Spreading From Tire Pressure

Velocity k525 MTB rim, used with a large-section tire at high pressure (nominal 45 mm (1.75"), ~4.1 bar (60  PSI)):

  

The rim has spread from a nominal new width of about 20.5 mm to about 22.0-22.4 mm, or about 7-9%. The rim section is splayed around the rim's circumference, indicating this is not the result of a local rim strike or artifact of sawing apart the rim. Thus, it is likely from high pressure in a large-section tire, leading to high hoop stress.

The rim is not especially worn, with thickness of about 0.8 mm on the left (bent) side, 0.9  on the flatter side. In comparison, the following Mavic 517 rim is as thin as 0.35 mm in the box and as thin as 0.8 mm between the tube bed and the hook, and is substantially less spread:

  

The Mavic 517 is spread, from a nominal new width of 21.6mm to about 22.5-22.7 mm (4-5%), although the brake track is worn out to the hook, so it may have spread more but also have worn more. The Velocity k525 was run with the pads sufficiently inboard that the rim is still anodized at the hook, indicating no wear there, and an accurate measurement.

Both rims were removed from service before failure.

Important differences that account for the Velocity k525's greater spreading likely include:

• Use of higher air pressure on the Velocity k525, about 4.1 bar (60 PSI) versus 3.0 bar (45 PSI) for the Mavic 517 rim.
• An unused Velocity k525 is about 20.5 mm versus 21.6 mm for the Mavic 517, or about 5% narrower. Thus the tire enters the Velocity k525 at a more acute angle and causes higher spreading force at any given tire pressure.
• The k525 is about 6.5 mm from hook to tube bed, while the other rim is about 3.5 mm. That difference means nearly double the bending force for any given bead force.

The effect of tire section is unknown. A similar size tire was used on the Mavic 517 for some of its service, but also a slightly smaller tire for some of its service.

The effect of material is unclear — the k525 is made of 6106-T6 aluminum, the Mavic's "Maxtal" material composition and properties are unknown.

The spreading force of the tire is roughly proportional to the tire pressure, the section circumference of the tire and rim, and is related nonlinearly to the angle the tire makes with the rim. For a small tire, the height inside the rim may also affect spreading force. Thus:

• Doubling the pressure doubles the spreading force.
• The total section circumfrence is both the tire casing (sidewalls and tread) and also the rim's distance between the hooks. For example, if a tire that is 60 mm bead-to-bead is mounted on a rim that is 16 mm between the hooks, the total circumference is about 76 mm (there is overlap between hooks and beads, so the exact distance depends on where you measure). If the same tire is mounted on a rim that is 22 mm between hooks, the circumference is about 82 mm or about 8% more. Although the spreading force depends also on the entry angle, the angle will be about the same for the two cases, so the same tire at the same inflation will have about 8% more spreading force.
• Spreading force also depends on the angle that the tire makes with the rim. A very large tire on a very narrow rim will pull almost directly sideways, so nearly all the force directly apart. In contrast, a narrow tire on a wide rim meets the hook almost parallel to the brake track, and so causes little spreading force. The important relationshipo is the sine of the angle between the casing and the rim sidewall.
• Air pressure inside the rim cavity also spreads the sidewalls. The taller the sidewalls inside the rim — that is, the larger distance between tube bed and hook — the larger the spreading force. For a large-section tire, the spreading force inside the rim is small compared to the other forces, but for a very small section tire, the spreading force may be significant.

Many riders assume low-pressure tires will not spread the rim sidewalls, but since low-pressure tires are typically also large-section tires, and since large-section tires typically meet the rim at a sharper angle than small-section tires, the spreading force from a 3 bar (45 PSI) tire often exceeds the spredaing force of narrow section 6 bar (90 PSI) tire.