I have been building my little girl a bike which is running 24" Halo Tornado rims. I ordered a pair of Schwalbe Table Top tyres but they were too big to fit between the chainstays. Looking through Chain Reaction, and found a pait of Intense Micro Knobby tyres in 24×1" 3/8. Ordered them and they were delivered yesterday. Went to fit them on and found that they were too big in diameter! Emailed Chain Reaction and they have been very helpful.
It turns out that imperial sized tyres, such as the 24x 1" 3/8 are designed to run on bigger rims (?) than say, 24×1.85. I just thought that a 24" tyre was a 24" tyre! Silly me. 😳

you want to try buying "24 inch" wheelchair tyres. ridiculously difficult!

I'm just in the process of building a 24" wheel bike for my son… I'd recommend Shwalbe Mow Joe 24×1.85 for a light grippy tyre.

Oh and I made that mistake before with some 16×1 1/8th tyres, ended up building some wheels to fit the tyres, although in hindsight it might have been easier to just get some tyres that fit.

Ah, tyre sizing, it's a nightmare, especially on older or unusual bikes… Look for the metric measurements and you can't make a mistake. All tyres have it written on the carcass, and it takes this form (XXX-XX). e.g. a 700c x 20mm is (622-20). The first number is the actual diameter of the rim in mm, the second is the tyre width.

What about Kenda SB8's, they are available in 24×2.0 – have them on my son's bike – fast, grippy and light.

adstick – close, but no cigar.

The ETRTO size specification 37-622 indicates the width of 37 mm and the tire inner diameter of 622 mm. This dimension is clear and allows for precise classification of the rim size.

(schwalbe tyres website)

Sheldon Brown has a very useful article about tyre sizing: http://www.sheldonbrown.com/tire-sizing.html
As it says, 24" can refer to a number of different sizes – 24x a decimal number is not the same as 24x a fractional number !
So always best to check the ISO/ETRTO size.

Chain Reaction are not very helpful, as they don't specify the ISO numbers for any tyres. Dotbike are a bit more useful as they do this for most tyres: http://www.dotbike.com/ProductsC137.aspx

24x 1/38 and 24 x 1/4 are old school sizes also done in 26 they are nothing to do with mtb rims .It can be very confusing