lookout

After seeing some arguments about anti roll bars over the last few weeks, I would like to try to give some correct information on the subject. It is not by any means comprehensive, as the subject is a bit large to fit in 1 post, but more to give a bit of clarity.

I hope that the following picture of a log, from Whiteline's website shows that fitting a larger diameter rear anti roll bar does not just change handling balance or reduce de rear axle's available grip.

It would be nice if a few more people would understand that anti roll bars can be too small, too big or about right for any given chassis set-up, tyres and driving style.

Just spouting that a big rear anti roll bar reduces rear end grip, while technically could be right in some instances, is not sufficient information. Based on a statement like that, it would be good to remove the rear anti roll bar altogether.

For each Impreza, there will be a 'right' rear anti roll bar for every driver. Hence the availability of adjustable bars in many diameters.

Having driven cars with most options, these are my opinions about wrx based cars (front wheel drive bias):

20mm rear bar produces more front end grip, more neutral handling, reduction in very fast corner roll oversteer.
22mm rear bar produces more front end grip, more neutral handling, reduction of roll oversteer and allows driver to induce wash-out oversteer easier.
24mm rear bar produces all of the above but makes oversteer even easier.
22mm front and 24mm rear bars produces a lot more grip with neutral handling and induceable oversteer. To allow this set up to produce the extra grip, decent tyres are needed.
24mm front and 27mm rear bars produces similar effects as 22/24mm front and rear bars, but needs racing tyres to work well.

Please note that I make a distinction between a car losing grip through roll and a car losing grip through wash-out. There is a big difference between the two in the way they are generated and experienced. Personally, I prefer a car that washes off it's grip over a car that rolls of it's grip. Generally, I find that roll oversteer is more unpredictable and more violent than a car that washes off it's rear grip. A car that rolls of it's front grip normally wears the outside edges of the front tyres very quickly and feels 'numb'.

In order to get a car that really handles well for the driver, you need to assess what part of the handling you want changed. Is it understeer when turning in, power understeer halfway through a corner or exit oversteer etc.etc.? With that information, you can choose the correct anti roll bars and any other chassis modifications.

Below is a G graph of a standard Impreza compared to the same car with just an uprated rear ARB

chiefs

Good post!

You’re definitely right is saying that each setting is driver specific. I haven’t upgraded my ARB yet as I’d rather wait until I’ve driven enough laps to confirm I can’t change my driving technique to accommodate the appeared greater rear end traction.

dunx

iTrader: (3)

Classic or New-age ?

"wash out" - please enlighten me further.

A good post, but as most of us use our cars on and off track on a variety of tyres, the choice is down to driver ability/style, imho.

R888's / T1S atm. 22mm F & 22 mm R + ALK.

dunx

lookout

Easy explanation without going too far, and eye glaze over :-)

A car that rolls off it's grip does so mainly because the camber angle has changed so much due to vehicle roll angles, that the contact patch disappears.

A car that washes of it's grip does so mainly because it comes to the limit of the grip that the proper tyre contact patch can deliver with the loading it has.

Hence, a car with too much bar on the front will wash out at the front upon turn in. A car with too little front bar will initially bite but as the front end starts to roll excessively, it will roll off it's grip as the outside front wheel gets more and more positive camber.

But, remember, this is dependent on the amount of grip available from the road surface. A car that has the ideal amount of bar at the front on nice dry tarmac, will understeer on turn-in if the same manoeuvre is tried on a wet surface. That why you see racecar engineers disconnecting bars for a wet set-up.

So, for a road car, a compromise has to be found that works ok for most conditions. Where you lay that compromise is personal taste.

I would expect that on your car on the T1S, your set-up is ideal for road use. Nice and 'pointy', without being nervous and a benign behaviour (if springs and dampers are not rated ridiculous, and damper allowing enough travel both in compression and rebound).

On the 888, on the track, I suspect that you will have a small amount of roll understeer, allowing the 888 to go off quite quickly, as well as a small amount of mid-corner push-on understeer, due to the fact that the back of the car has a lot of traction. If you are a smooth driver with slow to fast steering acceleration (rather than hacking the steering wheel), you might not feel this at all. There could be a case of needing to go up to a 24mm rear bar, but that depends on your own taste.

Anyhow, a very good case can be made for top mounts that increase caster. These will increase the negative camber on the outside front wheel and increase the loading on the inside front wheel during cornering.
Personally, I like the Impreza with between 6 and 7 degrees caster, providing you still run powersteering.

Oh yes, I am babbling again. I'll shut up now.......