View Single Post
Old 06-05-2016, 08:03 AM
harry's Avatar
harry harry is offline
Techno Guru! He's Good! He Knows Everyfink!
Join Date: May 2005
Location: Taunton
Posts: 3,399

Originally Posted by Friar Tuck View Post
I have no idea what is being talked about here, but I'm intrigued as to what it's all about, and waiting to see the outcome....
We talk about gear ratios, but what actually matters is how far you travel for each turn of the engine crankshaft.
This is a combination of the overall gear ratio (a combination of primary drive reduction gearbox reduction and final drive reduction) But then this is multiplied by the Diameter of the driving wheels.

The technical term is rolling radius and is roughly the distance from the ground to the centre of the wheel.

The tyre size is marked as (for example) 235/50-18. This means the tyre is 235mm wide (It is measured on a standard sized wheel rim and called a nominal size as it varies depending on the rim width.) 18 inches is the diameter of rim that it will fit. 50 is the aspect ratio and is the differnce between the tyre width and the distance between the rim and the tread expressed as a percentage.

So in the example above the distance from the ground to the hub centre will be half the rim diameter plus half the tyre width (50% aspect ratio) so is 9 inches plus 117.5 mm.

This is only a rough guide to actual rolling radius as tyre flex and things like tread squirm have an effect, but its good enough for what we need.
si is does non opportunus vos postulo a maior pango
Reply With Quote