Sizes and tyre codes through the ages

These (tyre sizes and codes) continue to cause confusion as at least four different approaches to quoting motorcycle tyre sizes have been employed since the start of motorcycling over 120 years ago. And modern tyre covers also quote speed ratings and load limits, though these are often quoted as codes rather than precise figures. I thought I’d covered these subjects in TCM ‘recently’… On checking, I discovered I’d penned this information 17 years ago! So here’s a brief résumé for road use tyres.

* Many beaded edge, some early wired-on and a few classic tyres available today are sized by width and outer diameter of tyre rather than rim diameter: eg 25x3 = 25 inch outer diameter and three inch width.

* Traditional classic and some modern lightweight tyres are sized by width and rim diameter: eg 3.00x19 = three inch width, 19 inch rim

* Wide section Imperially sized tyres: eg 3.60 x 19 = 3.60 inch width by approximately three inch balloon height by 19inch rim

* Metric width: eg 90/90x19 = 90mm wide with balloon height, 90% of width (some makers give this as a height measurement) by 19 inch rim, though in some instances rim diameter is quoted metrically. Thus 90x90 tyre measurements equate to 3.5inch inch width by approximately 3.15inch balloon height

Ranges of classic tyres are suitable for fitment to both front and rear wheels, but others are wheel specific. In some instances, this is an accepted standard: e.g. ribbed tyres are front wheel fitment only. Some tyres are universal, which may be indicated on the tyre, and some classic and lightweight tyres suit either wheel and can be fitted either way round, in which case there is often no indication marking. Yet other tyres may be suitable for either wheel but must be fitted as indicated by direction arrows, exampled here.

This is a genuine Avon RoadRider. Reference to Avon’s published information will detail type of compound and construction of the tyre cover. Beware, however, as fake replicas of good quality tyres have been offered on the market.

As well as offering published details of tyre construction, the subject Avon RoadRider carries much information on the tyre.