Confusion over 2022 Rim Sizes

[MoistureDaddy]

Hello, I recently developed an interest in F1 technology and modern suspension components, but have some confusion about the "18 inch wheels" on the 2022 cars.

As stated in the 2022 FIA F1 Technical Regulations section 10.7.2 Wheel Rim Dimensions:

The front wheel rim key dimensions are:
- Tyre mounting width = 335.3mm +/-0.5mm.
- Outer lip diameter = 490,6mm +/-1mm.
The rear wheel rim key dimensions are:
- Tyre mounting width = 429.3mm +/-0.5mm.
- Outer lip diameter = 490,6mm +/-1mm.

I am confused because the outer lip diameter of 490.6mm = ~19.3in, not 18in.

Any clarification would be much appreciated. Thanks!

[Tommy Cookers]

afaik
what the FIA presumably would call tyre mounting diameter will be 18"
the correct name of this in English is bead seat diameter

too many rubbish translations come from the FIA - and accepted by the rubbish english-language press

[jjn9128]

There's an overlap or the tyre bead and the wheel rim lip so the tyre is held in place. They're wheels for 18"tyres. The 13" tyres had 358mm rims.

[RGAEDA]

What a coincidence! I was thinking about this a few hours ago. Can anyone provide a diagrammatic representation putting this into perspective?

[jjn9128]

[chrisc90]

As above, the mounting width of the tyre where the bead sits will be narrower than the actual lip of the wheel.




If the bead/mounting was the same as the rim diameter the tyre wouldnt seal against the bead and would just slide off.

[RGAEDA]

Thanks guys.

[JordanMugen]

https://www.morrisminorforum.com/articl ... ection.jpg

I am confused because the outer lip diameter of 490.6mm = ~19.3in, not 18in.

Tyre diameters are measured from the inside of the bead AFAIK. I.e., the very inside of the tyre, the bit that overlaps and seals against the flange on the wheel.

I.e., the hole in the middle of the tyre is 18" I believe. Therefore the outer diameter of the wheel is bigger than 18", as it is strictly a wheel that fits 18" tyres despite being commonly called an '18" wheel'.



These and other tyre dimensions are certainly very confusing. Especially where 305mm width for example (e.g., a Ferrari 296 GTB rear tyre, 305/35ZR20) is the nominal sidewall-to-sidewall width, and many retail tyres do not conform exactly to the sidewall-to-sidewall width they claim to be!

Some models of tyre can be, in some cases, significantly wider than others despite being the same nominal tyre size. E.g., some one will say "the Hoosiers run wide". The nominal tyre size only strictly accounts for the inner and outer diameter conforming to the intended size I guess.