Originally Posted by
ksryder
Seeing that my original post was more than a year ago I've long since forgotten the specifics but I eventually got the numbers dialed in by biking a known distance and adjusting the wheel size until the numbers matched. On this particular bike I normally run a 700x38 Specialized Trigger Pro at anywhere from 35-45 psi, I forget exactly how wide they measure with calipers on a TB14 rim but it wasn't exactly 38mm. The wheel circumference I came up with that gave me accurate distance figures was 2120 mm. I think the "official" circumference in the Lezyne manual was something like 2192mm.
This is a good description of what you did.
The "official" number on the table is a guess.
The difference would produce a 3.3% error in the speed (0.5 miles at 15 mph; 1.65 miles at 50 mph).
The 72 mm difference in effective circumference is about 1/2 inch (11 mm) difference in the height of the axle from the road. (Keep in mind that the height we want is with a load on the bike.)
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Different tires, different rim widths, different pressure, different rider weights all mean that a table can't be expected to be very useful (unless you are lucky). The real width of tires is often different than the indicated width (38 mm, in your case).
You don't care about the width of the tire anyway (you care about the height of the tire under load).
The issue isn't the unit. The number (the distanced travelled) really has to be measured.
Originally Posted by
ksryder
I'm finding I have to tell the Lezyne that my wheel is a smaller size than it actually is .
No. The number you used from the table isn't what your wheel size actually is (it's just a guess).
You had to tell the unit the actual real size of your wheel to get the right speed/distance.
Originally Posted by
ksryder
You would think, since it's a GPS unit, they would have programmed in the capability for the computer to automatically figure out the wheel circumference and adjust it, but I guess for >$200 that's what you get. Maybe the new generation of the device does that, I don't know.
There might be inaccuracies and variation with that method too.