At the same time or soon after, Avon decided to do something about the fact that a tire designated "3.50" was actually a little over four inches wide.
This was a cheap, second rate racing tire- the Dunlop KR series "Triangulars" were essential for serious racing- but the GP was miles ahead of any street legal tyres available then.
So Norton decided to equip the new Commando with a version of Avon's 3.50-19 roadracing tire, the "GP". The lighter weight of the 3.50-19 was also advantageous- heavy wheels and tyres are flywheels and gyroscopes that make a bike hard to accelerate and maneuver. In the 1960s even a hard ridden street bike could stress a good quality 4.00-18 to the point of disintegration. (The biggest problem with racing tyres is usually operating temperature rather than traction- of course, traction fades as a tire overheats). These were the same rolling diameter, but the slimmer carcass of the 3.50-19 ran cooler. The standard big bike rear tire at the time was 4.00-18, but road racers still used 3.50-19. With the introduction of the Commando, the low profile movement took a giant step forward, and the manufacturers took the opportunity to introduce a new sizing system to draw attention to it. No one noticed, because the tire manufacturers didn't do anything to clue us in. Those 3.50 tyres were still three and a half inches high, but the width was creeping out towards four inches. But in the 1960s, the "low profile" concept began to affect motorcycle tyres. It was also three and a half inches tall, but we never really worried about that. But the rest gets complicated.Īt one time, after determining your wheel diameter, one number told you all there was to know: "3.50" meant a tire three and a half inches wide. The last part of a tire size, the wheel diameter, given in inches, is still simple, thank goodness. The first thing we must do is understand how tyres are sized. The trend to much smaller diameter and wider tyres on new motorcycles (which reduces the number of suitable tyres on the market) makes picking new tyres even harder. Selecting tyres for the Commando has always been tricky because of the relatively unusual 19 inch rear wheel and the transition to new and different tire size designation systems right at the time of the Commando's introduction.