Generally the best way to know what size tyre you have is to check the markings on your tyre. There can be multiple markings on a tyre each corresponding to a different system that has been in different areas of the world. This can be a bit daunting at first but a little bit of guidance should clear things up.
- ISO (ETRTO)
This system is the international standard that was created to regulate and guide the tire measurement system.
The markings for this system take the form of XX - XXX. An example being 32 - 622.
The above picture illustrates how the two digit number corresponds to the tyre width and the three digit number corresponds to the internal diameter of the tyre, otherwise known as the "bead diameter". The bead diameter is a very important measurement as it is what needs to match on tyre and rim in order to ensure a good fit. (All in mm)
The internal diameter is the relevant measurement for the Swytch kit and so if you can find the marking or measure this then you can safely ignore the rest of the markings on your tyre.
- Inch Markings
Inch marking are amore traditional measuring system. They are often what is referred to when people state that a wheel is a 28" wheel or 16" wheel etc.
The markings take the form of three measurements, each separated by an 'x'. E.g: 28 x 1 5/8 x 1 3/8
The first measurement is the outer diameter, the second the tyre height and the third the tyre width.
The outer diameter is useful to know as it narrows you down to what wheel you require however for the Swytch kit you will require the internal diameter.
This marking can also be found on rims. If you look on your rim and find the ISO/ETRTO marking then you should order a wheel to match the ETRTO three digit number.
Another traditional system that is often marked on tyres is the French system. This marking can be identified by a three digit number followed by a two digit number separated by an 'x'.
This is similar to the ISO system, however it measures the external diameter along with the tyre width. Important to note that this is not the internal diameter as with the ISO markings.
In order to know what wheel you need to order from Swytch you need the internal tyre diameter. If you cannot locate ISO markings as above on your tyre then you can measure it yourself.
When measuring to get measurements that correspond to the ISO bead diameter, place the tape from the centre of the wheel to the tyres’ inner edge. Then double it to obtain the diameter. This will likely not produce the bead diameter as the bead is where the tyre and rim meet which will not be visible from the outside. Therefore your measurement should be slightly bigger than the ETRTO number of the wheel. This should give you a good idea of what wheel to order however a marking is the best option and should be used where possible.
If you are unsure you can contact us at firstname.lastname@example.org