My understanding is it's related to tire size... so mine are 120/90/18 rear and 100/19/90 front.
The original warning sticker is still on my rear fender and that's the pressures I use. I know the pressures stated are for the old 'inch' sizes, but the above tires are very close to the OEM sizes.......
Anyone else have any more info ?
Modern metric tires require approximately 8 PSI more than their equivalent 1970s siblings.
Here is the EXACT way to figure your tire pressure:
Start with a nominal value, like 30 PSI, and cold tires (parked for at least 1 hour, out of the sun). Ride the bike about 10 miles, then check the tire pressures. If they rise by exactly 2 PSI, you have the right pressure. If it rises by more than 2 PSI, add 2 PSI to the cold pressure and repeat the test. If they rise by less than 2 PSI, deflate by 2 PSI and repeat the test. Always start with the tires cold, though, or this does not work.
I learned this from the Continental Motorcycle Tires chief engineer, when the Conti Twins first appeared (a knowledgeable German rider). It is the engineer's rule-of-thumb for inflating motorcycle tires with unknown loads.
An example: my CB750K2 with Vetter fairing and me winds up with 38 PSI front, 39 PSI rear, on Avon Road Riders 100/90-19 front and 110/90-18 rear.
