So, this is a seemingly simple question, what number do you enter into your bike computers if you ride 700c tires? I’m curious to know how many people actually sit on their bikes and measure the distance required to travel one complete revolution of their bikes’ tires. How many people just take 700 and multiply it by pi? How many people actually measure out the diameter of their wheels with the tire inflated to their preferred psi and then multiply by pi?
Seems simple, but depending on how you measure this number, it could have a pretty big impact on mileage and speed your computers display.
This is my exact question, do most people do rollouts? And, do most people actually change that number depending on whether they have on their race wheels or not, depending on conditions and tire pressures, etc?
I, personally, do a rollout when I get a computer and stick with that number and never think twice about it. Apparently, I should be changing that number depending on my psi and race wheels vs. not.
Your weight and the psi are the biggest part of the equation. Why are you changing psi on your race tires? You’re not one of those “over inflaters” are you? All you are getting is a harsher ride, smaller contact patch and less control, not less rolling resistance.
Yes, most people do the rollout. If you have two different wheels, write down the number or split the difference if they’re close and forget about it.
I, personally, do a rollout with me on the bike. It is usually only a few mm’s off from the “standard” distance from the owner’s manual. But, accuracy can sometimes be important.
I have a whole year of swim data that is off because I was swimming in a 50m pool and logged it all as yards.
Uh, there’s definitely a difference in the actual circumference between a 20c, 23c, or 25c tire, and they’ll even vary somewhat between manufacturers in the same labeled size. I don’t bother with a rollout to account for and weight/pressure differences, just start with whatever is listed in the chart that comes with the unit instructions, and then maybe adjust a couple of ticks up or down after a few rides if it becomes obvious it’s either consistently short or long when comparing against routes of known distance (which is pretty common).
I’m not sure how universal these are but the department of highways around here has some “speedometer test” areas with signs posted every kilometer for 5k. Back in the days of imperial measures it was easy, 1 mile at 60 mph was 1 minute. It’s not so straightforward now with kilometers, but being a gov’t project they keep setting up the signs. Anyway, there’s a test section like that just a little down the road from here. When I get new tires, I ride that section, take note of how closely the bike computer distance matches the posted 5k, and adjust until it’s reasonably close. I don’t get too fussy though because, as mentioned above, lots of stuff like inflation pressure or the weather will make it vary a bit.