205 Or 309 Track Front And Rear Track Widths.

[jonD6B 0]

The 309 beam enhances turn because of its stiffer arb and bars not the wider track. If you put a 309 beam with 19mm torsion bars and 19mm arb it would actually understeer more. Wider track front reduces weight transfer but the stiffer bars in the beam increase it more. stiffness increase of arbs - rough guide for torsion bars too..

on this link you can see the increase in %tage stiffness of different arbs. You can also use to work out torsion bars. 19mm torsion is roughly 100lb/in. So 23mm torsion bar is about 215lb/in

 

CW what offset are your wheels? If less than 19ET this could explain the tramlining?

 

 

O.K. If I was to uprate the 205 beam with larger ARB's this would give me the same benefit as changing to a 309 beam but the point that keeps coming into my mind is that then if I fit the 309 front shafts and arms to reduce track tyre wear and creative negative camber, I am increasing the width of the front to rear track even further and given the statement I quoted on the first post of this topic that's quite concerning. Do people agree with that actual statement in that more modern cars with a more equal track handle better?

[C_W 3 1 Cars]

CW what offset are your wheels? If less than 19ET this could explain the tramlining?

 

They're 7J ET11 . BUT, I had no noticeable problems before I changed to the 309 arms; on the motorway in the inside lane it can swing about in the tramlines.

[niklas 1]

O.K. If I was to uprate the 205 beam with larger ARB's this would give me the same benefit as changing to a 309 beam but the point that keeps coming into my mind is that then if I fit the 309 front shafts and arms to reduce track tyre wear and creative negative camber, I am increasing the width of the front to rear track even further and given the statement I quoted on the first post of this topic that's quite concerning. Do people agree with that actual statement in that more modern cars with a more equal track handle better?

 

That depends...

 

A car's balance is all about weight transfer. And weight transfer often comes down to roll stiffness.

Roll stiffness depends on the spring stiffness (antiroll bars and springs), chassi geometrics (roll centre), track width and weight (and CoG).

 

The weight is basically what is shifting and creates the weight transfer.

The geometrics, CoG and track width creates a lever arm on the suspension for the weight being transferred.

So basically we have weight * lever arm for the springs to work with.

The distance between the rollcentre and CoG creates a positive lever effect for the weight (don't know if this understandable english) where as the distance between the roll centre and the wheel (basically half the track width) creates a negative lever effect for the weight being transferred.

 

In short, a wider track decrease weight transfer.

 

Weight transfer is also affected by the roll stiffness ratio between front and rear. The side with the greater roll stiffness gets more weight transfer. Why that is, I cannot explain But I would be happy if anyone could fill that in!

If we increase the spring (spring and arb) stiffness in the front with 200% we will have to do the same in the rear to maintain balance. We want to increase the spring stiffness as much as possible, as then we will increase roll stiffness which will reduce camber loss and maintain grip. But we don't want too much spring stiffness as then we will loose grip on uneven surface. And we never want to increase the stiffness in the front so that we cannot get the same percentual increase in the rear! (for example if there doesn't exist thick enough arb and tb's)

I cannot explain the relationship that affects the choice of arb and spring stiffness. I would be happy if anyone could fill that in as well!

 

Conclusion:

- A wider track front and a narrower rear will reduce understeer.

- An increase of rear stiffness in relation to the front will reduce understeer.

 

Note: When I say spring I can mean both coil spring and torsion bars.

[veloce200 3]

and to continue where Niklas started, the pinned article has but one flaw and it is this statement:

 

" So you may want to measure the front/rear track and fit some spacers to make the rear track as wide as the front. Again this will create sharper and more stable turn in.

 

The 206/307 rear track width is far closer to that of the front track......in fact the 307 rear track is slightly wider than the front track.......as manafacturers now know the benefits of handling and stability this offers."

 

Granted many modern FWD cars do have front and rear track similar and TBH this is a sensible way to go. It does make the car stable. However the tendency will be towards understeer, but then nearly all modern cars are designed to understeer. An oversteering FWD car is much more dangerous than an oversteering RWD car so the makers make them "safe". For RWD usually the rear track is widened to provide stability in the opposite way to FWD. here are some examples:

 

Honda NSX (Senna no less did the handling)

Front track 1510

Rear track 1540

 

Honda S2000

F 1471

R 1509

 

205 GTI 1.9

F 1382

R 1339

 

Clio 1.6 2003 (Clio widely regarded as a modernn FWD car that handles unlike the Peugeot 307)

F 1392

R 1372

 

 

 

 

They're 7J ET11 . BUT, I had no noticeable problems before I changed to the 309 arms; on the motorway in the inside lane it can swing about in the tramlines.

 

The 309 arms definitely change the scrub radius but not by a lot. I think it almost becomes centrepoint steering (zero scrub) with std wheels which is bearable - but if you have another 9mm offsett from the wheels then that probably explains tramlining. So I think its the combination of the two together.

[jonD6B 0]

I am going to completely contradict what I had said previously about my proposed set up now. I am going to disregard the statement that started this whole topic off (no offence to its author) and go with what seems to me to be the general opinion and uprate the 205 beam with a thicker ARB and solid mounts and widen the front track. Although the shocks and springs currently fitted are of an unknown make I was told that they are uprated around 20% by a reliable source and seem in good condition. I will try and source some more adjustable rear shocks as I've got Spax adjustables on at the minute and feel that they contribute to a nervous back end. Probably head in the direction of Bilsteins. Having decided to opt for this set up could anyone add anything else that should be done whilst I'm undertaking this work or other opinions that will contribute?

 

Incidently, thanks alot for all this advice your all stars!

