Discussion
Was getting ready for work this morning when a random thought popped into my head.
Why are tyre sizes part metric part imperial?
I remember running an MG Metro years ago which ran metric measurement tyres, but on the whole, the majority of tyres still have the width in mm and the rim diameter in inches.
Anyone know how this came to be?
And why the beurocrats have not imposed a change yet?
Why are tyre sizes part metric part imperial?
I remember running an MG Metro years ago which ran metric measurement tyres, but on the whole, the majority of tyres still have the width in mm and the rim diameter in inches.
Anyone know how this came to be?
And why the beurocrats have not imposed a change yet?
kambites said:
I suppose they were both imperial once and it was OK for the width to be "close enough" so they could convert it, whereas even a small change in diameter would mean new wheels. Wheel widths are still measured in inches, I think.
Neat guess which applies to many things but not the case here. All tyres were cross ply and all dims were in inches back in the days when a third of the globe was shaded pink. Then Michelin invented radial tyres and being french the dims were metric but they were stuck with imperial rim dims or there would have been a very small market. Some cars like the 80s Jag did introduce 400mm dia wheels but these were quickly dropped.
kambites said:
Isn't that what I guessed then?
No! Allow me to remind you:kambites said:
I suppose they were both imperial once
Radial tyres were never imperialkambites said:
it was OK for the width to be "close enough"
It's not OK for widths to be close enough, the tyres fit the rims exactly.You can do the rest for yourself
What? Now I'm thoroughly confused. By "both", I meant both the width and the diameter.
Tyres used to be imperial.
Someone wanted tyres to be metric.
Tyres couldn't be completely metric because rims were already imperial diameter.
Therefore the person who wanted tyres to be metric made the width metric but left the diameter imperial.
What part of that is not true? Neither I nor the OP ever mentioned the transition from transverse to radial ply.
Tyres used to be imperial.
Someone wanted tyres to be metric.
Tyres couldn't be completely metric because rims were already imperial diameter.
Therefore the person who wanted tyres to be metric made the width metric but left the diameter imperial.
What part of that is not true? Neither I nor the OP ever mentioned the transition from transverse to radial ply.
kambites said:
What? Now I'm thoroughly confused.
Tyres used to be imperial.
Someone wanted tyres to be metric.
Tyres couldn't be completely metric because rims were already imperial diameter.
Therefore the person who wanted tyres to be metric made the width metric but left the diameter imperial.
What part of that is not true?
Read my first post.Tyres used to be imperial.
Someone wanted tyres to be metric.
Tyres couldn't be completely metric because rims were already imperial diameter.
Therefore the person who wanted tyres to be metric made the width metric but left the diameter imperial.
What part of that is not true?
JR said:
Read my first post.
I did. Several times. And I still think it's saying exactly the same thing as me. 
Tyres used to be fully imperial.
Michelin wanted to be metric.
Rims were imperial so they could only change the width, not the diameter.
The fact that all this happened at the transition from one construction method to another, is neither here nor there unless I'm missing something completely.
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