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PranK

Rear sway bar considerations

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I recently added a 27mm Whiteline front sway bar which feels great and stiffened the whole front up nicely.

Problem is, I'm still using the OEM 13mm sway bar on the rear which undoes a lot of the front bar benefits.

Now its time to upgrade the rear one and I'm a little undecided about the one to get. 

Whiteline have a 16mm and a 20mm rear for my E90. They said that 16mm is probably better because the 20mm might cause unwanted oversteer on the street. 16mm seems pissy though and I'm wondering if they are protecting themselves.

SuperPro have a 20mm 2-point adjustable one which feels like the better option. 

Its irritating that there is no standardised measurement of sway bars so you can get a better idea before buying, rather than just reading reviews. It'd be nice to say "well 16mm will be x times better than 13mm".  The OEM front one was hollow FFS, it was light as a feather. 

Theres no doubt that the 16mm Whiteline one is going to be much better than the OEM 13mm one, but will there be an appreciable difference between the 16 and 20 ? And, maybe the softer Superpro setting will be like the static 16mm Whiteline one ? 

So confused. 

Thanks guys!

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There is no way that a 20mm solid bar on its "soft" setting will be anywhere near as soft as the 16mm solid bar. The change in arm length from "soft" to "hard" on a typical bar is about 20mm, on an arm length that is about 300mm, maybe a bit less. So, in the order of 10-15% change.

The stiffness of solid bars varies with the FOURTH POWER of the thickness. Thus, the 20mm bar will be 244% of the stiffness of the 16mm bar. Or, 144% additional stiffness over the 16.

A +/-10-15% change on that still leaves the 20mm bar over double the stiffness of the 16.

When I put bars on my R32 I put adjustable 24mm on front and rear. Set stiff at the front, soft at the rear. Was too stiff at the rear. Replaced 24 rear with 22. 24mm bar is 141% of 22mm bar stiffness, or the other way to look at it is 22mm bar is only ~70% of 24mm bar. Damn near perfect. Could possibly handle being a 20mm, preferably adjustable.

'Twere I you, I would go with suggestion for 16mm bar. It will be 229% of the 13mm bar's stiffness (assuming the 13 is solid, which it probably is).

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Well, any time an ARB is too stiff at one end it will push that end towards loss of grip, particularly when the surface is slippery. So on the rear the trend is towards oversteer. Now, if you want the back end to be completely detached and you aim to drive it everywhere using spinning rear wheels to provide direction as well as thrust, that's good. But otherwise it can make the back end overly nervous, prone to mid-corner detachment, which is not fun.

A quite stiff front bar (like you have now) will generally make the car's turn-in behaviour good. It will react to steering quickly and start turning the car, without that rolly feeling you get on soft barred setups. But if it is too stiff that can quickly turn into mid corner understeer immediately afterwards.

When you put on a stiff front bar, you will generally need to beef up the rear bar too, otherwise the balance will very likely be shifted towards understeer. But going overboard generally leads to the situation in the 1st paragraph.

Big bars also reduce the independence of the suspension across the car. So a big rear bar means bumps on the inside wheel get transferred to the outside wheel, which also contributes to the problem of sudden slip.

The two philosophies of suspension are;

  1. Light springs, stiff bars. This is intended to allow full axle bumps to be absorbed (ie both wheels going up and down together because you've hit something in the road that covers the whole width of the axle) and the bars control body roll,
  2. Stiff springs, light bars. This means that the springs are responsible for controlling body roll, with the bars really only used for tuning the balance.
  3. Or, you could go anywhere in between. It's a spectrum baby.

 

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23 minutes ago, Rusty Nuts said:

 No need for stiffer rear sway bar; just apply throttle to rotate the rear. 👍

Or the brakes!

But generally yes the bars at either end affect how the pedals rotate the car 😛

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