Tyre sizes .. 20" for Dahon

[robgul]

I'm looking at buying a Dahon folder (the Speed 7 - cheap-ish and cheerful) ... 20" wheels, but I'm confused about tyre sizes (the off-the-shelf tyre size in the spec is quoted as 20" x 1.75") - I would be looking to fit something a bit narrower BUT there appears to be plethora of 20" tyres with all sorts of sizes ... I looked at Herr Schwalbe's range of tyres and got the list below.  The width figure I understand, it's the 400/406 etc that's unclear.

Can anyone give me some clarity on this please?  (BTW - I'm open to suggestions on suitable tyres ... although "my tyre of choice" for most of my bikes is the Marathon)

Rob

Schwalbe 20" tyres

54-400    20 x 2 x 1 3/4 20 x 2F4J    
28-406    20 x 1 1/8    
32-406    20 x 1.25    
35-406    20 x 1.35    
37-406    20 x 1 3/8    
40-406    20 x 1.50    
44-406    20 x 1.625    
47-406    20 x 1.75 20 x 1.90    
50-406    20 x 2.00    
54-406    20 x 2.00    
57-406    20 x 2.125    
54-428    20 x 2.00    
40-432    20 x 1 1/2    
37-438    20 x 1 3/8    500A
40-438    20 x 1 3/8    500 x 38A
28-440         500 x 28A 500A 500A Standard
40-440    20 x 1 1/2NL    500 x 38A
28-451    20 x 1 1/8    
37-451    20 x 1 3/8 B.S.    

[Biggsy]

If the "1.75" is correct, then you'll be able to fit any ISO 406 tyres that are wide enough for the rims.

What exact model of 7-speed Dahon are you considering?

[Biggsy]

There are a lot of references to 406 for Dahon.

[robgul]

If the "1.75" is correct, then you'll be able to fit any ISO 406 tyres that are wide enough for the rims.

What exact model of 7-speed Dahon are you considering?

It says in the OP - Speed 7 machine - the root of the question was really to find out why there are different numbers like 400/406 etc all purporting to be 20" ....

Rob

[Biggsy]

Obscure historical reasons.  Tyre diameter size codes, other than ISO, originally nominally referred to the overall diameter, rather than the diameter at the bead.  The trouble came when a range of widths, therefore depths, were made available.  Different countries and manufacturers came up with different naming systems, and there were overlaps and silliness.

This is why it's important to check the ISO (eg. 406mm) when in doubt, as this refers to the bead diameter.

It says in the OP - Speed 7 machine

Sorry I didn't pick that up - because it doesn't sound like a model name.  They could have thought of something more interesting! 

[Biggsy]

From http://sheldonbrown.com/tyre-sizing.html

Traditional Sizing Systems
The traditional sizing systems are based on a measurement of the outside diameter of a tire. This would usually be measured in inches (26", 27", etc.) or millimeters (650, 700, etc.).

Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. Here's how it works: Let's start with the 26 x 2.125 size that became popular on heavyweight "balloon tire" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tyre is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tires, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tires, marked 26 x 1.75 to fit the same rims. Although they are still called "26 inch", these tires are actually 25 5/8", not 26". This same rim size was adopted by the early pioneers of west-coast "klunkers", and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25 mm to fit these rims, so you wind up with a "26 inch" tyre that is more like 24 7/8" in actual diameter!

A second number or letter code would indicate the width of the tire. (26 x 1.75, 27 x 1 1/4...650B, 700C...)

Does Point Seven Five Equal Three Quarters?
Note that the inch-based designations sometimes express the width in a decimal (26 x 1.75) and sometimes as a common fraction (26 x 1 3/4). This is the most common cause of mismatches. Although these size designations are mathematically equal, they refer to different size tires, which are NOT interchangeable. It is dangerous to generalize when talking about tyre sizing, but I would confidently state the following:

Brown's Law Of Tyre Sizing:
If two tires are marked with sizes that are mathematically equal,
but one is expressed as a decimal and the other as a fraction,
these two tires will not be interchangeable.

[robgul]

I had looked at the Sheldon site before I posted ... but the mix of Imperial and Metric coupled with Sheldon's sense of humour (sorry "humor") doesn't really help.   [I've met the late man at Harris Cyclery in Boston in about 2002 , knowledgeable but I thought he was a bit up his own backside in terms of attitude ... eccentric is probably a fair word for him!]

Sounds like the answer is to look at the tyre wall, if I buy the machine, for the  4** number and replace with the same but narrower, and not worry about the logic!

Rob

[Mr Larrington]

They'll be 406 for certain.  451 are pretty rare and all the others are unlikely to be found in the wild except on something really esoteric.

[StuAff]

They'll be 406 for certain.  451 are pretty rare and all the others are unlikely to be found in the wild except on something really esoteric.

+1. All 20" Dahons are 406. Bike Friday's Pocket Rocket models (amongst others) use 451, though just to be difficult they do 406 models as well. 451, as the higher number implies, gives a slightly higher circumference, why they're not marked up as 21" or some such, pass....

[Biggsy]

BTW - I'm open to suggestions on suitable tyres ... although "my tyre of choice" for most of my bikes is the Marathon

Kojak for the summer months, Marathon Plus for the winter.

There are both folding and non-folding versions of the Kojak.  It's light, but still with substantially thicker rubber than you get on a true racing tyre, so is more practical than you might fear.  I have them on my Brompton.