Checking Distance

rsheard said:

The guy I ride with and I each have Trek trip computers, but we're getting pretty big discrepancies in our mileage, even though it appears both are set for the correct wheel size. Today's 35-mile ride had us with almost a mile difference. To complicate things, I sometimes log a ride with a GPS unit and/or check it in the car afterwards.

ALL FOUR methods come up with substantially different distances over a ride of 35 miles. What do YOU use to verify your distances?

Unfortunately, it's just one of those things in life you have to live with. It varys because of rider weight, tyre pressure, and EXACT distance among other little differences. If you get two tyres and input the circumference into two computers, one measurement from the instruction book and one from rotating the tyre and measuring from point a to point b, there's one difference. The same two tyres both inflated to 120psi. One guy weighing 260lbs and another weighing 120lbs. The heavier guy will cause more compression on his tyre thus reducing the overall diameter and circumference. Another difference. Two guys going through a corner, one takes the outside line, the other takes the inside line and you have yet another difference. All these little differences over a 35mile ride can make a hell of difference in distance. Throw a car with four big chunky tyres, more weight and a bigger turning circle into the pot and you'll certainly spoil the dinner!!!

So the obvious answer is to use whichever device gives us the longest distance!

😄

shannons dad said:

Unfortunately, it's just one of those things in life you have to live with. It varys because of rider weight, tyre pressure, and EXACT distance among other little differences. If you get two tyres and input the circumference into two computers, one measurement from the instruction book and one from rotating the tyre and measuring from point a to point b, there's one difference. The same two tyres both inflated to 120psi. One guy weighing 260lbs and another weighing 120lbs. The heavier guy will cause more compression on his tyre thus reducing the overall diameter and circumference. Another difference. Two guys going through a corner, one takes the outside line, the other takes the inside line and you have yet another difference. All these little differences over a 35mile ride can make a hell of difference in distance. Throw a car with four big chunky tyres, more weight and a bigger turning circle into the pot and you'll certainly spoil the dinner!!!

rsheard said:

So the obvious answer is to use whichever device gives us the longest distance!

😄

As I see it, exaggerating is completely different to lying.🙂

Could also be affected if one of you tends to cycle straighter whilst the other meanders a little bit from side to side - could add up to a bit over 35 miles. Not that I'm suggesting anyone was drunk!!

Well this all kinda intrigues me - I teach kayak navigation informally. On TOPO! pc mapping, I drew a live down my street end-to-end and it came up 0.99mi, then pedalled same it came up 0.99 mi and was really impressed. (Cateye Astrale 8 computer)

So I think that my Cateye is extremely accurate. Try this, if you can find a straightaway - say 1 mile if possible, using your GPS with waypoints at each end, check your computer. If you're using unit with WAAS accuracy, this 1 mile should be accurate within 10-15 feet.

I don't quite buy the wandering issue, or the riders weight. While I think theres merit to the physics of these I don't think those effects could be "considerable"

Let me know where you get with this. I'll be watching for your outcome. I think if anything, my computer is extremely accurate.

Peter - Plymouth, MA

about the wandering......if a rider wandered a perfect sinusoid that had an amplitude of 1 ft and had a period of 100 feet (that is, every 100 feet the cycle repeated, and it the maximum distance the rider was from the ideal straight line was 1 ft). Well, over 1 mile, that rider would have pedaled 6344.7 ft. I don't know that I'd want to ride w/ that guy, though, what with him wobbling back and forth 1 foot every 100 feet.