Problem understanding tire and rim sizes

[daniel]

Hello,

I'm currently building the Porsche RS Spyder in 3D and have some difficulties understanding wheel sizes.

http://www.mulsannescorner.com/PorscheRSSpyder-1.html

For example the front rim is 12,5 inch wide = 31,75cm and the wheels are wide 29/65-18
Afaik the rim is measured on the inside, roughly the grey part in my picture. The tire is 29 (cm) wide, the box on top the rim in my picture. That looks way to small...any ideas whats going on. Are racerims measured different...maybe the widhts is measured outside not inside like on road cars....

Thanks for any input.

[MPK]

The 29 stands for the width of the tread band width (cm), 65 tire external diameter (cm) and 18 rim diameter (inch).

[daniel]

Thank you. The rim diameter is the grey part in my picture, right ? Not the maximum outer rim size (the two yellow edges in the picture)

[JamesB]

Thank you. The rim diameter is the grey part in my picture, right ? Not the maximum outer rim size (the two yellow edges in the picture)

Exactly. The way you made the drawing is quite revealing and instructive. It took me ages to find this out.. finally I assumed that "wheels´inches" measured 27, not 25.4mm!!!
There´s not always a direct factor between title, inner diameter and visible, outer one. Though we end up knowing what the most usual are...

[daniel]

Thank you. The rim diameter is the grey part in my picture, right ? Not the maximum outer rim size (the two yellow edges in the picture)

Exactly. The way you made the drawing is quite revealing and instructive. It took me ages to find this out.. finally I assumed that "wheels´inches" measured 27, not 25.4mm!!!
There´s not always a direct factor between title, inner diameter and visible, outer one. Though we end up knowing what the most usual are...

also interesting that they mix cm and inches !?

[gp-models]

.... 65 tire external diameter (cm)

That`s not true, the 65 declares the hight in percentage to the width of the tread, 65cm in hight would be hilarious!

zB ein 215/35/17 straßenreifen hat eine Lauffläche von 215mm, die Höhe beträgt 35% von 215 (hab grad keinen Taschenrechner parat), die 17 geben den Felgendurchmesser an, sprich 17 Zoll.

Hier mal 2 Grafiken die das ganze erklären:

Bemaßung der Felge:


Bemaßung der Reifen:

[MPK]

I’d like to say I’m right with this one. Road and racing tire markings differ significantly.

[gp-models]

My mistake! Only had the street tyre dimensions in my head and overseen he is looking at the racing tyres.

[daniel]

My mistake! Only had the street tyre dimensions in my head and overseen he is looking at the racing tyres.

took me also some time to get the difference between racing and road

[DiemenRacer]

Racing tires go by the tire diameter rather than the aspect ration because diameter is very important for calculating gear ratios and ride heights.