Rear Beam Up Grade

[tommy gun 6]

Any advice would be good. Wat would be better a 205 gti beam with 25mm torsion bars, 306 gti arms, an 24mm arb, or a 309 gti rear beam?

ive seen both for sale an wonder witch one to go for? I run a gti6 engine bilstein b6,s an avo adjustable 306 shocks on the back.

Plus wat do the 306 gti arms do to the handling?

many thanks.

[Anthony 1,002]

Better for what exactly?

 

Heavily uprated 205 beam verses what I'm assuming to be a standard 309 GTi beam seems like an odd comparison, given that they are (or certainly should be) vastly different price points.

[tommy gun 6]

std 309 beam.£100

205 gti modded beam £300

I do track days an fast road.

[Miles 331 1 Cars]

309 beam will be sha**ed at that price and as I;ve always said and still do are a waste of time, The 205 beam is the better bet but make sure the bars are upto spec as there have been some low quality ones made in the past

[Cameron 16]

You won't get a decent 205 beam with uprated bars for that £300 either!

 

Uprated bars go for £100 each in good nick, so you might manage it if you got the beam for free along with some decent dampers. I'd say a more reasonable budget for the spec you listed (good beam, GTi6 arms, 25mmTB's, 24mmARB, UPGRADED DAMPERS ) would be more like £500-600.

 

309 beam will have standard bars which will be too soft for track, so you'd have to spend £££'s on bars to upgrade it anyway. I maintain that standard rear track width is enough, it isn't worth the extra cost and rubbing arches for whatever noticeable benefit a ~20mm track width increase is supposed to bring.

[petert 585]

A 25mm TB is physically impossible on a 205, as 25mm is greater than the spline base diameter.

 

GTi6 arms give 12mm wider track.

[Anthony 1,002]

A 25mm TB is physically impossible on a 205, as 25mm is greater than the spline base diameter.

No it isn't - I've personally built beams with 25mm torsion bars. 26mm and larger you need to start grinding bits of the tube away to give sufficient clearance though...

 

The main issue with torsion bars 24mm and larger is that you have to fit the torsion bar first, and then put the trailing arm on, because as you correctly point out, the main part torsion bar is too fat to fit through the splined hole in the torsion bar. This makes it a right faff to accurately set the ride height compared to 23mm and smaller bars, but it's entirely possible to do.

 

Remember the torsion bar splines are identical on 205's and 306's, and large torsion bars are far more common on the later.

[Anthony 1,002]

You won't get a decent 205 beam with uprated bars for that £300 either!

 

Uprated bars go for £100 each in good nick, so you might manage it if you got the beam for free along with some decent dampers. I'd say a more reasonable budget for the spec you listed (good beam, GTi6 arms, 25mmTB's, 24mmARB, UPGRADED DAMPERS ) would be more like £500-600.

If it's used, then it's a case of whatever the person sells it for - I agree that it *should* be more like £500-600, but if you're lucky you can occasionally grab a bargain if the person doesn't know what they're selling or they're desperate to get rid and have priced it insanely cheap. I've picked up a rebuilt beam with 21mm PTS bars, Group A mounts, ZX arms etc for less than £250 before now, and I know others that have picked up similar even cheaper.

 

309 beam will have standard bars which will be too soft for track, so you'd have to spend £££'s on bars to upgrade it anyway. I maintain that standard rear track width is enough, it isn't worth the extra cost and rubbing arches for whatever noticeable benefit a ~20mm track width increase is supposed to bring.

The advantage of 309 beams was always in the stiffer bars than the width IMO, although curiously most(?) people seem to buy them primarily because they're wider.

 

Remember that only a few years ago, uprated torsion bars were like hens teeth and expensive when they did crop up - 309 beams were comparitively plentiful, and hence it was a cheap way to a moderately stiffer bars. With uprated bars being far more readily available these days, and 309 beams being more scarce (and usually knackered) it makes much less sense.

 

The 20mm bars on the soft side for a dedicated track car though I agree - they match a typical road-going 205 with lowering springs well though, wheras standard 19mm bars always feel a bit too understeer-prone to me once you change the front springs - which makes sense given that most lowering springs are also stiffer, which would mean wanting to increase the rear spring rate to compensate.

[Anthony 1,002]

Also, as Miles says, you need to check the history and condition of the two beams as chances are that if they've never been rebuilt then they're probably knackered (or well on their way to being knackered), and even if they have been rebuilt, you're at the mercy of whoever did it - I've seen some absolute horrors from supposidly "reputable" companies, let alone your average have a go home mechanic. It's not an especially difficult job and there's plenty of people on here that can do the job properly if needs be, but make sure that it's factored into the price.

 

(to give you some idea, I stripped down the work of a claimed "Peugeot specialist" earlier this week to find out why the beam had failed after only 2k miles - the reason was very clear when I discovered that they hadn't fitted the cup to either shaft, effectively leaving the main crossmember tube nother to seal against, allowing water ingress and ultimately failure. Dreadful frankly)

[petert 585]

 

Remember the torsion bar splines are identical on 205's and 306's, and large torsion bars are far more common on the later.

 

And 206 and 405.

