Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community - FT86CLUB

Due to pretty much all production cars, including the 86, understeering (according to ISO test methods), I decided to test a few rear bars on my car and share my experiences.

Test environment were a variety of British roads. I am not talking about tracking, flying around a parking lot, or otherwise driving on a smooth, flat surface. British 'B' roads are usually narrow, bumpy, lots of camber changes, etc. A pretty good test for all-round driving. You arguably would want more bar than this on a flat surface, but that comes down to personal opinion.

Car = stock suspension, 225/45/17 AD08 tyres.

Stock (14mm)
Understeering monster. Entry into a bend, understeer. Once the car has rolled, understeer. Pushing out of the bend, understeer. Lifting off deep into the bend results in a mild reduction of understeer, but not neutral or oversteering. Very safe for the average consumer, what i'd expect.

WRX (16mm)
Much better. Car rotates better, less steering input required. Has more initial front bite going into the corner but still understeers on the limit. Heavy lift-off at < 40mph speeds results in almost a perfect sideways drift rather than oversteer. I did some high speed testing and lifting hard + turning at ~80 mph does result in over-steer, but the degree depends on your driving ability - light correction required. Generally fairly neutral when pushed hard. Allows rapid change of direction without upsetting the balance of the car. All in all a much better balance.

STI (18mm)
This is where personal taste will come into it. In my opinion this bar is too stiff and is throwing the balance of the car (with a 16) off. Initial entry into a bend now exhibits slightly more understeer than the 16, which tends to mean the rear bar is too stiff reducing grip on the front. As the car rolls more and more into a turn you notice the effect this bar has progressively. With this bar it doesn't take much throttle in the bend for to overload the rear tyres. It's still pretty safe with quick direction changes. You do get lift-off oversteer with this bar, if that's your bag. In a RWD car though it's not typically a characteristic you want, unlike FWD. You need the driven wheels to be able to pull you out the bend.

So I will most likely stick with the 16mm bar. I actually tested a 16mm whiteline bar using the OE drop links (perfectly acceptable, Subaru use the same end links for all their Impreza bars up to 19mm on a heavier car). I was using the middle hole on the bar as this gave the best geometry with the end-link and was closest to the OE mounting position. The Whiteline bar has a bit of a design fault though where they've extended the lever arm to the point where it can foul things on the car under serious suspension compression. I'll most likely grind off the end of the bar I don't need/want, unless I can find a cheap WRX bar quickly.

Some may argue that uprating the front bar may work well with the 18mm rear, but to be honest the car doesn't roll much (on the road) so it's a bit unnecessary.

I hope this information proves useful to some people :D. I'm not interested in FUD and bullshit from people who don't actually understand what understeer and oversteer is.