Tosion springs

[Mr Whippy]

Right.

My car has a 25.4mm rear anti-roll bar.

Now, if you were to make it 30mm, but the splined ends were still at stock size, and perhaps a small range beyond the splines before the bar could "fatten up" as it were, would the spring rate of the bar be effected like springs in series?

Ie, would the narrower bits between the spline at 25.4mm and the fatter bit at 30mm twist more than they did before, kind of offsetting the action of the thicker part of the bar?

Is it reasonable to calculate overall spring rate using the series spring method, and calculate the rate for the ends, and then the centre part, to get the overall rate, or do torsion springs work differently?


Any help would be much appreciated. Just wanting to uprate my rear torsion beam suspension, but not sure on what dimensions are needed for a specific resultant overall rate. The anti-roll bar will probably stay the same rate, but the action of the torsion beams is identical so the same would apply for those too.

Thanks for any advice.

Dave

[GreenV8S]

For the purpose of stiffness calculations, you should be able to treat the different sections of the torsion bar as springs in series. But if the narrow part is short compared to the wider bit, I would have thought the effects of the narrow section would be small enough to ignore.

[Avocet]

You might get big stress concentrations in the thin bit. Imagine a bar 30mm diameter everywhere but having a 15mm diameter bit 10mm long in the middle (for argument's sake).

[stew-typeR]

thats the diameter of the splined ends?

[Mr Whippy]

Splined ends are an inch I think, but obviously if I want to up the rate I have to make the bar thicker, or shorter.

Just not sure how I could make it shorter, because in doing so I just concentrate more stress into a shorter bar, although in theory it should be stiffer.

Or if I make the middle bit fatter. But like you say above it may cause stress points, so it'd need to be flared from the spline to the new fatter section ideally.

I'll have to do a few calc's on paper and see if series coil spring method looks to be working properly for torsion ones.

Cheers

Dave

[GreenV8S]

Mr Whippy said:

see if series coil spring method looks to be working properly for torsion ones.

It isn't to do with coil versus torsion - they're both linear springs and the arithmetic for combining springs in series is the same.

[Mr Whippy]

Oh ok

Makes it easier then

Already calculated the raw rate's for ARB's, but need to do it now for the torsion beams!

It's quite interesting how quickly the rate rises with just a few mm on an already quite thick bar, which is good for "stock" appearance

Just can't do with lowering if I can't up the rate too.

Does anyone know of any machinists who specialise in such things at all? I know you can specify/manufacture coil springs to desired parameters for example.

Dave