 

I have to get my head around this, but surely anything larger than the base diameter of the spline is having negligible effect on the stiffness of the bar?

[Anthony 1,002]

I have to get my head around this, but surely anything larger than the base diameter of the spline is having negligible effect on the stiffness of the bar?

I'll have to leave it to someone that's rather more adept at engineering calculations than me to come up with the figures, but as I understand it, increasing the thickness of the bar continues to increase its stiffness, although not quite at the same rate it would as if the splined ends were also of the same thickness (or larger) as the bar.

 

Remember that the splines themselves are locked in position and thus can't twist/bend with the rest of the bar and won't effect the stiffness, although it does leave a "weak" point where the bar size increases from the size of the spline to the larger thickness of the main bar. This is only a few millimeters long though, and as such, over the length of the bar doesn't have a notable impact as I understand it. As said, I'll have to leave it to someone else to come up with the actual figures - I'm merely the grease monkey

 

Certainly from a fitting/driving viewpoint, a 25mm bar is subjectively notably stiffer than a 23mm bar is, and needs to be built with a shorter shock-to-shock distance when unlaiden for a given ride height once it's dropped back on the floor.

 

If you look at it another way, if your theory was correct then ARB's larger than about 21mm or so wouldn't work otherwse when using 205 endplates, as the smaller splined end on an ARB is much smaller than a torsion bar one. They do though, and the difference between a (say) 21mm and 25mm ARB is massive when driving.

[Cameron 16]

I'll have to leave it to someone that's rather more adept at engineering calculations than me to come up with the figures, but as I understand it, increasing the thickness of the bar continues to increase its stiffness, although not quite at the same rate it would as if the splined ends were also of the same thickness (or larger) as the bar.

 

Pick me, pick me!

 

The amount something twists is dependant on the length (amongst other things) and since the length of bar that has the smaller diameter is so short (as Anthony said most of it is in the splines so doesn't twist at all) the difference it makes is negligible. It's safe to assume that the spring rate can be based entirely on the larger diameter.

Edited September 1, 2011 by Cameron

[welshpug 1,655]

slight tangent here...

 

but anyone able to calculate the spring rate of an ARB that is 25.4mm for 14", then 28mm for the rest, they are about 1170mm long iirc , minus the splined bits which are about 25mm each.

[jackherer 543]

This discussion has reminded me of an American patent I saw a while ago that was a very simple device to add stiffness to torsion bars that is also 'tunable'.

 

http://www.patentgenius.com/image/5354041-3.html

 

Could it be implemented on a 205? For the torsion bars and possibly ARB too? I suppose there would be clearance limitations but it might still be feasible.

 

The US patent expires in Dec 2012 and I don't think it is patented anywhere else anyway.

[Tom Fenton 1,540]

I can see how that could theoretically work. However I think the problem would be after the 20-year bars are cleaned off, the remaining dimension can vary quite a bit, as the clamp dimension has to be pretty exact to work this is now difficult as you would have to size each on individually.

[tommy gun 6]

will 25mm torsion bars be any good on 205 beam an 306 trailing arms. thanks

[Anthony 1,002]

will 25mm torsion bars be any good on 205 beam an 306 trailing arms. thanks

Depends.

 

Matched with appropriate dampers and front spring rates it will be very good on smooth tracks.

 

Start using rear bars that thick with standardish front suspension is will be awful though, and IMO they're a bit too stiff for a daily driver used on the road in all weathers.

 

With something that stiff, a rollcage wouldn't be a bad idea either, as otherwise that's high spring rates through a fairly flimsey shell, which will neither do the shell any favours long-term, nor allow you to get the most from the suspension. It will work in an un-caged carof course, just not as well as it could.

 

You really need to look at it as part of a complete package, aiming to match and compliment the rest of the car, rather than as an individual part

[tommy gun 6]

I got bilstein b6 front dampers lowered bout 30mm.

[tommy gun 6]

wat you ppl think?

[Paul_13 212 1 Cars]

wat you ppl think?

 

No bloody text talk, thats what I think!

[Anthony 1,002]

No then - the rear spring rate from 25mm torsion bars will be completely mis-matched with the soft spring rate from typical lower springs.

 

You'll want something more like 20 or 21mm torsion bars on the rear to match typical lowering springs

 

For 25mm torsion bars, you're looking at more like 300-350lb front springs, which pretty much means coilovers.

[Cameron 16]

It'll probably be higher than that, for 25mm bars I think you need something like 400-450lb springs. My 23mm bars are pretty well matched to 350lb up front.

[Anthony 1,002]

Will depend on tyres, engine weight, personal preference etc - probably should have said 300-350lb upwards

 

James_R's 205 for example was running 25's and 300lb front in its final incarnation, and that felt nice and well balanced on R888's

[Cameron 16]

James likes a lot of oversteer though doesn't he?

[Paul_13 212 1 Cars]

No need to tell me to watch my language by PM is there?

You need to write properly on here not in txt spk. Forum rules etc...

Firstly its bloody annoying to read

Secondly you can't take a bit of banter, instead you get your back up about it. Chill winston!

Edited September 5, 2011 by Paul_13