Crossply (or bias) tyres have a relatively simplestructure with sturdy sidewalls and are particularly suited to off-road use asthey resist impact well. They can’t be used at speeds over 150mph (240kmh) Radial tyres – which have an ‘R’ in the designation onthe side – have a casing that sits at 90° to the rolling direction, and a beltthat’s between around 0 and 25° off it. This belt, which sits under the tread(it’s what you see poking through on really badly worn tyres), adds stabilityand allows for far higher speeds as the deformation due to centripetal force isgreatly reduced. Because the sidewalls are thinner, the tyre heats upless, so high speed strength is improved. Modern motorcycles are geared to useradial tyres, as they only expand by a few millimetres at speed; a crossplytyre can expand by around 20mm at 130mph! The other tyre of note is the ‘bias-belted’ – effectivelya crossply with belts below the tread for additional support, and is suitablefor use up to 150mph. These tyres have a ‘B’ in their designation; in thepicture below you can see one on the 2018 Harley-Davidson Sport Glide.