so you guys are saying go to 180/60 in stead of 180/55 for quicker turning in?
Basically, given the same brand and model of tire, a "60" is taller than a "55". The 2nd number is the ratio of the width (180mm) to the height, expressed as a %. ie: a 180/55 would mean the height is 55% of 180 mm, or 99 mm. So a 180/60 would be 108 mm or about 9 mm taller (just under 1/2 cm). Since different tire brands and models already have different manufactured heights, it can be difficult to "read across" to different tire models, but you get the idea.
Same is true for a 170/60, which would be 102 mm tall. Not as big a difference, ie: only 3 mm, but the tire would also weigh a little less, and the carcass would be 10 mm narrower. All of these things will contribute to how the bike "feels".
http://bridgestone.com/products/motorcycle_tires/products/option/index.htmlAgain, this assumes the tire is mounted on the recommended rim size.
A quick look at Bridgestone's site, looking at the BT023 shows that they recommend a 5.0 rim for a 170/60 and a 5.5 inch rim for a 180/55. They show the diameter of the tire being 7 mm larger (or 3.5 mm radius, to bring it to the same criteria above). Note that you may have a harder time finding a 180/60 tire, as not all manufacturers make them.
http://bridgestone.com/products/motorcycle_tires/products/battlax/bt023.htmlIf you simply want to tighten up the steering by 1/2 cm or so, them I would recommend that you slide the fork tubes UP (drop the triple trees down) 3/8 of an inch or so. This is basically doing the samething as installing a larger diameter tire.