e34 rear swaybar question - sizes over 20mm

[BigKriss]

I emailed whiteline again today. www.whitline.com.au. I asked them if they can make bars over 20mm. Here is what they replied with


"The next available sizes up are 22mm or 24mm. We can make these sized sway bars, but we may not be able to supply 'D'-bushes to suit this size bar, nor do we accept any responsibility for the sway bar.

Another problem that you will find is that you may not be able to mount the ride height sensor as the U-bolt only just fits over the 20mm sway bar.

You have mentioned that no other manufacturer makes the sway bar larger than 20mm, may I suggest that you ponder why this would be the case, as I believe that if you increase the size of the rear sway bar too much you are going to suffer from o/steer"

My initial queries on this response was;
- get the 20mm polyurethane bushing and drill them out 2mm more by a shop
- the ride height sensor they talk about is only on the m5 with adaptive suspension - i thought my car did not have one

Is anything over 20mm too big, or can a 22mm or 24mm bar be appropriately fitted to the car?

Guys running with 19mm or 20mm bars, does it look like is appropriate clearence for a few more mm?

[bahnstormer]

u got room for 22's....i'll post pics soon

[genphreak]

have u checked them out?

[BigKriss]

wrote to KMAC today asking about a 22mm bar. They manufacture their own equipment. This is what they have to say?

"We can manufacture in any size - 20mm would recommend.

Every mm is 80% increase in strength going from 15 to 20mm is dramatic. 22mm would tend to lift rear with tyre cornering on its edge (ie. no droop at all)"

[Paul in NZ]

Kriss i can recomend the Mtech set up,my car corners very flat with little body roll,even a fairly enthusiastic slalom doesnt get the car rolling side to side too much.I cant say what my bar sizes are but i really like the way my car behaves.Very rigid bars will make the car quite jiggly even a little harsh on uneven roads,i wouldnt like bars more rigid than mine.

[pundit]

Kriss - What is the problem with your current setup?
If you are trying to correct understeer a stiffer rear bar will tend to reduce it BUT you need to diagnose all the factors that maybe effecting your cars handling. Standard factory E34's understeer. This was to prevent middle-aged lawyers and accountants from doing 180 degree spins around corners after consuming two bottles of red during client subsidised three hour lunch breaks in the late 80's and early 90's. :D
The larger front bar you already have will have acentuated understeer but that doesn't include any other mods you have made, springs, shockers, wheels & tyres etc.

Sometimes it's good to get right down to basics when trying to solve a problem. Without trying to appear like an idiot (:p) what is the purpose of the three main components in a cars suspension?... okay I'm an idiot! :D

SPRINGS (should actually be called shock absorbers!) - absorbs the 'shock' of striking bumps by converting the energy into heat. In an ideal situation the springs should totally prevent any transfer of energy into the vehicle itself.

SHOCK ABSORBERS (should correctly be called spring dampers) - dampens the oscillation of the spring after each impact in order to keep the wheel safely in contact with the road by coverting the energy into heat by forcing hydraulic fluid through a restriction (valve). The stiffer the spring the more dampening is required.

SWAY BAR (stabilser bar, anti-roll bar) - Reduces vehicle body lean when cornering. Should have no effect whatsover on vehicle when travelling in straight line on a smooth surface.

As mentioned assuming the bar is made of the same material every 1mm increase in diameter will have a greater effect than you may think. It's the old 'pi'-'r'-'squared' deal, the cross sectional area of the bar will increase at a much greater percentage rate than the diameter (if you get my drift).

A stiffer sway bar will have the effect of making your independent rear suspension less 'independent' by simply tying the left and right rear suspension together to the point where much of the suspension movement of one wheel will transfer across to the opposite wheel through the bar. If the bar is overdone it will cause excessive unloading of the inside wheel during hard cornering especially over bumps and may make the rear end more skittish and harsh as Paul suggests.

I believe you stated you are running standard 15" rims. These along with the standard profile tyres will significantly add to any understeer when pushing hard into corners due to sidewall roll. If you intend to track your car regularly I would look at either 17" or 18" rims first. (18" would be better for track work) Maybe just fit these for your weekend 'blasts' and keep the comfy 15"s for weekday commuting. Increased front negative camber will also help 'turn in' (camber adjusters required). Once you get a feel for how your car is handling with low profile tyres on the bigger rims then choose a rear bar just stiff enough to 'tune' out any remaining understeer you may have. (an adjustable bar would be helpful here) Just fitting a humungeous rear bar without carefully taking your wheels, tyres, camber etc. into account may cause more harm than good. - Good Luck