Tire air pressure vs. speedo-odo readout

[ExFire]

Just a reminder that the actual rolling radius/circumference of a tire changes with increased or decreased inflation.

An odo or speedo programmed to reflect distance traveled per revolution will be accurate ONLY at the actual rolling radius for which it is programmed. This can be shown by carefully measuring the axle center to ground with the bike loaded as when riding. (This measure X2 gives the EFFECTIVE "tire diameter", ...i.e. for an effective tire diameter of 20", the loaded radius would be 10"). Lower pressure (for softer ride?) equals shorter rolling radius, higher pressure equals longer rolling radius. To pick a nit, at some higher speed, centrifugal force may enter the picture(by expanding the rolling tire), but this would be difficult to ascertain, and (hopefully) is not much of an issue at our speeds. (This effect is quite apparent when watching top end dragsters accelerating off the line.)

If we want to be picky about speed/odo accuracy, ...then we have to be picky about calculator input ;-)

 

[Solarman]

With my XP's grossly inaccurate odometer only displaying/recording 82.5% of the actual (measured) miles I ride, the minute difference in tire circumference at varying tire pressure would be pretty insignificant!

 

[Handlebars]

My Spark bike has 26" tires and yet the display was set at 24" which gives an accurate reading of speed according to street radar. More stuff to not know about.

 

[AHicks]

My Spark bike has 26" tires and yet the display was set at 24" which gives an accurate reading of speed according to street radar. More stuff to not know about.

Leaving one wondering about the odometer's ability to record distance accurately?

 

[Handlebars]

Leaving one wondering about the odometer's ability to record distance accurately?

Indeed. But that seems OK too, although I haven't checked that nearly as much - the street radar is everywhere now. I think I will do a stricter test for the odometer in the next couple of days.