. Pin angle is usually defined from the axis of the throwing arm, positive forward, negative backwards. It's not common, but I've seen negative pin angles. If your ratio and sling length aren't totally weird, 30 degrees forward is a solid starting point, adjust from there.
Normal tuning process I go through: for a given projectile and counterweight, start at the 80% sling and 30 degree forward pin angle. Video directly from the side, making sure the entire sling motion and first part of release is visible in frame. What you are looking for in the video is for the counterweight to bottom out roughly at the time the arm hits 12 o'clock. On an HCW, the counterweight frequently lags a little, and will be 10-15 degrees short of getting to bottom-dead-center when the arm is vertical - totally normal. You want the sling to be around 45-50 degrees above the horizontal when the arm hits that 12 o'clock position. Adjust the sling length so the sling gets to that position when the arm does - long sling delays arrival, short gets there faster (short slings swing swiftly). Once you have the sling arriving in position at the right time, release pin angle is used to tweak the final trajectory.
One non tuning-related item that I've seen many HCWs not do right is making the hanger arm for the counterweight as LONG as possible for your setup - the weight should barely clear the bottom of your frame/trough/whatever. Don't make it short and leave a foot of clearance, you're hamstringing yourself if you do that! Longer hanger arms maximize your potential energy, which is the source of power for a trebuchet! That said, the arm should cock somewhere around 45 degrees above the ground - you can go a little steeper, but much more than, say, 55-60 degrees and you are making the throwing arm itself too short to do much good! Somewhere in the middle is likely the point of diminishing returns, but I've never done the research to determine it as it would need a bunch of arms, hangers, and general messing around that I've never had time for! 45-50 degrees cocking angle works pretty well in most situations though.