WHEEL & TIRE DY­NAM­ICS

Un­der­stand­ing the re­la­tion­ship be­tween rims and rub­ber

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To bet­ter ap­pre­ci­ate the dy­namic re­la­tion­ship be­tween a wheel and a tire you have to first un­der­stand the me­chan­i­cal char­ac­ter­is­tics of their uni­son. Let’s face it, there’s no other sin­gle com­po­nent more im­por­tant than your rims and tires since this is what ev­ery­thing else rests on. So to get a bet­ter ed­u­ca­tion on this ver­sa­tile sub­ject we spoke with Mark Chung of Yoko­hama Tires. At the heart of the con­ver­sa­tion was a de­tailed break­down of wheel and tire dy­nam­ics. We’ve also cre­ated a de­tailed out­line of the mea­sure­ments that you should bet­ter un­der­stand as both a con­sumer and en­thu­si­ast of af­ter­mar­ket wheels.

1. OVER­ALL WIDTH

This de­fines the over­all width of a tire, which is mea­sured from the in­ner to the outer side­wall. If your tire has a lip bead (or pro­tec­tive rib or raised let­ter­ing) then this is in­cluded in the mea­sure­ment.

2. SEC­TION WIDTH

This is the mea­sure­ment of the tire’s width from side­wall to side­wall (not in­clud­ing the pro­tec­tive ribs). It’s im­por­tant that this mea­sure­ment be taken when the tires are prop­erly mounted and in­flated with no load on the tires.

3. FREE RA­DIUS

This is the dis­tance from the wheel’s axle cen­ter to the outer tread sur­face of the un­loaded and prop­erly in­flated tire.

4. RIM WIDTH

This is the lin­ear dis­tance be­tween the outer and in­ner rim flanges on which the tire bead rests.

5. STATIC LOAD RA­DIUS

This mea­sure­ment is mea­sured with load on the tire and is taken from the axle cen­ter­line to the treat con­tact area or patch.

6. DEFLECTION

The mea­sured difference be­tween the tire’s free ra­dius and the loaded ra­dius when mounted on a rim that is in­flated to test pres­sure and un­der a pre­scribed or pre­de­ter­mined load.

7. TREAD WIDTH

The dis­tance from the outer edge to the in­ner edge of the tread.

8. TREAD RA­DIUS

The de­sign cur­va­ture or con­tour of the tread pro­file.

9. SEC­TION HEIGHT

This is mea­sured off of the ver­ti­cal dis­tance be­tween the tire’s bead seat and outer tread sur­face when prop­erly mounted and in­flated with no load on tire.

10. RAISED LET­TERS 11. NOM­I­NAL RIM DI­AM­E­TER

This is mea­sured as a lin­ear dis­tance be­tween the bead seats, which are taken at the widest point.

12. OVER­ALL DI­AM­E­TER

You prob­a­bly don’t need help un­der­stand­ing this but it’s the dis­tance from the tread sur­faces at its widest point. The tire should be aired to the pre­scribed air pres­sure and un­der no load.

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