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  • So I popped back into our local spring makers yesterday to get a quote on some small leaf springs. The last time I was there I wasn't 100% sure on the dimensions, so this time I was armed with a bit more info.

    What I wanted was 500mm eye to eye, 25mm wide and a thickness suitable to support about 50-75 kg. Unfortunately the largest flat stock they can handle is 19mm by 2.24mm thick, which is probably too light to take the load of a cyclekart and driver. I did manage to get a quote, which was $80 for a pair plus heat treatment (minimum $80) so the price ended up more than ordering them from the US.

    Figuring that I could probably make them myself and then get them heat treated, when I got home I did some calculations to work out exactly what was needed.

    Assuming that the weight of a driver and karts is somewhere around 200kg all up, the front wheels, will take just under half of that weight, and that will be split between the two front springs, which gives us roughly 50kg per spring. I know that the weight is more split towards the rear but decided on an arbitrary value to start with.

    The springs should also be 25mm wide so that they are easier to fit to the chassis (also 25mm wide), which means that to end up with a spring rate to support 50kg we need to determine the thickness and length of the spring.

    This was pretty easy using a formula in excel that I found online as it allowed me to easily and quickly try various sizes until I found one to suit.

    Here's what I ended up with:

    25mmm wide
    1 Leaf
    6mm thick
    500mm long
    This gives a rate of 65kg / inch deflection

    Note, this is a bit of a nonsense unit of measurement, simply as the original formula was in imperial measurements, but for me it's easier to understand. What it basically means is that for every 65kg you place on the spring it will deflect 25mm (1")

    I then decided to map out what this might look like for various combinations of spring thickness and length and ended up with a table that looks like the attached image.

    What you will note is that the shorter the spring, the higher the spring rate. Likewise a thicker spring will also result in a higher spring rate. By playing with these values it is possible to build a spring to suit. The greyed in areas are the 'might be suitable' combinations.

    So next up I decided to take a look at how I might make the spring. I made a quick test on rolling the ellipse, which was pretty simple on my bench mounted roller. This only leaves the eyes to make.

    One method is to make a former and then heat the flat bar to bend it into shape - a bit like making ironwork scrolls. The second is to make up a hossfeld style bender, which will allow the eyes to be bent cold. Each option has advantages and drawbacks.

    I quite like the idea of making up a hossfeld bender. It's a little bit of work but I do have other jobs that I could use it for, plus I figure that with a minimum charge for heat treating I might as well make up a few sets of springs and the bender would definitely be better for making multiple sets.

    As usual with most projects, it's a case of two step forwards and one backwards
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