er.. some useful info about the "thickness" lar.. "120/60 ZR15"
is a typical tyre rating. btw i got this from the singaporebikes
website.
the "120" refers to the width in mm, the "60" is the "aspect
ratio", which is the thickness of the tyre expressed as a
percentage of the width. so in the case above, a "120/60" tyre
would have a side profile of 72mm.
the "ZR" is a speed rating, which means the tyre is suited for
travelling at speeds greater than 240km/h, "Q" is up to 160km/h,
"T" is up to 190km/h and so on...
the last number is your rim diameter in inches lar... hope this
helps.. 
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rock hard... ride free...