Wheel/tyre size?

[KoffMixtures]

I recently bought a Sturmey-Archer wheel in very good condition, thinking it would go straight into one of my Raleigh Twenties.
It won't!
The (steel) rim is stamped 'RALEIGH 20 06 (20X1.75)'
Diameter is 16''
The vendor insists it's a 20'' wheel!
So I'm confused. What size of tyre would be suitable?
Could it be from a Mk 3 RSW 16?
Though I understand they had a drum brake/SA combo hub.

 

[Big John]

https://www.sheldonbrown.com/rim-sizing.html

This may or may not help.

 

[KoffMixtures]

Thanks BJ

From that I identified my ISO451mmm Raleigh 20 rim which takes a 20x1 3/8'' tyre.

However, my smaller wheel ISO measurement, as accurate as I can estimate, is ISO419mm

The nearest on that table is: ISO419mm = 20x1,75''

Now my brain hurts...the numbers say 20'' BUT the rim SHOUTS 16''

Still no wiser as to the correct tyre to fit!

 

[Chris S]

Thanks BJ

From that I identified my ISO451mmm Raleigh 20 rim which takes a 20x1 3/8'' tyre.

However, my smaller wheel ISO measurement, as accurate as I can estimate, is ISO419mm

The nearest on that table is: ISO419mm = 20x1,75''

Now my brain hurts...the numbers say 20'' BUT the rim SHOUTS 16''

Still no wiser as to the correct tyre to fit!

Metric tyre sizes refer to the bead diameter of the rim. Imperial tyre sizes refer to the approximate outside diameter of the wheel with the tyre fitted.
1 3/8 x 2 = 2.75"
451 / 25.4 = 17.75"
add them together and you get an outside wheel diameter of just over 20 inches

 

[KoffMixtures]

Thanks guy's.

I should mention I suffer from both Dyslexia and Dyscalculia.
Spelling and maths aversion!
Makes me wonder how I'm still here after nearly 70 years.
But here I am!

 

[KoffMixtures]

Eventually sorted my own query.

Uk Raleigh '20' was fitted with ISO 451 rims
Export Raleigh '20' was fitted with ISO 406 rims

A substantial difference in sizes.

 

[andrew_s]

But both 20".

As Chris S noted, tyres were originally sized by the outside/rolling diameter of the tyre, so what's now ISO 451 was originally 20"x1 1/8", and ISO 406 was originally 20"x2".
A substantial difference in width .

Getting new wheels when you wanted to use wider or narrower tyres isn't sustainable, so it wasn't long before tyre manufacturers started producing narrower or wider tyres to fit existing rim sizes, regardless of the fact that the outside tyre diameter would no longer be the same as the nominal size, so you'd have (for example) a tyre labelled 20" x 1 1/8" x 1 1/2" which fitted the rim for a 20 x 1 1/8" tyre, but which was 1 1/2" wide, with an outside diameter of 20 3/4".

There's a lot of scope for confusion with such a 3-part sizing system, so the ETRTO system (now ISO) of bead seat diameter and width in mm was introduced.

In general most of the rim sizes that have been used are semi-defunct, with only a very limited range of tyres available, and only one of each inch size still being relatively common, at least under its original designation.
So if you go into a random LBS and ask for a 20" tyre, you'll get ISO 406, rather than one of the other two still available 20" sizes, and if you ask for 26", you'll get ISO 559 rather than one of the other three 26" sizes (a 5th 26" size is now normally referred to as 650B or 27.5").