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24 Sep

Tyre sizing and rims

Posted by Malcolm Borg

The question gets asked alot as to why my road wheels are not wider than 19mm well it quite simple -ETRTO.

ETRTO is a standard organisation that lays down specification for tyres and rims. Standards for bead hook design are mandated and for tyres. My rims conform and all the major tyre manufacturers conform to these standards too.

Wider rims are often hookless as well. Hooked rims are recommended for all road and gravel width tyres. Often the rims used for road and gravel are the same and non of the major tyre manufacturers say its fine to use there road tyres with hookless rims. Some say explicitly you should not. Hookless rim are really an mtb creation for tyres inflated to 30 psi or under. While some claim pressures are fine much higher for road tyres remember to hookless carbon rim (its mostly carbon) exists solely because some rim manufacturers find it too expensive to make there bead hooks properly. Some never made them properly in the first place (ENVE bead hooks don't conform to ETRTO standards). I don't offer hookless rims. If your using one no IRC road tyre should be used with them. The gravel tyres I sell should be fine used tubeless and with pressures under 40 psi. 

There are many rim and wheel manufacturers now touting 23mm, 24mm and 25mm internal width rims and some say tyres from 25mm wide are fine. They do this because wider is better innit and it more aero. 

The problem is tyre manufacturers say different. At the extremes when say mounting narrower 25mm or 28mm tyre to a 23mm internal width rim one risk is tyre blow off. This is risk is higher with hookless rims than hooked rims. However this is the extreme for example the 28mm tyre pictured below mounted to 23mm internal width rim/28mm external width. This may not actually blow off but it shows another problem. Tyre shape.

Tyre manufacturers like IRC want there tyres to have a shape range. The top of the tyre should form a circular arc so when riding your on the centre of the tread and when corning you move progressively over the shoulders the more you lean into the bend. 

With a stretched tyre the shape is flattened at the top and this does affect how the tyre handles. You maybe more aero but slower in the bends. This especially true off road with off camber and undulating terrain. 

The side walls are also more exposed with stretched tyres to damage. Gravel and potholes on the road or all those roots, stones and holes off road are more likely to slice a side wall. Nothing slower that a blowout tyre. Tubeless tyre pinch flats are also more likely with over stretched tyres.

This is why i am not following the wider is better trend. It works to a point.

Suggested tyres rims for rim sections. These stretch the older ETRTO guidance a but new guidance was issued in 2020 which confirms my chart. You can actually go wider than i suggest safely but higher pressures would be needed to avoid tyre squirm in bends. This also affects tyre performance so the max width seem to be practical  maximiums. The minimum width really are minimums and with some tyres there may be pinch flat issues. With the wider rims the min tyre width is barely recommended as these are off road tyres and pinch flats are more likely at the bottom end of the tyre range.

Rim  internal width mm       tyre width mm

15mm                                     23mm to 32mm

17mm                                     23mm to 36mm

19mm                                     25mm to 42mm 

20mm                                     28mm to 45mm

21mm                                     32mm to 50mm 

23mm                                     36mm to 60mm

25mm                                      40mm to 65mm





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