First consideration is rim size. Starting from R25, to the Ninja 400, RC390 and 450SR mentioned, I believe all have 4 inch width rear rim. Options are 140/70 and 150/60.
» Click to show Spoiler - click again to hide... «
» Click to show Spoiler - click again to hide... «
Note: this is a general chart, some tyre models may vary
Key takeaway from fitment guides:
1. For the same size, going lower on the aspect ration (AR), from 80 to 70, a larger rim width is recommended
2. Bias ply vs radial tyres can have different fitment guides (example below)
» Click to show Spoiler - click again to hide... «
From Metzeler's book, a 140/70 is recommended for 3.75; whereas the 150/60 for a 4.25. Both will work on 4 inch rims, just the 140/70 will "expand" a little, while the 150/60 "pinched" a little. One way to do this, is to measure the tyre bead's distance from edge to edge before they are mounted.
My personal preference:
- high profile tyres such as bias ply, is better 'expanded' but can be pinched, because of the construction, either way is fine
- lower profile radial tyres should try avoid over-pinching because it affects the stress distribution at the radial belts which spans radially around the tyre
This post has been edited by alexei: Aug 26 2025, 10:28 AM