DIY adjustable anti-rollbar - using torsion spring shafts

[3point141]

I'm considering the design of a bespoke rear IRS for a Jag' XJ. Ignoring that a rear ARB can be had for these vehicles, I'm interested in designing a fully-adjustable rollbar setup utilising stock torsion-bar suspension springs as found on Pugs, beetles, et-al, not as suspension springs, but as a rollbar unit. Think of a bladed adjustable RB & you get the picture.

Any input is appreciated, if anyone has done, or thought of using torsion springs in this way. I could do with some stock dimensions of springs, if anyone happens to know where to get this info?

Cheers!

[ejenner]

won't work.

Roll bars operate on the principle of connecting both sides of the car together. If you change the bar from steel to spring metal it will be the same as fitting a really weak ARB. People usually go the other way when meddling with anti roll bars.

Here is an example, a 'piggy-back' bar which bolts to the original and adds additional strength.



The harder you corner the stiffer you want your ARB to be. So if you've stiffened the springs, fitted larger tyres and drive it on the track you'll want additional strength in the anti-roll-bar.

I'm not trying to talk you out of it, just highlighting the principle behind the operation and the typical train of thought.

[3point141]

Aug 20, 2015 13:52:10 GMT ejenner said:

won't work.

Roll bars operate on the principle of connecting both sides of the car together. If you change the bar from steel to spring metal it will be the same as fitting a really weak ARB. People usually go the other way when meddling with anti roll bars.

I'm not trying to talk you out of it, just highlighting the principle behind the operation and the typical train of thought.

Yes, I know how roll-bars work...

ARB material IS "spring steel". Look at roll-bar material grades & you'll find similar as for road springs, similar to half-shafts...

The particular properties of "spring steel" is that it has a high UTS, toughness & fatigue resistance to repeated loads, under which "steel" as you put it, would fail.

Given the youngs' modulus of steel, whatever the grade is almost the same, different steels have the same apparent load/deflection when wound into a spring & deflected or rotated by an angular displacement, as in the case of a rollbar or torsion spring.

With this in mind, a 1" OEM rollbar would have the same resistance to roll as a 1" torsion spring when operating at identical lever lengths.

[mk2cossie]

I to had thought of using the rear torsion bars from something PSA, but wondered if it would be any good as a blade type ARB.

Then I remebered that the rear setup has a straight ARB inside the axle tube with splined ends that would be perfect

[welshpug]

it will work just fine, the arb on a pug is just a slightly longer torsion bar "spring" bolted between both trailing arms.

[3point141]

Yeah, I think many trailing-arm rear-end designs link the sides with a shaft running between the arms.  Although, a typical PSA ARB might be a bit weedy, thus I'm looking at the actual suspension torque springs.

I estimate around Ø20mm diameter would be about sufficient for my needs, road-springs are about this diameter, the length might be somewhere in the ballpark, too.

One annoying feature tho', I think the splined ends on torque springs are often different sizes...

[welshpug]

I think youll find they are pretty chunky if you use the right ones, any sport chassis 306 uses a 24mm arb with a 20mm spring, the zx and xsara use 21mm spring and 24mm arb mostly on the top model, 22m on lower spec.

granted low spec 205's may not even have an arb at all

the 30/32 spline differences wont hinder anything, just use the matching end plates.

[waveman1500]

This is actually how the VW Type 3 front suspension works.
The Type 3 front end is quite different from the Beetle (Type 1).
In a Beetle, both torsion bar tubes contain flat leaves which are clamped in the middle and are therefore used as torsion springs to suspend the vehicle's weight.
On a Type 3, one of the tubes (I believe it's the top one) has torsion bars which run all the way through, and therefore act as an anti-roll bar.

[3point141]

Aug 21, 2015 11:43:20 GMT welshpug said:

I think youll find they are pretty chunky if you use the right ones, any sport chassis 306 uses a 24mm arb with a 20mm spring, the zx and xsara use 21mm spring and 24mm arb mostly on the top model, 22m on lower spec.

granted low spec 205's may not even have an arb at all

the 30/32 spline differences wont hinder anything, just use the matching end plates.

That's what i'm talking about!...          Sounds like this could be a go-er, 306 gti rear end now on thEbay watch list.

Cheers!

[composimmonite]

Am I missing something ? Jags have excellent handling and there are plenty of performance bits already available for those who want to push them far more than possible in normal road driving. So why the "reinventing of the wheel" ?
Is this just a theoretical exercise (which I do all the time so have no problem with that) or is the Jag IRS being fitted to another vehicle ?

Paul H

[3point141]

Aug 21, 2015 16:12:34 GMT composimmonite said:

Am I missing something ? Jags have excellent handling and there are plenty of performance bits already available for those who want to push them far more than possible in normal road driving. So why the "reinventing of the wheel" ?
Is this just a theoretical exercise (which I do all the time so have no problem with that) or is the Jag IRS being fitted to another vehicle ?

Paul H

Hi Paul, good question.. to quote myself in the root of the thread.. "I'm considering the design of a bespoke rear IRS for a Jag' XJ. Ignoring that a rear ARB can be had for these vehicles, I'm interested in designing a fully-adjustable rollbar setup.."

The rear end I intend to build will displace the original IRS completely, to include new suspension, differential etc..  The reason for this is mainly to overcome the following;

Jag diff's are not bulletproof, you can improve on their weaknesses, but at quite a high cost - they still end up not being that strong, ask the Cobra owners running big-blocks & sticky tyres..
Jag diff ratios are rubbish for racing, this is likely to prove somewhat a caustic remark to jag tuners, but its my 2p's worth & i'll stick with it, my rear ratio is a hilarious 2.88:1, so eventually will be finding a new home. you can buy new ring & pinion gear-sets, but at £700 quid for the ratio I want? no thanks!

Jag suspension geometry is a right curse-word to adjust, good when its done, don't get me wrong, but a pain to re-do for the kind of mixed-activity stuff I intend to do I would like ease of adjustment.
Then there's the roll-bar, or lack of at the back of mine. Yeah, I could buy a second-hand one & have a "standard" one, or invest more money again for a Harvey Bailey uprated one for racing (pricey!).. or do one myself, designed to work with the rear-end I'm alluding to above.  

Oh, and I just find the idea interesting...    

I've got the diff, brakes, uprights, axles & now an idea of the roll-bar, all I gotta do now is measure up & get some design underway. I'll post updates to this here & in my build thread. I think I'll go with the 306 derived torsion-bar design.

[froggy]

The downside of controlling rear roll with the bar is it pulls the inside wheel up as it twists causing traction issues .
I look after a double six coupe with t56 manual conversion and some decent brakes and suspension from paramount and they just spec a bigger front bar with their kit

[welshpug]

yeah, FWD, control with rear bar, RWD, control with front bar.

[3point141]

Yes, I totally agree with you both. My suspension setup will be actually quite soft, with the emphasis on damping, soft especially at the rear for hookup corner exit joy.

There's only so much you can control at the front, obviously that's where I'll start to get performance dialed in, after which, I expect I'll need something at the rear. At the moment the car has zero rear ARB, my "design" will have something. Whatever it is, it must be really easily adjustable.. & cheap!