Tyre Sizing

hfidgen

Hiya,

I'm currently on some Continental Twister Pros which have a size of "28x13/8x15/8" - whatever the heck that means.

I'm trying to get some Specialised Armadillos on in 28x700c but so far there's 2 snapped tyre levers so I'm coming to the conclusion that they are just too small! Can anyone shed any light on what I need here? (Other than some metal tyre levers).

EDIT Sheldonreckons that 28x700C are the ones by the looks of things - anyone willing to argue? Might have to MTFU and get them on there...

Thanks!

AndyOgy

For some strange reason, there are 2 sizes of 28" wheel. Same as there are 2 sizes of 26".

The difference is how the tyre width is measured. If it's imperial with a fraction (ie 1 3/8 or 1 1/2), you can not replace it with a tyre that has a metric or decimal measurement, such as; 23mm, 25mm, 1.2", 2.3".

To put it another way, 28 x 1 1/2 is not the same as 28 x 1.5

It doesn't make a lot of sense really, but that's bikes for you.

hfidgen

Hmm that's a bugger - I went and got them on (with a bit of pushing and shoving) then came back and read this!

The inside of the rim states:

18-622 for rims 622 X 15-18C/15C Offset max 145PSI

Which completely muddies the waters - I know those are the ISO standards but WTF is 15C or 18C?

Using a ruler, the inner measurement across the rim is 16mm and across the outside it's 21mm (IE the rim wall metalwork is 2.5mm thick )

hfidgen

Gah I hate this sh*t - why can't manufacuters just use ISO and have done with it. I had a chat with the Evans guy in Blackfriars this morning and it turns out that 28x700c should fit fine. I was a bit concerned that the bead wasn't locking in as I could see some of the red wall above the rim (especially on the back wheel) but he said that was normal!

If you hear of a cyclist down on Embankment due to slipping a tyre...

UndercoverElephant

No, that's not normal, not normal at all. There is a knack to fitting tyres, look at this:

http://www.youtube.com/watch?v=-XUFVrl0 ... r_embedded

The tyres are the right size, it's a matter of technique.

Wooliferkins

Manufacturing tolerances on tyres and rims can vary by up to 1mm so the accumulated difference can be 2mm. On your tyre somewhere will be the ertro number which is standard.

sketchley

UndercoverElephant wrote:

No, that's not normal, not normal at all. There is a knack to fitting tyres, look at this:

http://www.youtube.com/watch?v=-XUFVrl0 ... r_embedded

The tyres are the right size, it's a matter of technique.

I was worried about getting some SMP tyres and fitting myself until I watch that video. Easy and no tyre levers, all done by hand.

secretsam

There's a groovy little tyre fitting tool reviewed in C+ this month that gets 10/10 - made by VAR I think, only about £7 - might get one myself as tyres are a pain - it's not actually 'new' as I'm sure something similar was sold in the '80s!!!

vorsprung

hfidgen wrote:

Hmm that's a bugger - I went and got them on (with a bit of pushing and shoving) then came back and read this!

The inside of the rim states:

18-622 for rims 622 X 15-18C/15C Offset max 145PSI

Which completely muddies the waters - I know those are the ISO standards but WTF is 15C or 18C?

Using a ruler, the inner measurement across the rim is 16mm and across the outside it's 21mm (IE the rim wall metalwork is 2.5mm thick )

18 is the width of the rim in mm
This is not the same as the width of the tyre

Sheldon has a table of "safe" tyre widths that will fit on a particular sized rim
http://www.sheldonbrown.com/tire-sizing.html#width

NB this is just a "on the safe side" chart. I have 35mm Marathon Winters on a 15mm wide Ridga Chrina

An 18mm rim and a 28mm tyre is well in the safe zone I'd say

hfidgen

vorsprung wrote:

18 is the width of the rim in mm
This is not the same as the width of the tyre

Sheldon has a table of "safe" tyre widths that will fit on a particular sized rim
http://www.sheldonbrown.com/tire-sizing.html#width

Great! That's the bit of info I needed, suddenly it all makes sense

hfidgen

All sorted - they went on without levers and look a much better fit. There's still a little bit of red showing on the back wheel, but i'm confident the bead has locked in.