[SPGTi 2]

The 309 beam enhances turn because of its stiffer arb and bars not the wider track. If you put a 309 beam with 19mm torsion bars and 19mm arb it would actually understeer more. Wider track front reduces weight transfer but the stiffer bars in the beam increase it more. stiffness increase of arbs - rough guide for torsion bars too..

on this link you can see the increase in %tage stiffness of different arbs. You can also use to work out torsion bars. 19mm torsion is roughly 100lb/in. So 23mm torsion bar is about 215lb/in

 

CW what offset are your wheels? If less than 19ET this could explain the tramlining?

 

 

BUT the bars are different lengths from a 205 beam to a 309 beam so you you cannot do a comparison on thickness alone. A shorter bar doesn't need to be as thick as a longer to have the same resistance to twisting. To compare the stiffness a formula is required that takes account of the length as well as the thickness.

 

Steve

[niklas 1]

I made a beautiful picture in paint to make it a little bit easier to explain

I know it's not a perfect drawing but it'll do...

 

 

The red lines are suspension details: track control arms, rear beam, etc

The black lines are just symbolic, to extend the track control arms to where they would intersect if they were longer.

The green lines are the symbolic "lever arms".

And everything in blue just for description.

 

One interesting thing is that the lower the front, the longer the lever arm which requires firmer springs to maintain the front roll stiffness!

Also on our kind of rear beams the roll centre is always at the ground level.

[veloce200 3]

BUT the bars are different lengths from a 205 beam to a 309 beam so you you cannot do a comparison on thickness alone. A shorter bar doesn't need to be as thick as a longer to have the same resistance to twisting. To compare the stiffness a formula is required that takes account of the length as well as the thickness.

 

Steve

 

 

Agreed. I punched some rough figures in (not got the torsion bar with me in the lounge) but if I can be forgiven for being imprecise and using track widths as the basis - if we assume the 205 bar is a long as the track 1339mm and the 309 bar is 39mm wider then the extra width means the 309 bar is 1.46% less stiff.

 

(BAR LENGTH - The stiffness or rate of a torsion bar is inversely proportional to it's length. Given two bars of the same diameter and other specifications, one that is twice as long will have half the rate or stiffness. )

 

In terms of diameter the effect is much more dramatic, (the stiffness of a bar is proportional to the 4th power of the diameter. A bar that is twice as thick as another will be 16 times stiffer.) Therefore comparing a 19mm bar to a 20mm bar the 20mm bar will be 23% stiffer than the 19mm bar.

 

So it is the extra length is largely unimportant and explains why the 309 beam mod is so popular.

 

Don't get me wrong though I think the 309 beam and 309 front is a very good upgrade and ultimately it's all a case of personal preference - some like it "arsey" and some like a bit of "push" as my Oval man next door says !

[pug_ham 240 3 Cars]

BUT the bars are different lengths from a 205 beam to a 309 beam so you you cannot do a comparison on thickness alone. A shorter bar doesn't need to be as thick as a longer to have the same resistance to twisting. To compare the stiffness a formula is required that takes account of the length as well as the thickness.

Thanks Steve, this is the point I was trying to make earlier, its not a simple change in diameter as the length is also changed.

 

205 torsion bars are approx 991mm long & 309 torsion bars are approx 1028mm long.

 

205 ARB is 1095mm & 309 is 1130mm long.

 

If that helps some more accurate calculations.

 

Graham.

[veloce200 3]

Thanks Steve, this is the point I was trying to make earlier, its not a simple change in diameter as the length is also changed.

 

205 torsion bars are approx 991mm long & 309 torsion bars are approx 1028mm long.

 

205 ARB is 1095mm & 309 is 1130mm long.

 

If that helps some more accurate calculations.

 

Graham.

 

Thats makes stiffness increase of 20mm 309 torsion bar 23% and length reduction 1.87% - 20.91% - I'm assuming the 309 torsion is 20mm? I think thats right?

As for ARB the net increase is 21.18% so together the 309 rear beam gives over 42% increase roll stiffness - not surprising its a popular cheap mod. However simply adding a 23mm arb to a std 205 rear beam adds 115% to rear roll stiffness...

[pug_ham 240 3 Cars]

I'm assuming the 309 torsion is 20mm? I think thats right?

Yes, torsion & arb on 309 GTi are 20mm.

 

Do the torsion bars calculations work under the same principal as the antiroll bar with diameter changes?

 

Graham.

[niklas 1]

Do the torsion bars calculations work under the same principal as the antiroll bar with diameter changes?

 

Yes.

 

The formula looks something like this for all torsional springs:

[spring rate] = [diameter]^4 * [modulus of elasticity] * [length]

Edited February 2, 2006 by niklas

[SPGTi 2]

Resurecting this topic again.

 

After doing a bit more research into this can anyone confirm that the "arm length" ie the leverage force that is exerted onto the torsion bars is the same on a 205 and a 309 beam ?

 

This site has some good calculators

www.proshocks.com/calcs/torsionrate.htm

 

This site also has some good info near the bottom of the page

www.kisssoft.ch/english/manual/chap13.html

 

and this site has design tools for torsion bars

www.hexagon.de/fed8_e.htm

 

Steve

[B1ack_Mi16 67]

Just measured my 309 gti beam, the bars are 20mm diameter.

They are 970mm long between the splines.

 

This give 119lb/in spring-stiffness.

 

The 205 bars are according to the parts cd 18.9mm, and if I remember correct they're 50mm shorter than the 309 ones.

 

This give the 205 bars 100lbs/in spring-stiffness..

 

So 19% stiffer bars on the 309gti beam.