difference in speed 2cm makes

ddraver

hey guys - brought me a shiny new cat eye HRM computer which i have now smashed falling off

i can't change the tyre circumfirence to roadie length - stuck at 2068 now but actual length is 2086

now for some reason i can hardly do the maths - am i right - going say 30 kph with the computer set for the mtb wheel...ami going faster or slower on the roadie

my thinking - 1 mtb wheel revoloution = z
- 1 roadie wheel revoloution = z + 2cm
thus so many revoloutions per second (R) - speed = R x z or R x z + 2cm

thus i am going faster than my computer says (albeit not by much)

Have i tied myself in knots completly

fidbod

So your computer thinks your wheel has a circumference of 206.8 cm but actual circumference is 208.6.

for every rotation of the wheel you travel 1.8 cm more than the computer thinks

therefore your computer thinks you are travelling 0.8% slower than you actually are.

Given there are many variables that will affect the accuracy of you computer anyway (tyre pressure, tyre deflection due to rider weight....) I would sugget this level of accuracy is perfectly adequate

ddraver

good good...and if i'm struggling i can use that excuse!!