Re: Can't find replacement springs can i use shackles?
You need a shackle system on a leaf spring axle to handle the length change as the spring is compressed, on my Caulkins trailer the shackles bolt around the trailer frame tube and are fully adjustable to change the axle location front to rear. This is common to boat trailers as they are designed to be adjusted to fit a range of boat lengths and weight distribution and to correct the tongue weight when fully loaded.
Length of the shackle is related to the length of the spring. For the difference in length of your new spring I would not change the factory length shackle. In general your shackle should lie at 45 degrees to the frame fully loaded and be long enough to accomodate the spring length change as the spring flattens, it should be about 5-10 degrees from vertical unloaded but this is a variable depending on how you want the suspension to perform.
