Recently I started tracking a stock 2008 AP2. Only upgrade is 235/255 NT01 tires. Best way I can describe is a loose (over-steer) rear end in high speed corners. I have been able to correct it each time but my instructor mentioned I should really tighten up the rear end fearing I maybe in for a spin out. He mentioned "adjustable sway bars" would be a good idea. I am uncertain if I need to get the front, rear or both? Are there other options to tighten up the rear end of this car? I do feel the rear end comes out very progressively and relatively safe/easy to correct (vs the AP1 which had more snap over-steer and I can not easily catch it...)

 

First disconnect one side of the rear sway bar which effectively disconnects the whole sway bar and see how that feels to you. That's all I did to mine.

If you find you understeer too much then look into an adjustable rear bar or add a stiffer front to the stock set up with the rear sway bar connected.

 

You upset the handling balance by your tire sizing. Stock is 215/245, so by going 235/255, you add more front grip percentage wise which is causing the loose rear condition. So, you can either adjust your tire sizing or disconnect the rear sway bar as has been suggested.

 

or get a bigger FRONT sway bar (like Saner or Gendron), which is what your instructor was probably talking about.

 

As Tom suggests, the stiffer front anti-roll bar would probably be the better way to go. There are many FSBs out there that should fit your application, depending on your budget. OE CR or '00/'01 AP1 FSBs run somewhere around $50-$75 in the member classifieds or you can spend upwards of $1,000 for a top-of-the-line aftermarket FSB. A stiffer FSB will also help reduce overall body roll, which can have a positive impact on tire wear and transition response.

By disconnecting one of the RSB endlinks, you can test whether you like the balance change before dropping any money on a stiffer FSB. Obviously, that will only show you one data point with respect to front:rear roll stiffness bias.

 

if the problem is high speed corners a wing will definitely help. Of course some people don't like the looks of it.

 

Disconnecting the rear bar might make the balance better but it will make you slower overall. Upgrade your front bar to something stiffer and leave the stock bar as-is and you will correct your balance AND be faster.

 

Best mod I ever did for handling (besides great tires), was go to the CR shocks/springs. My car is very neutral now. It's almost impossible to spin it on purpose even on the skidpad. It's a great mod cause it's turnkey.

Also, make sure your rear tires have sufficient tread. The S2000 gets very loose if the rears get down to 3/32nds in my opinion