Brooklands Bentley

Did a bit more experimentation today. After yesterday's success, I decided to try and turn my curved bit of flat bar into an actual leaf spring with proper rolled eyes

I initially started out trying to figure out how to make a simple former, but then struck on the idea to simply clamp it in the vice and bend the eyelet around a bit of bar stock (with the help of a large hammer). I had to re-grip the work a few times, bending it around and then rotating the whole thing in the vice so that I could bend it into a full circle, but it was surprisingly easy and the results came out pretty good. A definite proof of concept. If you are going to have a go yourself, make sure that your flatbar is square to the vice by using an engineers square or protractor.

The material I used is 3mm thick by 50mm wide, a little wider than I want to use for my finished spring, and the length ended up a little too long as well (600mm eye-to-eye), but it turned out pretty good. It also allowed me to do some spring rate testing. The spring is obviously not heat treated and is only actually mild steel, so the elastic point (Youngs Modulus) is a lot less than it would be if it were say heat treated 5160 spring steel, but it still does have an elastic limit and so would actually work as a spring provided that it doesn't get bent past that point.

What I found was the spring was fairly weak and easy to bend by leaning on the upturned spring on my welding bench. I could easily deflect it 100mm and it would still return to its original shape but bending past this limit caused the spring to permanently deform.

I then decided to make some additional leaves for the spring and increase its strength. As these were offcuts that I had kept from a previous job, I already had another identical bit of flat bar in the scrap pile. So I rolled another curve and cut a third off of it to give me an additional two leaves of staggered dimensions. I clamped these to the first leaf in the centre and redid the test.

I found that with three leaves I could place a majority of my weight on the spring and it would deflect by approximately 100mm and still spring back into its original shape. It worked that well that you could almost forego the heat treating and just use mild steel.

Just for fun I punched the dimensions into the excel spreadsheet I made up yesterday to see what the extra 25mm of width did to the results. It pretty much doubled the results, which is unsurprising given that they are twice as wide.

I'm keen to get some 25mm wide flat bar and make up some proper springs now. I may even go for a thinner gauge of steel so that I can make it up with multiple leaves as I think that this would be kinda cooler than a single large spring.

Another consideration is using pre-treated 4140. A while back I was planning on making up an anti-roll bar for my car and read about a fella who made his from pre-treated 4140. He just heated up the ends to form the bends he needed and then re-quenched and tempered the steel. Given that there is not a lot of deflection in the area of the eyelets on a spring, it's probably not super important to get this area back to the same hardness / temper. The only thing I'm not 100% sure of is if it can be cold rolled to form the ellipse. I guess given enough force, anything can be bent.