Flat bar 'spring' with sideways torsion stiffness (Metal Noob)

[darkkevind]

Hello! First time posting, only just joined today but would like some help with my project please?

Please see the picture below. I'd like to create this 'spring' so that it give the skateboard a bit of 'suspension' but so that it also doesn't twist when the rider tries to turn. ie. It can still twist the trucks as it would do bolted to the deck.

The dimensions are somewhere in the region of 55-70mm wide flat bar 3mm thick. What I intend to do is heat up a length (just heat up a section about 25mm wide from edge to edge) until orange, then bend around a form to make the arc, then quench in oil. Heat up again to orange, then quench again.

Would this (in mild steel) give it enough strength to be a 'spring' of sorts, so that it can bend but return to it's shape, and also when the board leans, won't tortionly twist the flat bar out of shape?

Any help, feedback or suggestions are greatly appreciated

 

[SwinnyGG]

You'd be better off:

1) using an alloy designed to be used as a spring (i.e. spring steel of some kind)

2) making that bend a complete 180^o and putting the trucks where they normally go on a skateboard. Your board as shown doesn't allow the rider very much weight transfer between the front and rear axles, which is a thing you need on a skateboard.

 

[darkkevind]

@jgKRI thanks, I thought spring steel would be better once I thought about it a bit more, but I've already bought some mild steel so just wanted to see if I could do it with that.

When you say weight transfer, what do you mean? I have the trucks already bolted to the kick tails on some angled risers to get the trucks back to almost horizontal, and that at the moment is a great riding position, am able to ride and turn well with ease.

I was just hoping this 'spring' might be able to give a smoother ride...

 

[SwinnyGG]

The point of the kick tail is to allow you to lift the front axle off the ground, should that be necessary.

Why did you not mount the trucks where they normally go?

Maybe you're trying to accomplish something specific. In that case, carry on. It's just an odd looking skateboard.

 

[darkkevind]

I'm used to riding long boards, I never lift the board to turn or to do tricks etc.
I was given this board and now have 180mm trucks on it with 97mm flywheels, it's basically going to be for crusing and be electrified like my other board.

If the trucks are mounted where they normally go, with the large wheels and short wheel base it's crazy easy to accidently fall off backwards and not very stable for cruising, which is why I've mounted them on the tails, to create a longer wheel base.

 

[EdStainless]

Remember how springs work, the amount of flex will be determined by the thickness mostly, and by the width.
If they are the same size all steels will flex the same amount.
The higher strength the steel is the further it can flex without permanently deforming.
You can probably test with mild steel, but in service you will need something much stronger.
To make this tight of a bend in spring steel you will need to bend it hot, and then HT afterwards.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[darkkevind]

Thank you EdStainless, I've got hold of some 80 x 3mm mild steel which I intend to heat until orange about an inch wide across the width (80mm) of the bar, bend it round a wooden form and then quench in oil.

I then intend to heat the whole thing up until orange and do the same thing again (quench in oil) to anneal the whole thing.

How many times would you recommend I do that to make it keep it's shape whilst still being able to 'spring'?

 

[Tmoose]

Based on what I see in the picture, would it be possible to leave your leaf spring flat, or gently offset ?

 

[darkkevind]

Hi @Tmoose, do you mean, the part that connects to the truck slightly points upwards rather than horizontal to the ground?

 

[SwinnyGG]

I believe what he's asking is what I was also hinting at originally- that instead of bending your flat stock into a c-shape, leave it flat and use the existing mounting holes, with one end extended under the tail, putting your axle approximately where it already is, and allowing the fabrication of you spring to be extremely easy. You'd be taking a piece of flat stock and drilling some holes in it.

 

[darkkevind]

Oh! I see! So all flat bar coming out from under the deck rather than where I have the mount holes now, on the kick tail?

 

[SwinnyGG]

Yes. You'll get the same effect. You may wind up with a different spring rate for the same material than you would with the c-spring. But there's a benefit to the flat spring, in that you can stack multiple leaves together of the same or different thicknesses to change the spring rate very easily, quickly, and cheaply.