Any discussion of what tire width will fit what wheel width is meaningless without including the aspect ratio. A 130/90
tire will properly fit a 2.5" min rim. A 130/70 requires a rim at least 3.0" wide min.
what does proper or approved size mean. First of all there are tire/rim regs that are mandated by governmental agencies and
may or may not be enforced. These regs are worked out with the tire people and certain standards are agreed upon. A good ref
source for general Rim to Tyre applications:
tire mfg have their own recommended rim widths for each tire they make. I don't know of any tire mfg offhand that doesn't
approve the fitment of a 140/70 tire on our 3.5" wide rear rims, bias or radial. But also can't think of any that make radials
in a 130/70 or a 140/70 size. Most have a 140/80 radial that will fit the 3.5" rims and fit our bikes.
Most radials start at a min 150/70 size, some
no narrower than 160/70. The 150/70 is approved for our 3.5" rims by general tire regs, but is not approved by all tire mfg.
Metz Z4 and Conti Contiforce are approved, Dunlop D205, Avon AV45/46, and Bridgestone BT020 are not approved for fitment of
the 150/70 on the 3.5 rim. Even more interesting is the BT45 bias tire that is approved for this fitment for the H rated tire,
but not the V rated one.
The 150/70 is marginal for the GS500 rim and for clearance in fitting it to the bike. Depends on which 150/70 it is.
I would not be concerned
about much handling change in putting a 140/70 bias tire on my GS. The original 130 is the optimum size and will give good
handling. Tires keep getting wider and wider to handle the ever increasing HP of motorcycles. A 130 will more than handle
the power output of the GS. If you want better traction, then you want stickier rubber rather than wider tires.
The Sportecs are excellent sport tires. A very
good choice if you are willing to sacrifice some tire life for better handling. I'm not. The 150/60 will fit the 3.5 rim according
to general tire regs, Metz specifies a min 4.0 rim for it. Actually it measures only 143 mm wide on a 4.0 rim, less on the
3.5 rim. Looks like a good fitment to me for the GS500 along with a 110/70 front. I'd try them myself except for the tire
very happy with the Z4/Z2 combo last summer, but the new Avon Azarro AV45/46 ST tires are looking very good. Did you guys
see the brief write-up on them after trying them out in the latest Cycle World mag? I would use the 140/80 size approved by
Avon rather than the 150/70 they don't approve on my rim. So many decisions.