I needed to get figuring on how I'm going to suspend this thing on the springs, first thing I had to do is find out how "strong" the pair of springs I bought were. I measure the resting length of the spring at 4", I then hung a 5 gal pail of motor oil (36 lbs) from it an measure the length as 7". So the spring constant is 12 lbs/in. How did I figure that out?
My daughter (11 yr old Grade 6) has been watching what I've been doing, so when I started my figuring I decided I would teach her some simple algebra and physics. She's a keen student (proud Dad here) and has a great appetite for knowledge. I have this cool digitial pen that records what I sketch in a special notebook and then uploads the notes into the computer. I used it and here's the two pages from her physics and algebra lesson.
I had a bell crank style suspension in mind pulling on a regular spring, so I had to figure out how long the lever arm connected to the spring would be compared to the trailing arm length. Also to consider is how much wheel travel I wanted to allow (6") and how far I would need to stretch the spring etc... I just used 10" for my trailing arm length although I haven't measure the actual length. I'll do that, then substitute that number back in and see what comes out.
Summary, I need to use two springs like these ones on each side and they are mounted about 6 5/8" from the axle. Sounds good right? Wrong! This means at full load, the sprayer is back at stock ride height!! Then slowly goes up to 6" higher as it empties... I need a lot more spring rate to try and limit the full load travel to maybe 1/3 the overall travel or 2". (That's a rule of thumb I remember from dirt bike suspension setup).
Running the math again with that fact yields an 11 1/2" long lever arm for the spring... OK, need heavier springs or it won't be compact enough. Problem is, they don't print the spring constant on the stupid packaging so it's trial and error.
Alright, just ran some more numbers. If I arbitrarily decided I wanted a spring lever arm of 3" only and I wanted just one spring I would need a spring that would have a coefficient of 350 lb/in !! It would only need to stretch 5/8" to be balanced. Working even further backwards I find that this spring would only stretch 0.102" when my pail of oil hangs from it. SO! I'm going back to my hardware store tomorrow with some rope and I'm going to be picking up a 5gal pail of oil with different springs until I find one that is pretty short and stretches less than 1/8" when I pick up the pail.
Maybe I drank too much coffee today...
Edited by Chi11iwack, June 20, 2011 - 11:53 PM.
Make scribbles even bigger